RFI Mitigation for Radio Astronomy

1. RFI Mitigation for Radio Astronomy An Overview Rob Millenaar ASTRON/CRAF ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

2. Overview • Introduction • The Problem • Mitigation • methods • implementation • results • prospects • Summary & Conclusions ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

3. Introduction • Sensitivity • progressing science demands more sensitivity and more frequencies • and technology in principle makes this possible • FrequencyRange • radio astronomy has been blessed with reserved frequencies, but... • Nature/Physics • goes beyond the reserved ranges • Mankind • wants the passive service ranges and transmits in pretty much all of the rest ‘It is the astronomer’s duty to study the Universe. The radio astronomer does this by performing measurements at the highest sensitivity and at frequencies dictated by the natural phenomena being investigated, in spite of obstacles created by mankind.’ ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

4. The Problem • The Sensitivity Issue • Sensitivities needed to do astronomical observations - next generation instruments ~100x as sensitive • Levels of RFI sources around us are going up, up, ... ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

5. The Problem • Available spectrum • allocated only ~2% at cm wavelengths • suffering from o.o.b. radiations from other spectrum users ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

6. The Problem • Available spectrum, SKA frequency range: ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

7. The Problem • Future spectrum developments (not all is bad news) • migration to higher frequencies • digital tv, radio • lower eirp’s vs. more transmitter antennas • wide bands vs. spikes • satellites are becoming smarter - not higher power but smarter illumination (but there will be more of them) • licence free transmissions • wireless devices like Bluetooth, WLAN • UWB • SRR • PLC (!) ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

8. Characteristics of RFI • RFI to us, useful (legitimate) signal to others... • Signal characteristics: • basic properties • signal power (R/N ratio) • signal frequency • signal modulation • origin properties • internal • external • stationary • moving: airborne, space • coherency: direct, scattered (multipath) • time properties • stationary • nonstationary • cyclostationary ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

9. Mitigation • RFI Mitigation is about • prevention of the occurrence of unwanted signals • recognition of the unwanted signals once prevention has failed • and dealing with them by applying techniques and strategies that either • remove them • or minimize their influence on the wanted signal • while the wanted, astronomical signal • stays free from artefacts • and is observed as close to the theoretical sensitivity as possible. ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

10. Mitigation MethodsPrevention • Prevention of RFI • Quiet zones, siting • but no escape from LEO’s (in ever increasing numbers) • And all the other Regulatory measures • to Regulate or to Mitigate? ultimate sensitivityonly way for unprotected freq.  we need both • Prevent Self-Generated RFI • increasingly difficult ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

11. Mitigation MethodsPrevention Self-generated RFI: fouling up one’s nest ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

12. Mitigation MethodsPrevention • Preventing detrimental effects of RFI • Super linear RF circuitry • Filtering out strong signals while preserving as much of the spectrum as possible and at same sensitivity (High Temp. Superconducting Filters) • Use Multi-bit sampling (2) ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

13. Mitigation Methods - How Repairing the damage that is done. • How: In general we can apply a method in some domain (time/frequency/spatial): • excision • flagging/blanking • filtering • thresholding • beamforming/nulling in (sparse) arrays and fpa’s • higher order statistics of gaussian noise + (non gaussian) RFI input signal (probability distribution analysis) ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

14. Mitigation Methods - How Repairing the damage that is done. • How: In general we can apply a method in some domain (time/frequency/spatial): • canceling • adaptive interference cancellation (with ref. channels) • other • anticoincedence (interferometry) • targeted methods: Iridium, radar, bursts, DME ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

15. Mitigation Methods - Howadaptivecancellation Using reference antennas F. Briggs ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

16. Mitigation Methods - Howadaptivecancellation Without dedicated reference antennas ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

17. Mitigation Methods - Where • The various methods for RFI mitigation can be applied at many stages in the chain of signal processing parts: antenna receiver IF chain ADC correlator/tpd data processing calibration real-time analog digital software off-line • Some methods can be applied both early and late in the chain. ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

18. Mitigation Methods - Where • Where: • Pre-correlation • excision by higher order statistics • excision by thresholding • spatial filtering: beamforming and nulling • adaptive cancellation ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

19. Mitigation Methods - Where • Where: • Correlation • excision by blanking, spatial nulling • fast correlators - need high dump rate • the natural suppression of non-sidereally moving sources of RFI by fringe and delay steering ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

20. Mitigation Methods - Wherecorrelator • At the correlator RFI suppression naturally takes place because of the required fringe and delay steering to compensate for the Earth’s rotation (example: WSRT) =10° =80° ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

21. Mitigation Methods - Where • Where: • Post-correlation • adaptive cancellation • off-line editing (excision) • a calibration nightmare: RFI mitigation affects the observation by dynamically changing the: • frequency coverage • observation times • uv coverage • beamshapes ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

22. Mitigation Methods • Combining methods • time, frequency and spatial domain • some methods exclude others (esp. blanking, excision) • no silver bullets • need for a Mitigation Strategy Machine • database of RFI sources (know your enemy) • fixed and mobile terrestrial • satellites • perform monitoring operations • multidimensional parameter space • time • spectrum • modulation • track or position • artificial intelligence, neural networks ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

23. Mitigation Methods • Making the most of RFI Mitigation techniques: • sense the signals at strategic points in system • use data from RFI database and monitoring equipment • apply mitigation measures/algorithms at appropriate points • let some smart Mitigation Strategy Machine decide: • what technique(s) to apply • with what parameters • at which spot(s) in the system ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

24. Implementation • Technology • asic’s (application specific ic) • dsp chips (digital signal processing) • fpga (field programmable gate array) • using IP blocks (fft, fir filters, etc) • mmic’s (monolithic microwave integrated circuit) • digital radio • beamformers, analog and digital • Rapid prototype development/demonstrators through: • c.o.t.s. technology test boards • c.o.t.s. fpga boards ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

25. Implementation • Example RFI Mitigation System for the WSRT • how to implement RFI mitigation in an existing backend • pre-correlation processing system for one of 8 20 MHz wide bands has been built • future post-correlation mitigation sub-systems ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

26. Implementation Flexible algorithm implementation in fpga’s excision in freq. domain ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

27. Implementation Flexible algorithm implementation in fpga’s adaptive noise canceller ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

28. Resultsexcision in time and frequency • Effelsberg 100m single dish RT with ‘portable’ RFI mitigation system • Continuum observations at 1625MHz, 20MHz wide • Median filtering, excision in time and frequency domain using thresholding. ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

29. Resultsexcision in time and frequency • image built from 16 scans before after ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

30. Resultsadaptive cancellation • WSRT example, using neighboring telescopes as reference antennas • observation at 355MHz • autocorrelation RT5 ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

31. Resultsadaptive cancellation • WSRT example, using neighboring telescopes as reference antennas • observation at 355MHz • crosscorrelation RT5&6 ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

32. Resultsadaptive cancellation • WSRT example, using neighboring telescopes as reference antennas • map ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

33. Results • Despite the positive results, at this time that must be put into perspective: • There is a selection effect: we tend to attack problems that we think are solvable • In some case much handtuning goes into the algorithms • We can (often) deal with moderate strength RFI; now how about really strong RFI... • ... and really weak, broadband RFI (UWB)? • We need much more field experience with ‘real’ routine astronomy ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

34. Prospects • Moore’s law will allow more and faster signal processing • Much work on the theoretical foundation takes place now  next generation benefits • There is an increase in the exchange of ideas, implementations and results among groups worldwide: • through international fora and publications • URSI World assembly • (SKA) RFI workshops • (IUCAF/CRAF) Summer school • RadioNet initiatives • less obvious: the telecom industry contributes as well, by vigorous research in signal processing and cost effective implementation ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

35. Words of Warning • RFI mitigation is not without risks: • danger of throwing away the baby... (lose the signal of interest while suppressing the rfi) • generation of artefacts • reduction of sensitivity to the astronomical signal - dynamic range • reduces the chances of ‘discovery’ • complicates calibration • Need to Regulate AND Mitigate ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004

36. Summary & Conclusions • Apply the appropriate mitigation method for the: • type of instrument • type of observation • continuum • spectral • pulsar • transitional • type of RFI • And do all you can to protect: • RA769 • sites “RFI Mitigation results are always better than expected”Ron Ekers but seriously... ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004