A New Satellite Time Service

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

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

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

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

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

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

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

8. Satellite Time Service GEO LEO 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 A LORAN/NASA Operated System

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

10. Verification of Metrology 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.

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 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 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: -2.65 ns Looking across all three flight tests, the relativity prediction error statistics were:

12. Excerpt from Handbook on Relativistic Time Transfer Satellite orbital properties Satellite ISS TOPEX GPS Molniya GEO Tundra Semimajor axis km 6766 7715 26 562 26 562 42 164 42 164 Eccentricity 0.00 0.00 0.02 0.722 0.01 0.2684 Inclination deg 51.6 66.0 55 63.4 0.05 63.4 Argument of perigee deg 0 0 0 250 0 270 Apogee altitude km 388 1337 20 715 39 362 36 208 47 103 Perigee altitude km 388 1337 19 653 1006 35 364 24 469 Ascending node altitude km 388 1337 19 653 10 507 35 364 32 749 Period of revolution s 5539 6744 43 083 43 083 86 164 86 164 Mean motion mrad/s 1.134 0.932 0.146 0.146 0.0729 0.0729 rev/d 15.6 12.8 2.0 2.0 1.0 1.0 Mean velocity km/s 7.675 7.188 3.874 3.874 3.075 3.075 Clock effects Secular time dilation s/d -28 -25 -7 -7 -5 -5 Secular redshift s/d 3 10 46 46 51 51 Net secular effect s/d -25 -14 38 38 46 46 Amplitude of periodic effect due to eccentricity ns 0 0 46 1653 29 774 Peak-to-peak periodic effect due to eccentricity ns 0 0 92 3306 58 1549 Secular oblateness contribution to redshift ns/d 23.0 27.6 0.5 2.5 -0.1 0.2 Amplitude of periodic effect due to oblateness ps 256 286 38 167 0 27 Peak-to-peak periodic effect due to oblateness ps 512 572 76 334 0 54 Amplitude of periodic tidal effect of Moon ps 0.0 0.0 1.2 1.2 6.1 6.1 Amplitude of periodic tidal effect of Sun ps 0.0 0.0 0.5 0.5 2.7 2.7 Signal propagation Maximum Sagnac effect ns 12 22 136 234 218 275 Gravitational propagation delay along radius ps 0.8 2.5 -4.7 -4.7 -27.3 -27.3 Amplitude of periodic fractional Doppler shift 10-12 0.0 0.0 6.7 241.1 2.1 56.5

13. MODEL WEIGHT SIZE POWER EDM ~ 4 kg 4.35” x 5.75” x 5.0” 7.5 W PPM ~ 3 kg 4.35” x 5.33” x 4.68” 7.5 W VFM* ~ 3 kg 4.35” x 5.33” x 4.68” 5 W NASA/Goddard LPT

14. Enhanced and Extended LORANThree 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

15. Predicted Performance ofEnhanced and Extended LORAN

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

17. - + Basic Two-Way Time Transfer Measurement MEAS2 = T2 - (T1+TD) CLOCK 1 Time = T1 - MEAS1 = T1 - (T2+TD) CLOCK 2 Time=T2 + Where: T1= Time of Clock 1 T2= Time of Clock 2 TD= Propagation Delay Desired Measurement: T2 - T1 = .5*(MEAS2 - MEAS1) 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) Backup

18. Higher Noise due to lower bandwidth 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 REGIME Clock degraded REGIME Clock replaced REGIME Clock beginning to fail REGIME timing deviation due to new clock Backup

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