II Morrow Inc. Apollo NMC, Operating Manual

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GPS Reference
This section provides an overview of the Global Positioning System
(GPS), its limitations, advantages, and special features.
General Information
GPS, is a constellation of 24 satellites in six lanes with four satellites
per plane in 10,898 nm orbits. The orbit period is 12 hours, or two
orbits per day. Each satellite transmits a signal with special coding
unique to each satellite called a pseudorandom code and allows any
GPS receiver to identify each satellite. A precise time mark is provided
in the code that is used to determine the range from the satellite to the
receiver. Each satellite transmits at the same frequency (1575.42
MHz). The GPS equipment in the Apollo Navigation Management
System (NMS) only receives signals. The receiver looks for the
satellites by searching for the codes transmitted by each satellite. Each
satellite found is used to determine a position solution. An additional
satellite is used to determine the time differences between the time of
signal transmission and its reception. Each satellite transmits its signal
at precisely the same time. Each satellite has two cesium beam clocks
on board as a time reference to ensure accurate timing. Knowing the
length of time it takes for the signal to reach the GPS receiver allows it
to determine the distance to the satellite. This distance is referred to as
the range from the satellite.
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Theory GPS