Techniques for interference mitigation on RATAN-600 radio telescope in dm ranges A.B. Berlin, N. A. Nizhel'skij, M. G. Mingaliev, P. G. Tsybulev, D. V. Kratov, R.Y.Udovitskiy

RATAN-600 is one of the main of the Russian Academy of Sciences with a ring multi-elements 600-m antenna. RATAN-600 is one of the main the radio telescope of the Russian Academy of Sciences with a ring multi-elements 600-m antenna. Maximum angular resolution 2 arcsec Frequency range MHz Accuracy of coordinate determination 1-10 arcsec

The technique time-frequency method of interference mitigation is based on the following idea: Analysis - the input band of the radiometer is subdivided into several "narrow" frequency channels, with an independent detection section and rapid signal processing in each. Synthesis – the wide radiometric band is synthesized by summing of the narrow channels each of them cleaned from the short and large pulses in the real-time digital signal processing. The principals of Time – Frequency RFI suppression :analysis and synthesis

The typical appearance of pulsed interference in all "narrow" channels of the 31-cm radiometer. Shown the individual half-periods of modulation in all four channels of the radiometer in the "antenna" — “noise source" sequence for each channel. As can be seeing the interference appears in the “antenna” half-period only. This behavior is indicative of interference that is external with respect to the radiometer.

Block diagram of the anti-interference adapter to the 13-cm radiometer GHz band microwave; 2 - gates; 3 – signal divider by 2; 4 - amplifier(about 40 дB) for compensating subsequent division of signal by 8; 5 – signal divider by 8; 6 – a bank of 50 MHz wide microwave filters; 7 – preliminary low-frequency amplifier with quadratic detectors at the input; 8 – low-frequency filter to match the operating band of the analog-digital converter;9 - ADC(10-12 bits) wiyh a multiplexer at the input; 10 – parallel-to-serial code converter; 11 – link to a digital signal processor on an ISA PC bus MOD – signal of radiometer modulation; INPUT – input of the microwave signal. The method of time-frequency mitigation at the level of signal processing consists of two stages. The first stage involves on-line removal in the digital signal processor of pulses whose duration does not exceed 80% of the half-period of modulation of the radiometer and whose amplitude exceeds the given threshold. Signal counts in these intervals are substituted by the preceding "normal" count that met the 3сигма criterion. However, if the duration of interference exceeds the specified limit, this interference is considered to be "long" so that its removal would break sig­nificantly the statistics of the channel and the cor­responding interference is not removed. This deter­mines the boundary of applicability of the method. The second stage consists in off-line frequency in­terference mitigation. The central idea of the method consists in synthesizing the wide frequency band of the radiometer from "narrow" channels in the process of subsequent data reduction

The demonstrates the efficiency of on-line filtration of pulsed interference. Observations with the 13-cm radiometer were performed simultaneously with and without on-line filtration in the full band of the radiometer (400 MHz). The figure demonstrates complete suppression of radar-station pulses in record (b). Residual "artefacts" indicate that the filtration algorithm operated at the limits of its capability when the duration of the pulse exceeded the permissible duration and the algorithm switched off for a short

Illustration of the method of time-and-frequency mitigation of interference as applied to a 31 cm radio source (four “narrow” channels) The application of the time-frequency method of interference mitigation to the 31-cm radiometer. It should be pointed out that 13-and 31-cm radiometers allowed recording data in the full radiometric band with the possible switching off of on­line filtration of pulsed interference (record f). It is evident from this record that during the observation the radio source was recorded at the presents interferences that far exceeded the amplitude of the source. The nature of this interference can be explained as following: it is pulsed interference, because it has been successfully suppressed by filtration in the digital signal processor. As a result, all "narrow" channels proved to be suitable for synthesizing the full band of the radiometer (120 MHz), see record (e)

Record of signal from radio source at the input of the 13 cm radiometer а – the signal of the “wide-band”(400 MHz) channel without noise protection; b,c,d,f,g,h, i - signals in “narrow” channels(each with a 50 MHz bandwidth) of the anti-interference system; j - sum of the signals of the seven “narrow” channels uncontaminated by interference. A case of observation of a radio source with the 13-cm radiometer in "narrow" channels (b, c, d, e, f, g, h, i). The (off-line) reduction of observations involved the synthesis of the full band of radiometer (record (a)) and the synthesis of the band with rejection of channel (e), so we get record (j). Interference is apparent only in channel (e), thereby preventing observation of the source in the full band of the radiometer. The source can be seen clearly in record (j) and its parameters can be measured with high accuracy.

To study the electro magnetic environment, the wide band (0.5—3000 MHz) measurement system has been developed on RATAN-600. The detector fully covers the operating decimeter - waves on RATAN-600. However, the sensitivity of this detector combined with a two-meter parabolic antenna, is much lower than that of the modern radiometer of RATAN-600, and that is why for precision measurements we acquired and installed an additional low-noise amplifier at the detector input (+17 dB). Such a measurement-instrument kit can be used for visual recording of interference. To allow recording the detected interference to a personal computer (PC) and to receive measurement reports, we developed additional software, which allows controlling measurement modes and recording data to a personal computer (PC). Block diagram one of the possible interference-measurement setup at RATAN-600.

The another way to measure electromagnetic environment at the radio telescope site: the spectrum analyzer FSU-46 (0-46GHz) used as a radiometer baсk-end (after the band pass filter or before band pass filter ). That lets to carry on an absolute measurements in an absolute power scale (dBm, e.t.c.). The second measuring setup for the electro magnetic environment investigation at RATAN-600.

Radio-frequency emission detected at RATAN-600 using the electromagnetic environment measuring setup RATAN-600 frequency ranges (green zones): MHz (49 cm), MHz (31 cm), and MHz (13 cm). Red lines are subdivided narrow channels

Radio-frequency emission detected at RATAN-600 using the spectrum analyzer FSU-46 (0-46GHz) as a radiometer baсk-end MHZ MHZ MHZ MHZ

CONCLUSIONS Complex electromagnetic environment requires the use of modern hardware and software tools to success interference mitigation in radio astronomy. The time- frequency approach described in this talk proved to be an efficient method for the worst frequency ranges in radio astronomy. Further development of hardware and algorithms for quality radio-astronomical observations required. Also, we need to develop other approaches for interference mitigation in radio astronomical ranges. Ones must be as a technical (for example adaptive filtering) so an organizing ways.

Thank you !

Loss of Information in the Method of Time and Frequency Interference Mitigation Let the total band of a microwave radiometer be equal to ΔF. This band is subdivided into N adjacent "narrow" bands or frequency channels of different width δf i. In this case, if the root mean squared (RMS) noise of a radiometer with a transmission band ΔF is equal to up then, given that the RMS noise at the radiometer output is proportional to the square root of the microwave band width, the RMS noise in a δf i -wide channel is equal to: This means that in the case of "long" interference in N-1 channels where the algorithm of cutting pulsed interference in these channels ceases to work, the resulting synthesis can be performed only using one "narrow" channel with an root of N times lower sensitivity compared to the entire band of the radiometer. - The noise of the NG half-period- The dispersion for the "sky" noise In this case, as a result of subtraction of varying-accuracy data of half-periods, their dispersions would add up: Thus in the most unfavorable case where interfer­ence still does not prevent observations with the given radiometer the radiometer sensitivity should be about times lower than in the case of interference-free observations