Using GPS/GNSS The Realities of GPS/GNSS Measurements

1. Presented by James M. (Mike) Hart, LS, CFedS – Director of Surveying & Mapping WHPacific, Inc. Presented to International Right of Way Association Annual Conference Hartford Connecticut June 23, 2014 Using GPS/GNSSThe Realities of GPS/GNSS Measurements

2. About Hart • Licensed to practice land surveying in 10 states • Certified Federal Surveyor • Past President IRWA Chapter 11 San Diego • Incoming Chair of International Surveying and Engineering Community of Practice (ISECoP)

3. Today’s Goals • Increase your fundamental understanding of how GPS works • Address the statistics of measurement • Explore the various commonly used GPS receivers and their limitations • Draw some conclusions about using GPS

4. Ground Rules • Provide input if you can contribute – your comments are welcomed and encouraged • Ask questions in real time while the subject is hot and fresh – just break in and ask (we are all friends here)

5. GNSS~GPS • GPS • A term familiar to us all • Global Positioning System – US System • Developed in 70’s • GNSS • Global Navigation Satellite System – not so familiar – but emerging • Collective term to include GPS and those systems from other countries • GLONASS – Russian • European - Galileo • Japan – QZSS (piggybacks on GPS) • China - BeiDou

6. System Types • Recreational Grade – least accurate due primarily to autonomous positioning and antenna type - $80 ~ $250 • Mapping Grade – generally recreational grade with added capability to utilize differential correction and data dictionary - primary for GIS applications - $2500 ~ $8500 • Survey Grade – highest accuracy, high grade antenna and GPS engines; differential correction is key to the capabilities of these systems; VRS, etc. - $25,000 to $60,000 (system)

7. How it works • Distance = rate x time • Rate is the speed of light in a vacuum • Time is measured using highly accurate cesium clocks onboard the satellites that are synced with the atomic clocks on the surface • Satellites (SV) are moving platforms that are constantly tracked and the orbits of which are precisely known (and predicted) • Need four SVs to calculate position • Distance-Distance-Distance-Distance intersection

8. Autonomous Positioning • Autonomous = stand alone position • Primary error source is from the ionosphere • Other error sources include multi-path, timing, and others • An autonomous position (stand alone) can be determined to about 10 meters (33 feet) • Cannot do any better than this statistically

9. Differential Corrections • Tie two receivers together via time both of which can “hear” four or more common SVs • Hold one receiver fixed on a know position • Correct the “rover” using the time-tagged positional correction determined on a second-by-second basis • Result can be sub-centimeter

10. Virtual Reference Networks • Survey-grade system where the self-managed base GPS receiver is eliminated and replaced with a network “node” • Differential correction signal transmitted by cell phone dial up of internet address vs. self-managed broadcast radio • When you see a surveyor with a rover unit – he’s likely working from a self-managed base or from a VRN connection

11. Count vs. Measurement • A “count” is exact • A “measurement” is never exact – let’s explore why

12. Statistics of the measurements • Statistical terms • Mean (or average) • Standard deviation of the mean • Normal distribution of the population of observations • Measuring “errors” are • Mistakes or blunders – those must eliminated • Systematic – we can compensate for those • Random – those are ones we can predict and must understand

13. Normal Distribution Curve

14. System Specifications • Recreational Grade • Autonomous position – ~5 to 20 meters • WAAS corrected – 1 to 3 meters • Mapping Grade • Autonomous position - ~ 5 to 20 meters • WAAS corrected – 1 to 3 meters • Differential correction using self-managed components - decimeter • Survey Grade • Autonomous position - ~ 5 to 20 meters • WAAS corrected – 1 to 3 meters • Differential correction using self-managed components = H 1cm + 1ppm (1 sigma) • Virtual Reference Systems

15. Look at the Mfg’r’s system specs!! • Survey grade receivers, for example: • Horizontally: • 1 sigma (or +/- 1 Standard Deviation) = 1 cm + 1ppm (68.2% of the observations will fall in this “region”) • 2 sigma = +/- 1.97 * the 1 sigma value – therefore 2 cm + 1ppm (95% of the observations will fall in this region) • Vertically – about 2x the H error

16. “But I get positions that are closer than that…..” • Not really…..if you take enough “shots”, the normal distribution curve will form and you will have those outliers • Don’t ignore the statistics – they represent reality • Reality is often something with which we don’t want to contend – but remember….no measurement is exact – so you will NEVER be able to repeat perfectly (this is the Surveyor’s dilemma).

17. Position – Geographic Coordinates • Latitude • Angle measurement N or S of equator • 1” of latitude = approximately 100 feet • Longitude • Angle measurement E or W of the Prime Meridian (Royal Observatory in Greenwich England) • DMS • 1” of longitude = approximately 101 feet at the equator – decrease as the latitude increases • Both Lat and Lon are expressed in DDD.mmmsssformat generally – but can be DDD.ddd or DD.MMmm

18. Position – Cartesian Coordinates • State Plane Coordinates • Specific area of coverage • Native unit is the meter – but be careful when converting to feet – the conversion factor differs by jurisdiction and the positional difference will be significant if using the wrong unit • UTM – Universal Transverse Mercator Coordinates • E-W Zones • In North America – zones are

19. GPS into Google Earth • Kmz is the very best way – get the kmz files from your surveyor • Input individual positions – but pay close attention to the units and significant figures • Note the longitude value should be negative to indicate W of the Prime Meridian.

20. Conclusions • GNSS/GPS is a very powerful and affordable tool • Use the right tool for the job and positional accuracy requirements • If GPS outputs a position, question the capabilities of the system you are using as to accuracy – then proceed cautiously forward recognizing the limitations • Remember – NO measurement is exact!!!