GPS (Global Position System) pg. 2
GPS is the most advanced navigational application made for the average users. For most of
its uses, including location and weather, it is extremely accurate. So why would there be
a need for anything else? Well, taking human nature into account, its always been a
passion to want bigger and better tools. So some crafty engineers came up with
Differential GPS, a means to correct the various inaccuracies and built in imperfections
of the GPS system.
How it Works
Differential GPS or "DGPS" works a bit differently than regular GPS. It can
yield measurements well into to a couple of meters in moving applications and even better
in stationary situations. This improved accuracy has made a significant improvement in the
GPS system. With it, GPS becomes more than just a system for navigating stations and
satellites. In a sense it becomes a universal measurement system capable of positioning
things on a very precise scale, mapping you down to your exact coordinates. Differential
GPS involves the cooperation of two receivers, one that's stationary and another that's
roving around making position measurements.
Global Positioning Satellite
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The stationary receiver is the key to
DGPS, it allows for the satellite measurements to be tied into a single reference. GPS
receivers use timing signals from at least four satellites to establish a position. Each
of those timing signals is going to have some error or delay depending on weather
conditions or other atmospheric chaos as the signal is sent from the satellite the a
receiver. Since each of the timing signals that go into a position calculation has some
error, that calculation then becomes compounded as the system goes from one point to the
We are so far away from the satellites, that even a few hundred kilometers is a short
distance. The little distances we travel here on earth are insignificant when compared to
the vastness of space. So if two receivers are fairly close to each other, say within a
this hundred kilometers, the signals that reach both of them will have traveled through
virtually the same space in the atmosphere, and so will have virtually the same errors. In
essence, that's the idea behind differential GPS. One receiver measures the timing errors
and then provides correction information to the other receivers that are roving around.
That way virtually all errors can be eliminated from the system, as well as the Selective
Availability error that the DOD puts into the signal for national security reasons.
Basically what you do is ground the reference receiver in a place that has been well
surveyed and keep do not allow it to move. Basically it calculates the normal GPS equation
in reverse, and uses its position to calculate time. DGPD uses its known position to
calculate exact timing. It figures out what the travel time of the GPS signals should
theoretically be, and then compares the data to the actual statistics. One more difference
includes the correction of errors. That is, the receiver transmits the normal error
information to the roving receiver so it can use it to correct the measurements in
The receiver basically contacts all the satellites to fix the errors. It also encodes the
information into a standardized format, and eventually sends it to the roving satellite.
Basically its similar to synchronizing watches, or tuning into the same channel on a
network of radio receivers. The U.S. Coast Guard, as well as other agencies, are trying to
put up receiver stations around busy harbors and bays. Anyone in the local harbor or bay
area can receive DGPS corrections because the U.S. Coat Guard uses the radio frequencies
already in use in the area for Maritime vessels. Many new GPS receivers are being designed
to accept corrections, and some are even equipped with built-in radio receivers.
The possibilities for DGPS seem endless. One can monitor better tracking by enabling
ambulances with GPS receivers so that operators and other emergency personal can monitor a
situation with ease and precision. You can easily use a radio frequency for this, and it
would be running off the same GPS system as normal, accept now it would be subject to the
local corrections, making it more accurate.
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