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Time for Loran

Time for Loran. Demetrios Matsakis and Harold Chadsey U.S. Naval Observatory dnm@usno.navy.mil Chadsey.harold@usno.navy.mil. USNO Mission. Determine positions and motions of celestial bodies, Earth’s motion/orientation and precise time.

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Time for Loran

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  1. Time for Loran Demetrios Matsakis and Harold Chadsey U.S. Naval Observatory dnm@usno.navy.mil Chadsey.harold@usno.navy.mil

  2. USNO Mission • Determine positions and motions of celestial bodies, Earth’s motion/orientation and precise time. • Disseminate astrometry and timing data to DOD, the Navy, other agencies and the public. • Conduct research to improve these products

  3. USNO Time • CJCS Master Navigation and Timing Plan makes USNO responsible for DoD timing (CJCS INST 6130.01A) • Satisfy time/frequency requirements for C4I, navigation, and electronic warfare systems • The Federal Radionavigation Plan designates USNO as responsible for time.

  4. USNO Clock Ensemble • 73 High-Performance Cesiums • 17 Cavity-Tuned Hydrogen Masers • 19 environmental chambers • Distributed in three buildings and two cities • Cesium and Rubidium Fountains under development • JPL Trapped-Ion Mercury Standard under evaluation • Purchase 2 masers / 4 cesiums per year

  5. USNO’s Main Clock Vault

  6. USNO Master Clock and UTC Sep 2002 Feb 1997

  7. Low-precision users • 202-762-1400 telephone service 880,003/year • Leitch Clock System: 110,000/year • Modem: 710,000/year • Web Pages: 200,000 queries/year • NTP: ~100 million queries/day • about half via USNO-DC • 200+% more queries than last year

  8. Time From Loran

  9. LORAN • Excellent GPS backup where available • Need to expand role • USNO monitors LORAN at three sites • Washington, D.C. • Flagstaff, Arizona • Elmendorf, Alaska • Required to be within 100 ns of UTC • Public Law 100-223 (1987)

  10. Washington DC’s LORAN data Jan 2001 Sep 2002

  11. Arizona’s LORAN data Jan 1993 Sep 2002

  12. Alaska’s LORAN data Sep 2002 Apr 1990

  13. UTC(USNO) - GPS TimeSep 01 – Sep 02, RMS=4.1 ns

  14. Some Sources of Errorfor GPS and LORAN • Multipath, Type of Path • Calibration • Environment (temp, humidity, etc.) • Ionosphere & Troposphere • Position and Clock errors

  15. The three most important considerations for timekeeping • Calibration 2. Calibration 3. Calibration

  16. Calibration and Simultaneity • Typically, time is measured by edge of a voltage spike repeating at 1-pulse per second • Other means to represent time are ok as long as they are consistent • Time-transfer equipment must say spikes at two sites are simultaneous when they are simultaneous

  17. Calibration and LORAN • At point of reception • USNO monitor sites • Distorted by weather • At point of transmission • Near field/far field issues for LORAN • Several ways to calibrate time-tick • TTM (LSU) • Portable (calibration trip) • Cesium clock trips • GPS • Two Way Satellite Time Transfer (TWSTT)

  18. One GPS receiver’s bias Average Bias: -30.882 nanoseconds

  19. USNO’s GPS Antenna Array

  20. Antenna Mount’s Multipath Reduction Same Ant. RMS=0.1ns Diff. Ants. RMS=1.3 ns

  21. Two Way Satellite Time Transfer

  22. USNO TWSTT Earth Terminals USNO BASE STATION ANTENNAS USNO MOBILE EARTH STATION

  23. TWSTT Calibration • USNO routinely calibrates about 20 sites • Insensitive to • External multipath • Troposphere delay • Ionosphere at sub-ns level • Absolute calibration (because done relatively) • Sub-nanosecond repeatability over 6 months • 0.8 ns over 1000 days

  24. Summary Ultimate limit for LORAN’s calibration • By GPS • easy at 10’s of ns • possible at few ns • TWSTT • routine at 1 ns

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