Appendix 03 - 2
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. This is usually given by the first satellite the receiver acquires after switch on.
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.
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.
There is the possibility however of this being intentionally degraded (by feeding random errors into the
satellite clocks) to 100 meters for strategic reasons. This is called Selective Availability or S/A and can
be implemented at will by the DoD.
Day to day accuracy is monitored by Falcon Air Force Base, Colorado, who receive information from a
group of ground stations positioned at Ascension Island, Diego Garcia, Kwagale and Hawaii.
These stations receive the satellite signals and check them for timing accuracy. If any errors are
detected, error information is transmitted back to the satellite, which then corrects itself.
Reliability should be good when the full constellation of 24 satellites are in orbit.
To Next Page
Loading...