Common-View LORAN-C for Precision Time and Frequency Recovery

1. Common-View LORAN-Cfor Precision Time and Frequency Recovery Tom Celano, Timing Solutions Corp LT Kevin Carroll, USCG Loran Support Unit Michael Lombardi, NIST

2. Introduction • Timing Solutions is investigating the potential performance of time recovery using common-view LORAN-C • Study is funded by LSU as part of the LORAN Accuracy Panel (LORAPP) that is chaired by the USCG • Common-View time recovery has been used for years in the GPS community with good results • Common-View GPS has repeatedly demonstrated < 10 ns timing performance • Common-View LORAN-C provides a differential solution that will help to reduce the dominating propagation delay errors that drive LORAN-C timing performance • LORAPP study to determine feasibility and projected performance of common-view approach for Enhanced LORAN era • Can Enhanced LORAN be a viable backup to GPS?

3. Common View GPS • Common View GPS involves computing a relative time difference between two clocks by subtracting the GPS measurements collected at each site • Each site collects passive GPS data from the individual GPS SV’s • The common view difference is computed by subtracting the GPS data sets by SV • Common mode GPS noise (like ionospheric delay) cancels • If one of the two clocks has a known relationship to UTC, absolute time can be transferred GPS SV Site 1 Site 2 Clock 1 - GPS Clock 2 - GPS Clock 1 GPS Rx GPS Rx Local Clock - Clock 1 – Clock 2

4. Common View GPS (cont’d) • Ionospheric noise (and other systematic GPS noise) that is common to both sites cancels in the calculation, resulting in a lower noise, relative measurement between the two clocks • Absolute reference of measurement is lost if one of the clocks is not related to UTC • Precision of common view measurement is significantly better than passive measurement

5. Common View LORAN-C • Common view LORAN-C is the same idea as common view GPS with GPS satellites replaced by LORAN-C transmitters • The data from each LORAN-C transmitter is treated independently and is corrected using TOA monitor data • Data from all transmitters can be combined after common view differences are calculated • LORAPP timing study initiated in July 2003 in order to examine common view technique LORAN-C for time recovery • Primary goal of LORAPP timing study is to determine if common view LORAN-C can be considered as a backup to GPS for precision timing users • Performance to be analyzed using real-world data collection • Bound best case using short baseline • Gauge expected performance using long baseline • Determine requirements and candidate architecture for precision time recovery in Enhanced LORAN era

6. Experiment Set-up • Dual GPS/LORAN data collection systems are installed at three locations • TSC (Boulder, CO), NIST (Boulder, CO), LORSTA Gillette (Gillette, WY) • LORAN data and GPS data logged continuously against local UTC source • Common-view differences computed using both LORAN and GPS data • LORAN-C common-view performance compared to GPS common-view data 300 Miles

7. Hardware Configuration • Systems installed at each location consist of separate hardware for GPS and LORAN-C processing • Independent methods for timing computation • GPS data collected using NIST common-view service (TSC and NIST) and ONCORE based common-view computation (TSC and Gillette) • LORAN-C data produced by Peterson Integrated Geopositioning (PIG) software using data from a LOCUS LORAN-C receiver • External time interval counter and local UTC estimate used to compute TOA referenced to local time

8. “Passive” LORAN-C Timing Data • Uncorrected LORAN-C data is subject to diurnal and seasonal variations in propagation delay • Seasonal term dominates the time of arrival data and limits time recovery performance • Without differential correction, LORAN-C timing is limited to microsecond level performance • Correlation in propagation delay over different transmission paths allows for common view principles to be applied • Same concept as single frequency GPS with ground propagation replacing ionospheric delay as the common mode noise source

9. Correlated Effects in LORAN-C Data • Degree of correlation for propagation delay between monitor site and user site will drive performance • This will also drive the required number and density of monitor sites *Data has been externally calibrated by applying a bias to each LORAN transmitter

10. Common-View LORAN-C Data • Common-View LORAN computation removes a significant portion of the long term propagation delay variations • Like GPS case, common noise is apparent in passive LORAN-C data • Noise level of common-view data is related to proximity to monitor station • Short baseline data is slightly noisier than common view GPS • Longer baseline data shows higher noise level but is still considerably better than passive case *Data has been externally calibrated by applying a bias to each LORAN transmitter

11. Clock at TSC cold started Common View LORAN-C Short Baseline • Short baseline between TSC and NIST provides the best case scenario for common view LORAN-C • TSC and NIST only 5 miles apart • Data collected between TSC and NIST shows excellent precision and can be compared favorably to GPS common view • Data clearly shows cold start of TSC timing system (50 ns effect)

12. Common View LORAN-C Short Baseline (cont’d) • Best case scenario data ranges from 8 ns (RMS) to 25 ns (RMS) depending on distance from transmitter • Higher noise stations would not be used in real solution • Low noise stations can be combined to increase robustness

13. Common View LORAN-C Long Baseline • Long baseline between TSC and Gillette provides a more realistic operational scenario for common view LORAN-C • 300 mile baseline • Data collected between TSC and Gillette still shows excellent precision and a significant reduction in propagation delay effects

14. Could be served by Enhanced LORAN Timing User Spectrum 0.1 ns 1 ns 10 ns 100 ns 1 µs 10 µs 100 µs 1 ms 10 ms 100 ms 1 s PTTI/R&D - NIF Scientific/ Experimental • High Precision Military • GPS Monitor Stations • GPS Weapons • AT3 Airborne Geolocation Demo • Bistatic Radar • Various Classified National Timing Labs Advanced Comms • Power Systems • Fault Location • Phasor Meas • Data Sharing CDMA2000 - Base Stations • Low Precision Military • Ground Terminals • VHF Special Comms High Speed Photometry • Wide Area Data Logging • Seismic monitoring • Nuclear Blast Detection Astronomy • Digital Time Servers • NTP, etc • Authentication • Internet login Timing user survey not intendend to be a complete representation of all users. Requirements have been generalized and averaged over user groups Financial Transactions

15. LORAN-C for Frequency Recovery • Long baseline common view LORAN-C frequency recovery data shows Stratum I performance with less than 1 hour of averaging time • No significant difference from the passive case over the short term • Common view technique not as beneficial for frequency recovery in short term • Long term performance expected to show performance benefit but we don’t have the data

16. Common View Requirements • Common view LORAN-C time recovery will be enabled by Enhanced LORAN assuming that TOA operations become the norm • TOA monitoring, TOA receivers • SAM sites will require precision timing in order to compute TOA corrections • TD monitoring is not sufficient • A calibrated and delay stable TOA LORAN-C receiver is also required in order to recover and maintain absolute time • TOA receiver required to get an absolute measurement from the LORAN-C receiver at the <10 ns level • Delay stable over time, power cycling and temperature

17. On-going Work • Work is continuing on LORAPP timing experiment • Hardware to be left in place to continue to collect data • Goal is to collect long term data so that full seasonal effect can be seen in data and processed using common view • Current data collection procedures is not robust enough to facilitate long term data collection • Data logging is too easy to disrupt • Too much data being lost due to unattended operation • Attempt will be made to provide better visibility to data collection operations • Need to enable long, continuous data runs

18. Summary • Data collection continues on a common-view LORAN-C timing experiment between three sites • Preliminary results indicate that precision time recovery is possible using common view LORAN-C • Results over 300 mile baseline with initial data set show that 25-50 ns (RMS) time recovery is possible • In order to bound performance, long term data is required • Preliminary data points to common view LORAN-C is a viable method for precision time recovery backup to GPS • Only method to provide < 50 ns timing to anywhere in US • In order to realize precision timing performance, Enhanced LORAN-C baseline must include TOA based monitoring with timed monitor sites • Requires a change to the current SAM hardware configuration • GPS dependence can be addressed if required • Experiment will continue and results will be presented at upcoming conferences