In a recent statement by President Clinton, he announced that the US government has stopped the intentional degradation of the GPS (Global Positioning System) signals, known as Selective Availability. Selective Availability was used as a U.S. defense strategy to degrade a GPS fix, meaning that there could be errors of up to a 100 meter radius (300 feet). According to a study conducted by the University of Texas, these errors are reduced to approximately a 30-meter radius (100 feet) with Selective Availability turned off. This is great news for GPS owners and manufacturers everywhere, but where does this leave Differential GPS (DGPS)?

Accuracy Levels -- GPS vs. DGPS
GPS accuracy levels are stated as having a 95% Confidence Factor. This means that 95% of the time your GPS fix is within a 30-meter radius (100+ feet) of your position, but the other 5% of the time the GPS could show your position even further out of the 30-meter radius from your actual position. A 30 meter error may not make that big of a difference for those GPS units being used in cars or hiking, but on a boat it could be the difference between being in "safe water" and being grounded on a jetty or sand bar. DGPS is also used for many other applications that require pinpoint accuracy, such as wreck fishing, bottom and pinnacle fishing, recreational and professional diving, and even navigating in poor visibility or at night.

A DGPS receiver will reduce the frequency of these errors and dramatically improve the GPS accuracy. Now, with the Selective Availability off, DGPS is even more accurate and its variance is typically 1 to 2 meters (3 to 6 feet). As you can see, even when a DGPS position fix is in error, it brings you 24 to 27 meters closer than the same GPS position fix.

Error Variance Comparison Chart
Selective Availability On vs. Selective Availability Off

* Typical Error Variance of approximately 30 meters is according to a study conducted by the University of Texas.

DGPS Features Can Give Ease of Mind
Accuracy is obviously the biggest factor when trying to decide whether to go with GPS or DGPS, so let's discuss what causes these errors and what can be done about them. Ionospheric distortions in the atmos-phere are the primary cause of accuracy errors in GPS equipment. Ionospheric distortions are irregular and their level of distortion is totally unpredictable from moment to moment, which can greatly effect the GPS fix. DGPS completely eliminates the atmospheric distortion and dramatically improves the accuracy and performance of regular GPS by using the GPS position and calculating the error variance against the DGPS beacon station position.

In addition, GPS satellites send erroneous or bad data transmissions, and unfortunately GPS units do not have the ability to provide a real-time integrity check of the data. The only way to prevent the transmission of bad data from effecting the accuracy of GPS fixes is by using a DGPS receiver.

So has discontinuing Selective Availability eliminated the need for DGPS? Perhaps for certain situations, but it is important to keep in mind that if you are looking for reliable and extremely accurate GPS fixes, DGPS is still the best product to fit your needs.

Above information from Furuno USA © 2000