2.1.2 Introduction to GPS



Some characteristics of satellite-based positioning systems, past, operational and proposed, are:



NAVSTAR Global Positioning System





NAVSAT & other LEO systems

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TRANSIT Doppler System


The history of TRANSIT coincides with the start of the Space Age (4th October 1957). Sputnik I was launched and the Doppler shift of the signals were used to determine the satellite orbit. The method was inverted so that if the orbit were known, the position of the receiver could be determined (PARKINSON et al, 1995). Special features and milestones:

Figure 1. The TRANSIT Doppler satellite positioning system configuration.


ARGOS System


ARGOS is another satellite system which uses the Doppler principle for positioning. ARGOS is a cooperative project between the French Centre National d'Etudes (CNES), NASA, and the U.S. National Oceanic and Atmospheric Administration (NOAA), and was first deployed in 1978. Transmitters are operated by users on variety of "platforms" (buoys, animal tracking, radiosondes, etc.), and satellites act as receivers (one of two U.S. TIROS weather stations). CNES computes position and velocity of platform, sends information (and the bill!) to user. Other such "subscriber" systems in place include the COSPAS-SARSAT search & rescue system and GEOSTAR. The important distinction is that ARGOS is essentially a satellite-based tracking system, while many of the other systems (including GPS) are self-navigating systems.

The GLONASS System


Russian equivalent to the GPS system, having the following characteristics (KLEUSBERG, 1990; IVANOV & SALISTCHEV, 1991; PARKINSON et al, 1995):

Figure 2. The GLONASS satellite configuration.


Table Comparing GPS and GLONASS ( IVANON & SALISTCHEV, 1991 )
Ephemeris information presentation method 9 parameters of s/c motion in the gecentric rectangular rotated coordinate system Interpolation coefficients of satelite orbits
Geodesic coordinate system SGS 85 WGS 84
Referencing of the ranging signal phases To the timer of GLONASS system To the timer of GPS system
System time corrections relative to the universal coordinates time ( UTC ) UTC ( SU ) UTC ( USNO )
Duration of the almanac transmission 2.5 min 12.5 min
Number of satelites in the full operational system 21 + 3 apares 21 + 3 spares
Number of orbital planes 3 6
Inclination 64.8 55
Orbit altitude 19.100 km 20.180 km
Orbital period 11 h 15 min 12 h
Satelite signal division method Frequency division Code division
frequency band allocated 1602.5625-1615.5
0.5 MHz
1575.42 1 MHz
Type of ranging code PRN-sequence of maximal length Gold code
Number of code elements 511 1023
Timing frequency of code 0.511 MHz 1.023 MHz
Crosstalk level between two neighboring channels - 48 dB - 21.6 dB
Synchrocode repetition period 2 sec 6 sec
Symbol number in the synchrocode 30 8

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© Chris Rizos, SNAP-UNSW, 1999