Bamford, William

Lulich, Tyler

Price, Samuel

Winternitz, Luke

NASA Technical Reports Server

G O D D A R D   S P A C E   F L I G H T   C E N T E R 
Results from Navigator GPS Flight 
Testing for the Magnetospheric 
MultiScale Mission 
Tyler Lulich 
William Bamford 
Emergent Space Technologies 
 
Luke Winternitz 
Samuel Price 
NASA Goddard Space Flight Center 
https://ntrs.nasa.gov/search.jsp?R=20120016337 2019-08-30T22:57:32+00:00Z
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
Contents 
• Navigator GPS 
• MMS Mission description 
• FFTB Test Configuration and Environment 
• Results 
• Full System Tests 
• Benchmark Tests 
• Conclusion 
 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
NASA GSFC’s Navigator GPS program 
• NASA GSFC GN&C hardware components branch has supported spaceflight GPS since 
1990’s. Research in late 90’s led to the Navigator GPS Receiver program targeting high 
altitude, above the GPS constellation applications 
• Fully autonomous C/A code receiver, High sensitivity: <-178 dBW cold start acquisition, Fast 
acquisition, Radiation hardened, Integrated high-fidelity orbit determination filter (GEONS) 
Program highlights: 
• Experimental flight on Hubble Space Telescope Servicing Mission #4 
• Primary navigation for NASA’s Global Precipitation Measurement (GPM) Mission (2013) 
• Technology infusion on Constellation program/Orion crew vehicle for fast acquisition on re-
entry, provided augmentation/redundant navigation ability for lunar transfer orbits 
• Commercialize to Broad Reach Engineering through technology transfer (2009) 
• Active internal research and development program building the Next-Gen Navigator 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
• Mission Description 
– Four identical spacecraft  
– Formation flying in a tetrahedron 
– Spin stabilized at 3 RPM  
– Two year operational mission 
• Orbits 
– Highly elliptical earth orbits in 2 
phases 
– Phase 1:  1.2 x 12 RE 
– Phase 2:  1.2 x 25 RE 
Magnetospheric MultiScale (MMS) 
Mission Overview 
Objective:  
To employ a formation of four 
identical spacecraft that fly in a 
tetrahedral formation and make 
coordinated measurements of 
Earth’s magnetic field  
 
 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
Mission Specific Challenges 
• Spinning 
– 3 RPM 
– Handoff between RX antennas 
• Weak GPS signal at high altitude 
• Long GPS signal outages 
– 2.4 days in Phase 2 orbit 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
Formation Flying Testbed 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
GEONS 
• Goddard Enhanced Onboard Navigation System 
(GEONS) 
– Developed in house at GSFC 
– Kalman filter to process pseudorange measurements 
– Can handle sparse, incomplete, and noisy 
measurement data 
– Graceful degradation during outages 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
GEONS Parameters 
Parameter Truth Filter 
Non-Spherical Earth Gravity 
Model 21x21 EGM-96 13x13 JGM2 
Point Mass Gravity Sun, Moon using DE 405 ephemeris 
Sun, Moon using analytical fit to 
DE 404 ephemeris, with 30 sec 
min lunar update interval 
Atmospheric Drag* 
Jacchia Roberts, Schatten +2 
sigma prediction solar flux, CD 
of 2.2, Drag area of 7.1 m2 
Analytical fit to Harris Priester 
model, CD of 2.2, Drag area of 
7.1m2 
Solar Radiation Pressure * Spherical model, CR of 1.8, SRP area of 2.026712 m2 
Spherical model, CR of 1.8, SRP 
area of 2.02 m2 
Integrator  8(9) Variable Step Runge-Kutta 4
th Order Fixed Step Runge-Kutta 
Integration Stepsize  1 second 10 seconds 
Precession/Nutation 
Update Interval  1 second 10 seconds 
Maneuver Model  Finite burns 
Accelerometer measurements 
averaged over 10 seconds, 
including acceleration knowledge 
errors 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
Navigator Performance Requirements 
• TAI time shall be kept to within 325 μs  
• Semimajor axis error less than 100 m (above 3 RE) 
• Acquire signals at or below -175 dBW 
• Track signals at or below -172 dBW 
• Acquire 99% of signals with received power 
greater than -156 dBW, and 75% of signals with 
received power less than -156 dBW 
• Tracking dynamic range less than 15 dB  
• Measurement noise less than 30 m (3-sigma) 
 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
Full System Test – Phase 1 
Error in semimajor axis:  
• Requirement  <100 m above 3Re 
• Result:  ~7 m error 
Phase 1 HEO orbit @ 1.2x12 Re 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
Full System Test – Phase 2 
Error in semimajor axis:  
• Requirement  <100 m above 3Re 
• Result:  ~10 m error 
 
 Phase 2 HEO orbit @ 1.2x25 Re 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
Phase 1 and Phase 2 PPS Accuracy 
• Requirement 
– Receiver must maintain knowledge of GPS time to within 325μs 
• Procedure 
– Universal counter differences 1 PPS between Reference and DUT 
– PPS control loop utilizes GEONS clock estimate 
• Result 
– Average PPS error near 120ns for Phase 1 and Phase 2, except below 3RE 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
Benchmark Test – Acquisition Probability 
• Special SW build - 6 tracking and 6 acquiring channels 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
Benchmark Test – Measurement Noise 
• Requirements 
• Pseudorange measurement 
precision shall not exceed 
30m (3σ) 
• Procedure 
• Method similar to traditional 
double differencing of GPS 
measurements 
• Received signal power held 
constant and varied over 
multiple simulations 
• Chose all pairs of GPS 
satellites 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
Benchmark Test – Measurement Noise 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
Dynamic Range 
G O D D A R D   S P A C E   F L I G H T   C E N T E R 
Conclusion 
• Navigator passed all requirements during full 
system and benchmark testing 
• SMA error in Phase 1 approx. 7 meters max, in Phase 2 
approx. 10 meters max 
• Acquire 99% of strong and 75% of weak signals 
• Noise on the pseudorange measurements approx. 24 
meters max 
• Acquire weak signals in the presence of signals 15 dB 
stronger 


Results from Navigator GPS Flight Testing for the Magnetospheric MultiScale Mission

The recent delivery of the first Goddard Space Flight Center (GSFC) Navigator Global Positioning System (GPS) receivers to the Magnetospheric MultiScale (MMS) mission spacecraft is a high water mark crowning a decade of research and development in high-altitude space-based GPS. Preceding MMS delivery, the engineering team had developed receivers to support multiple missions and mission studies, such as Low Earth Orbit (LEO) navigation for the Global Precipitation Mission (GPM), above the constellation navigation for the Geostationary Operational Environmental Satellite (GOES) proof-of-concept studies, cis-Lunar navigation with rapid re-acquisition during re-entry for the Orion Project and an orbital demonstration on the Space Shuttle during the Hubble Servicing Mission (HSM-4)

Lulich, Tyler D.

Wintermitz, Luke M. B.

Bamford, William A.

Price, Samuel R.

Results from Navigator GPS Flight Testing for the Magnetospheric MultiScale Mission

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