How Does GPS Work?
Instead, an ingenious system is used whereby GPS receivers require to
be fitted with only reasonably accurate (and thus low-cost) clocks. This
involves two important techniques:
1) Code matching or synchronization. Each satellite transmits, for
civilian use, a unique 1023 bit code. This code is repeated every mil-
lisecond (one thousandth of a second).
In the receiver, circuitry generates an identical code that is then slid back
and forth in time until it exactly matches the code being received from the
satellite.
When a perfect match is achieved, the time at which the receiver is gen-
erating the start bit of the code is recorded. The difference in time (or
offset) between the receiver generating the code and the satellite gener-
ating the code is therefore measurable because the time at which the
satellite should have generated the code is known.
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 mil-
liseconds (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 erro-
neous. Once this fix is calculated, a second fix is calculated using satel-
lites 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 pre-
sent.
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
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Rev 1 Mar/2000
KMD 150 Pilot's Guide
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