Appendix 04 - 1
APPENDIX 04: Differential Functions (Skymap IIIC only)
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 03, 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 03. 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, to orbital errors and to 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 which
(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, which 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.
Uses Of DGPS
Standard GPS provides very adequate accuracy for most navigational tasks, but the additional precision
available through DGPS opens up other possibilities in aviation and in many other spheres of activity.
Once the aviation authorities are prepared to approve DGPS-based low-visibility approach and landing
systems, these could provide low-cost precision facilities worldwide. The benefits of this could be
especially useful at small airports where the expense of an ILS or MLS system could never be justified
and in developing countries where few if any navigation aids exist today.
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