GPS Accuracies in the Field A short and informative talk by A. Richard Vannozzi, PLS Assistant Professor of Civil Technology/Surveying and Mapping Thompson School of Applied Science University of New Hampshire March 11, 2014 Email: a.r.vannozzi@unh.edu 1 Reference for this lecture: GPS for Land Surveyors, Third Edition By Jan Van Sickle Note: Some images are from the second edition 2 1
Reference for this lecture: Presentation Entitled: GPS for Surveying Presented by: Jim Bosworth Matt Wellslager 3 GNSS vs. GPS Image from GPS for Land Surveyors, 3 nd Edition, Van Sickle, Jan., Ann Arbor Press, 2008 Chinese GNSS: Beidou 4 2
GPS System Components Space Segment NAVSTAR : NAVigation Satellite Time and Ranging 28Satellites 20200 Km Control Space User User Segment Receive Satellite Signal Control Segment 1 Master Station 5 Monitoring Stations 5 Space Segment 30 Satellites (8/1/2010) 4 satellites in 6 orbital planes inclined at 55 o 20,200 km above the Earth 12 hour orbits Same satellite in view for 4-5 hours Designed to last 7.5-10 years Different Classifications Block 2, 2A, 2R, 2R-M & 2F 55 o Equator 6 3
Monitoring Station Locations Image from GPS for Land Surveyors, 3 nd Edition, Van Sickle, Jan., Ann Arbor Press, 2008 7 User Segment 8 4
User Segment 9 User Segment 10 5
24 Satellites (minimum) Why GPS is Amazing 20,000 km (12,500 miles) above the earth (Diameter of the earth is approx. 8,000 miles) Travel around the earth twice per day 103,700 mile path x2 = 207,400 miles day 207,400 miles/24 hours = 4300 miles/hr. GPS timing reference: http://www.4timing.com/syncgps.pdf 11 Why GPS is Amazing GPS Clocks are synchronized within about 10 ns. NOTE: A nanosecond is 1/1,000,000,000 (Billionth) of a sec. 24 satellites going in different directions at 4300 mph In any 10ns instant the exact 3D geometry of the GPS constellation is known with in +/- 1 meter. 1 meter in 20,000 km (2,000,000 meters) is 1:2,000,000. Atomic clock error (1 sec. in 30,000,000 years) =.032ns/yr. GPS timing reference: http://www.4timing.com/syncgps.pdf 12 6
Why GPS is Amazing Imagine redoing a 24 point 3D traverse, every 10 ns, getting that traverse to close to 1:2,000,000 every 10 ns, all while your traverse points are hurling through space, in different directions, at 4300 mph. 13 Four Main Categories of GPS/GPS Surveys Code Ranging: (Handheld GPS, 10m) Differential GPS (Handheld with corrections, submeter) Carrier Phase Shift Measurement (Survey Grade, mm/cm) Static Real Time Kinematics (RTK) 14 7
Code Ranging: (Handheld GPS, 10m) Note: Ranges from satellites have unresolved errors (clock errors, atmospheric errors, etc.) so we call these ranges pseudoranges, or the technique: pseudoranging 15 Differential GPS (Handheld with corrections, submeter) Raw Code Ranging Data is corrected based on simultaneous observations at nearby known point Resource Grade Receivers WAAS for Aviation Coast Guard Beacons Private subscription services Corrections are broadcast from known point to the DGPS receiver and corrections made. (You can post process, too) 16 8
Static GPS (Survey Grade, mm/cm) Relative Positioning Single or Dual Frequency Receivers Multiple receivers and multiple satellites Carrier Phase Shift Measurement Takes longer: need to solve for integer ambiguity Accounts & Corrects for Atmospheric conditions more completely Uses receiver data to calculate 3D vectors between stations, very precisely If one station is a known point, we can then calculate X,Y, Z for all other points 17 RTK GPS (Survey Grade, mm/cm, Real Time) Relative Positioning Dual Frequency Receivers Multiple receivers and multiple satellites Carrier Phase Shift Measurement Accounts & Corrects for Atmospheric conditions more completely Uses receiver data to calculate 3D vectors between stations, very precisely If one station is a known point, we can then calculate X,Y, Z for all other points Corrections broadcast from base to receiver in nearly real time 18 9
RTK GPS Antenna Receiver Radio Controller (Data Collector) 19 Image from GPS for Land Surveyors, 3 nd Edition, Van Sickle, Jan., Ann Arbor Press, 2008 20 10
Code Ranging Experiment Handhelds How do Handhelds Stack Up? The results from seven (7) different handheld GPS devices were simultaneously recorded at points at UNH with published/known geographic coordinates 21 Static Experiment Plymouth, Mass to West Bridgewater, MA Static GPS: 5mm (0.0164 )+/- 5ppm= (1:200,000) (0.0164 +.000005 X 22.4 x 5280) = 0.60776 22 11
Static Experiment Plymouth, Mass to West Bridgewater, MA 23 Static Experiment Plymouth, Mass to West Bridgewater, MA 51.3 miles x 5,280 = 270,900 270,900 1000 per set up = 271 traverse points @ 1 hour/point: 7 weeks traversing Error propagation theory tells us: w/ 3mm (00984 ) +/- 3ppm EDM Constant Error:. = 0.1620 Scalar Error: (3ppm = 0.000003 x 270,900 = 0.8127 ) Total Distance error: ±.. = ± 0.829 (THIS IS JUST DISTANCE ERROR!) and we haven t discussed angles or tribrachs! 24 12
Static Experiment Plymouth, Mass to West Bridgewater, MA Experimental Procedure: Set point in Plymouth, drive to West Bridgewater Set receiver on a point in West Bridgewater Wait two hours Take down receiver, drive to Plymouth, take down that receiver Process baseline data See how long it took and how reliable the measurement was. 25 Static Experiment Plymouth, Mass to West Bridgewater, MA 26 13
Static Experiment Plymouth, Mass to West Bridgewater, MA GPS Project Time: 3 hours travel/set-up (1 person) 3 hours GPS (getting paid to sit) 2 hours screwing around with the computer 0.5 hours processing the data. Total: 8.5 hours of which 5 hours was less than productive 27 RTK Experiment RTK Topography vs. DEM RTK Topographic Survey (CT 233 Students) 28 14
RTK Experiment RTK Topography vs. LIDAR (DEM) Contours Derived from Lidar Based DEM 29 Experiment #10 RTK Topography vs. DEM RED=LIDAR Yellow=RTK 30 15
RTK Experiment RTK Topography vs. DEM RED=LIDAR Yellow=RTK 31 GPS Accuracies in the Field A short and informative talk by A. Richard Vannozzi, PLS Assistant Professor of Civil Technology/Surveying and Mapping Thompson School of Applied Science University of New Hampshire March 11, 2014 Email: a.r.vannozzi@unh.edu 32 16