Dynamic Accuracy of GPS Receivers in Citrus Orchards

Abstract

The accuracies of six commercially available GPS receivers were studied under static and dynamic conditions in different citrus orchards. The receivers were: Trimble AgGPS®106 Autonomous (T106A), Trimble AgGPS®106 with WAAS (T106W), Trimble AgGPS®132 with Beacon (T132B), Trimble AgGPS®132 with WAAS (T132W), Garmin®18 PC with WAAS at 1 Hz (G18W-1), and Garmin®18 with WAAS at 5Hz (G18W-5). The study evaluated the effects of GPS receiver type, antenna mounting height, and source of differential correction in orchards with three different sizes of trees. Absolute Mean Error (AME) and Relative Error (standard deviation of the cross-track error, SD) were used in describing GPS dynamic accuracy. The AME measures the offsets of the recorded position information from their true position (established by an RTK GPS system). In open field static tests, the accuracies of GPS receivers were similar to the accuracies reported by their manufacturers. However, in dynamic tests, the receivers performed differently under various test and orchard conditions. Overall, receiver type and mounting height had significant effects on GPS accuracies. The two T132 receivers showed the least amount of absolute error in small tree orchards, which agrees with the static test results. Generally, the WAAS differential correction system could provide an accuracy level comparable to the Coast Guard Beacon in most central Florida orchard conditions.