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GPS Architecture
| GPS | |
|---|---|
| Title | GPS Architecture |
| Author(s) | GMV |
| Level | Basic |
| Year of Publication | 2011 |
The GPS system is divided into three major segments: Space Segment, Control Segment and User Segment
The Space Segment
The main functions of the Space Segment are to generate and transmit code and carrier phase signals with a specific signal structure, and to store and retransmit the navigation message sent by the Control Segment. These transmissions are controlled by highly stable atomic clocks on board the satellites.
The GPS Space Segments are formed by satellite constellations with enough satellites to ensure that the users will have, at least, 4 simultaneous satellites in view from any point at the earth surface at any time.
In the case of GPS, the constellation consists of at least 24 satellites, arranged in 6 orbital planes, with an inclination of 55 degrees in relation to the equator with an additional three satellites in orbit as redundant backup.
Orbits are nearly circular, with eccentricity less than 0:02, a semi-major axis of 26 560 km, i.e. an altitude of 20 200 km, and a period of 12 sidereal hours (11 h 58m 2 s), repeating the geometry each sidereal day. This means that each GPS satellite orbits the Earth twice each day. There is a spare satellite slot in each orbital plane, being the system capable of supporting a constellation of up to 30 satellites on orbit.
The present configuration allows users to have a simultaneous observation of at least 4 satellites in view worldwide, with an elevation masking angle of 15º.
Highly accurate atomic clocks are installed on these satellites, operating at a fundamental frequency of 10.23MHz each. With the help of these clocks, signals are generated from the satellite, to be broadcast to the Earth. GPS uses NAVSTAR satellites manufactured by Rockwell International. Each NAVSTAR satellite is approximately 5 meters wide (with solar panels extended) and weighs approximately 900Kg.
The Control Segment
The Control Segment (also referred to as Ground Segment) is the responsible for the proper operation of the GNSS system. Its basic functions are:
- To control and maintain the status and configuration of the satellite constellation.
- To predict ephemeris and satellite clock evolution.
- To keep the corresponding GNSS time scale (through atomic clocks).
- To update the navigation messages for all the satellites.
- In the case of GPS, the Control Segment is also responsible for Selective Availability
- (S/A) activation in signal transmission.
The GPS Control Segment is composed by a network of Monitor Stations (MS), a Master Control Station (MCS) and the Ground Antennas (GA).
The Master Control Station, located at Colorado Springs, is the core of the Control segment. It is responsible for operating the system, providing command, control and maintenance services to the space segment.
The Monitor Stations are distributed around the world. They are equipped with atomic clocks standards and GPS receivers to continuously collect GPS data for all the satellites in view from their locations. The collected data is sent to the Master Control Station where it is processed to estimate satellite orbits (ephemerides) and clock errors, among other parameters, and to generate the Navigation Message.
Prior to the modernization program, the Monitor Stations network comprised five sites located in Hawaii, Colorado Springs (Colorado, US), Ascension Island (South Atlantic), Diego Garcia (Indian Ocean), and Kwajalein (North Pacific). Cape Canaveral (Florida, US) was incorporated in 2001.
The modernization program added six new stations to the network in 2005: Adelaide (Australia), Buenos Aires (Argentina), Hermitage (UK), Manama (Bahrain), Quito (Ecuador) and Washington DC (USA). Five more stations were added afterwards in 2006: Fairbanks (Alaska), Osan (South Korea), Papeete (Tahiti), Pretoria (South Africa) and Wellington (New Zealand). With this configuration, each satellite is seen from at least three monitor stations, which allows computing more precise orbits and ephemeris data, therefore improving system accuracy.
The Ground Antennas uplink data to the satellites via S-band radio signals. These data includes ephemerides and clock correction information transmitted within the Navigation Message, as well as command telemetry from the MCS. Every satellite can be uploaded three times per day, i.e. every 8 hours; nevertheless, it is usually updated just once a day. The ground antennas are co-located in four of the Monitor stations (Ascension Island, Cape Canaveral, Diego Garcia, and Kwajalein).
The User Segment
The user segment is composed by GPS receivers. Their main function is to receive GPS signals, determine pseudoranges (and other observables), and solve the navigation equations in order to obtain their coordinates and provide a very accurate time. Please refer to GNSS Receivers.