In the North-Western part of Europe, ASTRON is building the world-largest largest low-frequency radio telescope. The telescope is based on phased array principles and is known as the LOw Frequency ARray (LOFAR). LOFAR is optimized for detecting astronomical signals in the 30-80 MHz and 120-240 MHz frequency windows. Since this part of the spectrum is in extensive use by others, special care must be taken in the selection of possible out-station sites for LOFAR. RFI measurements were made to be sure that the radio spectrum at the potential out-station locations is suitable for LOFAR operations. An assessment procedure is presented here and specific issues are discussed, such as the impact of Digital Video Broadcast, and wind turbines. The results of\ud
the RFI observations has generated a list of specifications for (international) LOFAR out-stations

Bentum, M.J.

Boonstra, A.J.

Millenaar, R.P.

English

In the North-Western part of Europe, ASTRON is building the world-largest largest low-frequency radio telescope. The telescope is based on phased array principles and is known as the LOw Frequency ARray (LOFAR). LOFAR is optimized for detecting astronomical signals in the 30-80 MHz and 120-240 MHz frequency windows. Since this part of the spectrum is in extensive use by others, special care must be taken in the selection of possible out-station sites for LOFAR. RFI measurements were made to be sure that the radio spectrum at the potential out-station locations is suitable for LOFAR operations. An assessment procedure is presented here and specific issues are discussed, such as the impact of Digital Video Broadcast, and wind turbines. The results of the RFI observations has generated a list of specifications for (international) LOFAR out-stations

Bentum, Marinus Jan

NARCIS 

Assessment of rfi measurements for lofar

University of Twente Research Information

Assessment of RFI measurements for LOFARM.J. Bentum∗ASTRON, University of TwenteE-mail: m.j.bentum@utwente.nlA.J. BoonstraASTRONE-mail: boonstra@astron.nlR.P. MillenaarASTRONE-mail: millenaar@astron.nlIn the North-Western part of Europe, ASTRON is building the world-largest largest low-frequencyradio telescope. The telescope is based on phased array principles and is known as the LOwFrequency ARray (LOFAR). LOFAR is optimized for detecting astronomical signals in the 30-80MHz and 120-240 MHz frequency windows. Since this part of the spectrum is in extensive useby others, special care must be taken in the selection of possible out-station sites for LOFAR. RFImeasurements were made to be sure that the radio spectrum at the potential out-station locationsis suitable for LOFAR operations. An assessment procedure is presented here and specific issuesare discussed, such as the impact of Digital Video Broadcast, and wind turbines. The results ofthe RFI observations has generated a list of specifications for (international) LOFAR out-stations.RFI mitigation workshop - RFI2010,March 29-31, 2010Groningen, the Netherlands∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/Assessment of RFI measurements for LOFAR M.J. Bentum1. IntroductionThe Low Frequency Array (LOFAR) is a large radio telescope based on phased array princi-ples, distributed over several European countries with its central core in the Northern part of theNetherlands. LOFAR is optimized for detecting astronomical signals in the 30-80 MHz and 120-240 MHz frequency windows [1, 2, 3]. Since this part of the spectrum is in extensive use by others,special care must be taken for the selection of possible LOFAR sites. In this paper we present themethod of assessing possible LOFAR locations by looking at RFI measurements. In section 2 animpression of the current RFI situation is given, after which we discuss the impact of RFI on LO-FAR measurements in section 3. The measurement equipment, the mobile RFI monitoring station,the measurements and the assessment methodology will be discussed hereafter. This leads to aset of LOFAR site requirements, which are presented in section 7. We end this paper with someconclusions.2. RFI situationFor astronomical applications each LOFAR station will contain 96 Low Band Antennas (LBAs)and 48 to 96 High Band Antenna (HBA) tiles to cover the whole frequency range with sufficientsensitivity. The LBAs cover the spectrum from 30 to 80 MHz and the HBA tiles from 120 to 240MHz. The spectrum from roughly 30 to 240 MHz is in use by many other applications. In Figure 1,a typical spectrum is shown of the LOFAR band.40 60 80 100 120 140−200−180−160−140−120−100frequency (MHz)signal power (dBWm−2 Hz−1 ) DESY, 06−Feb−2009  df=3kHz, fstep=1.5kHz, Nt=2, Nfbins=2048, scantime=2.266s , integ.=1.11ms, ant.h.=4, receiver=ESMBantenna=VULP9118G, ant.coord.=53gr34.540min  9gr53.124min, location=Cal ??H000H090H180H270V000V090V180V270160 180 200 220 240 260−200−180−160−140−120−100frequency (MHz)signal power (dBWm−2 Hz−1 ) DESY, 06−Feb−2009  df=3kHz, fstep=1.5kHz, Nt=2, Nfbins=2048, scantime=2.266s , integ.=1.11ms, ant.h.=4, receiver=ESMBantenna=VULP9118G, ant.coord.=53gr34.540min  9gr53.124min, location=Cal ??H000H090H180H270V000V090V180V270Figure 1: Spectra of the LOFAR monitoring campaign in February 2009.2Assessment of RFI measurements for LOFAR M.J. BentumThe spectra were obtained by a special LOFAR monitoring van, which is described in sec-tion 4. The FM radio band is clearly visible in the figure. Also DVB-T and DAB-T signals in TVband III, from 202-230 MHz are prominent in the spectrum. As can be seen, the spectrum containsmany interferers. The spectrum is recorded in Desy, close to Hamburg in Germany. The site isclose to the Hamburg airport, which generates many signals in the aeronautic band that are seen inthe spectrum.3. Impact of RFI on LOFARKnowing the RFI situation, we can determine their impact on LOFAR observations. The use ofthe radio spectrum in terms of signal power and time-frequency occupancy is roughly known fromallocation tables, from monitoring observations, and for future spectrum developments from spec-trum management agencies. In order to estimate the maximum allowable RFI levels, the observedspectrum power needs to be related to the LOFAR sensitivity and system linearity:• Sensitivity . One of the key parameters of the LOFAR aperture synthesis mode is that LOFARwill be sky-noise dominated. The specifications set for LOFAR are a sensitivity of 2 mJy at10 MHz down to 0.03 mJy at 240 MHz for a 1 hour integration over a 4 MHz bandwidth. Fora 1 kHz bandwidth (the typical channel bandwidth of the correlator) this corresponds to 127mJy at 10 MHz down to 2.1 mJy at 240 MHz. In [4][5] this is related to the ITU-R RA769levels in bands allocated to radio astronomy.• Linearity . For bands which are not allocated to radio astronomy, there can be strong ob-served transmitter power present. These signals must not cause linearity problems for thereceiver systems. The strongest observed transmitter signal in the north-east part of theNetherlands, apart from the FM band which is filtered-out by the LOFAR receivers, is apager at 170 MHz. The LOFAR systems were designed such that a limited number of trans-mitters having the same observed field strength as the pager, would not affect the linearityof LOFAR. Also, the design was such that the induced spurious signals would be accept-able. Monitoring observations give a maximum observed signal strength for this pager of 65dBμVm−1 in a 3 kHz band. This corresponds to a maximum allowable flux ofS|dBWm2Hz−1 = S|dBμV/m−10log10(Δ f )−10log10((106)2)−10log10(120π)=−115 dBWm−2Hz−1(3.1)The considerations and criteria mentioned above are used for the assessment of suitability ofthe proposed sites for placing LOFAR stations in The Netherlands and elsewhere in Europe. Thefocus of the assessment is on the effects of the strongest observed RFI signals, and much less soon the spectrum occupancy of weak signals, as the sensitivity of a monitoring system is obviouslyrather limited when compared to the RA769 levels.4. RFI monitoring stationThe instrumentation needed for vetting potential LOFAR sites was specified once the maxi-mum allowable levels of RFI had been established. The equipment we used for the mobile mea-surements is fairly simple. A Rhode & Schwarz ESMB receiver is the data acquisition instrument,3Assessment of RFI measurements for LOFAR M.J. Bentumwhich connects to a notebook computer control and readout system. The input is connected toeither the Rhode & Schwarz HE010 active antenna, the Schwarzbeck Vulp9118G passive antennadirectly or the output of a LNA which connects to the Vulp9118G antenna. A Miteq 10-1000 MHzamplifier can be used, which has a noise figure ranging from 2.1 dB at 100 MHz to 1.14 dB at 610MHz to 1.2 dB at 1 GHz. The reason for having an option to bypass this LNA is to be able toquickly verify whether strong transmitter signals or RFI signals are forcing the LNA or the ESMBinto nonlinear operation. Nonlinear operation leads to intermodulation products and decreased sen-sitivity, and should be avoided. For this reason the amplifier gain is a rather moderate 18 dB. Inthe case of the current measurements, the LNA was always switched off. This choice was madebecause the focus of the measurement campaign is on the occurrence of strong interfering signals.These measurements are intended to survey the low and high band LOFAR ranges for signalsthat are present in the radio environment. Included are relatively fine frequency resolutions. Also asurvey of a larger frequency range at lower resolution is done to make an inventory of signals thatpotentially could drive the LOFAR electronics into non-linear regimes, even if those signals falloutside the nominal LOFAR bands.5. MeasurementsSeveral strong interferers were detected in the measurements shown in Figure 1. Digital audioand video broadcasting from DAB-T and DVB-T are some of the major challenges. We see thesebroadband signals popping up in all the measurements we took. The maximum in-band DAB/DVBsignal is −120dBWm−2Hz−1, which gives 60dBμVm−1 for a 3 kHz bandwidth. The maximum al-lowable signal strength is 65dBμV m−1, so no linearity problems are expected. DAB-T and DVB-Twill be one of the RFI sources with large impact. Although the transmitters will have one or twoorders of magnitude lower ERP than the old analogue transmitters, the dense network of trans-mitters will compensate for that. The integrated received power of a digital transmission will beof the same order of magnitude as the integrated received power of an analogue system, or less.The signals are spread spectrum signals with bandwidths up to 8 MHz. That makes it very diffi-cult to observe astronomical sources at the same frequency. In most cases the occupied band willbe flagged in the data (or the specific subband will not be chosen). But, there is another issue.Although the subband with the DVB-T signal is not taken into account, we need a good subbandfilter to eliminate this band. In LOFAR, the digital subband filter provides approximately 80 dB ofattenuation. So, if the level of a DVB-T signal for example is 100 dB above the noise floor, a 10%increase of overall system temperature will appear in all subbands. And this is generated from onlyone subband.That might give problems in the future, since the rollout of DVB-T and DAB-T will happensoon. That is one of the reasons why the LOFAR core is treated in the Netherlands as an RFIcoordination zone, which makes it possible to discuss optimal locations for new transmitters withthe operators of DVB-T networks.Wind turbines will have impact on the EM spectrum as well. One of the prominent effectsis the reflection of RFI due to the rotation of the blades of the windmill. This introduces timevariable multipath effects of the RFI at the LOFAR antenna. Close to the wind turbine we observedvariations of more than 4 dB in a FM radio signal. Since the position of a wind turbines is fixed, RFI4Assessment of RFI measurements for LOFAR M.J. Bentummitigation techniques at the station must be able to counteract this source of RFI. Wind turbinesare the subject of active RFI research at this moment.6. Assessment methodologyIn the assessment of the measurements we have to give answers to the following questions: Isit possible from an RFI point of view to place a LOFAR station at the measured sites? What is theranking of available and measured sites and what are the arguments for such a ranking? What arethe current limitations of the site(s) from an RFI point of view and what are future limitations?In the assessment several aspects are taken into account:• Linearity, in-band and outside LOFAR bands. Are there signals present which exceed thelimits calculated for the LOFAR system.• RFI summary. In this part we will elaborate a little more on the RFI situation at the measuredsites. In our assessments of potential LOFAR sites, we identify three frequency regions (30-80 MHz, 108-180 MHz and 180-270 MHz) and identify an objective number between 1 and 5for the usefulness of the band for LOFAR measurements: (1) unsuitable, (2) partial suitable,(3) suitable, (4) good, (5) excellent. We note that the assessments are based on statisticswith a limited sensitivity. We only measured for four hours in total. Although this is notan extensive RFI observation, it will give good information and will identify the potentialproblems in using the site for a LOFAR station.• Specific RFI issues. Are there specific issues at the locations or in the measurements to takeinto account.• Limitations of the measurements and additional comments.We studied various site locations for LOFAR. As an example of the process, we will look atone of the situations in Germany. There is funding for a LOFAR site, either in Juelich or nearHamburg in Germany. The first step in the process is to look at the sites. Is there enough space,clear views, etc? The next step is to measure the sites using the special mobile monitoring station.In the assessment the first step is to look at the linearity. Are the in-band interference levels belowour threshold? Interference levels in Juelich are considerable lower than those at the Hamburglocation. The number of interference signals is greater in Hamburg where the strongest interferenceis in the aerospace bands. Also the DVB/DAB signals are very strong. The strongest interference ismeasured at −120dBWm−2Hz−1 in a 3 kHz bandwidth this gives 60dBμV m−1. Table 1 providesan overview of the signal strengths in the data.The conclusion is that no linearity problem with the in-band interferers is expected at either theJuelich or the Hamburg sites. This includes the digital communication signals, DAB and DVB-T,and the measured aerospace signals.As can be seen in the table, the RFI levels from the FM radio transmitters, the digital com-munications at UHF and the GSM signals are all very strong. In the design, filters are placed toreduce the levels of out-of-band interferers. The FM radio signals may cause some (low-level)intermodulation product which can result in narrow-band interference in the LOFAR bands. In the5Assessment of RFI measurements for LOFAR M.J. BentumFrequency Site 1 - NorderSted Site 2 - DesyMin Max Avr Min Max AvrFM radio band 88-104 -140 -105 -124 -142 -110 -127Low frequency band 30-90 -195 -146 -173 -180 -140 -160Aerospace band 110-160 -165 -120 -140 -190 -127 -156Pager band 160-200 -180 -148 -164 -175 -132 -154Digital TV/Audio 200-400 -146 -138 -142 -150 -135 -143Mobilophone band 270-500 -158 -120 -137 -150 -130 -139UHF band 500-800 -145 -128 -138 -150 -135 -144GSM-band 900-1000 -145 -123 -134 -118 -100 -111Frequency Site 3 - Kirchwerder Site 4 - JuelichMin Max Avr Min Max AvrFM radio band 88-104 -135 -90 -116 -138 -100 -120Low frequency band 30-90 -180 -160 -170 -180 -145 -165Aerospace band 110-160 -170 -130 -135 -170 -135 -153Pager band 160-200 -180 -130 -152 -170 -135 -153Digital TV/Audio 200-400 -145 -120 -134 -165 -142 -154Mobilophone band 270-500 -150 -110 -132 -155 -118 -141UHF band 500-800 -137 -117 -127 -155 -138 -148GSM-band 900-1000 -132 -115 -121 -136 -112 -126Table 1: RFI levels in the measurements in dBWm−2Hz−1 for each of the frequency bands (in MHz). Inthe columns the data is given for respectively the minimum of the observed maximum levels of site, themaximum of the observed maximum levels of the site, and the average signal strength of the site.comparison between the four sites we see that the RFI levels of the Juelich site are on average lowerthan the three Hamburg sites.Looking at the RFI situation at the measured sites, we identify three frequency regions (30-80MHz, 108-180 MHz and 180-270 MHz) and identify an objective number between 1 and 5 forthe usefulness of the band for LOFAR measurements. We note that the assessments are based onstatistics with a limited sensitivity. We only measured for four hours in total. Although this is notan extensive RFI observation, it will give good information and will identify the potential problemsin using the site for a LOFAR station. The best results were obtained at the Juelich site.The conclusions of this assessment are limited to the current situation. The observations weremeasured during daytime in a four hour slot; daytime measurements probably see the worst sit-uation at the site, though not necessarily. The results should therefore be considered as a roughguide as to whether a LOFAR station at the site would actually contribute to the overall LOFARconfiguration (and be functional as a stand-alone station).7. LOFAR site requirementsFor practical reasons we made a list of requirements for potential LOFAR stations which is6Assessment of RFI measurements for LOFAR M.J. Bentumbased on experience and measurements.• 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 meters, to the south no trees within 500 meter• No other radio interference sources in the neighborhood• A location in or at the fringe of a nature reserve is favorable but requires good communicationwith environmentalists and nature organizations.This list of requirements is used for looking for possible LOFAR site locations. Once a site is iden-tified (or a few sites), measurements are done with the mobile monitoring station and an assessmentis performed as presented in the previous section. Note that RFI will still be present, even if theserequirements are fulfilled!8. ConclusionsRFI measurements should be done at potential LOFAR sites to assess their RFI burden. Inthis paper we describe the mobile LOFAR RFI monitoring station, which is available to measurethe RFI situation at possible LOFAR locations. We also presented the assessment methodologywe used to assess these possible LOFAR locations. LOFAR has been designed to cope with RFIsignals of up to 65dBμVm−1 successfully. Experience has suggested a list of practical LOFAR siterequirements.References[1] A.W. Gunst and M.J. Bentum, Signal Processing Aspects of the Low Frequency Array. IEEEInternational Conference on Signal Processing and Communications, p 600-603, 24-27 November2007, Dubai, United Arab Emirates.[2] A.W. Gunst and M.J. Bentum, The Current Design of the LOFAR Instrument. URSI-2008,Commission J8: Radio Astronomy, URSI General Assembly, Chicago, August 2008.[3] M.J. Bentum, A.W. Gunst and A.J. Boonstra, Low Frequency Array LOFAR - Potential andChallenge. book chapter in Applied Signal and Image Processing: Multidisciplinary Advancements,IGI publishers, 2010.[4] A.J. Boonstra, and S. van der Tol, Spatial filtering of interfering signals at the intial LOFAR phasedarray test station. Radio Science, 2005, Vol. 40.[5] A.J. Boonstra, Radio Frequency Interference Mitigation in Radio Astronomy, PhD thesis DelftUniversity of Technology, ISBN 90-805434-3-8, 2005.7

Low Frequency Array LOFAR - Potential and Challenge.

Radio Frequency Interference Mitigation in Radio Astronomy,

Signal Processing Aspects of the Low Frequency Array.

Spatial ﬁltering of interfering signals at the intial LOFAR phased array test station. Radio Science,

https://ris.utwente.nl/ws/files/5459736/RFI_LOFAR.pdf

Assessment of rfi measurements for lofar

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