GPS WAAS ACCURACY

 

The Wide Area Augmentation System (WAAS) is a form of differential GPS (DGPS) giving enhanced position accuracy developed primarily for aeronautical navigation but usable by other users.  Each Wide Area Reference Station (WRS) provides correction data to a Wide Area Master Station (WMS), which computes a grid of correction data to be uplinked to a geostationary satellite (GEO) via a Ground Earth Station (GES) in the Ground Uplink System (GUS).  The geostationary satellite transmits the correction data (and also navigation data) to the user on the L1 GPS navigation frequency (1575.42 MHz).  The user GPS receiver uses the downlink WAAS data to correct received navigation data.  The goal of WAAS is to obtain at least a 7-meter horizontal and vertical accuracy.

 

In the analysis reported here, a Garmin GPSMAP 76 receiver was used with a Garmin GA 29 pole mount GPS antenna.  The WAAS corrections were received from the INMARSAT 3F4 satellite at 54 degrees west, which is known as AOR-W.  This satellite has GPS PRN number 122.  Although some users have reported difficulty receiving the WAAS signals, they were copied 100% of the time during these tests. 

 

WAAS corrections are WGS84 rather than USCG DGPS, which is NAD83/NAVD88.  As the accuracy of the system is very good, this distinction is significant.  In the analysis presented here, the surveyed NAD83/NAVD88 position was converted to a WGS84 position using the NGS program HTDP (see the FAQ page for a link to obtain the software).

 

The plot and embedded table below show the distribution of horizontal and vertical errors that were obtained during the test session.

 

 

The plot below shows a comparison of WAAS with non-WAAS using the same receiver.  As only one such receiver was available, the WAAS and non-WAAS session were non-simultaneous; however, looking at sub-sessions, the accuracy of each mode seemed fairly stable during these observations.  The mean number of satellites received during the WAAS session was 8.43, the mean HDOP was 1.09, and the RMS of the HDOP was 1.11.  The mean number of satellites received during the non-WAAS session was 7.44, the mean HDOP was 1.27, and the RMS of the HDOP was 1.31.

 

 

The above plot and table show the improvement in both horizontal and vertical inaccuracy due to WAAS. 

 

WAAS accuracy performance using this type of GPS equipment is comparable to the accuracy obtained by using DGPS beacon stations as the plot below idicates.

 

 

Finally, the plot below shows the improvements in horizontal and vertical accuracy obtained by averaging a position over time.  Note that the NMEA reported position was averaged rather than using the receiver’s own waypoint averaging; this was done in order to obtain a sufficient sample size.  Also note that the plot is for RMS-errors and that the 95% error distances will generally be something less than twice the RMS-errors.

 

 

Although four days of data were collected, that is only perhaps sufficient to give reliable statistics for averaging up to 180 minutes (3 hours), as longer periods may yield too few disjoint periods.  The curve-fit extrapolation beyond 3 hours is only a model prediction for periods up to 480 minutes (8 hours).  This curve-fit is the one given by Levenburg-Marquadt non-linear regression on the measured data using the family:

 

 

The measured values of the constants were:

 

 

E1

L1

E2

L2

Horizontal

1.43

3.77

1.09

251.07

Vertical

2.37

5.24

2.11

210.07

 

The applicability of this fit to other receivers, times or locations is not know so the reader is cautioned that these numbers should only be considered as representative.

 

The above analysis shows that WAAS can improve the accuracy of position measurement.  WAAS gave 95% of the time horizontal position within 3.2 meters and vertical position within 6.0 meters in these tests.  Averaging for2 or 3 hours reduced this to 95% of the time horizontal position within 2 meters and vertical position within 4 meters.

 

The numbers presented here are only presented as being somewhat typical.  Position accuracy is a function not only of the GPS receiver and antenna, but also a function of the geometry and status of the satellites and the WAAS system, the surroundings of the antenna and ionosphere conditions/modeling.  At the same location with the same receiver and antenna, daily RMS error of horizontal and vertical positions have been seen to vary. 

 

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