OPUS Online Positioning User Service

1. OPUS Online Positioning User Service http://www.ngs.noaa.gov/OPUS/ opus@ngs.noaa.gov

2. WHAT IS OPUS? On-line Positioning User Service Provide GPS users faster & easier access to the National Spatial Reference System (NSRS)

3. HOW DOES OPUS WORK? Submit RINEX file through NGS web page Processed automatically with NGS computers & software With respect to 3 suitable National CORS Solution via email (usually in minutes)

4. OPUS USES 3 CORS SITES

5. HOW DO I USE OPUS? Go to OPUS web pagewww.ngs.noaa.gov/OPUS • - Enter your email address • - Enter/Select RINEX file • - Select antenna type from menu • - Enter antenna height in meters (defaults to ARP) • - Optional:State Plane Coordinates • - Select up to 3 base stations (optional) • - Upload File Check your email (usually only takes a few minutes)

6. WHAT ARE SOME OPUS GUIDELINES? Must submit dual-frequency (L1/L2) data Must submit at least 2 hrs of data No kinematic/Rapid Static No Glonass • Correct vertical requires: • antenna type • antenna height

8. GETTING TO OPUS

9. OPUS Web Page

10. OPUS - Select Reference Site(s)

11. ARP The height is measured vertically (NOT the slant height) from the mark to the ARP of the antenna. The height is measured in meters. The ARP is almost always the center of the bottom-most, permanently attached, surface of the antenna. See GPS Antenna Calibration for photo’s and diagrams that show where the ARP is on most antennas: http://www.ngs.noaa.gov/ANTCAL/ If the default height of 0.0000 is entered, OPUS will return the position of the ARP. MARK HOW IS THE ANTENNA HEIGHT MEASURED?

12. You do not need to know these offsets. They are passed to the processing software through the antenna type The antenna phase centers are located somewhere around here. The antenna offsets are the distance between the phase centers and the ARP If the user selects NONE as the antenna type, the offsets are set to 0.000 and the antenna phase center becomes the reference The Antenna Reference Point (ARP) is almost always located in the center of the bottom surface of the antenna. Incorrect or missing antenna type big vertical errors WHY DO I NEED THE ANTENNA TYPE?

13. Why NGS uses the ARP The ARP is a known point on the antenna The phase center varies depending on the elevation angle from the antenna to the satellite If there are 8 satellites, there are 8 phase centers that must be modeled and averaged The Phase center for L1 is different from the L2

14. Antenna Phase Center Variation . . . . . . . . . . . . SV 20 SV 20 SV 14 SV 14 Different Phase Patterns Note that SV elevation and varying phase patterns affect signal interpretation differently Antenna Type A Antenna Type B

15. ELECTRONIC PHASE CENTER • Phase Center Variation (mm) • Elevation Angle (deg.)

16. GPS Antenna Phase Pattern Calibrations http://www.grdl.noaa.gov/GRD/GPS/Projects/ANTCAL  Each antenna type has a unique phase pattern  GPS antennas must be calibrated  Mixing uncorrected antenna types can produce errors of up to 10 cm in the vertical component  NGS is at the forefront in relative field calibration of GPS antennas  NGS maintains a calibration database with parameters for 88 antenna types (03-26-01)

17. Antenna Calibration Facility in Corbin, Virginia

18. HOW ARE OPUS POSITIONS COMPUTED? NGS PAGES software Ionosphere free Tropospheric scale height adjusted Fixed ambiguities Average solution to 3 suitable CORS ITRF and NAD83 positions returned

19. WHAT DOES OPUS OUTPUT LOOK LIKE?

20. READING OPUS OUTPUT (input) The version of the PAGES software used for processing • The ephemeris used (OPUS will use the best available): • “igs” final post-fit orbit-better than 5 cm(14 days wait) • “igr” rapid post-fit orbit-better than 10 cm(2 days wait) • “igu” ultra-rapid predicted orbit-better than 25 cm(available immediately) The antenna name and antenna reference point height you entered

21. READING OPUS OUTPUT (process results) Start/end dates/times of your file Ratio and % of observations used in solution Ratio and % of fixed/total ambiguities Overall RMS of the solution

22. READING OPUS OUTPUT (coordinates) Reference Frames Independent NAD83 & ITRF solutions Peak to peak error is the difference between max and min error for individual solutions Peak to peak error may differ between NAD and ITRF solutions Orthometric ht. is based on current geoid model

23. READING OPUS OUTPUT (coordinates) Universal Transverse Mercator (UTM) coordinates US National Grid State Plane coordinates are given (if requested)

24. READING OPUS OUTPUT (control) Base Stations used in positioning The closest published station listed in the NGS IDB Disclaimer

25. WHAT IS A GOOD SOLUTION? No hard rules - only guidelines • Make sure antenna type and antenna height • are correct • Review statistics • should use 90% or more of your observations • at least 50% of the ambiguities should be fixed • overall RMS should seldom exceed 3.0 cm • peak to peak should seldom exceed 5.0 cm

26. HOW CAN I IMPROVE MY RESULTS? The best way to get more accurate results is to observe longer sessions Data sets of at least four hours have been shown to produce more reliable results

27. HOW DO I GET HELP? First use the Links on the OPUS page • detailed discussions of guidelines • description of processing techniques • description of output • guidelines for successful use Submit specific questions at OPUS web page http://www.ngs.noaa.gov/OPUS/

28. WHAT CHANGES ARE PLANNED IN THE FUTURE? Inclusion of Cooperative CORS in the base station selection process Better treatment of RINEX-2 header errors is being implemented. Some translators are not compliant with the RINEX-2 standard Single frequency data processing is being considered

29. QUESTIONS? http://www.ngs.noaa.gov/OPUS/ opus@ngs.noaa.gov

30. Demonstration Application II

31. CONTROLLING A BRIDGE SURVEY The accompanying slides were presented at the 2002 CORS Forum by Gary Thompson of the North Carolina Department of Transportation.

32. Using OPUS to control Bridges On a typical bridge job, NCDOT Sets an azimuth pair ( ) Uses approximately 6-7 control panels ( ) Controls the site with 2 receivers

33. TIP: B-9999 Place the Base Station over your first point and begin RTK survey ensuring that you are collecting Raw Data for at least 2 hours (This data will be sent to OPUS). We will now refer to this as OPUS1. B9999-1 B9999-2 Be sure to measure to opposite Azimuth Pair point. Start Rover and begin setting and controlling your panels P4 P1 P2 P5 P3 P6

34. TIP: B-9999 Move the Base Station over your second point and begin RTK survey ensuring that you are collecting Raw Data for 2 hours. (This data will also be sent to OPUS). We will now refer to this as OPUS2. Again, Be sure to measure to opposite Azimuth Pair point. Start Rover and begin controlling your panels from the second location. If you use one controller and name the points the same the controller will provide comparisons in the field. P4 P1 P2 P5 B9999-1 B9999-2 P3 P6

35. Field Work is now complete. The following steps need to be taken to finish the process:

36. Office Process • Download the Raw Data and RTK dc files • Convert both blocks of raw data to RINEX format using Trimble’s utility • Upload the files to: http://www.ngs.noaa.gov/OPUS/ • Receive the results from OPUS via email in minutes

37. Continued... • Import the dc file into Trimble Geomatics Office • Update the initial base position for the first base to the coordinates provided by OPUS1 • After a recompute, everything in the dc file should be corrected relative to the first base location (OPUS1)

38. Continued ... • The position for OPUS2 is only used for comparison to what was derived from OPUS1 • Coordinates can now be utilized as needed

39. Staff Hours Vehicles GPS Receivers Cell Phones Static 24 - 48 3 3 3 OPUS & RTK 6 - 12 1 1 *1 Savings 18 - 36 2 2 2 OPUS & RTK Savings to NCDOT * The cell phone listed in the OPUS & RTK surveying comparison was not used in the survey work, but was available for contacting the office.