Accuracy Characterizations of GPS Positions Collected under Non SA Conditions

Michael D. Londe, Ph.D.
BLM / WY 957
Cheyenne, WY

The results of two tests to determine the horizontal accuracy of GPS positions collected under non-SA conditions are shown below.  One test looked at the difference in value between the non-SA positions and the corresponding corrected positions. The second test looked at the difference in values between the non-SA positions and an established control point. In both cases the average horizontal difference was less than 2.5 meters.

Vertical differences were not looked at in these tests. However the absolute value range of vertical differences was from 0 to 12 meters.

A brief qualitative analysis of the data from these tests seems to indicate a link between the accuracy and PDOP. As PDOP increased the radial differences of the non-SA points from the corrected or known values increased.
 

Horizontal Characterization 1:

A Trimble GeoExplorer 3 with External antenna was used to collect a continuous line feature from Gillette, WY to Laramie, WY to Cheyenne, WY. The total distance was approximately 350 miles and time span was 6 hours. The raw SSF file was corrected against the University of Wyoming / BLM GPS Base Station. Delta X and Delta Y differences were calculated by subtracting the corrected position value from the raw Non SA value for each position. The radial difference in meters was then calculated from the deltas for each position set. These results are displayed in Figure 1. The average difference was 2.30 meters with a range from approximately 0.5 meters to 5 meters.

Figure 1

 

Horizontal Characterization 2:

A Trimble GeoExplorer 3 with external antenna on a 2 meter fixed height range rod was positioned over a control point established through ties to multiple CORS stations. Approximately 4 hours of data was collected. Delta X and Delta Y values in meters were calculated by subtracting the known coordinate values of the station from the raw Non SA positions.  Radial differences for each position set were then calculated from the delta values. These results are shown in Figure 2. The average difference was 2.47 meters for this test. The range was from approximately 0.5 meters to 6 meters.

Figure 2