F-2 Acutime 2000 Synchronization Kit User Guide
Theory of Operation F
F.1 GPS Satellite MessageEvery GPS satellite transmits the Coarse/Acquisition (C/A) code and
satellite data modulated onto the L1 carrier frequency
(1575.42 MHz). The C/A code is a unique pseudo-random sequence
for each satellite. The satellite data transmitted by each satellite
includes a satellite almanac for the entire GP S system, its own
satellite ephemeris, and its own clock correction.
The satellite data is transmitted in 30-second frames. Each frame
contains the clock correction and ephemeris for that specific satellite,
and two pages of the 50-page GPS system almanac. The time required
to transmit the complete system almanac is 12.5 minutes; the time to
transmit the satellite ephemeris is 30 second s.
The system almanac contains information about each of the satellites
in the constellation, ionospheric data, an d special system messages.
The ephemeris contains detailed orbital in formation for a specific
satellite. The GPS system almanac changes infrequently and is
typically valid for weeks. Ephemeris data changes hourly, but is valid
for up to four hours. The GPS control segment updates the system
almanac weekly and the ephemeris hourly through three ground-
based control stations. During normal operation, the smart antenna
updates the ephemeris stored in its memory at 30-minute intervals.
The performance of a GPS receiver at power-on is determined largely
by the availability and accuracy of the satellite ephemeris data and the
availability of a GPS system almanac.