An Investigation of GNSS Atomic Clock Behavior at Short ...wegc...An Investigation of GNSS Atomic...
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An Investigation of GNSS Atomic Clock Behavior at Short Time Intervals Erin Griggs, Rob Kursinski, Dennis Akos University of Colorado Moog Broad Reach September 5, 2013
Transcript of An Investigation of GNSS Atomic Clock Behavior at Short ...wegc...An Investigation of GNSS Atomic...
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
