Assessment of RFI measurements for LOFAR

1. Assessment of RFI measurements for LOFAR Mark Bentum, Albert-Jan Boonstra, Rob Millenaar ASTRON, The Netherlands Telecommunication Engineering, University of Twente, The Netherlands

2. Content • LOFAR • RFI situation • Impact of RFI on LOFAR • RFI monitoring station • Measurements • Assessment of the measurements • List of LOFAR site requirements • Conclusions

3. Low Frequency Array: LOFAR • Interferometer for the frequency range of 10 - 250 MHz • Array of 50 stations of 100 dipole antennas • Baselines of 10m to 150 km • Fully digital: received waves are digitized and sent to a central computer cluster • Ideal for observing transient events

4. Low Band Antenna (30-80 MHz)

5. High Band Antenna (120-250 MHz)

6. Germany Effelsberg Garching Tautenburg Potsdam .. UK Chilbolton .. Sweden – Onsala France – Nancay …. International

7. Typical RFI situation

8. Signal level considerations • Sensitivity • LOFAR will be sky noise dominated • 2 mJy at 10 MHz (1 hour integration time over 4 MHz bandwidth) and 0.03 mJy at 240 MHz • For a typical 1 kHz band, this leads to: 127 mJy at 10 Mhz and 2.1 mJy at 240 MHz • Studies indicated that there are relatively large fractions of the LOFAR band where the RFI environment allowed the production of good quality sky maps. • Linearity • Signals should not cause linearity problems for the receiver systems. • Maximum detected signal (NL) is 65 in a 3 kHz band • Gives a maximum allowable flux of -115 dBWm-2Hz-1

9. Out-of-band filters • Given maximum observed transmitted power, criteria can be made for the out-of-band filter attenuation factors • Soft spurious criterion • The integrated power of all spurious in the selected band should remain 10 dB below the integrated noise power of the selected frequency band after whitening the sky noise and before beam forming. • Spurious requirement related to strongest sky source • A strong source (eg. Cas.A) can be removed from LOFAR sky images • So, a (very) limited amount of spurious signals would be allowed, as long as they are not stronger than Cas.A. • Focus of the assessment is on the effects of the strongest observed RFI signals, and much less so on the spectrum occupancy of weak signals,

10. Digital subband filter (1) • Filter design such that adjacent subband RFI is less than CAS-A type signal. • CAS-A with resolution of 1 kHz: -40 dBμV/m • Maximum RFI is 65 dB μV/m  Stopband attenuation is 105 dBμV/m • By beam-forming an extra suppression of about 14 dB is gained, when the RFI is in the side lobe of the beam pattern.  So, filter requirements is 91 dB stopband attenuation

11. Digital subband filter (2) • In case of soft spurious requirements: • RFI signal is 65 dBμV/m • Sky noise power @ 170 MHz region is ~ -23 dBμV/m (1 kHz bandwidth) • Assuming subband width of 156 kHz and flat sky noise in the band: sky noise power in the subband is -1 dBμV/m • So, aliased RFI power must -11 dBμV/m (10% below Sky noise power) • So, stopband attenuation : 76 dB • Compensate for multiple aliased RFI : + 4 dB  80 dB stopband attenuation

12. Subband filters • 80 dB stopband attenuation requirement • 91 dB “nice to have” • Practice: > 90 dB  In the RFI measurements 65 dBμV/m is requirement

13. Process site location • Search for possible locations • RFI experts site visit and inventory • RFI measurements with LOFAR RFI monitoring station • Assessment of the measurements • Current limitations • Future limitations • Go/no-go

14. Antennas R&S HE010 : Active antenna 9 kHz – 80 MHz Single vertical polarization Schwarzbeck Vulp9118G: Single polarization log-periodic antenna 35-1500 MHz Receiver R&S ESMB monitor receiver Storage on PC Calibrated offline Mobile RFI monitoring station

15. Mobile RFI monitoring station Vulp9118G 35MHz-1500MHz 9kHz-80MHz

16. Antenna factor and gain

17. What did we measure • First 5 surveys are within the LOFAR frequency range for assessment of in-band signals • The last three lines survey a larger range at lower resolution to make an inventory of signals that potentially could drive the LOFAR electronics into non-linear regimes. • Measurements last about three hours

18. Measurement results • Observed several national and international sites • For the international sites multiple locations were identified, assessment was needed to rank the locations • Main sources of RFI: • Analogue TV (disappearing) • DVB/DAB • Aerospace • Pager signals

19. Torun - Poland

20. Onsala - Sweden

21. Potsdam - Germany

22. Jodrell Bank – UK

23. DAB and DVB signals - example • Maximum in-band signal is -120 dBWm-2Hz-1, • Gives 60 dBμV/m • Maximum allowable signal strength is 65 dBμV/m

24. Wind turbines measurements

25. Assessment • Linearity, in-band and outside LOFAR bands • RFI summary • Specific RFI issues • Limitations

26. One site to be placed in Juelich or in the Hamburg area. Assessment questions Is it possible from an RFI point of view to place a LOFAR station at the measured sites? What is the ranking of measured sites and what are the arguments for such a ranking? What are the current limitations of the site(s) from an RFI point of view? What are future limitations? Example – site in Germany

27. Step 1 : Locations in Juelich area

28. Step 1 : Locations in Hamburg area

29. Step 2 : RFI measurements

30. Step 2 – RFI measurements

31. Step 3 : Assessment • Linearity • In-band interferer levels are below the threshold • Interference levels in Juelich are considerable lower • Number of interference are more in Hamburg • Strongest interference: aerospace • Also very strong: digital signals (DVB/DAB) • Strongest interference: -120 dBWm-2Hz-1 with 3 kHz BW gives 60 dBμV/m

32. RFI - summary

33. RFI - summary

34. Site requirements - environment • Fairly isolated • No power lines within 2 km • No highway within 500 meters • No urban development within 500 meters • No railroad, tramway within 2 km • No windmills within 2 km • No forest or high trees within 100 meter. At south no trees within 500 meter • No other radio interference sources in the neighbourhood • A location in or at the fringe of a nature reserve is favourable but requires good communication with environmentalists and nature organizations.

35. The other way around • The LOFAR Stations are sensitive to RFI as we discussed before • The international stations are often built next to existing astronomical infrastructure. So, what is the radiated power of the LOFAR station itself?

36. Local (NL) shielding

37. International shielding

38. Conclusions • A mobile LOFAR RFI monitoring station is available to measure the RFI situation at possible LOFAR locations • LOFAR is designed such that RFI of ~ 65 dBμV/m can be handled successfully (linearity) • An assessment methodology is presented to assess the possible LOFAR locations • RFI created by the LOFAR equipment is attenuated using shielded cabinets • For international stations the requirements for RFI suppression are very high because of the co-existence with other astronomical instruments  special RFI container (> 100 dB)