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To Test the Positional Accuracy of Non-Differential, Low-Cost, Handheld GPS Receivers:

1. Mark a test station in the center of a 10 acre minimum square open field.

2. Set the receiver to UTM basis and record coordinates of the test station; the Universal Transverse Mercator (UTM) basis is preferred as northing and easting coordinates can be readily compared arithmetically. Note: A meter is equal to 3.2808 feet.

3. Turn-off receiver and pace about 300 feet/100 meters East, then turn-on receiver and walk back to test station and again record its coordinates.

4. Repeat step 3 to South, West and North; remember to use the receiver on/off sequence in order to achieve a valid test.

5. A comparison of the five(5) recorded UTM coordinates for the central test station location will normally result as follows:

The effect of Selective Availability (SA) induced by the Department of Defense (DOD) and other factors, will usually result in a position differential of about 100 meters. If the GPS receiver is left on during the test, the overall repeatability of the test station coordinates will normally result in a smaller difference as most of the handheld GPS receivers employ a built-in signal averaging feature, to hide or to mask the effects of selective availability.

The Department of Defense (DOD) autonomous GPS accuracy is normally 100 meters at a 95% confidence interval and 40 meters at a 50% confidence interval; excluding receivers employing real-time kinematic (RTK) or post-processed differential GPS capability.

Reference: Trimble Navigation, Limited/1997.

Prepared By: William C. Rohde, RLS
Date: 11-05-99

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