Contact: Dr. Lowrey A. Smith, USDA-ARS-APTRU, P.O. Box 36, Stoneville, MS 38776, email:  lasmith@ars.usda.gov

Abstract. A SATLOC AirStar M3 swathing system installed on an Air Tractor 402B aircraft was evaluated to determine the GPS (Global Positioning System) position latency associated with known ground coordinates and to determine the accuracy of GPS ground speed.  A photo-detector circuit was used to synchronize aircraft position with reference points on the ground from which vertical beams of sunlight were reflected upward.  When the aircraft flew over the light beam, the circuit responded by closing a solid-state relay.  This action caused an extra record to be inserted into the SATLOC log file with precise time and position information associated with the event.  Latency was determined by comparing the aircraft GPS position with the reference point GPS position.  GPS ground speed was calibrated by using two reference points to close and open the relay, respectively.  The difference between the time associated with the relay opening and closing could be used with the measured distance between the points to compute average ground speed.  Results indicated that position latency was positive for N-S flight directions and negative for E-W flight directions.  The magnitude of these latencies was relatively small (less than 9 m) considering that the aircraft was traveling at a nominal speed of 58 m/s.  GPS ground speed calibration results revealed errors ranging from 0.01% to 0.11% for a series of eight runs with speeds varying from 176 to 238 km/h.

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