GPS has 3 parts: the space segment, the user segment, and the control segment. The space segment consists of 24 satellites, each in its own orbit 11,000 nautical miles above the Earth. The user segment consists of receivers, which you can hold in your hand or mount in your car. The control segment consists of ground stations (five of them, located around the world) that make sure the satellites are working properly.
One trip around the Earth in space equals one orbit. The GPS satellites each take 12 hours to orbit the Earth. Each satellite is equipped with an accurate clock to let it broadcast signals coupled with a precise time message. The ground unit receives the satellite signal, which travels at the speed of light. Even at this speed, the signal takes a measurable amount of time to reach the receiver. The difference between the time the signal is sent and the time it is received, multiplied by the speed of light, enables the receiver to calculate the distance to the satellite. To measure precise latitude, longitude, and altitude, the receiver measures the time it took for the signals from four separate satellites to get to the receiver.
The GPS system can tell you your location anywhere on or above the Earth to within about 300 feet. Even greater accuracy, usually within less than three feet, can be obtained with corrections calculated by a GPS receiver at a known fixed location.
To help you understand the GPS system, let’s take the three parts of the system – the satellites, the receivers, and the ground control – and discuss them in more detail. Then we’ll look more closely at how GPS works.
- The Space Segment of the system consists of the GPS satellites. These space vehicles (SVs) send radio signals from space.
- The nominal GPS Operational Constellation consists of 24 satellites that orbit the earth in 12 hours. There are often more than 24 operational satellites as new ones are launched to replace older satellites. The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each day). There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. This constellation provides the user with between five and eight SVs visible from any point on the earth.
The first eleven spacecraft (GPS Block 1) were used to demonstrate the feasibility of the GPS system. The orbit inclination used for these satellites was 63 degrees, differing from the 55 degrees used for the operational system. The Block 2 spacecraft began the operational system. The Block 2A spacecraft (A = Advanced) were a slight improvement over the Block 2.
GPS Block 1 satellites formed the GPS Demonstration system and were followed by the Block 2 operational system.
GPS Block 1
3-Axis stabilized, nadir pointing using reaction wheels. Dual solar arrays supply over 400 watts (EOL). NiCd batteries. S-Band (SGLS) communications for control and telemetry. UHF cross-link between spacecraft. Hydrazine propulsion system.
Two L-Band navigation signals at 1575.42 MHz (L1) and 1227.60 MHz (L2)
GPS Block 2
GPS Block 2 is the Operational system, following the Demonstration system comprised of Block 1 spacecraft. The Block 2A are "Advanced" versions of this spacecraft. The complete constellation has 24 spacecraft in 6 high-altitude orbit planes.
3-Axis stabilized, nadir pointing using reaction wheels. Dual solar arrays supply 710 watts (EOL). S-Band (SGLS) communications for control and telemetry. UHF cross-link between spacecraft. Hydrazine propulsion system.
Two L-Band navigation signals at 1575.42 MHz (L1) and 1227.60 MHz (L2). Each spacecraft carries 2 rubidium and 2 cesium clocks. Also carry nuclear detonation detection sensors.
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