Transitioning to Time of Transmission Control in the U.S. Loran System

1. Transitioning to Time of Transmission Control in the U.S. Loran System ILA 2003 Boulder, CO Mr. Gene Schlechte CAPT Curtis Dubay, P.E. U. S. Coast Guard Navigation Center CDR John Macaluso U.S. Coast Guard Loran Support Unit

2. Disclaimer The views expressed herein are those of the authors and are not to be construed as official or reflecting the views of the Commandant or of the U. S. Coast Guard.

3. The Central Question What are the potential methodologies and projected effects of transitioning the U.S. Loran system to a Time of Transmission (TOT) method of control?

4. Overview • Presentation will specifically address: • Current method of using far-field System Area Monitors   • Potential methods to transition to TOT control • Changes that might be needed to Loran Signal Spec • Anticipated effects on legacy (hyperbolic) Loran receivers

5. North America Loran-C

6. Current Loran Control • Far-field System Area Monitors track (for hyperbolic navigation): • Time Difference (TD) • Pulse Shape (ECD) • Signal Strength (Gain)

7. Current Loran Control • Control Network has three main elements: • Control Station w/ Loran Consolidated Control System (LCCS) • Transmitting Station • Primary Chain Monitor Sites (SAM) • Acts as a real-time monitor of transmitting station data, alarms, and physical condition

8. Current Loran Control • Normal mode is Alpha Control: • TD (Master to Secondary) – standard sampling point 25 uSec into pulse (far field), based on CSTD determined during calibration at 30 usec point (at transmitting station) • Far Field ECD • Signal Strength

9. Current Loran Control • LCCS Time Difference Controller: • Automatic insertion of LPAs • Each LPA must be 40 nSec or less • No more than 2 LPAs /baseline /hour • Cum total of LPAs not to exceed 100 nSec /24 hours

10. Current Loran Control • Envelope to Cycle Difference (ECD) characterized by relationship between: • Phase of RF Carrier • Time origin of Envelope Waveform • Needed for proper acquistion of signal

11. Current Loran Control • Transmitting Station: • TINO • Peak Volts • Assigned/Nominal ECD

12. Current Loran Control • System Area Monitors (SAM) • Long term monitoring and control • Two Alpha monitor receiver sites (A-1 & A-2) • Alarm tolerances based on operations and seasonal variations

13. SAM to TOT Methodologies • Three switch-over methodologies possible: • Station by Station • Chain by Chain • Whole System

14. SAM to TOT Methodologies • Legacy Receiver Performance Factors: • Station geometry • Control point for grid (SAM receiver) • Distance from user to SAM • Accuracy of USCG provided ASFs • Accuracy of receiver TD measurement

15. SAM to TOT Methodologies • Modern Receiver Performance Factors: • Station geometry • Precision of TOT control to UTC • Accuracy of updated ASFs • Accuracy of receiver TOA measurement

16. SAM to TOT Methodologies • Two Important Points: • Master Loran Stations are already under TOT control (currently +/- 100 nSec) • New Timing & Frequency Equipment (TFE) designed for SAM or TOT, steering phase of signal will be possible instead of 20 nSec jumps)

17. SAM to TOT Methodologies • Station by Station: • Most gradual transition for legacy users • Incremental improvement for modern receivers • Increased complexity for control watchstander • Lowest risk - facilitates longest performance validation period

18. SAM to TOT Methodologies • Chain by Chain: • Less confusing for legacy users • Faster improvement for modern receivers (on regional basis) • Increased complexity for control watchstander • Medium risk - facilitates regional performance validation period

19. SAM to TOT Methodologies • Whole System: • Longest delay in impact on legacy users • Slowest improvement for modern receivers • Least complexity for control watchstander • Highest risk – does not facilitate performance validation period

20. SAM to TOT Methodologies • Methodology of Choice: Chain by Chain • Phase-in/out period for legacy/modern user • Moderate complexity for control watchstander • Facilitates performance validation in mid-2004 • Manageable transition • SAM will be used to monitor ECD and SS in far field

21. Changes to Signal Specification • Components of a modernized Loran: • Supportable into the future • TOT control • Data channel for broadcast of temporal correctors • Backwards compatible w/ legacy receivers • Compatible w/current tri-state PPM

22. Changes to Signal Specification • What might be changed: • Definition/implementation of “blink” • Addition of new pulse for Loran data channel • Data channel for broadcast of temporal correctors • Standards for spatial & temporal correction data • Quickened rise time to mitigate early skywave?

23. Anticipated Effect on Legacy Users • Studied using Double Range Difference Model, for: • 30 nSec Synchronzation • 100 nSec Synchronization • Repeatable accuracy predicted to degrade • 30 nSec case better than 100 nSec case

24. Anticipated Effect on Legacy Users • New TFE will allow 5 – 10 nSec Synchronization: • May mitigate some loss of repeatable accuracy • Data needs to be collected • Study needs to be re-visited • Emission Delays may need to be re-assigned • Use of historical means may help

25. Questions? Transitioning to Time of Transmission Control in the U.S. Loran System ILA 2003 Boulder, CO Mr. Gene Schlechte CAPT Curtis Dubay, P.E. U. S. Coast Guard Navigation Center CDR John Macaluso U.S. Coast Guard Loran Support Unit