This Briefing
is
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Re-tuning the GPS Constellation
Performance Analysis Working Group
(1999)
Capt Michael Violet
2 SOPS/DOAS
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Overview
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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
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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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GPS Constellation History
• Block I Constellation Buildup
• Block II Constellation Buildup (Phase 1)
Feb 1978 -- Nov 1985
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)
• Block II Constellation Buildup (Phase 2)
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Feb 1990
Jan 1990 -- Jul 1991
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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)
• Block II Constellation Buildup (Phase 3)
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Jul 1991 -- 1992
Feb 1992 -- Mar 1994
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GPS Constellation History
• 3rd Block II Rephasing (Optimal 21+3 Corrected)
• Block II Constellation Buildup (Phase 4)
Sep 1994
Mar 1996 -- Nov 1997
• Driven by Aug 1994 Presentation at PAWG
– Analysis by Aerospace (Dr. Paul Massatt and Ted Bujewski):
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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)
– 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
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Nov 1997 -- present
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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
2

3
 RE 
2

   n J2
 cosi  1  e
2
 a 
Earth (J2 only)

2
 0.00338 cosni   0.00154 cosni 
Moon
Sun
– Initiated Minor Rephasing of Constellation to Correct for these Errors
– Furthermore, Initiating Yearly LAN Tuning to Avoid Error Growth Altogether
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Constellation Tuning
• New Yearly Tuning Process
– October 1 (every year)
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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
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