This Briefing is

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Re-tuning the GPS Constellation

Performance Analysis Working Group (1999)

Capt Michael Violet2 SOPS/DOAS

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Overview

• GPS -- The Early Years

• GPS Constellation History

• 1999 Rephasing of GPS Constellation

• Constellation Tuning

• Conclusion

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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

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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

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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

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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

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Longitude of the Ascending Node

Longitude of the Ascending Node

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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

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• 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

GPS Constellation History

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• 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

GPS Constellation History

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• 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

GPS Constellation History

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• 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

So What Now??

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Earth (J2 only) Moon Sun

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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

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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