1. Demetrios Matsakis and Harold Chadsey
Time for Loran
Demetrios Matsakis and Harold Chadsey
U.S. Naval Observatory

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 A)
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 (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 Time Sep 01 – Sep 02, RMS=4.1 ns

14. Some Sources of Error for 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 At point of transmission
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: nanoseconds

19. USNO’s GPS Antenna Array

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

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 TWSTT
easy at 10’s of ns
possible at few ns
TWSTT
routine at 1 ns