5-7. Pseudorange. A pseudorange is the time delay between the satellite clock and the receiver clock,
as determined from C/A- or P-code pulses. This time difference equates to the range measurement but is
called a pseudorange since at the time of the measurement, the receiver clock is not synchronized to the
satellite clock. In most cases, an absolute 3D real-time navigation position can be obtained by observing
at least four simultaneous pseudoranges.
between the Doppler-shifted satellite and receiver frequencies. The phase differences are continuously
changing due to the changing satellite earth orbit geometry. However, such effects are resolved in the
receiver and subsequent data postprocessing. When carrier phase measurements are observed and
compared between two stations (differential mode), 3D baseline vector accuracy between the stations
below the centimeter level is attainable. New receiver technology and processing techniques have
allowed for carrier phase measurements to be used in real-time centimeter positioning.
5-9. Broadcast Messages and Ephemeris Data. Each NAVSTAR GPS satellite periodically
broadcasts data concerning clock corrections, system and satellite status and, most critically, position or
ephemeris data. There are two basic types of ephemeris data--broadcast and precise.
a. Broadcast Ephemerides. Broadcast ephemerides are predicted satellite positions broadcast
within navigation messages (transmitted from satellites in real time). The ephemerides can be acquired
in real time through a receiver capable of acquiring either a C/A- or P-code. Broadcast ephemerides are
computed using the past tracking data of satellites. The satellites are continuously tracked by the
monitor stations to obtain current data for use in orbit predictions. The data are analyzed by the master
control station, and new parameters for the satellite orbits are transmitted back to the satellites. This
upload is performed daily, and the new predicted orbital elements are transmitted every hour by the
navigation message.
b. Precise Ephemerides. Precise ephemerides are based on actual tracking data that are
postprocessed to obtain more accurate satellite positions. These ephemerides are delayed for processing
but are more accurate than the broadcast ephemerides because they are based on actual tracking data and
not predicted data. Nonmilitary users can obtain this information from the NGS or private sources that
maintain their own tracking networks. For most survey applications, broadcast ephemerides are
adequate to obtain needed accuracies.
EN0593
5-4
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