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

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

3. rpm 2004ESF 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 Frequency Range –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.’

4. rpm 2004ESF 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,...

5. rpm 2004ESF 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

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

7. rpm 2004ESF 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 (!)

8. rpm 2004ESF 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

9. rpm 2004ESF 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.

10. rpm 2004ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004 10 Mitigation Methods Prevention 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

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

12. rpm 2004ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004 12 Mitigation Methods Prevention 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)

13. rpm 2004ESF 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)

14. rpm 2004ESF 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

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

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

17. rpm 2004ESF 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 analog digital software real-time off-line Some methods can be applied both early and late in the chain.

18. rpm 2004ESF 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

19. rpm 2004ESF 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

20. rpm 2004ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004 20 Mitigation Methods - Where correlator 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°

21. rpm 2004ESF 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

22. rpm 2004ESF 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

23. rpm 2004ESF 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

24. rpm 2004ESF 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

25. rpm 2004ESF 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

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

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

28. rpm 2004ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004 28 Results excision 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.

29. rpm 2004ESF Workshop on Active Protection of Passive Radio Services Cagliari, 28 October 2004 29 Results excision in time and frequency image built from 16 scans beforeafter

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

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

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

33. rpm 2004ESF 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

34. rpm 2004ESF 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

35. rpm 2004ESF 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

36. rpm 2004ESF 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...