Time for Loran

Demetrios Matsakis and Harold Chadsey

U.S. Naval Observatory [email protected]

[email protected]

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 Feb 1997 Sep 2002

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

Jan 2001 Sep 2002

Arizona’s LORAN data

Jan 1993 Sep 2002

Alaska’s LORAN data

Apr 1990 Sep 2002

UTC(USNO) - GPS Time Sep 01 – Sep 02, RMS=4.1 ns

15 10 5 0 -5 -10 -15 52100 52150 52200 52250 52300 52350 MJD 52400 52450 52500 52550

Some Sources of Error for 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