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NGA’s Role in GPS

NGA’s Role in GPS. Barbara Wiley. Coauthors:. David Craig Dennis Manning John Novak Randall Taylor Leonard Weingarth. Overview. NGA’s GPS Mission WGS 84 GPS Monitor Station Network Network Assessment Clock Analysis Legacy Accuracy Improvement Initiative (L-Aii) Precise Ephemeris

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NGA’s Role in GPS

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  1. NGA’s Role in GPS Barbara Wiley

  2. Coauthors: • David Craig • Dennis Manning • John Novak • Randall Taylor • Leonard Weingarth

  3. Overview • NGA’s GPS Mission • WGS 84 • GPS Monitor Station Network • Network Assessment • Clock Analysis • Legacy Accuracy Improvement Initiative (L-Aii) • Precise Ephemeris • Earth Orientation • Summary

  4. NGA’s GPS Mission • 30+ years of satellite tracking • Precise orbits needed to produce precise positions • From the Master Positioning, Navigation and Timing Plan: • NGA is responsible for: • Services to support DoD navigation systems • Maintenance of GPS fixed site operations • Generation and distribution of GPS precise ephemerides

  5. WGS 84 Reference Frame Reference Frame: Global network of control stations that binds an Earth-centered, Earth-fixed 3-D coordinate system to the earth Control Station Position Accuracy Transit (1 - 2 m) Jan 1987 G730 (~10 cm) Jun 1994 Ensure the WGS 84 Reference Frame errors are negligible in the GPS ephemeris error budget G873 (~5 cm) Jun 1997 G1150 (~1 cm) Jan 2002

  6. Alaska United Kingdom St. Louis, MO Colorado Springs USNO Korea Hawaii Cape Canaveral Austin, TX Bahrain Kwajalein Ecuador Ascension Diego Garcia Tahiti South Africa Argentina Australia GPS Monitor Stations NGA Site (11) NGA Test Site (2) USAF Site (6) New Zealand GPS Monitor Station Network • GPS Wing (formerly Joint Program Office) designs and acquires the GPS space systems • USAF operates and maintains the GPS constellation • NGA provides data and analysis to USAF on GPS satellites referenced to WGS 84

  7. Network Assessment • High performance: Dual Redundancy

  8. Clock Analysis

  9. Legacy Accuracy Improvement Initiative (L-Aii) • L-Aii included: • Initial addition of 6 of NGA’s GPS monitor stations to the OCS Kalman filter • Model improvement to the OCS Kalman filter • Results: • OCS has visibility of all satellites at all times • 44% improvement to the ZAOD orbits • 10% improvement in SIS URE

  10. NGA GPS Network Tracking Data • Currently 8 stations sent in near-real-time to the GPS Master Control Station at Schriever AFB • Improves integrity monitoring • All satellites in view of at least two stations > 99% of the time • Improves accuracy • Most users accuracy on the ground is 3-10m depending on equipment and environment • Most users received 5-10% accuracy improvement • Remaining 3 NGA stations will be incorporated for more robustness

  11. Coverage by GPS Monitor Stations Assumes a 10 degree elevation mask

  12. NGA Precise Ephemeris for GPS is DoD Truth • Precise determination of where the satellites were • Precise GPS clock solutions • - Used for precise positioning and system analysis

  13. NGA Precise Ephemeris comparisons Range diff data from Phase Obs added Retuning and new tropo model Retuning Additional Tracking Stations Deployed G1150 implemented L-Aii begins

  14. Mean Axis of Rotation Axis of Figure North Pole Earth Orientation Prediction • Earth Orientation is used by the GPS Master Control Station to transform between Earth fixed coordinates to Earth inertial coordinates. • The transformation accuracy directly affects GPS accuracy

  15. Polar Motion

  16. UT1-UTC

  17. Summary • NGA supports the GPS MCS • With L-Aii, NGA’s role in GPS has changed • Now an integral partner in real-time GPS accuracy and integrity • Real-time operations • NGA’s GPS Monitor Station Network maintained at a high standard • GPS products available online • www.nga.mil • Email: gps@nga.mil

  18. NATIONAL GEOSPATIAL-INTELLIGENCE AGENCY Know the Earth…Show the Way Approved for Public Release NGA Case # 06-230

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