V-1 A New Satellite Time Service Enhancing and Extending LORAN-C Al Gifford National Institute of Standards and Technology James Doherty Institute for Defense Analysis Tom Bartholomew Northrop Grumman TASC

V-2 Overview The basic idea presented is the measurement of time differences between LORAN stations using a two-way-time-transfer device on NASA satellites A new LORAN S-band 2ns time service is proposed The technology, devices and development process will be discussed Projected performance goals of the enhanced and upgraded LORAN services are presented

V-3 Basic Idea LORAN-C is upgrading the three elements of timekeeping –Clocks upgraded to Agilent 5071s (approximately 100) –Clock measurement capability upgraded to state-of-the-art; includes and GPS receiver for cross-site measurement –Clock management will include ensembling of site clocks and a possible calculation of a system wide distributed time scale (steering to UTC(USNO) is the current plan) LORAN navigation and time services will be significantly enhanced This paper proposes an extension of these LORAN services Utilizing the LORAN state-of-art distributed timekeeping system, an Ultra High Precision (UHP) global time service operation at S-band from NASA satellites could be realized

V-4 Enhanced and Extended Services Regional Services Navigation Time (50-500ns) GPS Augmentation Global Services Time (<2ns) Ephemeris Upgraded LORAN-C Satellite Time Service

V-5 Why? UHP Time users with global applications are dependent solely on GPS USNO’s primary time transfer vehicle is GPS and its alternate to UHP users (Two-way Satellite Time & Frequency Transfer) is operated from only a single location A Backup to the DOD Positioning-Navigation-Timing (PNT) infrastructure is required DOD Instructions require backup for some applications (e.g. C4ISR) LORAN is a UHP user that would benefit from an alternate UHP time transfer service

V-6 Why Extend LORAN Services? LORAN has invested a significant amount in a distributed timekeeping system in order to provide a robust regional navigation and time service –Internally, time will be managed to the 15ns level via GPS direct broadcast There is a potential of utilizing GPS common view measurements to compute an independent time scale –The broadcast LORAN signals will provide UTC <500ns –This service meets Stratum I frequency requirements but is not suitable for UHP users –Time service will gradually degrade in the absence of GPS service The LORAN infrastructure could provide the basis for a UHP satellite service utilizing its distributed clock assets as a flywheel time scale –This LORAN capability coupled with recent technology developments in communications based time-transfer devices could enable a 2ns global time transfer system

V-7 How it could work LORAN would compute a distributed time scale with the cross-station measurements of clocks Using the GPS timekeeping model, LORAN system time would be steered to UTC through USNO or directly to UTC(BIPM) NASA would provide a time-based-comms device on several satellites which would be accessible to multiple users LORAN operators would schedule the collection times for satellite and ground assets and upload these schedules The comm devices would initiate the measurements and provide these clock time differences to the operators in real-time A Low Earth Orbiting (LEO) satellite would require an atomic clock in order to flywheel between station measurements A Geosynchronous (GEO) satellite could provide continuous regional measurements between sites

V-8 Satellite Time Service A LORAN/NASA Operated System LEO GEO A LEO Implementation would require an atomic clock of the type that is flown on GPS IIR. LEO coverage is global. A GEO Implementation would not require an atomic clock and could provide service continuously. GEO coverage is regional

V-9 The Technologies Metrology: independent verification of time transfer –Flight verification of metrology in DARPA AT3 program Two-way Time Transfer measurement Understanding and implementing physical principles –Handbook and simulator for relativistic time transfer –Relativistic transformation of satellite proper time to coordinated time The hardware devices –NASA/Goddard Low Power Transceiver (LPT) (supporting manned missions) –NASA/JPL BlackJack receiver (supporting science missions) The test and evaluation –Current aircraft testing underway Flight opportunities –LPT to fly on shuttle in early 03; time transfer mods to be complete in 04 –BlackJack is currently flying on NASA science missions –Both devices will be utilized for time transfer on the Space Station

V-10 Clock offset data for the entire test period. The AT3 PVTF risk-reduction flight tests were conducted on a T-39 aircraft flown by the 412 Test Wing at Edwards AFB, CA. Verification of Metrology

V-11 The estimated relativity effects during flight test 1 were: Gravity: 9.4 ns (fast) Velocity:-1.63 ns (slow) Sagnac:-0.1ns (slow) Total:7.66 ns (fast) Measured:5.97 ns (fast) Delta:1.69 ns The estimated relativity effects for flight test 3 are: Gravity: 5.83 ns (fast) Velocity:-1.19 ns (slow) Sagnac: 0.0 ns Total:4.64 ns (fast) Measured:7.29 ns (fast) Delta: ns Looking across all three flight tests, the relativity prediction error statistics were: The estimated relativity effects during flight test 2 were: Gravity: 8.96 ns (fast) Velocity:-1.64 ns (slow) Sagnac:-0.11 ns (slow) Total:7.21 ns (fast) Measured:5.2 ns (fast) Delta:2.01 ns

V-12 Satellite orbital properties Satellite ISSTOPEXGPSMolniyaGEOTundra Semimajor axis km Eccentricity Inclination deg Argument of perigee deg Apogee altitude km Perigee altitude km Ascending node altitude km Period of revolution s Mean motion mrad/s rev/d Mean velocity km/s Clock effects Secular time dilation  s/d Secular redshift  s/d Net secular effect  s/d Amplitude of periodic effect due to eccentricity ns Peak-to-peak periodic effect due to eccentricity ns Secular oblateness contribution to redshift ns/d Amplitude of periodic effect due to oblateness ps Peak-to-peak periodic effect due to oblateness ps Amplitude of periodic tidal effect of Moon ps Amplitude of periodic tidal effect of Sun ps Signal propagation Maximum Sagnac effect ns Gravitational propagation delay along radius ps Amplitude of periodic fractional Doppler shift 10  Excerpt from Handbook on Relativistic Time Transfer

V-13 NASA/Goddard LPT

V-14 Enhanced and Extended LORAN Three Levels of Configuration and Performance Core/GPS: LORAN station timing systems interoperating with direct GPS and GPS common view between stations Core/GPS/STS: Interoperating via STS satellite(s) with TWTT and direct GPS and common view Core/STS: Operation using only LORAN system time-scale as reference input to STS; time-scale available via LORAN-C and STS

V-15 Predicted Performance of Enhanced and Extended LORAN System Conf Time Transfer WRT UTC Frequency Transfer “UTC” Recovery without GPS Flywheel (F w ) Independent of GPS Comments CORE/ GPS ns1x ns + RSS F w Time <1 μs (days) Freq< (forever) CORE/ GPS/ STS <5ns (STS) <200ns (Loran) 2x x <15ns (STS) + RSS F w <200ns (Loran) + RSS F w Time < 100ns (years) Freq <1x (forever) STS implemented on LEO CORE/ STS (no GPS) <5ns (STS) <200ns (Loran) 2x x <15ns (STS) + RSS F w <200ns (Loran) + RSS F w Time < 100ns (years) Freq <1x (forever) STS implemented on LEO

V-16 Summary The basic idea presented in this presentation was the measurement of time differences between LORAN stations using a two-way-time-transfer device on NASA satellites A new LORAN time service would provide backup to GPS in UHP applications (including LORAN) The technology is mature enough to support this proposed Satellite Time Service

V-17 CLOCK 1 Time = T CLOCK 2 Time=T MEAS 2 = T 2 - (T 1 +T D ) MEAS 1 = T 1 - (T 2 +T D ) Desired Measurement: T 2 - T 1 =.5*(MEAS 2 - MEAS 1 ) Basic Two-Way Time Transfer Measurement Measurement Requirements 1) Event (pulse) to measure 2) Low noise measurement of event 3) Mechanism to exchange data between locations 4) Reciprocal Delay (over measurement interval) Where: T 1 = Time of Clock 1 T 2 = Time of Clock 2 T D = Propagation Delay Backup

V-18 Cross-Site Data via GEO  Daily data sets color coded  Standard Deviation of measurement noise is < 1ns  Long term variation in curve can be attributed to clock steering at RUNWAY and REGIME Higher Noise due to lower bandwidth REGIME Clock beginning to fail REGIME Clock degraded REGIME Clock replaced REGIME timing deviation due to new clock Backup

V-19 Two-way time transfer using Fiber Data collected in the lab from SONET fiber optic timing equipment (best case scenario) –17 ps rms over 12 hours Backup