Assessment of RFI measurements for LOFAR

Assessment of RFI measurements for LOFAR

Mark Bentum, Albert-Jan Boonstra, Rob MillenaarASTRON, The NetherlandsTelecommunication Engineering, University of Twente, The Netherlands

2Assessment of RFI measurements for LOFAR

Content

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


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


Low Band Antenna (30-80 MHz)


High Band Antenna (120-250 MHz)


International

Germany Effelsberg Garching Tautenburg Potsdam ..

UK Chilbolton ..

Sweden – Onsala France – Nancay ….


Typical RFI situation


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

1VmdB


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,


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


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


Subband filters

80 dB stopband attenuation requirement 91 dB “nice to have”

Practice: > 90 dB

In the RFI measurements 65 dBμV/m is requirement


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


Mobile RFI monitoring station

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


9kHz-80MHz35MHz-1500MHz

Mobile RFI monitoring station

Vulp9118G


Antenna factor and gain


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


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


Torun - Poland


Onsala - Sweden


Potsdam - Germany


Jodrell Bank – UK


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


Wind turbines measurements

30

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35

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37

38

Time (20 seconds in total)

Am

plit

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Assessment

Linearity, in-band and outside LOFAR bands RFI summary Specific RFI issues Limitations


Example – site in Germany

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?


Step 1 : Locations in Juelich area


Step 1 : Locations in Hamburg area


Step 2 : RFI measurements


Step 2 – RFI measurements


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 gives60 dBμV/m


RFI - summary


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.


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?


Local (NL) shielding


International shielding


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)

Assessment of RFI measurements for LOFAR

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