Satellite navigation system. General principles of functioning of satellite navigation systems
"Fundamentals of satellite navigation Theory and principles Systems and overview of applications Title Fundamentals of satellite navigation Subtitle Brief..."
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Basics of satellite navigation
Theory and principles
Systems and application overview
Name Fundamentals of satellite navigation
Subtitle
Quick Guide
document
Document ID GPS-X-02007-C
Modification Date Name Status / Comments
SBAS (WAAS, EGNOS)
GPS update
Highly sensitive GPS
AGPS errors and DOP
UTM projection
DGPS services
Data interfaces
GPS receivers
Introduction to satellite navigation
Satellite navigation made easy
Space segment
User segment
GPS message
Position calculation (equations)
DGPS services for real-time correction
Wide Area DGPS
Equipment interfaces
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We reserve all rights in this document and the information contained therein. Reproduction, use and transfer to third parties without permission is prohibited!
For further documents, please refer to www.u-blox.com The performance data given in this document is an estimate only and does not constitute a guarantee of product performance. u-blox does not support weapon related apps. u-blox' products are designed for civilian and commercial aviation and similar applications. In devices or systems where failure of this product could result in damage, use is at your own risk. u-blox reserves the right to change this product, including circuitry and software, to improve performance without prior notice.
u-blox makes no warranties of any kind for the performance in this document. u-blox will not accept claims for damages resulting from the use of this product as documented.
u-blox schemes, software and designs are protected by intellectual property law in Switzerland. u-blox, the u-blox logo, TIM-GPS module type, Antaris, SuperSense, "your position is our focus", NavLox, u-center, AssistNow, AlmanacPlus, FixNow and EKF are registered trademarks of u-blox AG. This product may be subject to intellectual protection in whole or in part. Please contact u-blox for more information. Copyright © 2007, u-blox AG.
Satellite Navigation Basics GPS-X-02007-C Contacts For further information, please refer to the following sources.
Headquarters u-blox AG Zuercherstrasse 68 CH-8800 Thalwil Switzerland Phone: +41 44 722 74 44 Fax: +41 44 722 74 47 E-mail: [email protected] www.u-blox.com Sales offices North, Central and South America Europe, Middle East, Africa Asia, Australia, Pacific
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Satellite Navigation Basics Contacts GPS-X-02007-C Foreword Where Am I on Earth?
The answer to this seemingly simple question can sometimes mean life or death. Look at an aviator trying to find a place to land safely, a captain on an emergency ship in need of help, a mountain hiker lost in bad weather. Your location on Earth is vital and may have different applications.
Despite the rarity of the above dramatic circumstances, there are situations of great importance in our daily life. How to find the required address? The potential applications and uses of location information are endless. Our position on the blue planet will always be very important and today it is something that we can get with amazing convenience.
Among the stunning technological developments in recent years, the developments in satellite navigation or Global Navigation Satellite Systems (GNSS) have been of great importance. Within a few years, satellite navigation has gone from science fiction to science fact with rapidly developing technology around the world dedicated to a reliable and easily accessible way to determine position.
Global leaders are rapidly changing the industry, u-blox AG adds a Swedish touch to precision and quality. The men and women of the company are enthusiastic about their work and their motto is “your position is our focus”. It is part of u-blox AG's commitment to you to provide this guide to help you explore the exciting world of satellite navigation.
The purpose of this book is to provide an overview of the systems running satellite navigation and applications using it. All the latest developments in this field will be considered. This document is intended for users interested in this technology as well as application developers. The document is structured in such a way that there is a gradual transition from simple concepts to complex concepts. The basic theory of satellite navigation will be supplemented with additional important details. This manual serves as an aid to understanding the technology of satellite navigation receivers, modules and ICs. The most important new developments will be presented in various sections. Understanding the various coordinate systems used in GNSS equipment is a difficult task. Therefore, a separate chapter is devoted to cartography.
We hope that this document will help you and you will be fascinated by this technology. Everything connected with it answers the question “where am I on Earth?”.
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In 1990, I traveled by train to the Swiss canton. I had several magazines with me. In one of them, I came across a special article on satellites, which described a new positioning and navigation system. Using several US satellites, this system, known as the Global Positioning System or GPS, could determine coordinates anywhere to within 100 m(*).
As an athlete and mountain lover, I have often found myself in situations where it was necessary to know my location, which becomes possible when using a GPS receiver. After reading the article, I was blown away by the accuracy of the GPS.
Then I began a detailed study of the global positioning system. I instilled my enthusiasm into the students at my university using GPS, and the result was a list of various term papers providing information about the subject. Feeling like a real GPS expert, I sent articles to various magazines and newspapers. Because of my enthusiasm, interest in the system grew.
Basically, a GPS receiver detects only 4 variables: longitude, latitude, altitude, and time.
Additional information (eg speed, direction, etc.) can be obtained from these four components. Evaluation of the development path in which the functions of the GPS system are necessary suggests the development of new attractive applications. If the technical side of the GPS system is well known, then it is possible to develop and use new equipment for navigation and positioning. This book also describes the limitations of the system, so don't expect too much from it.
Before you start, I must warn you about the presence of unknown GPS errors, so you are at risk!
How was this book written?
In 2000, I decided to shorten my lecture time at the university and turn my attention to another area. My goal was to work professionally with GPS and u-blox. The company commissioned me to design a brochure that they would give to their customers. This summary is the result of earlier articles and new chapters.
Sincere Wishes I wish each of you success in your GPS work and I believe that you will be able to easily manage your navigation through these exciting technical features. Enjoy reading!
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Foreword……………………………………………………………………………………….……4 Author’s Preface
1.1 Principle of measurement of transmission time
1.1.1 Basic principles of satellite navigation
1.1.2 Transit time
1.1.3 Position definition
1.1.4 Time error occurrence and correction
2 GNSS technology: GPS example
2.1 Description of the whole system
2.2 Space segment…………………………………………………………………………………………………………………………… ………..19 2.2.1 Distribution and movement of the satellite
2.2.2 GPS satellites……………………………………………………………………………………………………………………… ………….…22 2.2.3 Signal generation from satellite
2.3 Control Segment…………………………………………………………………………………………………………………….… ..27
2.4 User segment
2.5 GPS messages…………………………………………………………………………………………………………………………… ………..…31 2.5.1 Introduction……………………………………………………………………………………………………… ……………………………………..31 2.5.2 Navigation message structure
2.5.3 Information in subframes
2.5.4 TLM and HOW
2.5.5 Division into 25 pages
2.5.6 Comparison of ephemeris and almanac data
2.6 GPS update…………………………………………………………………………………………………………………………… ………..34 2.6.1 New modulation procedure, BOC
2.6.2 GPS Upgrade……………………………………………………………………………………………………………………… …..36
3 GLONASS and GALILEO
3.2 Russian system: GLONASS
3.2.1 Composition of GLONASS
3.3.1 Overview
3.3.2 Main GALILEO services
3.3.3 Accuracy
3.3.4 GALILEO technology
3.3.5 The most important properties of the three GNSS systems
4 Position calculation
4.1 Introduction
4.2 Position Calculation………………………………………………………………………………………………………………………. …48 4.2.1 Principle of signal transit time measurement (pseudo-range estimation) .................. 48 4.2.2 Linearization of the equation
4.2.3 Equation solution
4.2.5 Error analysis and DOP
5 Coordinate systems
5.1 Introduction
5.2 Geoids
5.3 Ellipsoid and data………………………………………………………………………………………………………………………… ...58 5.3.1 Ellipsoid
5.3.2 Modified local ellipsoids and data
5.3.3 National reference systems
5.3.4 WGS-84 Single Reference Ellipsoid
5.3.5 Transformation from local to single reference ellipsoid
5.3.6 Converting coordinate systems
5.4 Coordinates of regions on the plane, projection
5.4.1 Gauss-Krger projection (Transversal Mercator Projection)
5.4.2 UTM projection…………………………………………………………………………………………………………………… ..……….64 5.4.3 Swedish projection system (Conformal Double Projection)
5.4.4 Single coordinate transformation
6 GPS enhancements: DGPS, SBAS, A-GPS and HSGPS
6.1 Introduction
6.2 Sources of GPS error………………………………………………………………………………………………………………………… 68
6.3 Ways to reduce measurement error
6.3.1 DGPS based on propagation delay measurement
6.3.2 DGPS based on carrier phase measurement
Satellite Navigation Basics Contents GPS-X-02007-C 6.3.3 DGPS Post-Processing (Travel Time and Phase Measurement)
6.3.4 Transferring correction data
6.3.5 DGPS classification according to transmitted range
6.3.6 Standards for correction signaling
6.3.7 Overview of the various correction services
6.4 DGPS services for real-time correction
6.4.1 GBAS Services……………………………………………………………………………………………………………………… ………...77 6.4.2 European GBAS services
6.5 Wide Area DGPS (WADGPS)
6.5.1 Enhanced Systems Satellite, SBAS (WAAS, EGNOS)
6.5.2 DGPS satellite services using RTCM SC-104
6.6 Ultimate accuracy with DGPS and SBAS
6.7 Auxiliary-GPS (A-GPS)
6.7.1 Principle A-GPS
6.7.2 A-GPS with online additional data (Real-time A-GPS)
6.7.3 A-GPS with offline supplementary data (allowable orbits)
6.7.4 Reference network
6.8 High Sensitivity GPS (HSGPS)
6.8.1 Improved generator stability…
6.8.2 Antennas
6.8.3 Noise levels
6.8.4 Correlators and correlation time
6.9 GNSS amplifier or reradiating antenna……
6.10 Pseudo-satellites for domestic applications
7 Data formats and hardware interfaces
7.1 Introduction
7.2 Data interfaces
7.2.1 NMEA-0183 interface
7.2.2 DGPS correction data (RTCM SC-104)
7.2.3 Private data interfaces
7.3 Equipment interfaces…………………………………………………………………………………………………………………….105 7.3.1 Antennas
7.3.3 Clock pulse: 1PPS and time systems
7.3.4 Converting TTL level to RS-232
8 GNSS receivers
Satellite Navigation Basics Contents GPS-X-02007-C
8.2 GNSS receiver modules
8.2.1 GNSS module basic design
8.2.2 Example: u-blox 5
9 GNSS applications
9.1 Introduction
9.2 Description of various applications
9.2.2 Business and industry
9.2.3 Communication technology
9.2.4 Agriculture and forestry
9.2.5 Science and research
9.2.6 Tourism / Sport
9.2.7 War department
9.2.8 Time measurement
A Resources on the World Wide Web
A.2 Differential GPS………………………………………………………………………………………………………………….125 A.3 GPS institutes
A.4 GNSS antennas……………………………………………………………………………………………………………………… …………….126 A.5 GNSS groups and GNSS technical journal
B Pointer
B.1 List of Figures
B.2 List of tables
B.3 Sources
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1. First, precise positioning (longitude, latitude and altitude coordinates) is provided within a range of 20 m to approximately 1 mm.
2. Precision time (UTC), its accuracy ranges from 60 ns to about 5 ns.
The speed and direction of movement can be obtained from these coordinates. The values of coordinates and time are determined by the satellites of the Earth.
Fig.1 Main Function of Satellite Navigation In 2007, the Global Positioning System (GPS) developed by the United States Department of Defense (DoD) was the only full-fledged working GNSS system. The booming satellite navigation industry focuses mainly on the GPS system, which is why the terms GPS and satellite navigation are sometimes used interchangeably. This document will cover other GNSS systems.
GPS (full name: Navigation and Global Positioning System, NAVSTARGPS) was developed by U.S. Department of Defense (DoD) and can be used by both civilians and the military. The civilian SPS (standard positioning) signal can be used by everyone, while the military PPS (precision positioning) signal can only be used by special agents. The first satellite was placed into orbit on February 22, 1978, and there are currently 28 operational satellites at an altitude of 20,180 km in 6 different orbits. Their orbits deviate by 55 0 to the equator, while the last 4 satellites provide radio communication with any point on the planet. The orbit of each Earth satellite is approximately 12 hours, and it has 4 atomic clocks on the board
During the development of the GPS system, the main focus was on the following three aspects:
1. It should provide consumers with the ability to determine position, speed and time in motion or at rest.
2. It must provide continuous 3D positioning with a high degree of accuracy, regardless of the weather.
3. It must be able to be used by the civilian population.
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The brief guide will review the basic principles of satellite navigation and their application to applications and technologies. GPS will be the main focus due to the industry standard, and developments such as Differential-GPS (DGPS), Assisted-GPS (AGPS) and device interfaces will be covered in various sections. All this is done to provide the reader with fundamental information about such a fascinating field.
Rice. 2 GPS satellite launch Among them, aviation, satellite navigation takeoffs and landings become possible.
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1.1 Signal transit time measurement principle For a while during a thunderstorm night you no doubt wondered how far you were
By a flash of lightning. The distance can be set fairly easily (Fig. 3): distance = the moment the lightning flashes (start time) to the appearance of thunder (end time) multiplied by the speed of sound (approximately 330 m/s). The difference between the start and end times is the transit time.
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Fig.3 Determination of distance by lightning flash Distance = transit time * speed of sound The GPS system operates according to the same principle. In order to calculate the exact position, all you need to do is measure the signal transit time between the observation point and four other satellites whose positions are known.
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All satellite navigation systems use general principles for determining coordinates:
Satellites with a known position transmit a regular signal.
Based on the measurement of the propagation time of radio waves (electromagnetic signals propagate at the speed of light c = 300’000 km/s), the position of the receiver is calculated.
Here we see the principles most often applied in simple models. Imagine that we are in a car and want to determine our location on a long and straight street. There is a radio transmitter at the end of the street that sends out a clock pulse every second. The car has a clock that is synchronized with the radio transmitter's clock. By measuring the time from the transmitter to the car, we can determine our position on the street (Fig. 4).
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Fig.4 In the simplest case Distance is determined by the propagation time Distance D is calculated by multiplying the propagation time by the speed of light c.
D = c Since the clock synchronization in the car with the transmitter is not perfect, there is a difference between the calculated distance and the actual distance. In navigation, this incorrect value sounds like a pseudo-range. In our example, the timing error is 1 microsecond (1µs) and defines a pseudorange of 300m.
We could solve this problem by equipping our car with an accurate atomic clock, but this will significantly affect our budget. Another solution would be to use a second synchronized transmitter whose distance is known. By measuring both propagation times, the distance can be accurately determined despite the inaccurate onboard clock.
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Fig.5 With two transmitters, it is possible to calculate the exact position despite time errors As can be seen, in order to accurately calculate the position and time along the line (assuming that the line continues in only one direction), we need two time transmitters. From this we can draw the following conclusion: with an unsynchronized on-board clock used in calculating the position, the number of time transmitters needed exceeds the number of unknown measurements by one.
Example:
On a plane (two dimensions) we need three time transmitters.
In 3D space, we need four time transmitters.
Satellite navigation systems use satellites as transmitters of time signals. Communication with the last 4 satellites (Fig. 6) is necessary to determine the three-dimensional coordinates (Longitude, Latitude, Height) throughout the entire time. We will look at this in more detail in the following sections.
Fig.6 4 satellites are needed to determine the Longitude, Latitude, Altitude and Time
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Satellite navigation systems use high-lying satellites, which are placed in such a way that from any point n on the earth it is possible to draw a line to at least four satellites.
Each of these satellites has up to four atomic clocks on board. Atomic clocks are currently the most accurate instrument, losing a maximum of one second every 30,000 out of 1,000,000 years. To make them even more accurate, corrections or synchronizations are made from various control points on Earth. Each satellite transmits its exact position and exact time to Earth at a frequency of 1575.42 MHz. These signals travel at the speed of light (300,000 km/s) and therefore take approximately 67.3 ms to reach the earth's surface directly below the satellite. The signal needs 3.33 for each additional kilometer. If you want to establish your position on land (or at sea or elsewhere), all you need is an accurate clock. When comparing the time of receiving a satellite signal with the time of sending, it is possible to determine the transit time of this signal (Fig. 7).
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Fig.7 Determination of signal transit time
As in the car example, the Distance D to the satellite can be determined using the transit time:
Distance = travel time * speed of light
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Imagine that you are walking across a vast plateau and want to know where you are. The two satellites above you are broadcasting their ship times and positions. Using the signal transit times of both satellites, you can draw two circles with radii D1 and D2 around the satellites. Each radius corresponds to a distance calculated by the satellite. All possible distances to the satellite are located on the circumference of the circle. If the position above the satellites is excluded, the receiver position is at the intersection of the circles below the satellites (Fig. 8).
Two satellites are enough to determine the position on the X/Y plane.
Fig.8 Receiver position at the point of intersection of two circles
In reality, the position must be defined in 3D space, not on a plane.
The difference between a plane and a 3D space is the extra dimension (Z-height), an extra third satellite must be available to determine the actual position. If the distances to the three satellites are known, then all possible positions are located on the surface of three spheres whose radii correspond to the calculated distances. The desired position is the intersection of all three spheres (Fig. 9).
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We have assumed so far that the measurement of signal transit time was accurate. However, this is not true. The receiver needs synchronization to accurately measure time. If the transit time has an error of 1 ns, the positional error will be 300 m. The clocks on board all three satellites are synchronized, and the transit time measurement error is added. Mathematics is the only thing that can help us. Recall that if there are N unknown variables, then we need N independent equations.
If the time measurement is accompanied by a constant unknown error, we will have four unknown variables in the 3-space D:
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It follows from this that in 3-dimensional space 4 satellites are needed to determine the exact position.
Satellite navigation systems are designed in such a way that at least 4 satellites can be seen from any point on Earth (Fig. 10). Thus, despite the error of the receiver's clock and time errors, the position is calculated with an accuracy of approximately 5 - 10 m.
Fig.10 4 satellites are needed to determine the position in 3-D space
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If you like...
o understand why 3 different GPS segments are needed o know that each segment has a function o know how a GPS satellite is made o know what kind of information is transmitted to earth o understand how a satellite signal is generated o understand how signal transit time is determined o understand the importance of correlation o understand why a minimum period of GPS time is needed to work online o know what frames and subframes are then this chapter is for you!
2.1 Description of the system
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The global positioning system (GPS) includes 3 segments (Fig. 11):
Spatial segment (all working satellites)
Control segment (all ground stations of the system: main control and additional for control)
User segment (all civil and military GPS users)
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Rice. 11 Three GNSS segments As can be seen from Figure 11, there is a unidirectional relationship between the spatial segment and the user segment. Control stations on the ground have a bi-directional connection with the satellites.
2.2 Space Segment 2.2.1 Satellite Movement The space segment currently consists of 32 active satellites (Fig. 12) orbiting in 6 different planes (four to five satellites per plane). They are located at an altitude of 20.180 km above the Earth's surface and are inclined at 550 to the equator. Each satellite completes an orbit in 12 hours. Due to the rotation of the Earth, the satellite will be in its home position (Figure 13) after approximately 24 hours (23 hours 56 minutes to be exact).
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Satellite signals can be received within the effective range of the satellite. Rice. 13 shows the effective range (shaded area) of a satellite directly above the prime meridian.
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The distribution of satellites at any given time can be seen in Fig. 14. It is a consequence of the successful allocation of orbits at high altitude to ensure communication with at least 4 satellites at any time in the world.
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2.2.2.1 Satellite design All satellite time signals and data are synchronized by the atomic clocks on board on the same frequency (1575.42 MHz). The minimum signal length received on Earth is approximately -158dBW to OdBW [According to the specification, the maximum length is approximately -153dBW].
Rice. 15 GPS satellite 2.2.2.2 Link analysis Link analysis (Table 1) between the satellite and the user is needed to set the desired transmit power level. According to the specification, the minimum power should not be lower than -16OdBW (-13OdBm). To ensure this level is supported, the transmit power of the L1 satellite, modulated with the C/A code, must be 21.9 W.
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The following information (navigation message) is transmitted by the satellite at 50 bits per second.
Satellite timing and time signals
Accurate data (ephemeris)
Correction information for determining the exact time
Approximate satellite data (almanac)
Data on the ionosphere
Satellite Status Information The time it takes to transmit this information is 12.5 minutes. Using the navigation message, the receiver is able to determine the time of transmission of each signal and the exact position of the satellite at the time of transmission.
Each of the 28 satellites transmits a unique signature. This signature consists of an arbitrary sequence (Pseudo Random Code Noise, PRN) of 1023 zeros and ones (Fig. 17).
Fig.17 Pseudo-random noise
The last millisecond is a unique identifier that repeats continuously and has two purposes in relation to the receiver:
Identification: A unique signature means that the receiver knows which satellite it received the signal from.
Measuring Signal Transit Time 2.2.3 Satellite Signal Generation 2.2.3.1 Block Diagram The satellites carry four very precise atomic clocks. The following clock pulses and frequencies, necessary for daily operation, are derived from the resonant frequency of the atomic clock (Figs. 18 and 19):
Data frequency 50 Hz
C/A code pulse that modulates data using an exclusive process3 (in the range above 1 MHz)
L1 Civilian Carrier Frequency (1575.42 MHz) C/A code modulated data is in turn modulated on the L1 carrier using BiPhase-Shift-Keying (BPSK)4. With each change in the modulated data, there is a 1800 turn in the L1 carrier phase.
A logical operation with two operands that results in the boolean value true if and only if one of the operands is true.
A method of carrier wave modulation in which the broadcast data is rotated in phase by 90°.
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Satellite navigation signals are generated using a process known as DSSS (Direct Sequence Spread Spectrum) modulation. This is a procedure in which the nominal bandwidth (not to be confused with the bandwidth of the receiver chip) is deliberately wider, matching the higher signal frequency. This modulation principle was discovered in 1940 in the USA by actress Hedy Lamarr and pianist George Anthell. This process allows a closed radio channel to work in difficult environments.
The atomic clock on board the satellite has a stability of more than 2*10-13. The fundamental frequency of 10.23 MHz comes from the resonant frequency of one of the atomic clocks. In turn, the carrier frequency, data frequency, pseudo-random noise (PRN) generation time and C/A code are derived from the fundamental frequency (Figure 20). That is, all 28 satellites are transmitting at 1575.42 MHz using a process known as CDMA Multiplex5 (Code Division Multiple Access). Data is transmitted based on DSSS modulation. The C/A code generator has a frequency of 1023 MHz and a period of 1.0237 which corresponds to ms. The used C/A code (PRN code), which is similar to the golden code8 and has good correlation properties, is generated by the feedback shift register6.
The modulation process described above is called DSSS modulation, with the C/A code playing an important role in this process. Since all satellites transmit on the same frequency (1575.42 MHz), the C/A code contains the identification and information generated by each satellite. The C/A code is an arbitrary sequence of 1023 bits called pseudo-random noise (PRN). This signature, which lasts a millisecond and is unique to each satellite, is constantly repeated. Therefore, the satellite is always identified by the corresponding C/A code.
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Fig.20 Detailed GPS satellite block system A form of multiplexing that divides radio channels using a pseudo-random sequence for each user. CDMA is a form of "spreadspectrum" signal in which the modulated code signal has a greater frequency width than the transmitted data.
A shift register in which the input bit is a linear function of the previous state.
Transmission time for individual bits in a pseudo-range sequence.
The golden code is the setting of the binary sequencesT. Take two sequences of the same length n, such that they have only three common values. The set of n exclusive-ors operations of two sequences in different phases (that is, relative to all positions), together with the sequences themselves, is the Golden Code. An exclusive or operation of two Golden Codes will produce another Golden Code in some phase.
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The Control Segment (OCS) consists of a main control station located in Colorado, five control stations equipped with atomic clocks located around the globe near the equator, and three control ground stations that transmit information to satellites.
The most important tasks of the management segment:
Observation of the movement of satellites and processing of orbital data (ephemeris)
Monitoring the satellite clock and its operation
Satellite time synchronization
Retransmission of accurate orbital data received from satellites in communication
Retransmission of approximate orbital data received from all satellites (almanac)
Retransmission of further information including satellite status, time errors, etc.
The control segment also monitors artificial signal distortion (SA) in order to limit the positional accuracy of the system when used by civilians. The accuracy of the system is deliberately limited until May 2000 for political and tactical reasons by the US Department of Defense (DoD) for satellite operators. The restriction can be turned off during May 2000, but can be started again if necessary.
2.4 User segment
Signals transmitted by satellites take approximately 67 ms to reach the user.
Although the signals travel at the speed of light, their transit time depends on the distance between the satellites and the consumer.
Four different signals are generated at the receiver and have the same structure as those received from 4 satellites. When these signals are synchronized, a time shift t is formed (Fig. 21).
The measured time offsets t on all 4 satellite signals are used to determine the signal transit time.
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Receiver Signal (Synchronized Receiver Time Offset Fig.21 Measuring Signal Transit Time To determine the user's position, radio contact with four other satellites is required.
The distance to the satellites determines the transit time of the signals. The receiver then calculates the user's position: latitude, longitude, altitude h and time t from the range and known position of the four satellites. In mathematical terms, this means that the four unknown variables, h and t, are determined using the distance and position of these four satellites, although a fairly complex level of iteration is required, which will be important later on.
Satellite Navigation Basics GNSS Technology: GPS Example GPS-X-02007-C p.27 As mentioned earlier, all satellites transmit on the same frequency but with a different C/A code. This process is called Code Division Multiple Access (CDMA). Signal recovery and satellite identification occurs through correlation. Since the receiver can learn all the C/A codes that are in use, systematically shifting and comparing each code with all incoming signals from the satellite will result in a perfect type match (i.e., a correlation score of CF - 1), and the correlation point will be reached. (Fig. 22). The correlation point is used to measure the actual signal transit time and to identify the satellite.
Input signal from PRN -18 bits 11 - 40 Input signal from PRN -18 bits 1 - 30, initial
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Incoming signal from PRN -18 bits 21 to 50, last Cross signal from PRN -5 bits 11 to 40, in phase The CF value ranges from minus one to plus one and is positive only when the signal types (bit frequency and phase) match.
mB: number of all bits that matched uB: number of all bits that did not match N: total number of bits As a result of the Doppler effect (all satellites and receivers move relative to each other), it is possible for transmitted signals to shift by ±6000 Hz from the receiving point. Determining the signal transit time and recovering the data requires not only correlation with all possible codes and offset phases, but also identification of the correct carrier phase. Through a systematic shift and comparison with all codes (Fig. 22) and a carrier frequency with incoming satellite signals, we find the desired point (at which the correlation factor is 1) (Fig. 23). The desired position in the carrier frequency is known as binary.
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Rice. Fig. 23: Investigation of the maximum correlation of code and carrier frequency interval The power spectral density of the received GPS signal lies about 16 dB below the thermal noise (see Fig. 16).
Demodulation and concentration of the received GPS signal gives the system gain GG:
After concentration, the power density of the used signal becomes greater than the thermal noise (Fig. 24).
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Doubling Dwell Time increases the difference between signal and thermal noise (receiver sensitivity) by 3 dB. In practice, it is not a problem to increase the correlation time to 20 ms. If the value of the transmitted data is known, then this time can be increased even further.
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2.5.1 Introduction A message is a continuous stream of data transmitted at 50 bits per second. Each satellite transmits the following information to Earth:
System time and adjusted clock values
Proprietary high precision orbital data (ephemeris)
Approximate orbital data for all satellites (almanac)
System status, etc.
The navigation message is needed to calculate the current position of the satellites and to determine the transit time of the signal.
The data stream is modulated by the HF carrier wave of each individual satellite. Data is transferred in logically grouped blocks called frames or pages. Each frame is 1500 bits long and takes 30 seconds to transmit. Frames are divided into 5 subframes. Each subframe is 300 bits long and takes 6 seconds to transmit. It takes 25 different frames (or pages) to transmit the entire almanac. The transmission time for the almanac is 12.5 minutes. The GPS receiver must receive the entire almanac in order to function (eg for its initial initialization).
2.5.2 Navigation message structure
A frame of 1500 bits takes 30 seconds to transmit. The 1500 bits are divided into five 300 bit subframes (transmission time 6 seconds). Each subframe is in turn divided into 10 words, each of which is 30 bits long. Each subframe begins with a telemetry word and a handover (HOW) word. A complete navigation message consists of 25 frames (pages). The structure of the navigation message is illustrated in Fig. 25.
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2.5.4 TLM and HOW The first word of each frame, the telemetry word (TLM), contains an 8-bit preamble sequence (10001011) which is used for synchronization, the next 16 bits are reserved for registered users. As with all words, the final 6 bits of the telemetry word are parity bits.
The Handover (HOW) word immediately follows the telemetry word in each subframe. The Handover word has 17 bits (the range of values from 0 to 131071 can be represented by 17 bits) and contains within its structure the start time for the next subframe, which is transmitted as the time of the week (TOW). The TOW counter starts at 0 at the beginning of the GPS week (transition period from Saturday 23:59:59 to Sunday 00:00:00 hours) and increments by 1 every 6 seconds. Since there are 604.800 seconds in a week, the counter runs from 0 to 100.799, then resets to zero. The token inserts itself into the data stream every 6 seconds and transmits a HOW to synchronize with the P code. Bits 20..22 are used in the Handover word to identify the subframe just transmitted.
2.5.5 Split into 25 pages A complete navigation message requires 25 pages and takes 12.5 minutes. The page or frame is divided into five subframes. In the case of subframes 1..3, the content is the same for all 25 pages. This means that the receiver has all clock values and ephemeris data from the transmitting satellite every 30 seconds.
The only difference in the case of subframes 4 and 5 is in the organization of the transmitted information.
In the case of subframe 4, pages 2, 3, 4, 5, 7, 8, 9, and 10 relay almanac data from satellite numbers 25 to 32. In this case, one satellite's almanac data is transmitted per page.
Page 18 transmits correction values due to ionospheric scintillation as well as the difference between UTC and GPS time. Page 25 contains information about the configuration of all 32 satellites (i.e. block connection) and the status of satellites with numbers 25 ... 32.
In the case of subframe 5, pages 1…24 relay almanac data from satellites numbered 1 to
24. In this case, the almanac data for one satellite is transmitted on one page. Page 25 transmits information about the status of satellites 1…24 and the actual time of the almanac.
2.5.6 Comparison of almanac and ephemeris data
Using ephemeris and almanac data, satellites move in orbits and, therefore, the coordinates of a particular satellite can be found at any time. The difference between the values passed is mainly in the precision of the numbers. The following table (Table 2) compares the two numbers.
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Table 2 Comparison of almanac and ephemeris data For an explanation of the terms used in Table 2, see Fig. 2. 26.
Fig.26 Ephemeris terms Major semi-axis of the orbital ellipse: a Major semi-axis of the orbital ellipse: b Eccentricity of the orbital ellipse: e
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In order for all satellites to transmit the same frequency, GPS signals are modulated with a special code. This code consists of a Pseudo Random Noise Code (PRN) of 1023 zeros or ones and is known as a C/A code. A code with a period of 1 ms has a transmission rate of 1.023 Mbps. The code repeats continuously and, due to its unique structure, allows the receiver to identify its signal from each satellite.
Modulation of the data signal is achieved using the exclusive-or (EXOR) operation (Fig. 27). The result is called Binary Phase Shift Keying (BPSK(1)). The nominal or base frequency signal is generated by one of the atomic clocks and all satellite signals are derived from it. The nominal or base frequency is then modulated with the C/A Code at a rate of 1 1.023 Mbps.
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Rice. 27: With BPSK, the navigation data signal is the first code modulation In the future, GPS and European GALILEO systems will use a new modulation process called Binary Offset Code Modulation (BOC). With BOC BPSK the signal is further modulated. The modulation frequency is always a multiple of the base frequency of 1.023 MHz. The properties of this modulation are transmitted in a specific way. For example, BOC(10.5) means that the modulation frequency is 10 base frequencies (10 1.023 MHz) and the C/A code rate is five times the base frequency (5 1.023 Mbps) (Figure 28).
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Rice. 28: Modulation in the future: BOC(10.5) With BOC, the signal will spread better over the passband and the influence of the reflected signal on the navigation signal receiver will be less compared to BPSK. With the simultaneous use of BPSK(1) and BOC(1,1), their influence on each other is practically absent, since the power density maxima are separated (Fig. 29).
Power Spectral Density (dBm/Hz)
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Rice. Fig. 29: With BPSK(1) and BOC(1,1) signal maxima are separated (signal intensity is 1 W) Satellite navigation basics GNSS technology: GPS example GPS-X-02007-C p.35 2.6.2 GPS upgrade
Since the activation of the GPS system in 1978, all satellites have been transmitting the following three signals to Earth:
On L1 frequency (1575.42 MHz): one civilian signal (SPS service with C/A signal, BPSK(1)) and one military signal (PPS service with P(Y) signal, BPSK(10))
On the L2 frequency (1227.60 MHz): the second military signal.
U.S.DoD plans to improve the structure of the GPS signal (Fig. 30). For civilian applications, the introduction of the second and third frequencies is very important; then more frequencies can be used to establish the position, while the influence of the ionosphere on the signal transit time can be reduced or even eliminated. This compensation is possible because the ionospheric transmission rate c is frequency dependent. In addition to the two new signals, the GPS upgrade will provide an increase in signal strength for civilian users, providing capabilities similar to military applications.
On September 25, 2005, the first of eight new IIRM satellites (Block 2, Replenishment and Military) was delivered into orbit. On December 16, 2005, the satellite was ready for transmission. The launch of the remaining seven satellites will begin before 2006. These new satellites additionally transmit the following:
The new civil signal is 1227.60 MHz, the so-called L2C frequency.
Auxiliary military signals 1575.42 MHz and 1227.60 MHz: M signals using BOC(10.5) modulation.
A new generation of satellites is planned for the end of this decade. The new series will be designated IIF (Block 2, Follow-ON) and III (Block 3).
Below are the most important characteristics of these satellites:
New civil signal 1176.45 MHz (L5 frequency). This signal is more stable and can be used in aviation during critical landings.
Increasing signal intensity M (= M+) through the use of concentrating beam antennas.
Improvement of C/A signal structure for civil frequency L1. (defined as L1C).
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Rice. 30: With the upgrade, the number of available GPS frequencies has been increased GPS ground stations will also be upgraded. This development should be fully completed and put into operation by the middle of the next decade. New signals will become available to users.
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On December 28, 2005, the first GALILEO satellite was launched into orbit. The satellite, called GIOVE-A, began a new era. At first, satellite navigation was also actively introduced in Europe. GPS must have competition: there will probably be three independent GNSS systems available within the next five or six years. USA will continue to support GPS, Russia and the European Union will use their GLONASS and GALILEO systems. With three functioning GNSS systems, we will have the possibility of more accurate positioning, and there will also be differences.
GPS will also be upgraded in the foreseeable future and become more accessible (see 2.6).
This chapter looks at the not yet fully operational GLONASS system and Europe's future GALILEO system.
Basics of satellite navigation GLONASS and GALILEO GPS-X-02007-C p.37
3.2 Russian system: GLONASS GLONASS is the abbreviation for the GNSS system currently used by the Russian Ministry of War. The name GLONASS stands for Global Navigation Satellite System. The program first started in the Soviet Union and is now under the Commonwealth of Independent States (CIS). The first three test satellites were launched into orbit on October 12, 1982. The most important characteristics of this system are:
24 planned satellites (21 standard + 3 standby). There has never been such a number. The relatively short lifetime of individual satellites of 3 to 4 years prevented the completion of the system.
3 orbital levels with an angle of 64.8° from the equator (this is the highest angle of all GNSS systems, it allows you to have good reception in the polar regions.
Orbital altitude 19,100 km
Orbital period 11 h 15.8 min
Each GLONASS satellite transmits two codes (C/A and P-code) on two frequencies. Each satellite transmits the same codes (PRN) but at different frequencies between 1602 MHz and 1246 MHz. These associated frequencies must be changed subsequently.
3.2.1 Composition of GLONASS
The GLONASS system requires 24 working satellites. Due to political instability in the Soviet Union and other delays and problems, on August 18, 2006, only 14 operational satellites are in orbit. The CIS plans to finalize the system by the end of 2008. Three replacement satellites were successfully launched on December 25, 2005. Two of the three satellites belong to the M series, which has a lifetime of 7-8 years. These new satellites transmit two civilian signals. After 2007, the first K-series satellite will be launched. Its life span has been extended to 10-12 years, and it transmits three civilian signals.
Rice. 31: GLONASS-M satellite (Source ESA) 32: Proton Carrier Launch
Basics of satellite navigation GLONASS and GALILEO GPS-X-02007-C p.38
3.3 GALILEO 3.3.1 Overview GALILEO is a European GNSS system. The European Union (EU) together with the European Space Agency (ESA) developed this system. The EU and ESA together form an umbrella organization: the GALILEO Joint Venture (GJU, headquartered in Brussels). GJU oversees and coordinates all stages of development, testing and implementation. GJU guarantees its responsibility for the administration of this program. The governments of Germany, Italy, France, UK, Spain and Belgium together bear approximately 85% of the costs.
GALILEO will consist of a constellation of satellites in three cyclic orbits at an altitude of 23.616 km above the Earth. These satellites are supported by a network of ground stations.
Key arguments for the introduction of GALILEO:
Achieving independence from the United States. There are only two satellite navigation systems: American GPS and Russian GLONASS. Both were conceived with military purposes. At the moment, the Russian system does not have civilian applications available, so the European GALILEO will be the only alternative to the monopoly of GPS and American industry. GPS is controlled by the US government, which can limit or deactivate the system in the event of a crisis. This submission to the Americans does not suit the Europeans. However, the US military has said it is ready to destroy the GALILEO system in the event of a threat to US security.
Increased positioning accuracy. GALILEO is planned to be more accurate than GPS. This will expand the capabilities, provide accuracy in the range from 4 to 15 m. The security service will have an accuracy of 4 to 6 m. The sensitivity of receiving the reflected signal will also be reduced. This advantage will be achieved through BOC modulation. GPS will also add BOC after upgrading.
Obtaining only a civil navigation system. GALILEO will be developed according to civilian purposes; however, it can also provide security features. In contrast to the military-oriented GPS, GALILEO guarantees the functionality of private services.
Providing more options. GALILEO will have five different roles. For comparison, GPS currently has only two. In the case of modernization, the number of GPS capabilities for the civilian population will increase.
Providing a search and rescue function. Search and rescue functions have already been offered by other organizations. New in GALILEO - alarm confirmation.
Increased security through message integration. GALILEO will be more reliable as it has message integration. This will immediately notify the user about errors in the work. Above that is a guarantee of availability. For an open service, there was no guarantee of availability or messaging integration. These features are only available through EGNOS.
Employment. Experts estimate that by 2020 the European GALILEO satellite system will provide 130,000-180,000 jobs. With an initial investment of six billion euros (originally estimated at three), GALILEO will pay for itself and bring in a profit of seventy-four billion euros.
GNSS know-how. Most manufacturers of satellite navigation systems are located in the United States. Satellites and accessories for them, navigation receivers, measuring instruments, etc. mainly developed and sold outside of Europe. With GALILEO, Europe must gain experience and provide domestic industry with qualified personnel.
Improved coverage of satellite signals. GALILEO will provide better reception in cities with high latitude. This is possible because the GALILEO satellites have orbits at an angle of 56° from the equator at an altitude of 23.616 km.
European Geostationary Navigation Overlay Service GLONASS and GALILEO Satellite Navigation Basics GPS-X-02007-C p.39 In addition, modern GNSS receivers can distinguish between GPS and GALILEO signals. This increases the number of visible satellites from which signals can be received, thereby expanding coverage and improving accuracy.
3.3.2 Planned GALILEO Services For critical applications, GALILEO will provide system health information to ensure positioning accuracy. Health refers to the reliability of the data provided. Users will be promptly (within 6 seconds) alerted when the system's accuracy falls below this minimum. GALILEO operators believe that these warnings are sufficient even for critical applications (eg aircraft landings). Each service provides different requirements for functionality, accuracy, availability, health, and other parameters.
3.3.2.1 Open Service, OS Open Service (OS) is offered for mass applications. It provides free signals to determine position and time. Applications with low accuracy requirements will use inexpensive single frequency receivers. Since the transmitted frequencies from GALILEO and GPS (L1) are the same for such an application, the navigation receivers will combine the signals. Due to the increase in the number of satellite signals, the reception level will be improved even in poor conditions (for example, in a city). OS does not provide system health information, and GALILEO operators do not guarantee availability and cannot be held liable.
3.3.2.2 Commercial service, CS Commercial service is provided for market applications with higher requirements than OS. CS is designed to provide a range of useful services to its users in exchange for a fee. Typical examples of such applications are services with high data rates, guaranteed availability, accurate service times, and local signal correction for maximum positioning accuracy.
3.3.2.3 Safety Service, SoL This service is intended for transport applications where navigation degradation without warning is life threatening. The first difference from OS is the high level of system integrity information provided for applications such as maritime navigation, aviation and railways. This service is only available with a certified dual frequency receiver. SoL will use aeronautical links (L1 and E5) to achieve the necessary signal protection.
3.3.2.4 General Regulated Service, PRS GALILEO is a civilian system that will also provide stability and protection services for government (including military) purposes. General Regulated Service (PRS) is available to clients such as police, fire departments and border patrols. Access to the service is restricted and controlled by a civilian agency. The PRS must always be available regardless of the conditions, especially during crisis situations in which other services can be destroyed. PRS will be independent from other services and will have a high level of signal stability. The PRS will also be immune to electronic jamming.
3.3.2.5 Search and Rescue, SAR The SAR service will be used for search and rescue. Emergency transmitters and satellites will show the location of individuals, vehicles, on land and water in emergency situations. In the late 1970s, the United States, Canada, the USSR, and France developed a satellite system to house distress beacons. The system was called SARSAT (Search And Rescue Satellite Aided Tracking). The Russian name for the system is “COSPAS”. The COSPAS-SARSAT system uses six LEO (Low Earth Orbit) and five GEO (geostationary) satellites. The GALILEO-SAR service plans to expand and improve the existing COSPAS-SARSAT system[x] in the following ways:
Immediate reception of an alarm signal from anywhere on Earth (there are now delays of about an hour).
Basics of satellite navigation GLONASS and GALILEO GPS-X-02007-C p.40
Precise determination of the position of emergency beacons (up to meters, and not with the current accuracy of 5 km).
Improvement of space segment efficiency by increasing the number of available satellites to overcome interference (30 GALILEO satellites in medium orbits will be added to the existing LEO and GEO satellites of the COSPAS-SARSAT system).
GALILEO will introduce a new SAR feature; repeated alarm (from SAR operator via emergency radio). This should make rescue easier and reduce false alarms.
The GALILEO SAR service will work in conjunction with the COSPAS-SARSAT system, with features and functions administered by IMO (International Maritime Organization) and ICAO (International Civil Aviation Organization).
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Depending on the service, GALILEO will provide different levels of accuracy. When using a dual frequency receiver, accuracy will be better by compensating for signal transit time errors due to ionospheric conditions. When using local measurements (i.e. DGPS), accuracy can increase to centimeters. Table 3 shows the expected accuracy in 95% of measurements.
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Rice. 34: Constellation of GALILEO satellites (image: ESA-J.Huart) GALILEO satellites have a lifetime of 15 years. The required power of 1500 W will be generated by large solar panels. To support current navigation data, the satellites will have radio contact with the ground segment of the system at intervals of 100 minutes.
Rice. 35: GALILEO satellite (picture: ESA-J.Huart)
The ground segment of the system will consist of a series of control centers forming a global network of stations for various tasks. This includes monitoring signal integrity and coordinating search and rescue services.
Control centers are planned for navigation and control of satellites. The core of the ground segment will consist of two GALILEO control centers in Germany and Italy.
GLONASS and GALILEO Satellite Navigation Basics GPS-X-02007-C p.43 The German Aerospace (DLR) Center at Oberpfaffenhofen will be the main control center. From here, control over the normal operation of 30 satellites is planned for 20 years. The second center with its specific responsibilities for monitoring the operation of the satellites will be the center in Fucino in Italy. It is also a backup in case of any problems on the main one. Positioning control for 30 satellites will be shared between the European Satellite Control Center (ESA/ESOC) in Darmstadt, Germany and the French National Space Studies Center (CNES) in Toulouse, France. A chain of 30 monitoring stations (IMS) located in all countries will manage the integrity of the satellite signals. The two control centers will evaluate the IMS information and give an alarm if there is a strong deviation of the position data.
Three Arianne 5 rockets are planned, each carrying eight satellites (Fig. 36), and three Soyuz rockets, each carrying two GALILEO satellites, the rockets transport the satellites to the mid-Earth orbit (MEO).
Rice. Figure 36: Ariane 5 rocket delivers 8 GALILEO satellites into space (GALILEO-industries.net) 3.3.4.1 Signal Frequencies There will be different frequencies, modulation shapes and data rates depending on the service (See Table 4 and Figure 37). The principal modulation forms will be BPSK and BOC. The exceptions are E5a and E5b, which use a modified version of the BOC modulation called AltBOC.
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Additionally, E5a, E5b, E6 and L1 transmit a pilot channel. The pilot channel has no navigation data and is out of phase by 90° with respect to other signals. This reduces the satellite acquisition time.
Above the L1 band, GALILEO and GPS need to divide the frequencies. In this band, GPS has 3 signals: C/A signal, P(Y) signal and new M signal. GALILEO will only use two signals: the L1B signal and the E2/E1 pair. The shared use of this frequency band created intense conflict at the time. This was until, in June 2004, the US and the EU came to an agreement on the forms of communication and modulation on these frequencies.
On Fig. 38 shows the power density of the signal at the frequency L1, we assume that the power of all signals is the same (standard value is 1 W).
Rice. Figure 38: L1 band will be heavily used by GALILEO and GPS (Power density is typ. 1W per signal) 3.3.4.2 Planned dates On 26 June 2004, after difficult negotiations, the US and EU reached an agreement in Dublin. The purpose of the agreement was the cooperation and compatibility of GALILEO and American GPS. Controversial issues such as frequency assignment and modulation form have also been adjusted. All this made the coexistence of GALILEO and GPS possible. On December 10, 2004, in a European Commission recommendation, the European Council confirmed the technical characteristics of the system (with an emphasis on the services provided). The Council addressed the transition from the implementation phase to the operational phase and confirmed the participation of the EU in financing both phases. Accordingly, GALILEO should become operational in 2008. Commercial operations will begin no earlier than 2012.
The corporation's service will be in Toulouse and London.
Basics of satellite navigation GLONASS and GALILEO GPS-X-02007-C p.45
The system design involves four phases:
Design Definition: The purpose of the definition phase is to establish the basic parameters and characteristics of the system. The phase ended in 2003.
On-orbit development and testing: On December 28, 2005, the first experimental satellite GIOVE-A was launched from the Russian Baikonur Cosmodrome in Kazakhstan (Fig. 39).
GIOVE is an acronym for GALILEO In-Orbit Validation Element. On January 12, 2006, GIOVEA transmitted the first signals. The signals were recorded and analyzed by the Atmospheric and Radio Wave Observing Station at Chilbolton in Britain, and by the ESA ground station in Belgium. The second experimental satellite will be launched into orbit at the end of 2007. With GIOVE-A and B, the EU will provide the bands for GALILEO operation and determine the orbits for the test phase of the satellites. These satellites will test an important technology, atomic clocks, in the harsh conditions of space. GIOVE-A has two rubidium atomic clocks (with a reliability of approximately 10 ns per day) and GIOVE-B has two passive hydrogen quantum clocks (with a reliability of less than 1 ns per day) on board. Following the successful completion of the pilot phase with the GIOVE-A and GIOVE-B satellites, four satellites will be launched into orbit for testing (the satellites were ordered on December 21, 2004). With this "minimum constellation" scientists can see if it is possible to determine the exact position and location on earth. The entire test phase is to be completed in 2008, and the total cost of the definition and test phases is estimated at around €1.1 billion ($US 1.4 billion).
Implementation and launch of the entire system: if the first two phases are successful, the system will be finalized for final operation. The remaining satellites (four currently operating) will be completed and launched into orbit, and the necessary ground stations will also be completed. This phase is planned for 2011 and will cost around €2.1 billion (US$2.75 billion). 1/3 is financed by the state and 2/3 by private investors.
Usage: once all satellites are in orbit, the system will start working. At the end of the phase, there will be 27 operational and 3 standby satellites in orbit. Ground stations will be established, as well as local and regional service stations. The annual cost is estimated at €220 million ($US288 million), of which the government's share will be a record €500 million ($US655 million) during system launch. In subsequent years, the main costs will fall on the shoulders of private investors.
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Table 5: Comparison of properties of GPS, GLONASS and GALILEO systems Deviation from specified UTC Code identification: the code is different for each satellite Frequency identification: the frequency is different for each satellite
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4.1 Introduction GNSS systems combine sophisticated satellite and radio technology to provide navigation to receivers with radio signals, and display a lot of data during transmission, and identify the transmitting satellite. Calculating position using these signals requires math, which will be described in this chapter.
4.2 Position calculation 4.2.1 Signal transit time measurement principle (pseudo-range estimation) In order for a GPS receiver to determine its position, it must receive time signals from four other satellites (SP 1 ... SP 4) to calculate the signal transit time t1.. .t4 (Fig.
Fig. 40 Signals from 4 satellites must be received Calculations are made in Cartesian three-dimensional coordinate system with geocentric origin (Fig. 41).
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Fig.41 Three-dimensional coordinate system Due to the atomic clocks on board the satellites, the transmission time of the signal from the satellite is known with great accuracy. All satellite clocks are corrected or synchronized with one another and with universal time. In contrast, the user's clock is not synchronized with UTC and therefore runs slower or faster at t0. The sign of t0 is positive if the user's clock is faster. The resulting time error t0 causes errors in the measurement of signal transit time and distance R. The result is an incorrect distance, known as the pseudo-distance or pseudo-range PSR.
R: actual range from satellite to user C: speed of light t1: signal transit time from satellite to user t0: difference between satellite and user clocks PSR: pseudo-range
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Substituting (4a) into (3a) In order to determine the four unknown variables (t0, XAnw, YAnw, ZAnw), four independent equations are needed The following is true for 4 satellites (i= 1…4) 4.2.2 Linearization of the equation The four equations 6a represent is a non-linear set of equations. To solve it, you need to make the root function linear according to the Taylor model, using only the first part (Fig. 42).
Fig.42 Taylor sequence conversion Basic (with x = x-x0) Simplified (only 1 part) To linearize the four equations (6a), an arbitrarily assumed x0 value must be substituted for x.
For the GPS system, this means that instead of directly calculating XUser, YUser and ZUser, the estimated position of XTotal, YTotal and ZTotal is used (Fig. 43).
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Fig.43 Position Estimation Estimated position includes error due to unknown variables x, y and z.
The distance from 4 satellites to the estimated position can be calculated using the following equation:
We combine equation (9a) with (6a) and (7a) and get:
After partial differentiation, this will give the following:
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After transposing four equations (11 a) (for i = 1… 4) for four variables (x, y, z and t0), the rules of linear algebra can be applied:
Getting x, y, z is used to recalculate the estimated position of XTotal, YTotal and ZTotal according to equation (8a).
The estimated values XTotal_New, YTotal_New and ZTotal_New can now be entered into equations (13a) using the normal iterative process until x, y and z are smaller than the desired error (eg 0.1 m). Depending on the starting position, three to five iterations are needed to make the error less than 1 cm.
4.2.4 Summary To determine the position of the user (or his software), the last measured value or assumed new position for which iterations achieve the desired x, y and z error will be used.
The resulting value of t0 corresponds to the user's time error and can be used to correct his clock.
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4.2.5.1 Error analysis Error components in calculations have not yet been taken into account. In the case of a GNSS system, several causes contribute to the overall error:
Satellite clocks: Although each satellite has four atomic clocks on board, a time error of 10 ns results in an error of the order of 3 m.
Satellite orbits: The position of a satellite is usually known within 1 to 5 m.
Speed of Light: Signals from the satellite to the user travel at the speed of light. But the speed drops when moving through the ionosphere and troposphere and cannot be considered a constant.
Signal transit time measurement: the consumer can only determine the point in time when a signal is received from the satellite with limited accuracy.
Reflection of signals: the level of error increases due to the reception of reflected signals.
Satellite Geometry: The ability to determine a position is degraded if the four satellites used in the measurements are obscured. The effect of satellite geometry on measurement accuracy is called DOP (see Table 6).
Errors are caused by various factors, which are detailed in Table 1, which includes information on horizontal errors depending on the source.
By applying corrective measurements (Differential GPS, DGPS), errors can be reduced or eliminated.
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The accuracy with which a position can be determined by GNSS in navigation mode depends, on the one hand, on the accuracy of the pseudo-measurement range, and, on the other hand, on the geometric configuration of the satellites used. It is expressed as a scalar quantity, which is called DOP in the navigation literature.
There are several DOP notations in modern usage:
GDOP: Geometric DOP (position in 3-D space, time deviation in solution)
PDOP: Positional DOP (position in 3-D space)
HDOP: Horizontal DOP (position on the plane)
VDOP: Vertical DOP (height only) The accuracy of any measurement is proportional to the DOP value. This means that if the DOP is doubled, then the error in determining the position will also double.
Fig.44 Satellite geometry and PDOP The DOP value is the inverse of the volume of the square formed by the satellite and user positions (Fig.44 and Fig.45). Best geometric arrangement for maximum volume, and therefore minimum PDOP.
Rice. Figure 45: Influence of satellite position on DOP value
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In mountainous areas and in forests, DOP values play an important role in the measurement of unfavorable geometrical situations.
Therefore, it is necessary to plan measurements according to DOP values (eg HDOP) or evaluate the final accuracy accordingly, especially when different DOP values appear within a few minutes.
All planning and calculation programs show DOP values. Rice. 47 shows an example HDOP course when there is no shading (maximum HDOP is about 1.9). Rice. 48 shows an example of an HDOP course in the presence of shading (the maximum value of HDOP is 20 times higher!). The area between 180° and 270° is covered by skyscrapers and the area between 270° and 180° is covered by mountains.
Fig.47: HDOP value over a period of 24 hours without shading (max. value 1.9)
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In the case of extensive shading, there are several moments with a possible favorable DOP value (less than 2). Time points with DOP values greater than 6 are cancelled.
4.2.5.3 Total error The measurement accuracy is proportional to the DOP value. This means that doubling the DOP also doubles the error.
Typically: Error (1) = 1 Total RMS Value DOP Value Error (2) = 2 Total RMS Value DOP Value Table 7 gives sigma 1 (1 = 68%) and sigma 2 (2 = 95%) values. These values are correct for the average satellite placement at HDOP = 1.3. The implementation of convenient correction methods (such as interconnected satellites (Differential GPS, DGPS (see chapter 6)) can reduce or eliminate the number of sources of error (typically by 1...2 m, 1 sigma value).
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Table 7: Total error at HDOP = 1.3 Usually the accuracy is better than what is given in the table. The US-Federal Aviation Administration showed that in 95% of all measurements the horizontal error was less than 7.4 m and the vertical error was less than 9.0 m. The time period was 24 hours.
The U.S.DoD guarantees that the system will provide standard civilian applications with 13m horizontal, 22m vertical and ~40ns time accuracy. When applying special measures, such as DGPS, increasing the measurement time and technical methods (phase measurement), the position accuracy can increase to a centimeter.
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5.1 Introduction An important issue in the use of GNSS systems is the multitude of coordinate systems in the world.
As a result, the position measured and calculated by the GNSS system does not always match the intended position.
In order to understand how the GNSS system functions, it is necessary to turn to the fundamentals of the science that deals with the observation and distribution of the Earth's surface, geodesy. Without this basic knowledge, it's hard to understand why there are 100 different systems and approximately 10 different grids to choose from. If you make the wrong choice, the position error can be several hundred meters.
5.2 Geoids We have known that the Earth is round since the time of Columbus. But is it really a circle? Describing the shape of the blue planet has always been an inexact science. Several other methods have been trying for centuries to accurately describe the shape of the real Earth. The geoid is an approximation of this shape.
In an ideal situation, the smooth sea surface forms part of the level surface, which in a geometrical sense means the "surface" of the Earth. By analogy with the Greek word for Earth, this surface is called the geoid (Fig. 27).
The geoid can be defined as a mathematical figure with a limited degree of precision, and not without a few arbitrary assumptions. The fact is that the distribution of the mass of the Earth is odd and, as a result, the level surfaces of the oceans and the sea do not lie on the surface of a geometrically defined shape; therefore, approximations are needed.
Unlike the actual shape of the Earth, the geoid is a theoretical body whose surface is intersected by lines of gravity everywhere at right angles.
The geoid is often used as a surface for measuring altitude. The control point is located in Switzerland - "Repere Pierre du Niton (RPN, 373.600 m) in the Geneva Basin. From this height, the points of subsequent measurements are counted to indicate the size of the port of Marseilles (height above sea level 0.00 m).
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Rice. 49 The geoid is an approximation of the surface of the Earth
5.3 Ellipsoid and Data 5.3.1 Spheroid The geoid is, however, a very difficult shape to compute. For daily observations, a simpler form is needed. This shape is known as a spheroid. If the surface of an ellipse is rotated around its symmetrical north-south axis, the result is a spheroid. (Fig. 50).
The spheroid is defined by two parameters:
Semi-major axis a (on the equatorial plane)
Semi-minor axis b (axis of the north and south poles) The amount by which the shape deviates from the ideal sphere is called alignment (f).
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5.3.2.1 Local ellipsoids When working with a spheroid, be careful that the natural perpendicular does not intersect vertically at a point with the ellipsoid, but intersect with the geoid. Normal ellipsoidal and natural perpendiculars are different in "vertical deviation" (Fig. 52), that is, there are points on the Earth's surface that are designed incorrectly. To keep this deviation to a minimum, each country created its own non-geocentric spheroid as a surface to observe (Fig. 51). Semiaxes a and b and the midpoint are chosen so that the geoid and ellipsoid types of the national territories are as accurate as possible.
5.3.2.2 Data, chart systems National or international chart systems based on certain types of ellipsoids are called bases. Depending on the map used by the GPS receivers, it must be checked that the required map system is entered into the receiver.
Some examples of these systems are over 120 - CH-1903 for Switzerland, WGS-84 the global standard, and NAD83 for North America.
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Rice. 51 Local Ellipsoids A spheroid is well suited for describing the positional coordinates of a point in degrees of longitude and latitude.
The elevation information is based on either the geoid or the local ellipsoid. The difference between the measured orthometric height H, i.e. based on the geoid, and the height of the local ellipsoid h is called geoid ondulation N (Fig. 30)
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Other reference systems are used in Europe, and each system used for technical applications in the form of observation has its own name. The non-geocentric ellipsoids that form the basis are summarized in the following table (Table 8). These ellipsoids differ slightly from country to country.
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5.3.4 WGS-84 World Ellipsoid Details displayed and calculations made by the GPS receiver include the WGS-84 datum.
The WGS-84 coordinate system is geocentrically positioned relative to the center of the Earth. Such a system is called ECEF. The WGS-84 coordinate system is a three-dimensional, right-handed, Cartesian coordinate system with its center of mass (=geocentric) ellipsoid that approximates the total mass of the Earth.
The positive X-axis of the ellipsoid (Fig. 53) lies on the equatorial plane (an imaginary surface that is spanned by the equator) and passes through the center of mass through the point where the equator intersects the Greenwich meridian (0 meridian). The Y-axis also lies on the equatorial plane and is offset by 900 to the east of the X-axis. The Z-axis lies perpendicular to the X and Y axes and passes through the geographic north pole.
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6,378,137.00 298.257223563 6,356,’752.31 Table 9 WGS-84 Ellipsoid Ellipsoidal coordinates (, h), which are better than Cartesian coordinates (X, Y, Z), are commonly used for further processing (Figure 54). corresponds to latitude, - longitude and h ellipsoidal height, that is, the length of the vertical line P in the ellipsoid.
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Table 54. Illustration of ellipsoidal coordinates 5.3.5 Transformation from local to global ellipsoid 5.3.5.1 Geodetic data In general, local reference systems are better than geocentric ellipsoids. The relationship between the local (eg CH-1903) and the global geocentric system (eg WGS-84) is called geodetic data. In case the axes of the local and global ellipsoid are parallel, or can be considered as such for applications within the local area, then all that is needed for the datum transition is three displacement parameters, called the displacement constants X, Y, Z.
Next, the three rotation angles x, y, z and the scaling factor m (Fig. 55) can be added so that the complete transformation formula contains 7 parameters. Geodetic data defines the position of a local 3D Cartesian coordinate system in relation to the global system.
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Fig.55 Geodetic data The following table (Table 10) shows examples of various data parameters.
Additional values can be found in .
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Table 10 Data parameters 5.3.5.2 Data conversion Data conversion means the transformation of a 3D Cartesian coordinate system (eg.
WGS-84) to another (eg CH-1 903) through 3D translation, rotation and expansion.
The survey data must be known for this transformation. Comprehensive conversion formulas can be found in the specialized literature, or the conversion can be done directly via the Internet. Once the transformation has taken place, Cartesian coordinates can be transformed into ellipsoidal coordinates.
5.3.6 Converting Coordinate Systems 5.3.6.1 Converting Cartesian to Ellipsoidal Coordinates Cartesian and ellipsoidal coordinates can be converted from one to the other.
The transformation depends on the quadrant. As an example, consider the transformation for central Europe. This means that x, y and z are all positive.
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Ellipsoidal coordinates can be converted to Cartesian.
5.4 Planar coordinates, projection Normally, when surveying a surface, the position of point P on the Earth's surface is described by ellipsoidal coordinates of latitude and longitude (ellipsoid based) and altitude (ellipsoid or geoid based).
Geodetic calculations (eg, the distance between two constructions) in an ellipsoid are numerically very inconvenient, so ellipsoidal projections onto a plane are used for technical methods. This results in right-handed X and Y coordinates. Most maps have a grid that fits the point to any terrain. Planar coordinates are projections of ellipsoidal coordinates onto a mathematical plane. Projecting an ellipsoid onto a plane is not possible without distortion, but is acceptable if the distortion is minimal.
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5.4.1 (Transverse Mercator Projection) The Gauss-Kruger projection is a tangential, conformal, transverse Mercator projection. The elliptical cylinder is positioned around the spheroid, the body of the cylinder comes into contact with the ellipsoid along the Greenwich meridian and near the poles. In order to keep longitudinal and surface distortion to a minimum, three zones are taken from the Bessel ellipsoid at latitude 30. The width of the zone is positioned around the prime meridian. The cylinder is located at an angle to the ellipsoid, that is, rotated by 900 (Fig. 56). To minimize surface distortion, 3° wide ellipsoid rotation zones are used. The width of the zone is fixed around the central meridian. Different central meridians are used depending on the region (e.g. 6°, 9°, 12°, 15°,....).
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Values in north/south direction are calculated from the equator. To avoid negative values in the west/east direction, the central meridian is assumed to be +500000 m (Offset). The number of degrees of the central meridian is divided by three and placed in front of this value.
Position example:
Ellipsoid coordinates: N:46.86154° E 9.51280° Gauss-Krger (Central meridian: 9°): N-S: 5191454 W-E: 3539097 Position is the distance 5191454 m from the equator and 399097 m from the central meridian (9°).
5.4.2 UTM projection In contrast to Gauss-Krger, the UTM (Universal Transversal Mercator) projection is projected onto almost the entire surface of the Earth on 6020 = 1200 planes. The actual projection of the rotation of the ellipsoid onto the transversal cylinder is performed in accordance with the processes in the Gauss-Krger projection.
The UTM system is often based on the WGS84 ellipsoid. However, it only specifies the projection and coordinate system, not the ellipsoid and geodetic data.
The UTM system divides the entire world into 6° wide longitudinal zones (Fig. 57). Those numbered 1 to 60 begin at 180° W and end at 180° E. If, for example, zone 1 is between 180° W and 174° W, the central meridian of that zone is at 177° W, zone 2 is located from 174° W to 168°, the central meridian of zone 2 is at 171° W, etc.
Satellite Navigation Basics Coordinate Systems GPS-X-02007-C p.64 The central meridians for each projection area are 3°, 9°, 15°, 21°, 27°, 33°, 39°, 45°, 51°, 57 °, 63°, 69°, 75°, 81°, 87°, 93°, 99°, 105°, 111°, 11 7°, 123°, 129°, 135°, 141°, 147°, 153° , 159°, 165°, 171°, 177° east (E) and west (W) (longitudes) (Figure 58).
North-south directions (to the poles) are further subdivided into zones, with the exception of 8° latitude and are identified by names starting with C. The zone is only allowed between 80° south longitude and 84° north longitude. The line from 80°S to 72°N is referred to as Sector C, the line from 72°S to 64°S as Sector D, and so on. The exception is the area known as Latitude X between 72°N and 84°N. This is 12° latitude.
Rice. Fig. 57: Principle of projection of one zone (out of six) Fig. 58: Designation of zones using UTM with examples
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5.4.3 Swedish projection system (conformal double projection) The conformal projection of a Bessel ellipsoid onto a plane has two steps. The ellipsoid is initially projected onto a sphere, and then the sphere is projected onto a plane by means of a cylinder set at an acute angle. This process is known as double projection (Fig.
59). The main point of the ellipsoid (Old Observatory in Bern) is projected onto the plane using the original coordinate system (offset: YOst = 600.000 m and XNord = 200.000 m).
Two different coordinate settings are highlighted on the map of Switzerland (scale 1:25000):
Surface coordinates (X and Y in kilometers) projected onto a gridded plane
Geographic coordinates (longitude and latitude in degrees and seconds) based on the Bessel ellipsoid
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Fig.59 Dual projection principle Transit time of signals from 4 satellites must be known to obtain position coordinates.
Only after the main calculation and transformation, the position coordinates in Sweden will correspond to reality (Fig. 60).
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There are several possibilities on the Internet for converting one coordinate system to another.
5.4.4.1 Convert WGS-84 coordinates to CH-1903 coordinates as an example.
(Taken from "Bezugssysteme in der Praxis" (practical reference systems) by Urs Marti and Dieter Egger, Federal Office for National Topography).
Attention! accuracy is about 1 meter!
1.Longitude and latitude conversion Longitude and latitude in WGS-84 must be converted to sexagesimal seconds[‘’]
Example:
1. After converting latitude 46° 2’ 38.87” (WGS-84) will become 165758.87”. This is denoted as B: B = 165758.87”.
2. After conversion, longitude 8° 43’ 49.79” (WGS-84) will become 31429.79”. This is denoted by L: L = 31429.79”.
2. Calculation of auxiliary quantities
Example:
3. Calculation of the abscissa (W…E): y
Example:
4. Calculation of the ordinate (S…N):x
Example:
5. Calculation of height H:
Example:
After conversion, height WGS-84= 650.60 m gives H = 600 m
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6.1 Introduction The forerunner of all GNSS systems was the GPS system. In fact, it has been used so often that the term satellite navigation refers specifically to GPS. The developed GPS has some limitations that required improvements in technology. This chapter examines the technological advances in GPS that have become standards for GNSS as well.
Originally developed for military purposes, today the GPS system is mainly used for civilian applications such as surveillance, navigation, positioning, speed measurement, timing, monitoring, etc. GPS was not intended for applications with high requirements for accuracy, security measures, or indoor operation. For this reason, later changes were required.
Increased positioning accuracy, Differential-GPS (D-GPS) appeared.
Improving positioning accuracy and reliability (stability is very important for security applications), SBAS (Satellite Based Augmentation System) such as EGNOS and WAAS were created for this.
Improving sensitivity in enclosed spaces or reducing acquisition time, an Assisted-GPS (A-GPS) version has been proposed.
Improving the reception quality of GPS receivers has been continuously improved, and the sensitivity of the receivers has also been improved using High Sensitivity GPS (HSGPS).
6.2 Sources of GPS error
The positioning accuracy is approximately 13 m for 95% of all measurements (with HDOP the accuracy will be 1.3 m) and as discussed in the previous chapter, this accuracy is insufficient for applications. To increase the accuracy to a meter and better, extra efforts are needed. Various sources add error to GPS measurements. These cases and sources are shown in Table 11. These values are indicative and may vary from receiver to receiver.
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Rice. Fig. 61: Influence of measurement time on reflections
Receiver Influence: Further errors occur due to GPS receiver measurement noise and receiver time delays. Modern technology can reduce this effect.
Satellite position effects including shadows (DOP): This effect will be discussed in detail in chapter 4.2.5.2.
6.3 Possibilities for Reducing Measurement Error Reducing the effect of measurement errors will result in improved positioning accuracy. Various variants are used for this and are often combined. The most common are:
Dual frequency measurement: L1/L2 signals are used to compensate for the effect of the ionosphere. These receivers measure GPS signals at the L1 and L2 frequencies. If a radio signal is transmitted through the ionosphere, then it slows down in proportion to its frequency. By comparing the obtained times of both signals, the delay and hence the ionization effect can be determined.
Geophysical corrective models. Such models are used for the primary compensation of the effects of the ionosphere and troposphere. Corrective factors are used only in special and limited areas.
Differential GPS (DGPS): When considering one or more base stations, different errors have to be corrected. Evaluation of correction data from these stations is possible both after processing and in real time (RT). Real Time (RT DGPS) solutions require communication data between the base station and the mobile receiver. DGPS employs a number of other processes:
RT DGPS, based on RTCM SC104 standard
DGPS based on signal transit time delay measurement (Pseudo-range corrections, attainable accuracy of 1 m)
DGPS based on carrier phase measurement (achievable accuracy 1cm)
Post-processing (consecutive correction and data processing).
Choice of location and time of measurement to improve visibility or line of direct contact with satellites (See DOP 4.2.5 for explanations).
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In theory, the achievable level of accuracy is about 15 - 20 m. For operations requiring an accuracy of the order of 1 cm, the accuracy should be higher. The industry has found a simple and reliable solution to this problem: Differential GPS (DGPS). The principle of DGPS is very simple. The GPS reference station is set at a point with known coordinates. A GPS reference station determines a person's position using four satellites. Since the exact position of the station is known, any deviation from the measured actual position can be calculated. This deviation (differential position) is also valid for any GPS receivers within a radius of 200 km from the station.
The differential position can be used to correct the position measured by other GPS receivers (Fig. 62). Any deviation from the position can be transmitted directly by radio or its correction is possible after receiving the measurements. Based on this principle, the accuracy can increase to several millimeters. It is important that the correction is based on pseudo-range values and not on position deviation from the GPS reference station. The deviations are based on the pseudo ranges of certain satellites and can vary greatly depending on the position the satellite is using. Correction based on a simple deviation from the position of the reference base station is not taken into account and will give false results.
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Rice. 62 Principle of operation of GPS with reference station 6.3.1.1 Detailed method of operation Ionospheric effects directly affect data inaccuracy In DGPS technology, most of these errors can be compensated for.
Compensation occurs in three phases:
1. Determination of correction values from the reference station
2. Transmission of correction values from the station to the GPS user
3. Correction of user-measured pseudo-range 6.3.1.2 Determination of correction values A reference station, whose coordinates are known exactly, measures the L1 transit time of the signal from all visible GPS satellites (Fig. 63) and determines the pseudo-range of this variable (actual value). Since the position of the station is known exactly, the actual distance (target value) to each GPS satellite can be calculated.
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Fig.63 Definition of correction values 6.3.1.3 Transmission of correction values Since correction values can be used over a wide area to correct the measured pseudo-range, they can be transmitted without delay using any suitable device (transmitter, telephone, radio, etc.). etc.) to other GPS users (Fig. 39).
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Fig.64 Transmitting correction values 6.3.1.4 Correcting the measured pseudo-range After receiving the correction values, the GPS user can determine the actual distance using the measured pseudo-range (Fig. 65). The exact position of the user can now be calculated from this value. All causes of the error have been eliminated except for receiver noise and echoes.
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Fig.65 Pseudo-range measurement correction 6.3.2 DGPS based on carrier phase measurement In pseudo-range measurement, an accuracy of about 1 meter is not sufficient to solve surveillance problems. To increase the accuracy to several millimeters, it is necessary to estimate the phase of the carrier signal. The carrier wavelength is approximately 19 cm. The satellite range can be determined using the following method (Fig. 66).
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Fig.66 Phase measurement principle Phase measurement is an uncertain process because N is unknown. By observing several satellites at different times and continuously comparing the user's receiver with the reference (during or after the measurement), it is possible to determine the position to within a few millimeters after solving numerous sets of equations.
6.3.3 DGPS post-processing (signal transit time and phase measurement)
DGPS post-processing searches for corrective factors using appropriate software after taking measurements. The reading data is taken either from private reading stations or from public server systems. The disadvantage is that a data problem (poor satellite reception, corrupted files, etc.) is sometimes not detected after the correction values are calculated and transmitted, requiring the whole process to be repeated.
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Table 12: Transmission process of various signals (for code and phase measurements) Most countries have their own systems for transmitting correction data. A detailed description of these systems is beyond the scope of this manual. Some individual systems will be discussed below.
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6.3.6 Transmission standards for correction signals DGPS translators transmit signal transit time and carrier phase corrections. For most GBAS and some satellites based on WADGPS systems (LandStarDGPS, MSAT, Omnistar or Starfire), DGPS correction data is transmitted in accordance with the RTCM SC-104 standard. As a rule, the receiver must be equipped with a special decoder to receive and process the data. Satellite based Augmentation Systems such as WAAS, EGNOS and MSAS uses the RTCA DO-229 standard. Because RTCA frequencies and data formats are compatible with all GPS, modern GNSS receivers can compute RTCA data without additional hardware, unlike RTCM (Figure 67).
Table 13 lists the standards used for DGPS correction signals as well as references related to GNSS.
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6.4.1 GBAS services Among the large number of terrestrial DGPS services, such services as Ground Based Augmentation Services (GBAS) are known, we will describe them here in more detail. Many countries use similar systems. The following list describes some of the GBAS services available in Europe.
6.4.2 European GBAS services
SAPOS: (German Surveying and Mapping Administration Satellite Positioning Service) is a DGP S ongoing service. This service is available throughout Germany. The main system is a network of GPS reference stations. For real-time correction of values, data is transmitted using VHF radio, long wave, GSM and own two-meter (VHF) frequencies. VHF radio transmitters broadcast correction data signal in RASANT (Radio Aided Satellite Navigation Technique) format. This is an RTCM 2.0 conversion for data transmission in Radio Data System (RDS) format using VHF audio broadcast. SAPOS includes four services with different characteristics and accuracy:
SAPOS EPS: Real-time positioning service o SAPOS HEPS: Real-time high precision positioning service o SAPOS GPPS: Geodetic precision positioning service o SAPOS GHPS: Geodetic high precision positioning service o
ALF: (Accurate Positioning by Low Frequency) correction value translators with 50kW output from Mainflingen, Germany (near Frankfurt). The DCF42 longwave translator (LW, 123.7 kHz) transmits correction values over a distance of 600–1000 km. This upper sideband (USB) is phase modulated (Bi-Phase-ShiftKeying BPSK). The German Federal Center for Cartography and Geodesy together with the German Telecom service (DTAG) provide this service. When buying the necessary decoder, the user pays once. Due to the fact that correction data propagates at long wavelengths, they can be obtained in the presence of obscuration.
AMDS: (Amplitude Modulated Data System) is designed for digital transmission at medium and long frequencies using existing radio transmitters. The data is phase modulated and is transmitted over a distance of 600 - 1000 km.
Swipos-NAV: (Swiss Positioning Service) presents correction data using FM-RDS or GSM. Radio Data System RDS is a European standard for digital data using a network of VHF transmitters (FM, 87-108 MHz). RDS was designed to provide information above the VHF band. RDS data is modulated at 57 kHz on an FM carrier. The user needs an RDS decoder to expand the DGPS correction values. Direct contact with a VHF transmitter is essential to ensure good reception. Users of this service can pay once a year or once upon purchase.
Radio beacons: Radio beacons are navigation facilities and are usually installed along coasts. DGPS correction signals are typically transmitted at a frequency of approximately 300 kHz. The signal speed varies depending on the translator and is 100 and 200 bits per second.
Satellite Navigation Basics GPS Add-ons: DGPS, SBAS, A-GPS and HSGPS GPS-X-02007-C p.77
6.5 Global Area DGPS (WADGPS) 6.5.1 Satellite Based Augmentation Systems, SBAS (WAAS, EGNOS) 6.5.1.1 Introduction Satellite Based Augmentation Systems (SBAS) is used to enhance the functions of GPS, GLONASS and GALILEO (once it becomes operational). Corrective and reliable data for GPS or GLONASS are broadcast from geostationary satellites over the GNSS frequency.
6.5.1.2 The most important functions of SBAS SBAS is much better than GPS because of better positioning accuracy and reliability. SBAS, unlike GPS, carries out additional transmission of Signals from various geostationary satellites.
Improve Positioning Accuracy Using Correction Data: SBAS provides various correction data that improve GNSS positioning accuracy. Ionospheric error due to signal delay has been corrected. The ionospheric error varies with time of day and terrain. To check the global correctness of the data, it is necessary to process the network of ground stations to calculate the ionospheric error. In addition to ionospheric values, SBAS will check correction information regarding satellite position (ephemeris) and time measurement.
Improve reliability and safety: SBAS checks every GNSS satellite and notifies the user when an error or failure occurs within 6 seconds. Yes/no information is transmitted only if the quality of the received signals is below certain limits.
Increase availability through the broadcast of navigation information: SBAS geostationary satellites transmit signals similar to those of GNSS, although they miss accurate time data. GNSS can determine position from these signals using a procedure called “pseudoranging”.
6.5.1.3 Overview of existing and planned systems Although all Satellite Based Augmentation Systems (SBAS) include large regions (eg Europe), they must be compatible with each other and SBAS providers must cooperate and agree on common operating principles. Compatibility is guaranteed using the RTCA DO-229C standard. Currently, SBAS systems are defined for the areas below that are operational or in development and are compatible (Figure 68):
North America (WAAS, Wide Area Augmentation System): The US Federal Aviation Administration (FAA) is developing the Wide Area Augmentation System (WAAS), which covers most of the continental United States, as well as Alaska and Canada. WAAS operates with POR and AOR-W satellites. These satellites should become active in 2007/2008. Continuous operation of this service will be achieved with two new satellites located at 133°W and 107°W. It is planned to expand the service to Canada using the Canadian “CWAAS” system.
Europe (EGNOS, European Geostationary Overlay Service): The European Group of Three, including ESA, the European Union and EUROCONTROL, is developing EGNOS, the European Geostationary Navigation Overlay Service. EGNOS is for the European Civil Aviation Conference (ECAC) region. In June 2006, EGNOS was not fully approved for high security applications (eg aviation). The final release of the system is scheduled for 2007/2008. The current state of the EGNOS satellites can be viewed at.
Japan (MSAS, Multifunctional Satellite Based Augmentation System): The Japanese office of Civil Aviation is developing an MTSAT based Augmentation System (MSAS) that will cover all of Japan.
Satellite Navigation Basics GPS Add-ons: DGPS, SBAS, A-GPS and HSGPS GPS-X-02007-C p.78
India (GAGAN, GPS and GEO Augmented Navigation): Indian Space Research Organization (ISRO) is trying to develop a system compatible with other SBAS systems. This system will be started by 4 GSAT satellites, which are planned to be launched in 2007. It is planned to create an independent GNSS system for India called the Indian Regional Navigational Stellite System (IRNSS).
China (Beidou): The Beidou system includes three geostationary satellites (140°E, 110.5°E and 80°E) owned by the Chinese government, the system was conceived as a regional extension of China's COMPASS navigation system. The final commissioning of the system is unknown.
Rice. 68: Position and means of WAAS, EGNOS, GAGAN and MSAS
Geostationary satellites (Table 14) broadcast signals from about 36,000 km above the equator towards the area of use. Pseudo Random Number (PRN) is defined for each satellite. The signal broadcast frequency is the same as GPS (L1, 1575.42 MHz).
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Rice. 69: Principle All Satellite Based Augmentation Systems SBAS
Reference station: In the SBAS area there are several base reference stations connected in a common network. Base stations receive GNSS. They are carefully studied in relation to the position. Each base station determines the difference between the actual and calculated position relative to the satellites (pseudo range). The data is then transmitted to the control center.
Control center: Control centers evaluate correction data from base reference stations, determine the accuracy of all GNSS signals received from each reference station, calculate errors, the possibility of ionospheric turbulence, and verify the validity of the GNSS system. Data changes are integrated into the signal and transmitted via satellite ground stations.
Satellite ground stations: These stations broadcast signals to various geostationary satellites.
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6.5.2 DGPS satellite services using RTCM SC-104 Several geostationary satellites continuously broadcast correction data. Below are some of these services. These services use the RTCM SC-104 standard and require a special decoder.
MSAT: Developed by the National Research Council in Canada, this service broadcasts CanadaWide DGPS (CDGPS) signals using two geostationary satellites.
Omnistar (Fugro Group) and LandStar-DGPS, (Thales Company) independently broadcast correction data via 6 GEO satellites (Figure 70). Services are paid and users must have access to a special receiver/decoder to use it. Omnistar and Landstar broadcast information on the L-band frequency (1-2 GHz) to the ground. Base stations are spread all over the world. Geostationary satellites are located at the central latitude low above the horizon (10° ... 30°). Being within line of sight is necessary for radio contact.
Rice. Figure 70: LandStar-DGPS and Omnistar distribution area
Starfire Property of NavCom Technology, Inc. broadcasts correction data via 3 Inmarsat GEO satellites. The service is paid and the user must have access to a special receiver/decoder to use it. Starfire broadcasts information on the Lband frequency (1-2 GHz) to the ground. Corresponding base stations are spread all over the world. The service is available in the range from ± 76° latitude.
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Table 16: Positioning accuracy without and with DGPS/SBAS
6.7 Assisted-GPS (A-GPS) 6.7.1 A-GPS principle Let's assume that devices for Location Based Services (LBS, see 9.2.1) do not always work. Especially in cases where the location is obtained from GNSS, as battery operation is terminated during long stationary periods to minimize power consumption. Since the GNSS device does not always work, there is a possibility that information regarding the position of the satellite may not be available. If inactive for 2 or more hours, satellite orbital data must be loaded to launch. The GNSS receiver typically needs the last 8-36 seconds to acquire orbital data and calculate a position. In difficult reception conditions (in a city where tall buildings obscure the direct view of the sky), it may take several minutes to calculate the first position.
In the absence of GNSS orbital data, the receivers must search for available satellites, download the data, and calculate the position. Searching for GPS satellites (for example) by Code-Frequency-Level takes a lot of time. The correlation time is typically 1 ms (1 C/A period) per code-frequency-level position. If the frequency range is broken down into 50 steps (i.e. the frequency interval is (2 x 6000 / 50 Hz = 240 Hz), then 1023 x 50 = 51.150 positions would need to be traversed, which would take 51 seconds. See section 6.8.
This problem can be corrected by obtaining satellite orbital data and additional available GNSS information using other communication channels, such as GSM, GPRS, CDMA or UMTS. This solution is called Aiding (auxiliary) and is implemented using Assisted-GPS. Assisted-GPS (or A-GPS) is a feature or service that uses assisted data to obtain a position. The GNSS receiver receives assistance data via mobile communications or directly over the Internet.
Supporting data includes the following information:
Satellite location (Almanac)
Accurate orbital data (Ephemeris, orbits)
Time Information
Doppler Frequency and Frequency Offset (Error) of the GNSS Receiver Satellite Navigation Basics GPS Augments: DGPS, SBAS, A-GPS and HSGPS GPS-X-02007-C p.82 With the available GNSS assistance information, the receiver can quickly calculate a position even under poor conditions. Depending on the complexity and content of the supporting information, the start time can vary greatly. The start time also depends on the strength of the GNSS signal. Usually this is true, but still, the more complete the supporting information, the faster the launch. A mobile transmitting station with a built-in GNSS device also requires four satellites within view. To use AGPS GNSS receivers need an interface to receive additional data.
You can save time by eliminating the reception of orbital data. In addition, it is possible to shorten the search area if the Doppler frequency and frequency offset of the GNSS receiver are known (Figure 71). This results in faster signal capture and saves time.
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Supporting information is collected from the GNSS Reference Network located around the world.
The box diagram below illustrates a typical A-GPS system (Figure 72) consisting of a global system of GNSS receivers, a central server that provides assistance data, and A-GPS receivers (GNSS endpoints). GNSS receivers of the global network receive the corresponding satellite information and transmit it to the server. The server calculates assistance data and transmits it (via mobile or internet) as requested by GNSS end devices to quickly calculate the first position.
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Rice. Fig. 72: Assisted-GPS 6.7.2 A-GPS with online assistance data (Real-time A-GPS) With the online or real-time principle, the assistance data is directly downloaded from the server as needed and is valid for a short time. The disadvantage of this principle is the relatively slow connection (GPRS, for example, requires 30 seconds) or the unavailability of Internet access.
6.7.3 A-GPS with Offline Assistance Data (Predicted Orbits)
A-GPS with Offline Assistance Data is a system that provides a GNSS receiver with Predicted Orbits. The receiver saves this information and communication with the server is terminated. The next time the GNSS receiver runs the stored information to use it to determine the current orbital information for navigation. Therefore, there is no need to wait for all this information to be downloaded from the satellites and the receiver can start navigating immediately. Depending on the provider, the assistance data may be correct for up to 10 days, but over time, positioning accuracy decreases.
6.7.4 Network
Predetermining orbits, which are transmitted in real time by A-GPS, requires a worldwide network of monitoring stations that continuously and accurately track satellite movements. A powerful server uses this data to determine the orbits for the next few days. An example of such a network is the International GNSS-Service (IGS, or International GPSService ), which forms a network around the world (Figure 73).
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6.8 High Sensitivity GPS (HSGPS) While urgent calling or Location Based Services applications requiring good reception in buildings and urban canyons are in demand, the reception quality of GNSS receivers is constantly improving. The main efforts are aimed at:
Increasing signal sensitivity
Quick search on receiver activation (First Fix Time, TTFF)
Reducing susceptibility to interference (reflected or electromagnetic interference) Different manufacturers use different strategies to improve products. Most of these are discussed in this chapter, including:
Increasing generator stability
Antennas
Interference analysis
Increasing correlators and correlation time 6.8.1 Increasing oscillator stability The development and use of oscillators with increased stability makes it possible to reduce or compensate for the dependence of quartz on temperature, in turn, to reduce the signal search time in the required frequency domains. This includes temperature compensated crystal oscillators (TCXO).
In addition, studies have shown that conventional crystal oscillators produce
9 microvariations at a frequency of 10 Hz. The reason for these variations is the imprecise structure of the quartz crystal. Due to these frequency changes, the search time may increase since the search for the FrequencyCode-Level during the correlation process is broken. The development of crystal oscillators with a reduced tendency to microvariations will result in less interference.
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6.8.3 Interference analysis Interference (NF) is a quantity that indicates that the signal-to-noise ratio of an incoming signal is reduced by the addition of noise from the receiver itself.
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Rice. Fig. 74: Block Diagram of Input States With conventional noise in the first and subsequent gain stages of 20dB and 1.6dB respectively, only marginal improvements are possible with new technologies, developments . Further progress in this area is almost impossible.
6.8.4 Correlators and correlation time The power spectral density of received GNSS signals is about 16 dB lower than the thermal noise density (see Fig. 16). Demodulating and concentrating the received GNSS signals results in a GG system gain of 43 dB (see Fig. 24).
Increasing the correlation time (integration time or time interval) increases the sensitivity of the GNSS module. The longer correlator is at a special frequency level, below the required GNSS signal strength for the antenna.
With an increase in the correlation time by the value k, the increase in GR separately from thermal noise will be:
GR = log10 (k) Doubling the correlation time results in a signal-to-noise separation of 3 dB. In practice, increasing the correlation time to 20 ms is not a problem. If the value of the transmitted data bits is known, this time may increase further. Otherwise, it is possible through non-coherent integration to increase the correlation time to 1 second, which, however, will lead to a loss of several dB.
To increase the sensitivity of the search, the number of additional correlators has been increased.
Modern GNSS receivers typically have a sensitivity of approximately 160 dBm. This GPS operator (US Department of Defense) guarantees a signal strength of 130 dBm, GNSS receivers, therefore, can work in buildings with signal attenuation up to 30 dB.
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Rice. 75: GNSS amplifier (external antenna, electrical adapter and power wire, amplifier and internal antenna)
6.10 Pseudo-satellites for indoor applications Pseudo-satellites are transmitters on the ground that function similarly to GNSS satellites. Pseudo-satellites are often used in aviation during landings. This procedure is usually not used for internal applications because some of the required components are very expensive.
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7.1 Introduction GNSS receivers require different signals to function (Figure 76). These variables are passed after the position and time have been successfully computed and determined.
For various portable product types, there are either international standards for data exchange (NMEA and RTCM), or the manufacturer provides predefined formats and protocols.
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For transmitting calculated GNSS variables such as position, speed, heading, etc. to a peripheral device (eg computer, screen, transceiver), GPS modules have a serial interface (TTL or RS-232 levels). The most important piece of receiver information is transmitted through this interface in a special data format. This format is certified by the National Marine Electronics Association (NMEA) so data exchange is seamless. Data is currently transmitted in accordance with the NMEA-0183 specification. NMEA defines datasets for various applications eg. GNSS (Global Navigation Satellite System), GPS, Loran, Omega, Transit and for various manufacturers.
The following seven datasets are widely used by GNSS modules to communicate GNSS information:
1. GGA(GPS Fix data, data for GPS system)
2. GLL(geographical position - latitude/longitude)
6. VTG(heading over planet and planet speed, horizontal heading and horizontal speed)
7. ZDA(time and data)
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In the case of NMEA, the baud rate is 4800 baud using 8-bit ASCII characters.
The transmission starts with a start bit (logical zero), followed by eight data bits and a stop bit (logical one). Parity bits are not used.
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Fig.77 NMEA format (TTL or RS-232 levels) Different levels should be taken into account, depending on the levels used by the GNSS receiver - TTL or RS-232 (Fig.
In case of TTL level interface, logic zero corresponds to approximately 0V and logic one corresponds to system operating voltage (+3.3V...+5V)
In case of RS-232 interface, logic zero corresponds to positive voltage (+3V...+15V) and logic one to negative voltage (-3V...-15V).
If a GPS module with a TTL level interface is connected to a device with an RS-232 interface, then a level conversion must be performed (see 7.3.4).
Some GPS modules allow you to transmit at speeds up to 38400 bits per second.
GPS data has the following structure:
$GPDTS,inf_1,inf_2, inf_3,inf_4,inf_5,inf_6,inf_n*CSCRLF Individual symbol functions or symbol values are shown in Table 17.
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Table 17. NMEA DATA SET Individual Block Descriptions
