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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8Motivation Approach Results Implications

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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9Motivation Approach Results Implications

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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13Motivation Approach Results Implications

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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15Motivation Approach Results Implications

Allan Deviation Results

GPS GLONASS

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16Motivation Approach Results Implications

GPS/GLONASS Comparison

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17Motivation Approach Results Implications

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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19Motivation Approach Results Implications

GLONASS Corrections

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20Motivation Approach Results Implications

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