Wisconsin DOT Experiences Wisconsin DOT Experiences with Low Altitude Flights usingwith Low Altitude Flights using

Aircraft with GPS Aircraft with GPS

MR-RST Technology Exchange Meeting

Pyle Center, University of Wisconsin

14-15 April 2003

Wisconsin Department of Transportation

Geodetic Surveys Unit

15 April 2003 Low Altitude ABGPS 2

IssueIssue

Determine if Airborne Global Positioning System Determine if Airborne Global Positioning System (ABGPS) can Produce Accuracies as Comparable to (ABGPS) can Produce Accuracies as Comparable to

Conventional Equipment and Procedures for Low Conventional Equipment and Procedures for Low Altitude Flights.Altitude Flights.

15 April 2003 Low Altitude ABGPS 3

Test Approach CriteriaTest Approach Criteria

Utilize ABGPS on a 1:3600 large scale block mapping Utilize ABGPS on a 1:3600 large scale block mapping project around Campbellsport, WI.project around Campbellsport, WI.

Targets were placed at defined locations and surveyed to Targets were placed at defined locations and surveyed to established horizontal and vertical positions.established horizontal and vertical positions.

Trimble 4000 SSi’s were used to collect data in the aircraft Trimble 4000 SSi’s were used to collect data in the aircraft and simultaneously on the ground at three locations and simultaneously on the ground at three locations (Juneau, Dundee, and Campbellsport, WI).(Juneau, Dundee, and Campbellsport, WI).

One (1) second data collection rate was used.One (1) second data collection rate was used. Data was processed by WisDOT using Trimble Geomatics Data was processed by WisDOT using Trimble Geomatics

Office (TGO) software and by Woolpert using TGO and Office (TGO) software and by Woolpert using TGO and GrafNav softwares.GrafNav softwares.

Compare camera photo centers differences between Compare camera photo centers differences between ABGPS and Analytical Triangulation (AT) solutions. ABGPS and Analytical Triangulation (AT) solutions.

15 April 2003 Low Altitude ABGPS 4

Project LocationProject Location

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”

N

15 April 2003 Low Altitude ABGPS 5

Comparison ViewpointComparison Viewpoint

WisDOT processed data from Juneau GPS, Dundee GPS, WisDOT processed data from Juneau GPS, Dundee GPS, and T124 mobile base stations using TGO.and T124 mobile base stations using TGO.

Woolpert processed data from same mobile base stations Woolpert processed data from same mobile base stations using TGO and GrafNav.using TGO and GrafNav.

Comparisons were made to AT solutions and differences Comparisons were made to AT solutions and differences between WisDOT and Woolpert indicate similar results.between WisDOT and Woolpert indicate similar results.

15 April 2003 Low Altitude ABGPS 6

ResultsResults

Mobile Base Station CommentComputed Rank w/

X, Y, & O-Ht

Computed Rank w/ X, Y, &

O-Ht & offset

X DIF Mean

value (m)

Y DIF Mean

value (m)

O-HT DIF Mean

value (m)

O-HT w/ offset

T124 On Site 4.0 3.7 -0.111 0.292 -1.277 0.064DUNDEE GPS 9.2 km NE 4.7 3.0 -0.127 0.280 -1.286 0.055JUNEAU GPS 39.8 km SW 5.3 5.3 -0.217 -0.079 -4.115 -2.774

T124 & DUNDEE GPS On Site & Closest 4.7 3.7 -0.119 0.286 -1.281 0.060T124 & JUNEAU GPS On Site & Farthest 2.3 4.0 0.130 0.052 -0.357 0.984DUNDEE GPS & JUNEAU GPS

Off Site3.3 4.3

-0.201 -0.079 -0.748 0.593

T124, DUNDEE GPS &JUNEAU GPS

All Base Stations

3.3 3.7

-0.114 0.285 -1.269 0.072

15 April 2003 Low Altitude ABGPS 7

Analysis of Data SetsAnalysis of Data Sets

PDOP plots found to contain spikes indicating some PDOP plots found to contain spikes indicating some interference.interference.

Number of satellites was sufficient before, during, and after Number of satellites was sufficient before, during, and after the flight.the flight.

Continuous tracking of satellites between aircraft and base Continuous tracking of satellites between aircraft and base stations indicated cycle slips.stations indicated cycle slips.

Cycle slips occurred systematically during turns before Cycle slips occurred systematically during turns before and after flight lines.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.

15 April 2003 Low Altitude ABGPS 8

Flight TrajectoryFlight Trajectory

15 April 2003 Low Altitude ABGPS 9

DOP PlotsDOP Plots

15 April 2003 Low Altitude ABGPS 10

Considerations for ImprovementConsiderations for Improvement

The pilots need to hold the plane as flat as possible and to The pilots need to hold the plane as flat as possible and to graduate into a flatter turn before and after each flight line.graduate into a flatter turn before and after each flight line.

Investigate using a different GPS antenna instead of the Investigate using a different GPS antenna instead of the Tecom antenna.Tecom antenna.

Investigate using a different kinematic processor that is Investigate using a different kinematic processor that is more robust thus allowing the user to manipulate the data more robust thus allowing the user to manipulate the data with greater options.with greater options.

Use base stations less than 25 km to ensure higher Use base stations less than 25 km to ensure higher probability of quality coordinate determination. probability of quality coordinate determination.

15 April 2003 Low Altitude ABGPS 11

Future ConsiderationsFuture Considerations

Perform more trials as time allows.Perform more trials as time allows. Investigate cost benefit analysis of low altitude flights Investigate cost benefit analysis of low altitude flights

versus present day base control and positioning tools.versus present day base control and positioning tools. Success might be limited to small scale photogrammetric Success might be limited to small scale photogrammetric

projects in block configuration.projects in block configuration.

15 April 2003 Low Altitude ABGPS 12

Questions/Comments?Questions/Comments?