BendixKing SKYMAP IIIC - Manual (Page 136)

DIFFERENTIAL FUNCTIONS (Skymap IIIC only)
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APPENDIX E DIFFERENTIAL FUNCTIONS (Skymap IIIC only)
DIFFERENTIAL GPS
Your Bendix/King unit has been designed to enable it to make use of Differential GPS
(DGPS).
What Is DGPS?
GPS on its own is, as we have seen in APPENDIX D , an extremely accurate system for
all general navigation purposes. It is not, however, perfect. The most significant source of
error in GPS positional calculations is the deliberate downgrading of accuracy known as
Selective Availability (SA) described in APPENDIX D . Satellite clocks, although superbly
accurate, also have tiny errors, which can affect the accuracy of a fix. So also does any
slight drift from a satellite's predicted orbit. The signals from satellites have to pass
through the earth's atmosphere and, although radio signals travel at the speed of light, this
is only a constant through a vacuum. Things like water vapour in the troposphere and
charged particles in the ionosphere do slow down satellite signals and cause slight delays
that translate into errors in position. And finally it must be admitted that GPS receivers are
not perfect either. These can themselves induce further small positional errors, mainly as
a result of receiver noise or clock inaccuracy.
DGPS can virtually cancel out all of the errors due to SA, orbital errors and satellite clock
inaccuracy. It also greatly reduces any error caused by the atmospheric delay to radio
signals. What DGPS cannot correct for are only the inaccuracies due to the GPS receiver
itself and possible “multi-path” errors that (like “ghosting” on a TV) are due to secondary
signals being received after reflection off local obstructions. The net result of all this is that
while standard GPS can experience errors of between 15 and 100 metres (depending largely
on whether SA is active or not) DGPS is already capable of accuracy’s down to 1.3 metres
horizontally and 2.0 metres vertically.
How Does DGPS Work?
Any two GPS receivers within a few hundred miles of each other will receive satellite
signals that have been affected by virtually the same errors as each other. The signals
emanate from the same group of satellites, travel through the same portion of the earth's
atmosphere and so will be subjected to the same delays. Thus, all the errors (except
those due to multi-pathing and to the receivers themselves) will be common to both. So if
one of these receivers is located at a fixed point, which has been extremely and precisely
surveyed, it can be used as a reference. Instead of using the signals to calculate its
position, since it already knows its position it can calculate the errors in the incoming
signals.
Having established the magnitude of the errors all that is then needed is an associated
transmitter broadcasting signals to tell any mobile GPS receivers in the area exactly what
these errors are. The mobile GPS's are then able to use this data to correct for the errors
in their raw calculations and to come up with the positional accuracy’s mentioned above.
DGPS even removes the deliberate degrading of accuracy associated with the DoD's
Selective Availability. It is essential to process the data from the reference receiver very
rapidly and to transmit the correction data to the mobile GPS receivers almost
instantaneously since, especially if SA is active, these errors are constantly varying.
Transmitters and receivers have to be designed to work at high baud (bits per second)
rates to transfer the necessary data before it becomes out of date.
Rev 9 Jul 2006 E-1 Skymap/Tracker IIIC Pilot's Guide