This Briefing is UNCLASSIFIED

1. This Briefing is UNCLASSIFIED Re-tuning the GPS Constellation Performance Analysis Working Group (1999) Capt Michael Violet 2 SOPS/DOAS

2. Overview • GPS -- The Early Years • GPS Constellation History • 1999 Rephasing of GPS Constellation • Constellation Tuning • Conclusion

3. GPS - The Early Years (< 1974) • GPS Grew Out of Other Programs • US Navy’s Transit/Timation Programs • US Air Force’s 621B Program • Early Constellation Designs Were Varied • Elliptical Geosynchronous Orbits (“Eggbeater”, “Rotating X”, “Rotating Y”) • Multiple Walker Delta Patterns at Different Altitudes • Rosette Constellations • Repeating Ground Tracks for Table Lookup of Positions • Medium Earth Orbit (MEO) Constellation (Semisynchronous) • Walker Delta Pattern 24/3/2 (T/P/V) with a 63 deg inclination • 3 Planes Spaced Equally About Equator • 8 Satellites per Plane Spaced Equally Within Plane

4. GPS - The Early Years (< 1974) • Proof of Concept -- 6 Block I Satellites in 2 Planes • Eventually Build Up to 24 Block II Satellites in 3 Planes • 3 Plane Approach Allowed Easy Replenishment (only need 3 spares) • Constellation Buildup Simplified Due to Multiple Launch Capability on Shuttle

5. GPS - The Early Years (< 1989) • In 1980, Funding Constraints Drove Changes • Studies Performed to Determine Absolute Minimum Satellites • Constellation Reduced to 18 Satellites (+ 3 spares) • Extensive Analysis Selected 6-Plane Constellation • Walker Delta Pattern 18/6/2 (equal spacing within plane) • Inclination Decreased to 55 degrees due to Launch Vehicle Constraints • Spares Located in Every Other Plane to Ensure 0.98 Availability

6. Right Ascension of the Ascending Node •  is the Right Ascension of the Ascending Node • The Vernal Equinox is an inertial direction, meaning it does not rotate with the Earth

7. Longitude of the Ascending Node Longitude of the Ascending Node

8. GPS - The Early Years (< 1989) • Constellation Improved Once More Prior to 1st Block II Launch • Dr. Paul Massatt (Aerospace) Came Up With Asymmetrical Design • Design Removes 95% of Degraded Coverage Found in Baseline 18+3 system • Improved Robustness in the Event of Satellite Failures • Satellites are no longer equally spaced within the plane

9. GPS Constellation History • Block I Constellation Buildup Feb 1978 -- Nov 1985 • Block II Constellation Buildup (Phase 1) Feb 1989 -- Feb 1990 • Optimal 21 Constellation (21 satellites with 3 spares) • Funding Allowed for Return to Original Number of Satellites • 21 Considered Minimal Number of Satellites Necessary for Adequate Coverage • Ensures Constellation Value Does Not Drop Below 0.996 • CV -- % of Earth/Time where 4 satellites are available with PDOP < 10 • 1st Block II Rephasing (Optimal 21) Feb 1990 • Block II Constellation Buildup (Phase 2) Jan 1990 -- Jul 1991

10. GPS Constellation History • Dr. Rhodus and Dr. Massatt (Aerospace) Modified Asymmetrical Design Again • Less Sensitivity to Satellite Drift & More Robustness during Multiple Satellite Failures • Used Steepest Descent Optimization Approach, Subject to Constraints • PDOP < 6 with all satellites operating • PDOP < 10 with worst-case single-satellite failure • July 1991 the US Air Force Directed Transition to the Optimal 21+3 Constellation • 2nd Block II Rephasing (Optimal 21+3) Jul 1991 -- 1992 • Block II Constellation Buildup (Phase 3) Feb 1992 -- Mar 1994

11. GPS Constellation History • 3rd Block II Rephasing (Optimal 21+3 Corrected) Sep 1994 • Block II Constellation Buildup (Phase 4) Mar 1996 -- Nov 1997 • Driven by Aug 1994 Presentation at PAWG • Analysis by Aerospace (Dr. Paul Massatt and Ted Bujewski): • Right Ascension (RA) errors can slightly degrade constellation coverage • Degraded coverage can be corrected by slightly modifying the target LAN positions • Modified LAN positions also improve coverage with on-orbit satellite failures or outages • Analysis recommended regular adjustments of target LANs for these RA errors • AFSPC/DO directed rephasing constellation to corrected target LANs in Oct 1994 • One-Time-Only Rephasing • Individual Satellite Rephasings (Due to Failures) Nov 1997 -- present • SVN 16 was replaced by SVN 40 (16 was moved to E5) • SVN 20 failed and was replaced by SVN 30 • SVN 28 failed and was replaced by SVN 33

12. So What Now?? • Fourth GPS Constellation Rephasing • In five years, the RA errors have grown again, such that coverage is slightly degraded • 11 satellites are more than 2 degrees out of optimal (corrected) LAN positions • Initiated Minor Rephasing of Constellation to Correct for these Errors • Furthermore, Initiating Yearly LAN Tuning to Avoid Error Growth Altogether Earth (J2 only) Moon Sun

18. Constellation Tuning • New Yearly Tuning Process • October 1 (every year) • obtain latest vector ephemeris for the constellation • propagate to 1 July of following year • calculate how far off Right Ascension is from its spec value • tune the target LAN by the Right Ascension error value • Create a new Delta-V Projection Letter IAW new values • Jan 1 (every year) • New Target LAN values take effect • Can perform maneuvers at new target value from Oct - Dec if prudent

19. Conclusions • Constellation Rephasing requires minimal ops effort • 5-6 extra maneuvers between Aug 1999 and Jan 2000 • Approximately 2-3 maneuvers per month • Constellation Rephasing will slightly improve coverage to users • New Annual Tuning Procedures Will Control Constellation Entropy