SUPRESSION OF LORAN-C NAVIGATION SIGNAL IN DIGITAL CAVE RADIOS (AN EXPERIMENTAL APPROACH)

1. SUPRESSION OF LORAN-C NAVIGATION SIGNAL IN DIGITAL CAVE RADIOS (AN EXPERIMENTAL APPROACH) BCRA Cave Technology Symposium Mr. Antonio Muñoz Group of Technologies in hostile Environments (GTE) University of Zaragoza (Spain)

2. http://gte.unizar.es

3. http://gte.unizar.es

4. Outline • Loran-C • Digital Radios (SDR) • Supression algorithm • Experimental results • Conclusions http://gte.unizar.es

5. Loran-C: introduction • Radionavigation signal • Position is computed by carefully estimating pulse arrival times • Operated in chains identified by its GRI (Group Repetition Interval) • Stations power ranging from 11kW up to 1MW • Signal properties • Uses 20 kHz Bandwidth • Estimated dynamic range: 100 dB http://gte.unizar.es

6. Loran-C: why eliminate it? • User perspective • Introduces a great deal of distortion in voice communication • Disturbing noise in absence of communication • Contributes to battery discharge • Designers perspective • Input dynamic range • Dificulties AGC control • Band has no other source of noise • Loran-C perspective • Valuable position determination • Helps to evaluate the electrode coupling/wiring problems http://gte.unizar.es

7. Loran-C: signal description (I) • Built arround a deterministic pulse envelope • Each chain has a master and several slaves • Master: 9 equally spaced pulses (1 ms) except for the last (2 ms) • Slave: 8 equally spaced pulses (1 ms) • Pulses have also phase coding to enhance chain identification http://gte.unizar.es

8. Loran-C: signal description (II) Master burst 6731 Lessay Slave bursts Loran-C http://gte.unizar.es

9. Loran-C: some math • Known facts • GRI times are between 50 and 100 ms • Chains have from 3 to 5 stations • Pulse amplitude is reduced to ~0.1% @ 400 us • Received Loran-C signal >> received voice link signal • Pulse is “active” 400 us / 1 ms (40 %) • In each GRI (50 ms) there are 25 (9 + 8 + 8) pulses • Channel is “used” 20% of the time (25 * 0.4 / 50) in worst case • Pulses are like gaps in audio signal • To reconstruct it perfectly  BWmax < 1250 Hz (FS = 1/400us) http://gte.unizar.es

10. SDR: topology • SSB Modulation uses Weaver scheme • Simple implementation • Suitable for SDR systems • Frequencies choosen to avoid in band auto generated interferences (PWM armonics) IF 88450Hz 1500Hz http://gte.unizar.es

11. SDR: signals (I) http://gte.unizar.es

12. SDR: signals (II) http://gte.unizar.es

13. Supression Algorithm • Where goes the algorithm? • RF  Signal information is complete(!), processing requirements HIGH (Power consumption) • IF  Some signal information is lost, processing req. MEDIUM • BB  Most of signal information is gone, processing req. LOW • Separate mixing structure? • Does it have to be accurate? • How Loran-C removal can be done? http://gte.unizar.es

14. Algorithm: implementation • No separate mixing structure • Implemented in IF/BB (48/24 ksps) • Must be simple (low resources) • No Loran-C synchronisation needed • Uses signal power to implement signal detection http://gte.unizar.es

15. Algorithm: signal processing (I) • Wideband signal is mixed in RF with 88.450 Hz, “folding” the spectrum arround Fmix Fmix = 88540 Hz http://gte.unizar.es

16. Algorithm: signal processing (II) • Phase & Quadrature signals are decimated to get desired IF/BB frequency DCF77 Audio information http://gte.unizar.es

17. Algorithm: signal processing (III) • High pass filter IQ signals (to remove audio information) • Compute power of the filtered IQ signals and make two averages, one fast and one slow http://gte.unizar.es

18. Algorithm: signal processing (IV) • If fast average is n times greater than slow average, then blank signal starting at the point which fast average was greater than slow average Detail (zoomed version) http://gte.unizar.es

19. Algorithm: resources • Are very dependant of sampling speed • Case of BB sampled at 24 ksps: • Delay lines for I & Q channels (64 samples) • Compute IQ Power (2 multiplications + 1 add) • Delay line for computed power (32 samples) • Fast and slow power averages (2 adds + 2 substractions) http://gte.unizar.es

20. Experimental Results SNR is enhanced, but improvement has a strong dependance with relative signal amplitudes (Loran-C vs. Voice) Proposed algorithm introduces little distortion while eliminates some of the annoying noise. As the human hearing has logarithmic behaviour with perceived power, algorithm has to be very accurate to completely eliminate Loran-C perception. Audio samples http://gte.unizar.es

21. Conclusions This is our first approach to Loran-C supression Pulse correlation combined with confort noise would greatly enhance the results Use of other bands http://gte.unizar.es

22. Thank you!! Antonio Muñozanmunoz@unizar.es http://gte.unizar.es

23. SUPRESSION OF LORAN-C NAVIGATION SIGNAL IN DIGITAL CAVE RADIOS (AN EXPERIMENTAL APPROACH) BCRA Cave Technology Symposium Mr. Antonio Muñoz Group of Technologies in hostile Environments (GTE) University of Zaragoza (Spain)