The Geoscience Australia’s Online GPS Processing Service (AUSPOS) Operation, Limitations

1. The Geoscience Australia’s Online GPS Processing Service (AUSPOS) Operation, Limitations and Best Practices Gary Johnston and John Dawson Geoscience Australia Earth Monitoring Group, Canberra, Australia

2. Outline • GPS processing options • AUSPos • Principles • Submitting data • Reading and understanding AUSPos results • GPS error Sources • AUSPos recommended practices • Reference frames and Datums • ITRF, WGS84, GDA94 • AHD, AUSGeoid98

3. GPS Processing Options Baseline from existing geodetic infrastructure • disadvantages most propriety based software has limited modeling and suits baselines < 100 km marks can be difficult to find/access requires two GPS receivers • advantages not reliant on external data or processing services

4. GPS Processing Options cont. Baseline from IGS/ARGN • disadvantages user must find and download IGS/ARGN data require Internet connection need to model antennas correctly IGS/ARGN station may be > 1000km away • advantages not reliant on processing services requires one GPS receiver

5. GPS Processing Options cont. Use AUSPOS • disadvantages require Internet connection requires > 6 hours of data • advantages AUSPOS automatically collects IGS/ARGN data works anywhere in the world requires one GPS receiver

6. AUSPOS • Internet web application • users submit geodetic quality GPS via web-browser • rapid turn-around precise positions via email • cm level coordinates anywhere in the world in an absolute sense • position quality depends on data quantity/quality • local datum within Australia (GDA94) • ITRF coordinates outside of Australia • FREE service

7. AUSPOS Applications :- • DGPS reference station positioning • Remote GPS station positioning • Ultra-long GPS baseline positioning • GPS connections to IGS and ARGN stations • High accuracy vertical GPS positioning • In the field high-accuracy processing • GPS network quality control

8. AUSPOS • Relative GPS Positioning • cm level positioning with two receivers • requires existing ground geodetic infrastructure ie. coordinated ground marks

9. AUSPOS • Absolute GPS Positioning • cm level positioning with one receiver • geodetic infrastructure ‘invisible’ to the user

10. AUSPOS What do you Need? • GPS data • dual frequency data • RINEX format only • > 6 hrs of continuous GPS data • Internet web browser • i.e. netscape, Internet Explorer • Email address

11. AUSPOS Overview Input and Output User GPS Data Processing Results Coordinates (email)

12. AUSPOS Overview What do you Submit? • GPS observational data (RINEX) • Your email address • the height of your GPS antenna • the type of your GPS antenna What happens? • Data is uploaded to GA • processed • results emailed and available by ftp **AUSPOS Where do you submit it? http://www.ga.gov.au/nmd/geodesy/sgc/wwwgps/

13. AUSPOS Overview Speed and accuracy • 6 hour data file • results delivered in ~ 3 minutes • 20 mm horizontal, 50 mm vertical • 24 hour data file • results delivered in ~15 minutes • <10 mm horizontal and 10-20 mm vertical

14. IGS IGS User GPS IGS AUSPOS Overview Operations • ‘invisible’ Geodetic infrastructure • International GPS Service (IGS), worldwide network of permanent GPS receivers • ‘baseline’ from three closest IGS GPS • station to station distances • typically <1000 km, up to 3500 km • IGS analysis products • precise orbits • precise Earth Orientation Parameters

15. AUSPOS Overview Software MicroCosm • commercial version of the Goddard Space Flight Centre (GSFC) software GEODYN • capable of multiple technique data processing • GA currently uses MicroCosm for GPS, Satellite Laser Ranging (SLR) and DORIS http://www.vmsi-microcosm.com Processing standards • full implementation of the International Earth Rotation Service (IERS) 1996 computation standards • http://www.iers.org/

16. Submitting GPS data to AUSPOS http://www.ga.gov.au/bin/gps.pl

17. AUSPOS Data Submission

18. Reading and understanding AUSPOS results

19. Reading and understanding AUSPOS results

20. Reading and understanding AUSPOS results

21. Reading and understanding AUSPOS results

22. Reading and understanding AUSPOS results

23. Reading and understanding AUSPOS results

24. GPS Processing • GPS Processing Software • Modelling and Error Sources • Satellite Orbits • Satellite and Receiver Clock Error • Tropospheric Refraction • Ionospheric Refraction • Antenna Phase Centre Variations • Multipath

25. Geodetic GPS Observations • double difference observation • commonly used observation • used by AUSPOS GPS Baseline

26. GPS Processing Software short baselines (< 100 km) • errors sources tend to cancel in the double difference • propriety processing software usually adequate long baselines (>100 km) • many error sources become significant • good observation modelling is essential • requires sophisticated software systems E.g. MicroCosm, Bernese, Gispy, Gamit, Epos, page5

27. Geodetic GPS Errors and Modelling Issues Satellite Orbits Orbit modelling • Accelerations on the satellite • Solar radiation pressure • other accelerations acting on GPS satellites **AUSPOS uses these IGS precise orbits

28. Geodetic GPS Errors and Modelling Issues Satellite Orbits Orbit error example :- • Precise IGS orbit versus Broadcast orbit • GPS Satellite 27, 1st January 2000 Along track Cross track Radial

29. Geodetic GPS Errors and Modelling Issues Satellite and Receiver Clock Error • can be eliminated by double difference GPS receiver clock error example :- • Cocos Island Receiver Clock (AOA SNR-12 ACT)

30. Tropospheric Refraction • troposphere is non-dispersive for GPS signals, extending to a height of about 10km • both frequencies impacted identically • total delay due to the troposphere is about 2.3m • relative tropospheric error can impact station heights • 1 cm error in troposphere signal delay produces around 3 cm error in height • the troposphere can be modeled using a standard atmosphere but these models have limitations • can overcome model limitations by additional parameter estimation of site specific troposphere parameters **AUSPOS estimates a tropospheric scale factor every two hours at every site used in the processing

31. Ionospheric Refraction • ionosphere is dispersive for GPS signals • both frequencies impacted differently, the delay is approximately proportional to the frequency -2 • GPS signal is delayed in the ionosphere due to the interaction with free electrons • the total delay can vary from 1 to 20m • on short baseline the ionosphere doesn’t need to be accounted for • on long baseline the scale of the baseline can be impacted • for the most part the impact of the ionosphere can be eliminated by combining GPS observations from both frequencies **AUSPOS uses an ionosphere corrected L1 observation

32. Modelling Earth tides example :- Earth Tides, Suva Fiji **AUSPOS makes the IERS recommended tidal corrections

33. GPS Antenna Height and Modeling • GPS heights can now be determined very accurately • accurate antenna heights are important • correct antenna phase centre modeling is important • incorrect identification of antenna make and model can impact the computed coordinate significantly! • Up to 0.1 metre in height regardless of baseline length **AUSPOS models most commonly used antenna types which can be selected when submitting data

34. GPS Antenna Height • Antenna Reference Point (ARP) - the point from which phase centre offsets are measured • the Bottom of the Ground Plane (BGP) is the usual point for measuring slope heights • Top of Ground Plane (TGP) is also often used • Vertical Height to ARP = • **AUSPOS accepts only vertical height to the ARP

35. ARP to BGP offset Radius Slope Height to BGP Vertical Height to ARP GPS Antenna Height

36. GPS Antenna effects Antenna Phase Center offset • Consists of two components • First is the mean offset from the Antenna Reference Point • Second is the variable component around this mean • Second component depends on azimuth and elevation of incoming signal • Second component can cause errors in height of up to 0.1m even over very short lines • IGS phase center variation models eliminate the majority of this error • **AUSPOS uses the IGS models

37. Trimble: TR GEOD L1/L2 GP (Mod. 22020, compact, with groundplane) TR GEOD L1/L2 W/O GP (Mod. 22020, compact, without groundplane) ----+---- <-- 0.0625 L1/L2 / + \ ++----------------+-----------------+-----------------++ <-- 0.0591 TGP ++---------------+-------------------+----------------++ <-- 0.0556 BGP | | | | | | +------x------+ <-- 0.000 ARP=BPA <-- 0.467 --> NOTCHES <-- 0.483 --> EDGE Leica SR299E/SR399E: EXTERNAL WITH GP EXTERNAL WITHOUT GP / \ / \ / \ / + \ <-- 0.039 L1/L2 +-----------------------------------------------+ +-----------------------------------------------+ \ ---------+ +x+ +---------/ <-- 0.000 ARP=TOP | > < | ===| |=== ===| |=== -+ +- | | | | Ashtech: GEODETIC III L1/L2 _____________ / \ +--- + ---+ <-- 0.072 L1=L2 +------------------------------------------------------+ <-- 0.065 TGP +-------------------+-------------+--------------------+ | | =| | +-------------+ <-- 0.018 | | +--+--+ <-- 0.000 ARP <-- 0.368 --> Antenna constant phase center offsets examples :-

38. GPS Signal Characteristics • Right Hand Circular Polarised • L1 wavelength 19.05cm • L2 wavelength 24.45 cm • receipt characteristic depend on signal azimuth and elevation, as well antenna element

39. Phase Center Offset Diagram (ASHTECH)

40. Phase Center Offset Diagram (DMT)

41. Effect of Phase Center Variations

42. Effect of Phase Center Variations

43. Impact of Phase variations, example :- • May 1995 NTF campaign with geodetic quality GPS equipment • differences between solution that has no variable model applied and solutions that have IGS phase models applied • difference shows very little effect in horizontal position. • vertical difference varies from one antenna type to another, and one location to another • Differences are generally the same for like antennae in close proximity to each other

44. Impact of Phase variations, example :-

45. Multipath • multipath is the result of GPS signals that are reflected from a surface near to the antenna. The GPS antenna receives both the direct and indirect signal • systematic biases can result • up to 20 m in pseudorange • up to several centimeters in the carrier phase measurements • multipath effects greater for low elevation satellites • multipath is difficult to model because it depends on the antenna environment **AUSPOS tip -- when observing GPS take care to avoid high multipath environments

46. AUSPos recommended practices Positional Uncertainty versus GPS data attributes for “absolute” positioning.

47. AUSPos guidelines • Positional Uncertainty is a 95% confidence value, in metres, with respect to the GDA94. • Outside Australia results are ITRF at the epoch of the survey. • Refer to the IGS product guidelines at http://igscb.jpl.nasa.gov/components/prods.html • Some hand-held receivers may provide phase & code, but the quality of their data cannot be guaranteed for this type of processing

48. AUSPos Guidelines • Must apply antenna phase centre variation • Receiver Independent EXchange format (RINEX) is required • Most processes use 30 second data, but will accept any sampling rate less than 30 seconds that can be stripped back to 30 seconds (e.g. 1, 3, 5, 6, 10, 15, 30 sec). • Each session should be entirely within a UT day. Repeat shorter duration sessions should be observed at different times of the UT day to minimise systematic effects from the GPS system and ambient site conditions (e.g. similar satellite constellation). • Multiple sessions are recommended to ensure repeatability and hence confidence in the result. Re setup equipment for each session

49. AUSPos guidelines • Transformation to the GDA94 is required. The time-varying ITRF-GDA94 transformation parameters published by Geoscience Australia are recommended • Must examine coordinate precisions and observation fits to ensure acceptability • The calibration for an antenna can be different, even for the same brand with only slight variations in the model. Exact identification is essential to ensure that the correct calibration is applied (see note 5). • Ensure correct antenna heights used • Check operation of nearest 3 ARGN stations for critical

50. Reference Frames and Datums The International Terrestrial Reference Frame (ITRF) ITRF Definition ITRF History Relationships to the IGS realisation of the ITRF The World Geodetic System 1984 (WGS84) The Geocentric Datum of Australia (GDA) The relationship between ITRF, WGS84 and GDA GPS heighting issues