DIFFERENTIAL GPS

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DGPS mode using a beacon receiver

All Garmin receivers support DGPS except the Geko 101. All current Magellan receivers support DGPS except the GPS Companion as do all current Lowrance units. Many other models and manufacturers support DGPS as well.

In DGPS operation a station, often called a beacon, transmits correction data in real time that is received by a separate box, called a beacon receiver, which sends the correction information to the GPS receiver. In principle this is quite simple. A GPS receiver normally calculates it position by measuring the time it takes for a signal from a satellite to reach its position. By knowing where the satellite is, how long it takes to send the signal, and knowing the speed of the signal it can compute what is called a pseudo range (distance) to the satellite. This range must be corrected before it is used to compute the final position. Corrections such as compensation for ionospheric errors due to the fact that the ionosphere slows down the speed of travel of the radio wave is one form of correction that can be applied. A DGPS beacon transmitter site has already calculated all of the pseudo range correction data based on the fact that it already knows exactly where it is and can compute the errors in the satellite computed position from its known location. Once the pseudo range correction data is computed it is sent to the GPS and used to compute a more accurate fix. The data is sent at either 100 baud or 200 baud depending on the station and this can result in a typical delay of 2 to 5 seconds between the computation of the correction and the application of the correction. However, since most errors are slow moving this time delay is not usually a problem.

Each beacon transmitter is autonomous and computes its own corrections based on its reception of GPS signals. It then packages the correction data in groups of 3 satellites and sends the data to the GPS receiver. Note that the design of a beacon DGPS transmitter will send corrections for up to 9 satellites and these are only those at least 7.5 degrees above the horizon. The assumption is that the GPS receiver will be close enough to have the same sort of errors that the beacon station saw and they can be applied without modification to any SV's that they share a view of. This works well in practice since most of the error sources would be common between the two locations. Beacon sites have some ability to improve the system integrity as well, however there is no standard that is defined as to exactly what they can identify. They can easily identify a satellite where the required corrections exceed a prescribed value and should not be used.

It seems that Garmin will favor differentially corrected satellites when at least four exist to the exclusion of regular satellites. If the four are in a poor geometric relationship the epe number, and possibly the accuracy of the solution, can be worse than it was with a regular GPS solution.

While the major source of DGPS corrections are done via beacon transmitters operating in the 300KHz band this is not the only source of correction data. It is possible to get data from any source that can be received at your location. Some sources include FM radios using the subcarrier capability of these transmitters, the internet, and even satellites. In all cases a custom receiver (or software) is used to assemble the data in a form that is acceptable to the GPS receiver, which by standard is RTCM-104. Data conforming to this standard is then sent via the serial port to the GPS receiver on a cable. Even WAAS/EGNOS data could be massaged and delivered via standard DGPS techniques. One such system is SisNet which supplies corections via the internet.

DGPS beacon receivers capable of receiving the government corretion signal can be obtained from Garmin , from StarLink, or from Lowrance. There are probably others as well.

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