NAVIOR-24S
™
CHAPTER IV
Operation and Performance
52
NAVIOR-24S ™
Revision
V1.00 20.05.2008
GPS Satellite Message
Every GPS satellite transmits the Coarse/Acquisition (C/A) code and satellite data modulated onto
the L1 carrier frequency (1575.42 MHz). The satellite data, transmitted by each satellite, includes a
satellite almanac for the entire GPS 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
almanac is repeated every 12.5 minutes. The ephemeris is repeated every 30 seconds.
The system almanac contains information about each of the satellites in the constellation,
ionospheric data, and special system messages. The GPS system almanac is updated weekly and is
typically valid for months. The ephemeris contains detailed orbital information for a specific
satellite. 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 NAVIOR-24S GPS/GLONASS receiver module updates its
ephemeris and almanac as needed.
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.
