Sect_2.1.2

2.1.2 Introduction to GPS

SATELLITE-BASED POSITIONING SYSTEMS

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

TRANSIT Doppler (NNSS)

Operational since 1964
Typically 4-6 useable satellites at 1075km altitude in polar orbits
U.S. military navigation system, but open to all users (with restrictions)

TSIKADA

Russian equivalent to TRANSIT Doppler system

NAVSTAR Global Positioning System

Multi-satellite system to replace TRANSIT
24 satellite constellation at 20200km altitude
U.S. military navigation system, but open to all users (with restrictions)

GLONASS

Russian equivalent to GPS

STARFIX

Commercial positioning system for continental U.S.

GEOSTAR/LOCSTAR

Electronic package for 2-way information transfer on communication satellites
Limited coverage for subscribers
U.S. and French initiative

ARGOS

Doppler system from CNES (France) and NOAA (U.S.)
Transmitters on ground, receivers in two weather satellites
Subscriber service for a variety of platforms

NAVSAT & other LEO systems

ESA proposal for two-way GPS civilian system
18 satellites in mix of HEO & GEO orbits
Some may be communication satellites (GEO & LEO)

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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:

Development of TRANSIT began 1959, with first satellite launched in 1961.
Became operative for military use in 1964 and was released for civil use in 1967.
Two transmission frequencies (400MHz & 150MHz), with phase modulation of the navigation message.
Small number of satellites (4-6) and low altitude (1075km) means coverage is not time continuous.
In polar orbit.
Russian Republic (formerly the USSR) operates a similar system known as Tsikada.
To be phased out in 1997.

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):

21 satellites + 3 active spares.
3 planes, 8 satellites per plane.
64.8 inclination, 19,100km altitude (11hr 15min period).
Dual-frequency (1597-1617MHz, 1240-1260MHz).
Each satellite transmits a different frequency.
Spread-spectrum PRN code signal structure.
Global coverage for navigation based on simultaneous pseudo-ranges, with an accuracy of better than 100m horizontal, 150m vertical.
Considered an important complement to GPS for integrity monitoring and quality control purposes.

Figure 2. The GLONASS satellite configuration.

Table Comparing GPS and GLONASS ( IVANON & SALISTCHEV, 1991 )
Parameter GLONASS GPS

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

Table Comparing GPS and GLONASS ( IVANON & SALISTCHEV, 1991 )
Parameter	GLONASS	GPS
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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