Demetrios Matsakis and Harold Chadsey 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. 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 2002 Feb 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 Jan 2001 Sep 2002

Arizona’s LORAN data Jan 1993 Sep 2002

Alaska’s LORAN data Sep 2002 Apr 1990

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

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 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 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)

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