A COMPARISON

OF DIFFERENTIAL AND NON-DIFFERENTIAL GPS HORIZONTAL ACCURACY

The plot below shows several DPGS and non-DGPS configurations; only horizontal errors are considered here. There were no thunderstorms (which can make receiving the beacon difficult) in the area during the tests. A few non-differentially corrected fixes were discarded. The Garmin 12XL curves are "jagged" due to the 0.001-minute latitude and longitude resolution; all other receivers in the plot had 0.0001-minute latitude and longitude resolution. For both the Oncore and Garmin 12XL, two days with DGPS corrections were interleaved with two days without DGPS corrections to attempt to partially remove any time-varying other errors. The same external GPS antenna (Micropulse) was used for the Oncore and Garmin 12XL tests. The USCG differential corrections were provided by a Lowrance receiver using an 8-foot whip monitoring the USCG beacon station at Driver, Virginia that is 105 US miles (169 km) away. The Garmin 12XL is a consumer-grade12-channel handheld receiver. The Motorola Oncore VP is a discontinued OEM 8-channel board capable of providing pseudo-range and carrier-phase information for post-processing giving survey-grade accuracy. Four days of Starlink Invicta data was collected in late 1999 using the Driver beacon for differential corrections. The Starlink Invicta is an integrated DGPS system with an H-field beacon receiver and GPS antenna in a surprisingly small unit feeding a control/interface box that provides RS-232 output. In regard to the Invicta, Starlink states its "DGPS receivers are designed for professional users where performance is more important than cost. Our engineers have worked very hard to make the receiver provide the best possible performance."  The Omnistar was an Omnistar 7000 with five days of data collected in 1997; newer Omnistar receivers/data may perform better. Omnistar provides subscription C-band differential corrections using a combined antenna (for both GPS and their C-band service) and receiver. Omnistar has the advantage of providing differentially corrected fixes at locations not serviced by USCG (or other agency) low-frequency beacon differential corrections.

The table below shows some error statistics for the data used in the above plot. Distances are in meters. Mean error is the arithmetic mean or average error.

Receiver/mode	RMS error	Mean error	CEP (50%) error	95% error
Garmin 12XL/non-diff.	5.49	4.45	3.96	9.62
Garmin 12XL/diff.	4.48	3.44	2.60	8.65
Motorola Oncore VP/non-diff.	4.87	4.11	3.57	9.02
Motorola Oncore VP/diff.	5.01	3.40	2.40	9.30
Omnistar/diff.	3.56	2.51	1.91	5.91
Starlink Invicta/diff.	1.51	1.05	0.81	2.62

The analysis presented here is not meant to be definitive; however, some tentative conclusions can be derived. At the approximately 100 mile distance from the USCG differential corrections beacon site, no significant benefit by using differential corrections is noted in the 95% error distance level (the distance which will includes 95% of the error) with either a Garmin 12XL or a Motorola Oncore. (For smaller baselines, that is, shorter distance between the GPS and the differential correction site, the differential corrections certainly should have a greater impact in reducing the error.) At the 50% error distance level (CEP), when using the Garmin 12XL or Motorola Oncore VP, some benefit with differential is noted; however, the amount of benefit is probably of little practical value. For all practical purposes, the performance of the Garmin 12XL and Motorola Oncore VP were essentially the same in these tests. The Omnistar unit gave about half the CEP error of the non-differentially corrected Garmin 12XL and Motorola Oncore VP and also gave a better 95% error distance (about 6 meters versus about 9 meters). Finally, the Starlink Invicta gave the best performance-even though it used the same distant differential correction site as the Garmin 12XL and Motorola Oncore VP differential tests. It is clear that there are different levels of accuracy obtainable by using differential beacons. As higher-end receivers that perform better depend on proprietary methods, it is difficult to further analyze the reasons for the differences in performance.

In conclusion, DGPS should better improve accuracy when using consumer-grade GPS receivers on shorter baselines than the long one tested here. With higher-end survey grade GPS/DGPS receivers, DGPS gives very good accuracy even on long baselines. Note that caution should be used in comparing the above numbers to manufacturers' specifications. In some circles, manufacturers do not use the mathematical definition of RMS error but instead use the error probability (63%) that would correspond to RMS error if the distribution were exactly Rayleigh. In those circles that number has become a reference value for comparison that has some benefits over the mathematically defined RMS error since the error distribution may not be exactly Rayleigh. Do not forget that the error depends on the latitude, due to its influence on HDOP. The test position is near latitude 38 degrees and the horizontal error is believed to perhaps be at maximum at something a little over 40 degrees latitude. Finally, the accuracy of the source of the DGPS corrections will affect the accuracy of those using it. All these factors can cause differences in measured error statistics.

Addendum

Differential and non-differential data was later collected using an Eagle Explorer. The plot and table below summarize the measured errors.

As these measurements were at a later time, extreme caution should be applied in comparing these Eagle Explorer errors to the Garmin 12XL errors; however, they are of the same magnitudes. Again, on the long baseline, there was little difference between differential and non-differential.

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