- Composed of satellites that transmit signals from space, on the basis of which time and position of the user is measured.
- Set of satellites is called as constellation.
- GPS uses two satellite constellations i.e. NAVSTAR and GLONASS.
- NAVSTAR (Navigation satellite timing and ranging)
- NAVSTAR composed of 24 satellites, arrayed in 6 orbital planes, inclined 55 degrees to the equator and with a 12 hours period.
- They orbit at altitudes of about 20,200km each.
- Each satellite contains four precise atomic clocks, only one of which is in used at a time.
Fig: Space Segment
- Control segment consists of a group of 5 ground based monitor stations, three antennas and a master control station.
- The Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado.
- The monitor stations measure signals from the SVs continuously and provides data to the master control station.
- The master control station calculates satellite ephemeris and clock correction coefficients and forwards them to an antenna.
- The antenna transmit the data to each satellite at least once a day. The SVs then send subsets of the orbital ephemeris to GPS receivers over radio signals
- GPS User Segment consists of the GPS receivers and the user community.
- The typical receiver is composed of an antenna and pre-amplifier, radio signal microprocessor, control and display device, data recording unit, and power supply.
- GPS receivers convert SV signals into position, velocity, and time estimates. A minimum of four satellites are required to compute the four dimensions of X, Y, Z (position) and Time.
GPS Working Mechanism
Each of the 24 satellites emits signals to receivers that determine their location or range by computing the difference between the time that a signal is sent and the time it is received. The signal contains data that a receiver uses to compute the locations of the satellites and to make other adjustments needed for accurate positioning.
The receiver must account for propagation delays, or decreases in the signal's speed caused by the ionosphere and the troposphere. GPS satellites carry atomic clocks that provide extremely accurate time. The time information is placed in the codes broadcast by the satellite so that a receiver can continuously determine the time the signal was broadcast. With information about the ranges to three satellites and the location of the satellite when the signal was sent, the receiver can compute its own three-dimensional position. An atomic clock synchronized to GPS is required in order to compute ranges from these three signals. However, by taking a measurement from a fourth satellite, the receiver avoids the need for an atomic clock. Thus, the receiver uses four satellites to compute latitude, longitude, altitude, and time.
MEASUREMENTS OF DISTANCE:
- Satellites broadcast radio signals (EM radiation)
- Simple distance calculation d = v * t
- Velocity = velocity of light 186,000 miles per seconds
- time is known (difference between send & receive)
- distance is calculated