Specifications

The Enhancement of a Multi-Terrain Mechatron for Autonomous Outdoor Applications
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In order to make this measurement both the satellite and receiver require a precise
system of time keeping down to the nano-second. Atomic clocks using rubidium (and
later caesium) are installed in each satellite to achieve accuracy and long term
stability of a few parts in 10
14
per day (about 1 sec in 3,000,00 years) . Costing
between US$50 000 and $100 000 each and weighing more than 20 kg, atomic clocks
are not suitable for the receiver equipment so quartz crystal clocks are used. They
offer excellent short term stability therefore the receiver uses the timing signals from
at least 4 satellites to update the onboard clock, effectively getting atomic clock
accuracy for a fraction of the cost.
The GPS system consists of three segments as illustrated is Figure 4.19. The space
segment consists of a constellation of 24 (27 including 3 backups) orbiting satellites
and the signals they broadcast. They orbit on six equally spaced planes at an altitude
of about 20200 km and period 11 hours 58 minutes guaranteeing at least four satellites
are visible at anytime from any location.
Figure 4.19 Three GPS operation segments
The control segment consists of five ground stations, including the master control at
Schriever Air Force Base in Colorado Springs. These facilities provide satellite health
monitoring, data up-linking, telemetry, satellite command and control, and the
calculations of satellite orbit and clock offset. By tracking each satellite and
calculating the ephemeris (satellite position) updated clock and repositioning data can
be sent to the satellites to compensate for errors caused by gravitation forces and solar
radiation pressure.
Space Segment
User Segment
Control Segment