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 calcula-
tions and to come up with the positional accuracy's mentioned previ-
ously. DGPS even removes the deliberate degrading of accuracy asso-
ciated with the DOD's Selective Availability. It is essential to process the
data from the reference receiver very rapidly and to transmit the correc-
tion 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out of date.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-visi-
bility approach and landing systems, these could provide low-cost preci-
sion facilities world-wide. 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 naviga-
tion aids exist today.
DGPS has also already become an essential tool of the off-shore oil
industry, enabling drill bits to be placed with more precision than any
other system. Marine navigation, survey work and cartography, emer-
gency services, agriculture and forestry and road transport fleet manage-
ment are just some of the non-military applications already relying more
and more upon the accuracy of DGPS.
Rev 1 Mar/2000
KMD 150 Pilot's Guide