Wisconsin DOT Experiences with Low Altitude Flights using Aircraft with GPS

1. Wisconsin DOT Experiences with Low Altitude Flights usingAircraft with GPS MR-RST Technology Exchange Meeting Pyle Center, University of Wisconsin 14-15 April 2003 Wisconsin Department of Transportation Geodetic Surveys Unit

2. Issue Determine if Airborne Global Positioning System (ABGPS) can Produce Accuracies as Comparable to Conventional Equipment and Procedures for Low Altitude Flights. Low Altitude ABGPS

3. Test Approach Criteria • Utilize ABGPS on a 1:3600 large scale block mapping project around Campbellsport, WI. • Targets were placed at defined locations and surveyed to established horizontal and vertical positions. • Trimble 4000 SSi’s were used to collect data in the aircraft and simultaneously on the ground at three locations (Juneau, Dundee, and Campbellsport, WI). • One (1) second data collection rate was used. • Data was processed by WisDOT using Trimble Geomatics Office (TGO) software and by Woolpert using TGO and GrafNav softwares. • Compare camera photo centers differences between ABGPS and Analytical Triangulation (AT) solutions. Low Altitude ABGPS

4. Project Location N • Base Station Locations • Juneau GPS (40 km SW of site) • Dundee GPS (10 km NE of site) • T124 (on site) Event Markers are Blue “Diamonds” Low Altitude ABGPS

5. Comparison Viewpoint • WisDOT processed data from Juneau GPS, Dundee GPS, and T124 mobile base stations using TGO. • Woolpert processed data from same mobile base stations using TGO and GrafNav. • Comparisons were made to AT solutions and differences between WisDOT and Woolpert indicate similar results. Low Altitude ABGPS

6. Results Low Altitude ABGPS

7. Analysis of Data Sets • PDOP plots found to contain spikes indicating some interference. • Number of satellites was sufficient before, during, and after the flight. • Continuous tracking of satellites between aircraft and base stations indicated cycle slips. • Cycle slips occurred systematically during turns before and after flight lines. Loss of lock occurred at a rate that was too frequent thus the processor was unable to properly fix the correct set of integers resulting in poor GPS derived coordinates. Low Altitude ABGPS

8. Flight Trajectory Low Altitude ABGPS

9. DOP Plots Low Altitude ABGPS

10. Considerations for Improvement • The pilots need to hold the plane as flat as possible and to graduate into a flatter turn before and after each flight line. • Investigate using a different GPS antenna instead of the Tecom antenna. • Investigate using a different kinematic processor that is more robust thus allowing the user to manipulate the data with greater options. • Use base stations less than 25 km to ensure higher probability of quality coordinate determination. Low Altitude ABGPS

11. Future Considerations • Perform more trials as time allows. • Investigate cost benefit analysis of low altitude flights versus present day base control and positioning tools. • Success might be limited to small scale photogrammetric projects in block configuration. Low Altitude ABGPS

12. Questions/Comments? Low Altitude ABGPS