1. Re-tuning the GPS Constellation Performance Analysis Working Group
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

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 Nov 1985
Block II Constellation Buildup (Phase 1) Feb 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 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
Block II Constellation Buildup (Phase 3) Feb Mar 1994

11. GPS Constellation History
3rd Block II Rephasing (Optimal 21+3 Corrected) Sep 1994
Block II Constellation Buildup (Phase 4) Mar 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 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