Time for Loran

Time for LoranDemetrios 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.

• 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