Submitted to: American Society of Agricultural Engineers Meetings Papers
Publication Type: Proceedings
Publication Acceptance Date: 9/9/2004
Publication Date: 9/10/2004
Citation: Thomson, S.J., Smith, L.A., Sui, R. 2004. Performance evaluation of low cost GPS and WAAS-corrected swathing systems on agricultural aircraft using precise position triggering. American Society of Agricultural Engineers Meetings Papers. 2004. Paper No. 041062. Correct pub date when received. Interpretive Summary: A variable-rate aerial application system is being developed for agricultural aircraft. Along with flow control, ground location (latitude and longitude) obtained from the aircraft's Global Positioning System (GPS) will be required to target field locations to spray. Once accuracy of this GPS-based guidance system is evaluated, it can also become a reference by which to evaluate other GPS systems used on the aircraft. These 'parallel' GPS systems are being used for remote sensing and a proposed digital camera triggering system. For remote sensing, field images are obtained to determine areas that require chemical application. Images need to be geographically referenced so areas for treatment can be located. The digital camera trigger will activate a shutter when a GPS location corresponding to the desired field area is passed. For both of these applications, accuracy and repeatability of the GPS units must be evaluated. Two GPS units were tested in the airplane and compared with positioning data from the airplane's guidance system, which was tested against a geographically referenced ground point using a ground-based triggering device. The GPS units exhibited inconsistent position updating speed, depending on the day the test was run and method of data acquisition. One GPS unit showed a significant effect of aircraft speed, so it was not judged suitable for use on the airplane. Further testing will be required over many days to determine causes for updating inconsistencies so a program can be written for camera triggering.
Technical Abstract: A Satloc GPS-based aircraft guidance system and two stand-alone GPS units were tested for position updating accuracy. The Satloc was tested for suitability in a proposed variable-rate aerial application system and provided baseline information for assessing the stand-alone units. To run the accuracy test, a ground-based mirror system reflected sunlight to the belly of the aircraft, triggering a position event in the Satloc output file to 1/100-s accuracy when a ground-referenced point was passed. The stand-alone units were evaluated for use in georeferencing remote sensing images and for a proposed automatic still-digital camera triggering system. For the stand-alone units, the number of output sentences and method of acquiring data were changed to see the effect on position updating. Tests showed the Satloc to be within 6-m of geographically referenced ground position. The Lowrance Airmap stand-alone unit updated position 1.2- to 2.25-s ahead of the Satloc, but differences seen between different runs did not appear to be related to different output sentence configurations. Position updating was about 0.3-s faster when a video mapping system (VMS) was used as the data acquisition system rather than one using a custom-programmed Palmtop. Updating speed for the Lowrance unit was inconsistent over two days of testing, but position was not appreciably affected by changes in ground speed. A Garmin 76S stand-alone unit was found to update 0.5-s slower than the Lowrance on one day of testing. For the Garmin unit, output position changed linearly with increasing ground speed. Over ground speeds ranging from 38 to 69 m/s, output position changed by 27-m. This 27-m difference was equivalent to about a 0.5-s difference in position updating. The Lowrance Airmap appears to be a more suitable choice for use as a parallel GPS, but it is also designed for use in an airplane. More tests on different days need to be conducted to determine reasons for position updating inconsistencies in the stand-alone units. This will be necessary if either of them is to be used successfully for image georeferencing or automatic camera triggering.