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GPS Time Transfer

GPS Time Transfer. Dayne Olmstead 12/9/11. Current GPS Fleet. 31 Satellites currently in operation Medium Earth Orbit (20,180 km)– 6 orbital Planes: (4 satellites each + backups) Current operational satellites are exclusively Block II

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GPS Time Transfer

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  1. GPS Time Transfer Dayne Olmstead 12/9/11

  2. Current GPS Fleet • 31 Satellites currently in operation • Medium Earth Orbit (20,180 km)– 6 orbital Planes: (4 satellites each + backups) • Current operational satellites are exclusively Block II • Dual Use Fleet (Military and Civilian) • Planned fleet modernization with Block III satellites (Initial entry-to-service planned for 2014)

  3. GPS IIIR/M Satellite lockheedmartin.com/products/GPS/

  4. GPS III Improvements • Improved Position, timing, and anti-jamming system security • Increased transmission power (implement spot-beam directional transmission) • L1 and L2: -157 dBW for C/A and -160dBW for P(Y) signal • Addition of L5 (Search-and-Rescue) signal at -154 dBW

  5. GPS Signaling • Course Acquisition Code (C/A Code): 1575.45 MHz carrier --Indexes satellite for identification --CDMA format (1,023 bit pseudorandom noise). Repeats once every millisecond. (1.023 Mbit/s) Gives unique PRN code for each satellite. • Precision Code (P-Code): 1227.60 MHz --Extremely long message (6.1871 x 1012 bits long, repeats once per week), encrypted code for military applications. Combined with ‘W-Code’ (classified specifications) to generate ‘Y-Code’ called P(Y)

  6. GPS Signaling (2) • Navigation Message: Modulated over C/A and P(Y) signal at 50 bit/s. 1500 bit data ‘frames’ are divided into five 300 bit subframes. Each subframe requires 6 seconds of transmission time for reception. 25 total frames transmit the entire 1500 bit almanac Total navigation message requires 12.5 minutes to transmit a single complete GPS system almanac.

  7. GPS Signaling (3) • Individual Frame Contents: (Wikipedia, 2011)

  8. Onboard Timekeeping • Each Block II / IIA satellite has 2 Cesium and 2 Rubidium atomic clocks onboard. • Each Block IIIR satellite will carry 3 Rubidum atomic clocks • Current transmission specification is 100ns unaided UTC time transfer from existing satellite fleet

  9. Typical Unaided GPS Accuracy • Horizontal Accuracy 100m (95%) 300m (99.99%) • Vertical Accuracy 150m (95%) 450m (99.99%) • Time Accuracy 340 ns

  10. Factors Affecting GPS Accuracy • Geometric position error (PDOP) • Satellite Clock Error • Ephemeris error • Ionospheric and Tropospheric conditions • Multipath effects • Receiver inaccuracies

  11. Selective Availability http://www.gps.gov/systems/gps/modernization/sa/data/

  12. Typical GPS Timing Uncertainties

  13. One Way GPS Time Transfer • Satellite broadcast time code provides GPS system time and UTC(USNO) to receiver • GPS clock signal is compared to the local clock oscillator with a Time Interval Counter (TIC) • True position of receiver must be known to ~1m. • Signal Propagation delay variability limits theoretical accuracy to between 3 and 6 ns Image from NIST: http://tf.nist.gov/time/oneway.htm

  14. Single Channel Common-View • Difference between two clocks is determined by simultaneous observation of a GPS clock signal A - B -(dA - dB) = [A - GPS- dA] - [B - GPS - dB] • Results are independent of the GPS clock! (Accuracy 1-10 ns) Image from NIST: http://tf.nist.gov/time/commonviewgps.htm

  15. Single Channel Common-View (2) • Must follow a published satellite tracking schedule • Data collection requires a minimum 13 minute reception time for both stations to acquire a complete almanac • Satellite orbital schedule limits common-view technique to 48 common-view tracks per day

  16. Multi-Channel Common-View • Eliminates the need for a common-view satellite schedule • Allows seamless comparison of time standards between multi-channel users within a reasonable distance ( Same continent) • Typically 5-6 satellites are above 15°elevation angle at any given time • Standard protocol is 16 minutes per track (2 minutes satellite acquisition, 13 minutes data logging, 1 minute separation between tracks) • Allows roughly 450 16 minute satellite tracks per day

  17. Multi-Channel Common-View (2)

  18. Multi-Channel Common-View (3) • Requires a significant amount of digital computation • Demonstrates path losses from environmental effects in signal correction

  19. Common View Summary • Optimal performance occurs when stations are within 3,000 km baseline distance • Degraded performance = one-way measurement accuracy • Identical equipment should be used at both stations • Expected timing uncertainty (2σ) should be <10 ns at 1 day • 2.5 to 2.8 ns accuracy values frequently reported in publications

  20. Carrier Phase GPS Measurement • Primarily used for frequency transfer only • Uses L1 and L2 carrier frequencies in lieu of transmitted code data • Potential resolution is higher since carrier frequency is 1000x higher than C/A frequency • Requires extensive post processing and multiple (typ. 6) receiving sites • Time accuracy < 500 ps can be achieved

  21. Carrier Phase GPS Measurement (2) Where, = carrier wavelength, c/f = carrier phase observable for satellite S and receiver R = geometric range, , = satellite clock error = receiver clock error = propagation delay due to troposphere = propagation delay due to ionosphere = multipath error = unmodelled errors and receiver noise = carrier phase ambiguity or bias

  22. WAAS • Clock, Ephemeris, and ionospheric models are transmitted by geostationary satellite link to compatible receivers • Correction Data is computed by 24 ground stations throughout the United States • Similar system is the Japanese Multi-Functional Satellite Augmentation System (MSAS)

  23. WAAS (2) • Accuracy WAAS: -Spec: 7.6m (95%) -Act: 1.0m lateral 1.5m vertical MSAS: -Act: 1.5m -2.0m lateral and vertical http://www.environmental-studies.de/Precision_Farming/EGNOS_WAAS__E/3E.html

  24. WAAS Time Transfer? • The technique has been tested and published • Results have been less accurate than GPS Common-View (+20 / -25 ns 1 day error), (+6 / -15 ns 2-day average) • Improvement is expected with the addition of atmospheric modeling and multiple WAAS satellite inputs

  25. GPS in Instrument Approaches • Even with WAAS, GPS does not currently meet the FAA requirements for precision approaches under Instrument Flight Rules: CAT I: lateral accuracy, 910.5 m; vertical accuracy, 91.1 m. CAT II: lateral accuracy, 97.5 m; vertical accuracy, 91.1 m. CAT III: lateral accuracy, 93.0 m; vertical accuracy, 90.6 m. • Use of GPS for non-precision (RNAV) and RNP approaches continues to grow rapidly throughout the world

  26. AFLD SFC CONDITIONS NOT MONITORED BETWEEN THE HOURS OF 2300 AND 0700. • NOT RCMDD FOR NIGHT USE OR IN MARGINAL WEATHER BY UNFAMILIAR PILOTS DUE TO MOUNTAINOUS TERRAIN.

  27. References • [1] Lombardi, Michael A., Lisa M. Nelson, Andrew N. Novick, and Victor S. Zhang. "Time and Frequency Measurements Using the Global Positioning System." International Journal of Metrology. (2001): 26-33.   • [2] Weiss, M.A., P. Fenton, R. Pelletier, and E. Powers. United States. NIST Time and Frequency Division.Time and Frequency Transfer Using a WAAS Satellite with L1 and L5 Code and Carrier. IEEE, 2008.   • [3] Lechner, Wolfgang, and Stefan Baumann. "Global navigation satellite systems." Computers and electronics in agriculture. 25 (2000): 67-85.   • [4] Yang, Xu-hai, Yong-hui Hu, Zhi-gang Li, Xiao-hua Li, and Xing-wuZheng. "An Algorithm for a Near Real-time Data Processing of GPS Common-view Observations." Chinese Astronomy and Astrophysics. 27 (2003): 470-480. Print. • [5] Zhang, Victor S., Thomas E. Parker, Marc A. Weiss, and Francine M. Vannicola. United States. NIST. Multi-Channel GPS/GLONASS Common-View Between NIST and USNO. Boulder: National Institution of Standards and Technology, 2000. Print. • [6] Gifford, Al, Scott Pace, and Jules McNeff. "One-Way GPS Time Transfer." 32nd Annual Precise Time and Time Interval (PTTI) Meeting. Boulder: National Institute of Standards and Technology, 2000. 137-146.   • [7] Lahaye, Francois, Giancarlo Cerretto, and PatriziaTavella. "GNSS geodetic techniques for time and frequency transfer applications." Advances in Space Research. 47 (2011): 253-264.   • [8] Klepczynski, William J., Fenton, Pat, and Powers, Ed. "Time Distribution Capabilities of the Wide Area Augmentation System (WAAS)." 33rd Annual Precise Time and Time Interval (PTTI) Meeting. Boulder: National Institute of Standards and Technology, 2001. 111-120.

  28. References (2) • [9] Global Positioning System. (2011, December 6). In Wikipedia, The Free Encyclopedia. Retrieved 06:38, December 9, 2011, from http://en.wikipedia.org/w/index.php?title=Global_Positioning_System&oldid=464375525 • [10] Wide Area Augmentation System. (2011, November 22). In Wikipedia, The Free Encyclopedia. Retrieved 06:39, December 9, 2011, from http://en.wikipedia.org/w/index.php?title=Wide_Area_Augmentation_System&oldid=461877266 • [11] Multi-functional Satellite Augmentation System. (2011, August 11). In Wikipedia, The Free Encyclopedia. Retrieved 06:40, December 9, 2011, from http://en.wikipedia.org/w/index.php?title=Multi-functional_Satellite_Augmentation_System&oldid=444321461 • [12] "Official U.S. Government information about the Global Positioning System (GPS) and related topics."GPS.gov. National Coordination Office for Space-Based Positioning, Navigation, and Timing, 12/7/2011. Web. 12/9/2011. <www.gps.gov>. • [13] "NIST Time and Frequency Transfer Using Common-View GPS." NIST. NIST, n.d. Web. 9 Dec 2011. <http://tf.nist.gov/time/commonviewgps.htm>.

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