Lassen SQ, Справочное руководство

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Lassen SQ GPS Receiver 4 9
Operation and Performance 4
4.2
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 Lassen SQ GPS 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.