Viavi 87339, Operation Manual

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Principals of Operation
6 -13
Refer to Fig. 6-16. Due to the atomic clocks onboard the satellites, the time at which the
satellite signal is transmitted is known very precisely. All satellite clocks are adjusted to
be synchronized with each other and UTC (universal time coordinated). In contrast, the
receiver clock is not synchronized to UTC and is therefore slow or fast by Δ t0 (clock
bias). The sign Δ t0 is positive when the user clock is fast. The resultant time error Δ t0,
causes inaccuracies in the measurement of signal travel time and the distance R. As a
result, an incorrect distance is measured that is known as a pseudorange.
Fig. 6-16 GPS Pseudo Range
6.1.4.D Obtain Satellite Positions
GPS receivers download an almanac into memory which defines where in the sky each
satellite is, moment by moment. GPS satellites are constantly monitored by the U.S.
Department of Defense. Slight orbital errors are caused by gravitational pulls from the
moon and sun and by the pressure of solar radiation on the satellites. Radar is used to
check each satellite's exact altitude, position and speed. This information (ephemeris
data), is then relayed back up to the respective satellite, which in turn transmits the
ephemeris data in Nav Data subframes 2 and 3. Ephemeris data is updated every two
hours and is valid for four hours.