DIFFERENTIAL GPS

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This article covers the use of differential corrections on GPS receivers. While the example of a Garmin receiver is used the article should be generally applicable to other receivers as well. There is some data on other GPS receivers in the discussion when they differ from the way Garmin works.

Frequently Asked Questions

1. Q: What is DGPS?

   A: DGPS is a method of improving the accuracy of your receiver by adding a local reference station to augment the information available from the satellites. It also improves the integrity of the whole GPS system by identifying certain errors.

2. Q: Why would I need one?

   A: You might not, but if you want to improve accuracy beyond what is available for a consumer grade GPS receiver or want to improve integrity by knowing when to believe the receiver, one of these solutions may be right for you.

3. Q: Just how accurate is my GPS receiver any way?

   A: Most GPS manufacturers quote 17 meters (49 feet) as the accuracy of horizontal positions anywhere on earth. However, independent testing has shown that modern receivers can achieve 10 meters fairly reliably with a clear sky view.

4. Q: But my receiver reports errors much less than that. Do I have an exceptional unit?

   A: A GPS can only estimate the accuracy and many manufacturers are a bit optimistic in the numbers they quote. To be more precise the Accuracy number presented is often based on a 50% to 60% probability rather than the 95% probability that is usually considered in a scientific evaluation. Note that no receiver can guarantee a particular level of accuracy without stating a probability and one of the features of some of the systems described below is to help identify when the data in the GPS might have a higher than average amount of error.

5. Q: What are the sources of this error?

   A: They are well understood and the dominant contributors are listed in this table along with the likely amount that they contribute (assuming a good sky view and reasonable satellite geometry). Note that in real life the errors may be higher than those listed in the table.

   Error	Value
   Ionosphere	4.0 meters
   Clock	2.1 meters
   Ephemeris	2.1 meters
   Troposphere	0.7 meters
   Receiver	0.5 meters
   Multipath	1.0 meter
   Total	10.4 meters

6. Q: So what improvement can I expect?

   A: A beacon based DGPS system is somewhat dependent on how close you are to the beacon, but can achieve 1 to 5 meters overall accuracy, while a WAAS system can achieve an accuracy of under 3 meters if you are located in the area where ionospheric correction data is available. A LAAS is even more accurate and can get an accuracy in the neighborhood of 1.5 meters. These are still not as accurate as survey grade receivers that receive dual frequencies, have a DGPS reference station located very close to the survey site, and use post processing techniques to reduce errors even further.

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