Albert-Jan Boonstra

MULTICHANNEL INTERFERENCE MITIGATION TECHNIQUES IN RADIO ASTRONOMY

Radio-astronomical observations are increasingly corrupted by radio frequency interference, and on-line detection and filtering algorithms are becoming essential. To facilitate the introduction of such techniques into radio astronomy, we formulate the astronomical problem in an array signal processing language and give an introduction to some elementary algorithms from that field. We consider two topics in detail: interference detection by rank estimation of short-term covariance matrices and spatial filtering by subspace estimation and projection. We discuss experimental data collected at the Westerbork Synthesis Radio Telescope and illustrate the effectiveness of the spacetime detection and blanking process on the recovery of a 3C 48 absorption line in the presence of GSM mobile telephony interference.

Spatial filtering of RF interference in radio astronomy

The contamination of radio astronomical measurements by manmade Radio Frequency Interference (RFI) is becoming an increasingly serious problem and therefore the application of interference mitigation techniques is essential. Most current techniques address impulsive or intermittent interference and are based on timefrequency detection and blanking. Continually present interferers cannot be cut out in the time-frequency plane and have to be removed using spatial filtering. This technique is based on the estimation of the spatial signature vector of the interferer from shortterm spatial covariance matrices followed by a subspace projection to remove that dimension from the covariance matrix, and by further averaging. The projections will also modify the astronomical data, and hence a correction has to be applied to the long-term average to compensate for this. In this paper we analyse the performance of this spatial filteringalgorithm.

Signal processing for radio astronomical arrays

Radio astronomy forms an interesting application area for array signal processing techniques. Current synthesis imaging telescopes consist of a small number of identical dishes, which track a fixed patch in the sky and produce estimates of the time-varying spatial covariance matrix. The observations are distorted by RFI, e.g., radio, TV, radar and satellite signals. We describe some of the tools that array signal processing offers to filter out the interference, based on eigenvalue decompositions and factor analysis, a more general technique applicable to partially calibrated arrays. We consider spatial filtering techniques using projections, and discuss how a reference antenna pointed at the interferer can improve the performance. We also consider image formation and its relation to beamforming. Finally, we briefly discuss some future radio telescopes, which will consist of distributed phased arrays with 10,000s to 100,000s of elements.

Calibration, sensitivity and RFI mitigation requirements for LOFAR

In radio astronomy, cosmic sources are observed which are many orders of magnitude weaker than the telescope system noise level. The necessary sensitivity is achieved by large telescope collecting areas, long integration times, and large bandwidths. In the coming two decades, telescopes are planned which are even one to two orders of magnitude more sensitive than the current generation. Examples are the Low Frequency Array (LOFAR), currently under construction in the Netherlands, and the Square Kilometer Array, for which the envisaged start of construction is in 2012. For this next generation of telescopes, a dynamic range in the sky maps of over 10/sup 6/ is required. In order to reach these numbers, accurate calibration is needed. As these telescopes will observe with relatively large bandwidths, and because of the changing spectrum environment, interference mitigation techniques become increasingly important. Approaches for calibration and interference mitigation are presented, and results from the LOFAR initial phased array test station (ITS) are given.

A Multisource Calibration Method for Phased Array Radio Telescopes

For low frequency observations (< 300 MHz) the radio astronomical community is currently developing a number of new instruments such as the low frequency array (LOFAR), the Mileura wide field array (MWA) and the primeval structure telescope (PaST). These telescopes require new calibration algorithms since phased arrays require an all-sky approach to imaging and calibration. This implies that the calibration will generally have to deal with a multitude of sources. In this paper we will discuss a multisource calibration method to estimate the gains and phases of the receiving elements without having to solve for the receiver noise powers at the same time. Our method is able to handle arbitrary known source models. We will show that the proposed algorithm is asymptotically efficient by comparing the results from Monte Carlo simulations with the Cramer-Rao bound for the underlying data model

Oblique projection beamforming for RFI mitigation in radio astronomy

Radio astronomical observations are increasingly corrupted by radio frequency interference (RFI). Phased antenna array radio telescopes allow the recovering of spatial information of RFI and cosmic sources. Using this information, spatial signal processing techniques can limit the impact of the incoming interferences. In this article, we present an RFI mitigation technique, based on an oblique projector.

The effect of blanking of TDMA interference on radio-astronomical observations: experimental results

The fast growth of the wireless communication industry poses severe limitations on radio-astronomical observations. This is largely due to the fact that in radio astronomy, in contrast to communication systems, the signals of interest are many orders of magnitude below the receiver noise power levels. The structure of some communication signals opens the possibility to reduce the effect on radio-astronomical observations using advanced array processing techniques. One such structure is time slots, used in TDMA communication systems such as the Iridium system and the GSM system. We present the results of blanking of time-slotted interfering signals measured at the Westerbork Synthesis Radio Telescope.

Space-based aperture array for ultra-long wavelength radio astronomy

The past decade has seen the advent of various radio astronomy arrays, particularly for low-frequency observations below 100 MHz. These developments have been primarily driven by interesting and fundamental scientific questions, such as studying the dark ages and epoch of re-ionization, by detecting the highly red-shifted 21 cm line emission. However, Earth-based radio astronomy observations at frequencies below 30 MHz are severely restricted due to man-made interference, ionospheric distortion and almost complete non-transparency of the ionosphere below 10 MHz. Therefore, this narrow spectral band remains possibly the last unexplored frequency range in radio astronomy. A straightforward solution to study the universe at these frequencies is to deploy a space-based antenna array far away from Earths’ ionosphere. In the past, such space-based radio astronomy studies were principally limited by technology and computing resources, however current processing and communication trends indicate otherwise. Furthermore, successful space-based missions which mapped the sky in this frequency regime, such as the lunar orbiter RAE-2, were restricted by very poor spatial resolution. Recently concluded studies, such as DARIS (Disturbuted Aperture Array for Radio Astronomy In Space) have shown the ready feasibility of a 9 satellite constellation using off the shelf components. The aim of this article is to discuss the current trends and technologies towards the feasibility of a space-based aperture array for astronomical observations in the Ultra-Long Wavelength (ULW) regime of greater than 10 m i.e., below 30 MHz. We briefly present the achievable science cases, and discuss the system design for selected scenarios such as extra-galactic surveys. An extensive discussion is presented on various sub-systems of the potential satellite array, such as radio astronomical antenna design, the on-board signal processing, communication architectures and joint space-time estimation of the satellite network. In light of a scalable array and to avert single point of failure, we propose both centralized and distributed solutions for the ULW space-based array. We highlight the benefits of various deployment locations and summarize the technological challenges for future space-based radio arrays.

Spatial filtering of interfering signals at the initial Low Frequency Array (LOFAR) phased array test station

The Low Frequency Array (LOFAR) is a radio telescope currently being designed. Its targeted observational frequency window lies in the range of 10–250 MHz. In frequency bands in which there is interference, the sensitivity of LOFAR can be enhanced by interference mitigation techniques. In this paper we demonstrate spatial filtering capabilities at the LOFAR initial test station (ITS) and relate it to the LOFAR radio frequency interference mitigation strategy. We show that in frequency ranges which are occupied with moderate?intensity man?made radio signals, the strongest observed astronomical sky sources can be recovered by spatial filtering. We also show that under certain conditions, intermodulation products of point?like interfering sources remain point sources. This means that intermodulation product filtering can be done in the same way as for “direct” interference. We further discuss some of the ITS system properties such as cross?talk and sky noise limited observations. Finally, we demonstrate the use of several beam former types for ITS.

Distributed correlators for interferometry in space

New and interesting science drivers have triggered a renewed interest in radio astronomy at ultra long wavelengths. However, at longer wavelengths (beyond 10 meters) ground-based radio astronomy is severely limited by earths ionosphere, in addition to man-made interferences and solar flares. An unequivocal solution to the problem is to establish a space based observatory for ultra low frequency (0.3MHz-30MHz) observations. In ground-based radio astronomy, interferometers comprising of widely spaced antennas are employed to enhance the sensitivity and angular resolution of the observations. The signals received from the antennas are pre-processed, phase corrected independently and then cross correlated with one another using a centralized correlator to estimate the coherence function. However, a space based array, in addition to several other obstacles, presents new challenges for both communication and processing. In this paper, we discuss various conventional correlator architectures, such as XF, FX and HFX. In addition, the importance of a distributed correlator is emphasized for a space based array, in particular Frequency distributed correlator. We compute transmission, reception and processing requirements for both centralized and distributed architecture. Finally, as a demonstration, we present 2 projects were these signal processing estimates are applied.