Data acquisition and integration 3.
Global Navigation Satellite System
Bányai László
Data acquisition and integration 3.: Global Navigation Satellite System
Bányai László
Lector: Árpád Barsi
This module was created within TÁMOP - 4.1.2-08/1/A-2009-0027 "Tananyagfejlesztéssel a GEO-ért" ("Educational material development for GEO") project. The project was funded by the European Union and the Hungarian Government to the amount of HUF 44,706,488.
v 1.0
Publication date 2010
Copyright © 2010 University of West Hungary Faculty of Geoinformatics
Abstract
Main characteristics of the first and second generation GNSS systems are summarised. Signal structures, data acquisition and basic positioning concepts are presented on the bases of GPS system experiences. Error budget, receiver types, observation and data processing methods are explained in details. Complementary systems of the first generation GNSS systems and the possible fields of practical application are also summarised. Last the perspectives of the second generation GNSS systems are revealed.
The right to this intellectual property is protected by the 1999/LXXVI copyright law. Any unauthorized use of this material is prohibited. No part of this product may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system without express written permission from the author/publisher.
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DAI3Table of Contents
3. Global Navigation Satellite System...... 0
1. 3.1 Introduction...... 0
2. 3.2 General description...... 0
2.1. 3.2.1 Prologue...... 0
2.2. 3.2.2 Overview of GNSS...... 0
2.3. 3.2.3 Practical implementations...... 0
3. 3.3 Elements of satellite navigation...... 0
3.1. 3.3.1 Signal structures...... 0
3.2. 3.3.2 Data acquisition...... 0
3.3. 3.3.3 Positioning concepts...... 0
4. 3.4 Error budget of positioning...... 0
5. 3.5 Basic receiver types...... 0
6. 3.6 Observation and data processing methods...... 0
7. 3.7 Complementary systems...... 0
8. 3.8 Practical applications...... 0
9. 3.9 Summary and perspectives...... 0
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DAI3List of Tables
3-1. táblázat Approximate error sources of navigational applications (m). Approximate error sources of navigational applications (m) 0
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Global Navigation Satellite SystemChapter3.Global Navigation Satellite System
1.3.1 Introduction
The main purpose of this part is to introduce and summarize the basic principles of global navigation satellite systems (GNSSs). Receiver types, observation and data processing methods are outlined. The presented positioning accuracies can help to choose from the proper devices and services for different fields of applications.
The overview of GNSS concepts is followed by a short description of existing systems and those that are under development in Chapter 3.2.
The elements of satellite positioning − signal structures, data acquisition and positioning concepts − are introduced according to the most frequently used global positioning system (GPS) in Chapter 3.3.
The error budget of satellite positioning and the expected accuracies are summarised in Chapter 3.4.
Chapter 3.5 introduces the basic receiver types in the following categories: navigational, geodetic, CORS and GIS data collectors.
The observation and data processing methods of different receiver categories are discussed in details in Chapter 3.6.
The integrity, availability and accuracy of first generation GNSS systems are improved by complementary systems; they are summarised in Chapter 3.7.
For the sake of completeness Chapter 3.8 lists the fields of possible applications.
The last Chapter summarises the most important features and the perspectives of the near future of GNSS applications.
2.3.2 General description
2.1.3.2.1 Prologue
The first successful Earth-orbiting artificial satellite, the Sputnik-1 of the former Soviet Union was launched on 4th October, 1957. This date is the beginning of the space research and the Space Age. The spherical satellite, with 0.58 m diameter and 83.6 kg mass, transmitted on two frequencies (20.005 and 40.002 MHz) with 1W power. The orbit height was between 220 and 940 km above the sea level. During its 92 days of life time it revolved the Earth more than 1400 times before finally burnt in the atmosphere on 4th January, 1958.
According to the technologies of those times, the observation of the satellite’s orbit was carried out by using a photographic method with the help of special cameras. However the observation of the Doppler shifts and the time signals emitted by this satellite opened an unpredictable perspective in the development of global navigational positioning systems, too.
2.2.3.2.2 Overview of GNSS
In the case of Sputnik-1 all the elements of modern satellite navigation systems were present. The experiences generated a huge development of satellite positioning that is still in progress.
The general purpose of the Global Navigation Satellite System (GNSS) is the continuous supply of the users by enhanced positioning capacity and other required information. Such a system can be divided into three mean segments:
•space segment,
•control segment and
•user segment.
The space segment consists of several satellites distributed on properly designed global constellation. The satellites are equipped with radio transceivers, high precision atomic clocks (or frequency standard), on-board computers, solar panels for power supply and propulsion system for orbit maintenance and stabilisation.
The satellites are launched on nearly circular geocentric orbits, which can be characterised by:
•inclination (the angle between the orbit- and the equatorial plane),
•average height above the Earth surface and
•orbital period (or time of revolution).
The useful navigational orbit types are:
•polar orbit (inclination is very near to 90 degrees),
•semi-synchronous inclined orbit (approximately 12-hour period time, the configuration is repeated in every day),
•Geostationary orbit (inclination is very near to 0 degree and the period time is approximately 24-hour; observed from the Earth it appears as “motionless”).
The applied nearly semi-synchronous inclined orbits are (theoretically) evenly distributed along the equatorial plane, and more satellites are evenly distributed along one orbit plane to fulfil the planned navigational requirements.
Geostationary orbit is useful for both navigation and complementary telecommunication purposes as well.
Due to perturbing accelerations the Keplerian orbits of the satellites are only instantaneous. The motion of satellites can be described in the Earth Centred Inertial System by modified Keplerian orbit elements. For positioning purposes the satellite positions have to be transformed to Earth Centred Earth Fixed (ECEF) coordinate systems.
The task of the official control segment is the continuous monitoring of the satellites. Based on its observations, the master control station determines the orbits and the satellite clock errors. The orbits and clock parameters are predicted for the next time period and uploaded to the satellites. Along with the clock synchronisation the necessary satellite manoeuvre can be performed.
Parallel to the official control segment there are several local, regional or worldwide organizations that independently monitor the GNSS systems for civilian or scientific purposes. The post-processed precise orbits, clock parameters and the additional by-products can be used to enhance the accuracy and the integrity of the positioning systems. They can significantly contribute to different fields of geosciences as well.
The user segment means the various categories of permitted customers (military, state or civilian) and their different types of receiver.
Similarly to the satellites, these receivers have antenna, radio module, clock (or frequency standard) and built-in processor. They receive and decode the signals and the information, determine the positions and store the raw data if it is necessary. (Later it will be discussed in details.)
2.3.3.2.3 Practical implementations
For the sake of historical fidelity we have to mention that the first operational navigation satellite system was the TRANSIT or NAVSAT − Navy Navigation Satellite System (NNSS) − popularly called as NNSS Doppler after its observation and data processing method. It was designed to update the inertial navigation systems used on US Navy’s Polaris atomic submarines.
The satellites were launched on individual polar orbits at approximately 1100 km above the Earth surface with a period of 106-109 minutes. The 4-6 orbits were almost evenly distributed along the equator (Fig. 3-1.).
Based on nominal 5 MHz quartz oscillator two stable coherent frequencies (≈150 MHz and ≈400 MHz) were generated. The predicted orbit elements, navigation and other military information were phase modulated on both frequencies. The timing signals were transmitted in every even minute. The application of two frequencies permitted corrections for ionospheric refraction.
Approximately in every hour one satellite was observed for 15-20 minutes that allowed the single navigational accuracy of 200 m. The positions were derived from the Doppler shift of the received frequencies, measured as Doppler counts.
The NNSS was opened to civil users in 1967. During the geodetic applications the relative accuracy was improved to the level of about 20 cm using the method of multi-location. This system did not fulfil continuous positioning requirement of GNSS systems, but it was proper for navy's navigations. The system was ceased in 1996. (There are no sufficient information on secret Soviet version of Doppler system called “Cikada”.)
The first generation GNSS systems are the NAVSTAR Global Positioning System (GPS) and the similar Russian version called GLONASS.
The GPS replaced the TRANSIT system. It was realised by the US Department of Defence. The civilian use was allowed by the US Congress. The system was declared fully operational in 1994.
Theoretically 24 satellites are planned on six semi-synchronous orbits (Fig. 3-2.). The orbit inclination is 55 degrees, the average height is 20200 km and the period is 12 hours. The orbits and the satellites are distributed in a way that at any time and from any place on the Earth surface at least four satellites can be simultaneously observed. Because of the persistent satellite upgrades and replacements the number of satellites is more than 24 at the present.
Two carriers are generated - L1 = 1575.42 MHz and L2 = 1227.60 MHz – which are based on 10.23 MHz fundamental frequency. Different measuring and data codes are modulated on the carriers. Code division multiple access (CDMA) method is applied. The different satellites are identified by the code series.
The original positioning concept is based on the measurement of the travel time of code series that allows faster and more frequent positioning than in the case of TRANSIT. The phase information proved to be the means of high precision relative positioning. (The details will be discussed later.)
The GLONASS system is very similar to the GPS. The originally military system was opened for civilian use in 2007.
Theoretically 24 satellites are planned on only three near semi-synchronous orbital planes. The orbit inclination is 64.8 degrees, the average height is 19100 km and the period is 11.3 hours.
Unlike the GPS coding, in this system the frequency-division multiplexing (FDM) method was chosen. All the satellites have different frequencies around L1=1.602 GHz and L2=1.246 GHz.
In spite of the fact that the GPS, GLONASS and other supplementary systems fulfil the requirements of global navigational demands, the second generation GNSSs means modernised and newly developed systems.
In the case of GPS, new measuring codes are planned for military and civilian for users. The introduction of the third additional L5=1176.45 MHz frequency improve the performance of the system.
The introduction of CDMA coding instead of FDM, and the use of 30 satellites instead of 24 upgrades the capacity of the GLONASS system.
The Galileo Positioning System of the European Union (EU) and the European Space Agency (ESA) will be the first non-military system, which is independent of GPS but at the same time the systems will supplement each other. Although the Galileo is in preparation, the concept and the time schedules may be changed according to the available budget in the frame of Public Private Partnership (PPP).
Originally 30 satellites were planned in three near semi-synchronous orbits. The orbit inclination is 56 degrees, the average height is 23600 km and the period is 14 hours. At the moment two test satellites are in orbit.
Four carrier frequencies of the system − E2(L1)=1572.42 MHz, E5a(L5)=1176.45 MHz, E5b=1207.14 MHz and E6= 1278.75 MHz − partly overlap with the GPS carriers. Ten different signals will be coded on the carriers using the CDMA method. The interference with GPS signals have to be avoided.
Based on different signals and carriers five different services with different navigational accuracies will be adopted. The first less accurate is free of charge; all the other more precise services have to be purchased.
The Chinese COMPASS (or Beidou-2) is similar to modernized GPS and Galileo systems. 30 satellites are planned in near semi-synchronous orbits. The orbit inclination is 55.5 degrees, the average height is 21150 km and the period is 12.6 hours. 5 additional satellites are planed on geostationary orbit (36000 km height). At the moment 5 satellites are operational.
The used carrier frequencies are B1=1.561098 GHz, B1-2=1.589742 GHz, B2=1.207.14 GHz and B3=1.26852 GHz. The signals are based on CDMA method.
The system is basically designed for military and state use. China officially joined to the Galileo system as well.
The three inclined orbit planes of GLONASS and Galileo together with the geostationary orbit of COMPASS (dashed line) are shown in Fig. 3-3.
Regional navigation systems (Beidoi-1 − China, IRNSS − India, QZSS − Japan) are not subjects of this study.
Figure 3-1.: Four polar orbits of NNSS
Figure 3-2.: The six orbit planes of GPS
Figure 3-3.: The three orbit planes of GLONASS and Galileo, geostationary orbit of COMPASS
3.3.3 Elements of satellite navigation
In spite of the evident differences the introduced navigation systems are very similar to each other. Therefore the well known and most frequently applied GPS system is used to introduce the main elements of satellite navigation. The basic ideas and equations can be adapted to all other systems.
The numerical qualification of the positioning performance is a common requirement; therefore the next measures are shortly introduced.
In the geodetic error theory usually the expression is used, where σAis the measure of accuracy and σPis the measure of precision, c is the average systematic error which cannot be distinguished from the estimated parameters. In this sense the estimated value (including c) and σPquantities are the analogues of the expected value and dispersion in probability theory; furthermore, they are the analogues of the mean and standard deviation in mathematical statistics. The σP which is estimated by least squares adjustment is frequently called mean squares error.
The probability that the errorless parameter is inside the -3σA and +3σA interval around the estimated parameter is 99.7%.
The adjustment theory is not subject of this section. In the following Chapters only σ notation will be used and classified as accuracy or precision.
3.1.3.3.1 Signal structures
The carrier frequencies L1 and L2 are the multiples of the f0=10.23 MHz fundamental frequency:
L1: f1= 154×10.23 MHz = 1575.42 MHz , λ1=0.19029 m,
L2: f2= 120×10.23 MHz = 1227.60 MHz , λ2=0.24421 m,
Where f1 and f2 are the frequencies and λ1 and λ2 are the corresponding wavelengths.
Different measuring codes (or ranging signals) and data codes (orbital, timing and other military information) are modulated on the carrier waves using binary biphase modulation. The principle is shown in Fig. 3-4. Using the code sequences -1 and +1 (according to binary 0 and 1) the phase is shifted by 180 degrees (if multiplied by -1) or not (if multiplied by +1). The ranging codes are classified as pseudo-random noise (PRN).
The frequency of C/A (coarse/acquisition or civil/access) code modulation is f0/10=1.023MHz. The PRN codes are repeated in every millisecond. The code length is 1023 chips (bites) and the chipping rate multiplied by the speed of light equals to approximately 300 m chip length. Every satellite has unique C/A code sequence.
Figure 3-4.: The principle of binary biphase modulation
The frequency of P (precision or protected) code modulation is f0=10.23 MHz. This code is repeated approximately in every 266.4 days. The chipping sequence is divided into 37 weeks and each satellite has its own weekly sequence. These sequences are repeated in every week. The chip length is approximately 30 m.
The secret P code was opened because of the first Gulf War. To encrypt the P code the Y=P·W code was introduced in the case of newly launched satellites. It is called anti-spoofing (A-S). The frequency of W modulation is f0/20 Mhz.
The data code (D) modulation frequency is f0/204600=50 Hz. The total message contains 1500 bites which are divided into five subframes. The subframes are divided into ten words. One word consists of ten bites.
The navigation messages contain the almanac data (less accurate orbital data), the broadcast ephemerides (BE) and the satellite clock corrections. Each satellite broadcasts its own BE ephemeris and the almanac of all other satellites.
The BE contains modified Keplerian orbit elements that are necessary for position computation. (The GLONASS provides three positions and three velocities in ECEF coordinate system quarter-hourly. This is equivalent to instantaneous Keplerian orbit.)
The GPS modulation can be represented by the equations
, (1)
where a indicates the amplitudes, C replaces the C/A code and t is the time. The C/A and P codes have a quadrature phase relationship on L1.
During GPS modernisation the C/A will be coded on L2 as well, and a new L5 carrier and safer military (M) code will also be introduced.