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An Investigation of GNSS Atomic Clock Behavior at Short Time
Intervals
Erin Griggs, Rob Kursinski, Dennis AkosUniversity of Colorado Moog Broad ReachSeptember 5, 2013
Agenda
• Motivation• Approach• Results• Implications
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MOTIVATION
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Future of Radio Occultation• Utilization of multiple constellations– More occultations– Denser coverage
• Satellite clock contributions are significant to carrier phase– How stable are the GNSS clocks for time intervals of interest for RO?
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Source: GNSS, GPS, GLONASS and Galileo, www.kelloggreport.com
Motivation Approach Results Implications
Clock Stability
• Short‐term clock stability necessary for RO– Sample rates of 50 Hz used by current missions– ~100 second occultation duration
• Investigate GLONASS clock performance– Compare with GPS clock results– Time intervals between 0.4 and 100 seconds
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To utilize GLONASS satellites for RO, is ground‐based compensation required to eliminate satellite clock instability?
Motivation Approach Results Implications
Why GLONASS?
• Fully active constellation– 24 operational satellites
• Chipping rate– ½ of GPS (0.511 MHz)– Wider correlation peak– Potentially higher SNR
• FDMA• Less cross‐correlation
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Correlation Function
GLONASS
GPS
Motivation Approach Results Implications
APPROACH
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Clock Data
Must isolate the clock variation to the carrier phase of the GNSS signal
• Orbital effects– Interpolated IGS orbital data products
• Receiver clock– Single difference, three‐cornered hat
• Receiver noise– Linear model of white phase noise
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Receiver Clock
• Single differencing– Differencing the carrier phase observations between pairs of GNSS satellites
– Removes the mutual receiver clock error
– Does not reveal individual satellite clock behavior
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Clock Offsets, PRN 7/PRN 13
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Isolating a Single Clock• Three‐cornered hat technique– Statistically isolate carrier phase from a satellite by multiplying two pairs of single differences
– Assumes independence between individual clocks
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Motivation Approach Results Implications
How to measure clock stability
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Source: Standard terminology for fundamental frequency and time metrology (Allan, et al. 1988)
Allan Deviation Lesson
Motivation Approach Results Implications
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Receiver Noise
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τ‐3/2
Underlying satellite clock stability
RESULTS
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Data Collected
GPS
ClockGLONASS
ALM SVNPRN7
SVN48 Rb
BlockIIR‐M 11 723
ClockCs
8 38 Cs IIA 21 725 Cs13 43 Rb IIR 10 717 Cs23 60 Rb IIR 22 731 Cs19 59 Rb IIR 20 719 Cs3 33 Cs IIA
• Obtain clock phase by removing– Orbits– Receiver clock– Thermal contribution
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Allan Deviation Results
GPS GLONASS
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GPS/GLONASS Comparison
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IMPLICATIONS
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Estimate of Error in Phase Observations
• Linear interpolation of clock phase between two phase measurements
• Error in interpolated value
• Similar in form to Allan deviation definition
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GLONASS Corrections
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PRN 3, 30 s correctionsPRN 8, 30 s corrections
PRN 25, 30 s corrections
High rate compensation necessary to match measured GPS performance
Future work• Relate errors in carrier phase to RO data products– Refractivity, temperature, pressure
– Extend mapping from Kursinski et al. (1997)
• Data collection– High gain antenna, stable receiver clock
– Expand to other satellite blocks/constellations
• GPS IIF, Galileo, Compass
– Extend collection duration
• Provide more certainty in longer term results
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Acknowledgements
• Dr. Dennis Akos and Dr. Rob Kursinski• Moog Broad Reach• University of Colorado
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Questions
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ReferencesAllan D, Hellwig H, Kartaschoff P, Vanier J, Vig J, Winkler G, Yannoni N (1988)
Standard Terminology for Fundamental Frequency and Time Metrology, Proc. 42nd Annu. Freq. Control Symp., pp.419‐425 1988 :IEEE Press
Hauschild A, Montenbruck O, Steigenberger P (2012) Short‐term analysis of GNSS clocks, GPS Solut. 17(3):295‐307, doi :10.1007/s10291‐012‐0278‐4
Kursinski R et al. (1997) Observing Earth’s atmosphere with radio occultation measurements using the Global Positioning System, J. Geophys. Research, Vol. 102, No.D19 pp.23429‐23465
Rochat P et al. (2005) The Onboard Galileo Rubidium and Passive Maser, Status & Performance, in Proc. IEEE Freq. Contr. Symp. PTTI Syst. Applicat. Meeting, Vancouver, BC, Canada
Senior K, Beard R, Ray J (2008) Characterization of Periodic Variations in the GPS Satellite Clocks, GPS Solut. 12(3):211‐225
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BACK UP SLIDES
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Galileo RAFS Allan Deviation
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Galileo PHM Allan Deviation
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Comparison to Future Constellations
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GLONASS Corrections
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PRN 3PRN 8
PRN 25
High rate compensation necessary to match GPS and Galileo performance
Galileo RAFS
Galileo PHM
Scintillation Spec