1. GPS Satellite System Description
The Global Positioning System (GPS) satellites consist of a constellation of
satellites orbiting at an altitude of 10,400 miles above the earth, each satellite
completing one orbit every 12 hours. Ground-based control/monitor stations
monitor the health of each of the satellites, as well as providing precision
timing updates on a regular basis.
The satellite system comprises 24 satellites in six orbital planes inclined at
around 60 degrees to the equator, and provides positioning, velocity, and time
to user equipment worldwide.
GPS satellite positions are known to an accuracy of a few meters, and each
satellite carries an ensemble of atomic clocks that are maintained to an
accuracy of a few nanoseconds. Now that the selective availability function of
the GPS satellites has been disabled, this allows receiver position to be
calculated to an accuracy of approximately 10 meters
The signal transmitted from each satellite consists of two carrier frequencies,
referred to as L1 and L2. The L1 frequency is 1575.42 MHz and the L2
frequency is 1227.6 MHz. To provide enhanced performance for military and
other (US Government Authorized) users the L1 signal is modulated with a
precision (P) code, and also a coarse acquisition (C/A) code.
The GlobalTyme operates on the C/A code transmitted from each satellite.
This C/A code, which is unique for each satellite, contains information on the
satellite identity for acquisition and tracking. The C/A pseudo random number
(PRN) code is a 1023 bit code that repeats at the rate of every millisecond.
Operating on the L1 band and utilizing the C/A code transmissions, the ptf
3203A GlobalTyme determines time and frequency by measuring the time of
arrival of a precise timing mark transmitted by each of the satellites, and
computing the time against it’s known (previously determined and entered)
position. This is the basis for the GlobalTyme’s one pulse per second (1PPS)
output.
As the GlobalTyme time and frequency outputs are determined from satellite
transmissions calculated and referenced to the United States Naval
Observatory (USNO) through the GPS Master Clock System, then traceability
to International Time Scales is provided through data provided by the USA’s
National Institute of Standards and Technology (NIST) and the international
timekeeper, the Bureau Internationale des Poids et Measures (BIPM) in
France.
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