The Accuracy of Current GNSS Signal Sources for Radio Occultation Missions Erin Griggs - [email protected] Rob Kursinski, Dennis Akos University of Colorado at Boulder, Moog Advanced Missions and Science Data Collection & PostAnalysis Processing Allan Deviation Motivation The Modified Allan Deviation (MADEV), as described by Allan et. al (1981)1, is the metric used to characterize the stability of all AFS onboard the different GNSS constellations, and is computed as 3 300 3 (kilometers) 30 300 GLONASS 3 (kilometers) 30 300 (kilometers) 30 300 Galileo BeiDou Hardware Configuration Active hydrogen masers at NIST and JPL were used as the AFS for the receiver to characterize signals from all GNSS constellations and satellite blocks for use in future occultation missions. The block diagram below shows the hardware setup for the NIST collection campaign. 3 (kilometers) 30 GPS Future RO missions desire to utilize all navigation satellite systems as signal sources for atmospheric remote sensing. The stability of the onboard atomic frequency standard (AFS) is critical to the quality of the derived occultation profiles from the occultation measurements. Because of the variety of the AFS used by the different GNSS constellations, a comprehensive study was conducted to characterize the stability of the signal sources at time scales relevant to RO. Data Validation 1Allan, Conclusions D.W., J.A. Barnes, A Modified “Allan Variance” with Increased Oscillator Characterization Ability, Proc. 35 th Ann. Freq. Control Symposium, USAERADCOM, Ft. Monmouth, NJ, May1981 Correction Rates for Comparable Results • 10,000 second arcs of 50 Hz L1 carrier phase data were collected with a Trimble NetR9 receiver, typically at night to minimize ionospheric disturbances • Data collected • 37 days at NIST • 24 days at JPL • The use of hydrogen masers allow for isolation of the individual clock phases • Analysis of the Galileo and new Block IIF RFS is now possible without additional noise from three-cornered hat method2 2Griggs, E., R. Kursinski, D. Akos, An investigation of GNSS atomic clock behavior at short time intervals, GPS Solut, DOI 10.1007/s10291-013-0343-7, Sep. 2013 POSTER TEMPLATE BY: www.PosterPresentations.com GPS Block (Clock Type) IIF (Cs) IIF (Rb) IIR-M (Rb) IIR (Rb) IIA (Rb) IIA (Cs) 10.4 1.0 5.8 5.2 2.4 3.6 10.4 2.2 8.7 5.9 6.6 5.3 The maximum phase error from transmitter clock is shown above for the GPS constellation using 30-second clock corrections (typical correction interval from the IGS). Note the large errors for the IIF cesium reference in PRN 24, which requires 5-second corrections to have comparable performance to the IIF RFS. The figure on the right shows the equivalent maximum phase error values with the clock data from the GLONASS constellation. 1-4 seconds clock correction rates are necessary to have comparable stability to the Block IIF rubidium clocks. Acknowledgements Typical GPS Maximum Phase Error with 30-second Clock Corrections Constellation/Block Worst Case (~9 mm) Avg. Case (~5 mm) Best Case (~1 mm) BeiDou 200 s 79 s 2s Galileo 325 s 128 s 3s GLONASS 14 s 4s 1s GPS Block IIF (Cs) 23 s 10 s 2s Comparison of worst case satellites from other GNSS constellations to GPS phase errors with existing 30second IGS clock corrections Special thanks to Judah Levine, Stephan Esterhuizen, and William Diener for their assistance with the data collection efforts. National Institute of Standards and Technology Moog Advanced Missions and Science Jet Propulsion Lab