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For example, the GPS receiver might determine that it is exactly
12,000 miles from satellite A, 12,700 miles from satellite B, and
13,100 miles from satellite C. At the same time, the aircraft’s encod-
ing altimeter might be indicating an altitude of 9,500 feet MSL. There
is only one point in space that satisfies these four measurements.
GPS DATA SIGNALS
Two of the primary types of signals that the GPS satellites broadcast
are almanac and ephemeris data. These signals enable the GPS
receiver to quickly lock on to the satellites in view. Otherwise, the
receiver would have to look for each of the 24 satellites to determine
which ones could be used.
Almanac data is very crude data which describes the approximate
orbital position of the satellites. Each of the 24 satellites transmits the
almanac data for all satellites, so a GPS receiver has only to listen to
one satellite in order to know which satellites are “visible” (in the sky)
at that particular time. Almanac data is good for about six months, so
when you turn the receiver off, then back on a month later, it will
know what satellites to look for.
Ephemeris data is very precise data which each satellite transmits to
tell the GPS receiver exactly where it is and what its orbital parame-
ters will be for about the next four hours. Each satellite transmits its
own unique ephemeris data.
Appendix F
F-2
GPS Primer
Appendix F
12,000 NM
12,700 NM
13,100 NM
9500 FT
MSL
NOT TO SCALE
Ç
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