Signal processing for radio astronomy

 

Low-Frequency Radio Astronomy

LOFAR is a radio telescope currently being built in The Netherlands and neighboring countries. It is a large phased array consisting of some 50 stations, each with some 100 antennas. LOFAR will observe in one of the last unexplored frequency ranges in radio astronomy: the frequency band below 100 MHz. Because of the ionospheric scintillation, observations are severely corrupted. Estimating and removing ionospheric phase changes from the data is essential for high-resolution imaging. Also, the electronic gains and phases of all antenna elements are initially unknown and need to be calibrated.

LOFAR: The effect of the ionosphere

The ionospheric phase screen adds a varying phase to the geometric delays. At first order, each radio source appears to have a shifted position; at second order, it is also blurred. The ionosphere is turbulent: varying over time and space. In principle, there is one unknown parameter for each look direction for each station--clearly more than can be identified. We require a simple but accurate model that poses a sufficient number of constraints on the parameters.

Contributions:

Statistical model based on Kolmogorov turbulence: the covariance of the phase is modeled by a power law as function of distance.

Reduced-order fitting and interpolation using Karhunen-Loeve basis

Improved model order selection

Implemented as part of a new calibration software (SPAM)

Partners:

ASTRON

RU Leiden (Sterrenwacht)

LOFAR Station Calibration

An important problem in radio astronomy is the calibration of large antenna arrays. Initially the antennas of a LOFAR station are not calibrated, i.e., the antenna gain and phase offsets are imprecisely known. To enable beamforming, these parameters have to be estimated. Calibration is more involved if these parameters are also direction dependent, and multiple calibrator sources are simultaneously present. Typically the locations of the calibrator sources are well known, but due to ionospheric refraction, it is possible that thay appear to be shifted.

Contributions:

Algorithms based on weighted least squares, that iteratively solve for the direction independent complex gains of the array elements, their noise powers and their gains in the direction of the calibrator sources.

Extensions, to also estimate the location of the calibrator sources.

The methods are asymptotically statistically efficient and converge within two iterations even in cases of low SNR.

The algorithms are implemented in the LOFAR station calibration system

Interference Cancellation for Radio Astronomy

The band 30--240 MHz in which LOFAR operates contains many sources of RFI (radio frequency interference). The band 88-108 MHz is occupied by FM radio transmitters, and actually this band is given up in the LOFAR design. Apart from this, there are TV broadcasts, digital audio broadcasts, etc. LOFAR will be the first radio telescope in which RFI mitigation techniques will (necessarily) form an integral part of the system design.

Phased arrays are particularly interesting for their capabilities to do interference suppression. If the same interfering signal is received by several antenna elements, then by properly phasing and combining the antennas, that signal can be nulled. In practice, there are several complications: there are many interfering signals; the signals nor their directions are known; they are often time-varying; and signal nulling will also disturb the reception of the sky signals.

Following the LOFAR hierarchy, there will be two levels of RFI mitigation: at the station level and at the central level. At the station level, the 200 antennas in a station can be combined to modify the beamshape such that an interferer is nulled. This will at the same time modify the reception of the sky signals, and hence the resulting beamshape has to be known at the central correlator, which is complicated by the irregular structure of the array. This form of spatial filtering can be used to null strong local interference. At the central level, the signals from each station can be combined to null any remaining interference that is received by several stations simultaneously. A correction is needed to take the disturbance of the sky signals into account.

Partners:

ASTRON

OLFAR: Low frequency radio telescope in space

One of the last unexplored frequency ranges in radio astronomy is the frequency band below 30 MHz. This band is scientifically interesting for exploring the early cosmos at high hydrogen redshifts, the so-called dark-ages. This frequency range is also well-suited for discovery of planetary and solar bursts in other solar systems, for obtaining a tomographic view of space weather, and for many other astronomical areas of interest. Because of the ionospheric scintillation below 30 MHz and the opaqueness of the ionosphere below 15 MHz, earth-bound radio astronomy observations in those bands would be severely limited in sensitivity and spatial resolution, or would be entirely impossible. A radio telescope in space would not be hampered by the earth's ionosphere, but up to now such a telescope was technologically not feasible. However, extrapolation of current technological advancements in signal processing and nano/femto satellite systems imply that distributed low frequency radio telescopes in space could be feasible in about 10 years time.

To achieve sufficient spatial resolution, a low frequency telescope in space needs to have an aperture diameter of over 10 to 100 km. Clearly, only a distributed aperture synthesis telescope-array would be a practical solution. In addition, there are great reliability and scalability advantages by distributing the control and signal processing over the entire telescope array. The aim of the OLFAR project is to design a concept study on an autonomous sensor system in space to explore this new frequency band for radio astronomy. The project will develop scalable autonomous nano-satellite prototypes, demonstrated in the lab. The results will be validated by three flight units, which can be launched into space and work as a formation flying radio-astronomy array.

Partners:

ASTRON http://www.astron.nl

TU Delft, Fac. Aerospace Engineering

Univ. Twente

ISIS, SystematIC, Axiom, Aemics, Dutch Space, National Semiconductors

Publication list on Radio Astronomy

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Contact address

Mail:	prof.dr.ir. Alle-Jan van der Veen
	Delft University of Technology
	Fac. EWI/Electrical Engineering
	Mekelweg 4
	2628 CD Delft
	The Netherlands
Phone:	(+31 15) 2786240
Fax:	(+31 15) 2786190
E-mail:	allejan@cas.et.tudelft.nl