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Time for LoranDemetrios Matsakis and Harold Chadsey
U.S. Naval Observatory
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
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.
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
USNO’s Main Clock Vault
USNO Master Clock and UTC
Sep 2002Feb 1997
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
Time From Loran
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)
Washington DC’s LORAN data
Sep 2002Jan 2001
Arizona’s LORAN data
Jan 1993 Sep 2002
Alaska’s LORAN data
Apr 1990 Sep 2002
UTC(USNO) - GPS TimeSep 01 – Sep 02, RMS=4.1 ns
-15
-10
-5
0
5
10
15
52100 52150 52200 52250 52300 52350 52400 52450 52500 52550
MJD
Nanoseconds
Some Sources of Errorfor GPS and LORAN
• Multipath, Type of Path
• Calibration
• Environment (temp, humidity, etc.)
• Ionosphere & Troposphere
• Position and Clock errors
The three most important considerations for timekeeping
1. Calibration
2. Calibration
3. Calibration
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
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)
One GPS receiver’s bias
Average Bias: -30.882 nanoseconds
USNO’s GPS Antenna Array
Antenna Mount’s Multipath Reduction
Diff. Ants. RMS=1.3 ns Same Ant. RMS=0.1ns
Two Way Satellite Time Transfer
USNO TWSTT Earth Terminals
USNO BASE STATION ANTENNAS USNO MOBILE EARTH STATION
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
Summary
Ultimate limit for LORAN’s calibration– By GPS
• easy at 10’s of ns• possible at few ns
– TWSTT • routine at 1 ns