BendixKing SKYMAP IIIC - Manual (Page 133)

HOW DOES GPS WORK?
This gives a very accurate way of measuring the time differences between transmission
and reception of the signals generated by the satellites, but does not help with the need for
an absolute time reference. One must bear in mind that if the clock in the receiver were
only 100 milliseconds (one tenth of a second) out with respect to universal time, the
calculated distance from the satellite would have an error of 1,860 miles.
This problem is overcome by a second technique:
2) Making use of an extra satellite. In order to obtain a positional fix in three
dimensional space, it is necessary to know the exact distance to each of three satellites.
The exact distance is however not known because of the presence of an offset or error in
the receiver's clock with respect to universal time.
A fix is nevertheless calculated using the three satellites, all be it erroneous. Once this fix
is calculated, a second fix is calculated using satellites 2 and 3 in conjunction with a fourth
satellite. This is repeated for satellites 3, 4 and 1 then 4, 1 and 2. This gives four
calculated points in space.
If the four calculated points do not overlap, a timing error must be present.
A slight correction is then fed into the receiver's clock and the points are calculated again.
They move closer to each other.
This process is repeated until all the calculations tie up correctly at the same point. The
exact position in three-dimensional space is then known and the offset in the receiver's
clock eliminated.
Similarly, a two-dimensional fix can be established from three satellites.
As a result of this technique the receiver does not initially need to know absolute universal
time. All that is needed is a reasonably stable clock for taking relative measurements over
the short periods between the signals received from each satellite.
An approximate knowledge of universal time is required, to know which satellite to expect
in which part of the sky. The first satellite the receiver acquires after switch on usually
gives this.
The exact universal time is transmitted as part of the almanac and health status data by
each satellite. This is then corrected by the receiver for signal transition delay as part of
the position fix calculations.
APPE
NDICES
Accuracy and Reliability
All GPS satellites transmit two separate pseudo-random codes on two different
frequencies.
The most accurate is the P code, which is transmitted at both 1.2276 GHz (1.2276
thousand million cycles per second) and 1.57542 GHz.
The P code is extremely complex and has a repetition rate of one week. This code is
restricted to military use only, is capable of supreme accuracy and is supposedly
unbreakable.
The signal available to civilian users is transmitted on 1.57542 GHz only and is called the
C/A code. C/A stands for Coarse Acquisition.
This code is relatively simple compared to the P code and has a repetition rate of one
millisecond.
The present maximum accuracy for receivers using the C/A code is 15 meters.
Rev 9 Jul 2006 D-2 Skymap/Tracker IIIC Pilot's Guide