Sebastiaan van der Tol

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LOFAR is a low-frequency radio astronomical array currently under development in The Netherlands. It is designed to produce synthesis images of the most distant celestial objects yet observed. Due to high redshift levels, observations must be at unusually low frequencies (30-240 MHz), over large apertures (100 km), using thousands of antennas. At these(More)
Instruments for radio astronomical observations have come a long way. While the first telescopes were based on very large dishes and two-antenna interferometers, current Instruments consist of dozens of steerable dishes, whereas future instruments will be even larger distributed sensor arrays with a hierarchy of phased array elements. For such arrays to(More)
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(More)
10 [1] The Low Frequency Array (LOFAR) is a radio telescope currently being designed. Its 11 targeted observational frequency window lies in the range of 10–250 MHz. In frequency 12 bands in which there is interference, the sensitivity of LOFAR can be enhanced by 13 interference mitigation techniques. In this paper we demonstrate spatial filtering 14(More)
— One of the challenges in the design of the LOFAR radio telescope is the calibration of the ionosphere which, at low frequencies, is not uniform and can change within minutes. The number of unknown parameters quickly approaches the number of measurements and hence, structural assumptions on the ionosphere must be made, in time, frequency, and space. Using(More)
The LOFAR astronomical array currently under development in the Netherlands produce synthesis images of the most distant (and thus oldest) space objects by observing at unusually low frequencies (30-250 MHz) over a large aperture (100 km) using many antennas (on the order of 10,000). This presents some significant challenges for sensor calibration because(More)
Radio telescopes based on large phased arrays form an interesting application area for array signal processing. LOFAR is a large low frequency (10Mz–240MHz) array consisting of 13,000 antennas grouped into 50 stations, currently under construction in the Netherlands. Data from a 60-element test station of LOFAR is available to evaluate the performance of(More)
— In radio astronomy images are made of astronomical objects as they appear at radio frequencies using a technique called aperture synthesis. Signals from several antennas are correlated and integrated over time. The data collected over several hours are further processed to calibrate the instrument and to form an image or intensity map. The calibration and(More)
The contamination of radio astronomical measurements by man-made 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 time-frequency detection and blanking.(More)
Several low frequency radio astronomy arrays are currently under development. For example the LOFAR array (with 13,000 dipole elements) will operate in the frequency range of 20MHz-240Mhz. At these frequencies the effect of the ionosphere cannot be ignored. Due to the fact that the size of the array is larger than the size of the irregularities in the(More)