The Low Frequency Array (LOFAR) is a radio telescope currently being designed and planned. Its targeted observational frequency window lies in the range 10-250 MHz. The initial test station (ITS) is a full scale prototype of a LOFAR station, and it became operational in December 2003. It consists of 60 sky noise limited dipoles, configured in a five-armed spiral, which are connected to a digital receiver backend. ITS operates in the frequency band 10-40 MHz, and the observed signals are directly digitized without the use of mixers. The data can be stored either as time series or as covariance matrices.
To enhance the sensitivity of LOFAR, time-continuous man-made radio signals can be suppressed by applying spatial filtering techniques. In order not to deteriorate the LOFAR calibration processes, it is required that the station beamshapes vary only very slowly. In practice this means that spatial filters are limited to semi-fixed filters (fixed nulls).
In this paper we demonstrate spatial filtering capabilities at the LOFAR ITS test station, and relate it to the LOFAR RFI mitigation strategy. We show the effect of spatial filters by applying them to antenna covariance matrices, and by applying different beamforming scenarios. We show that in frequency ranges which are occupied with man-made radio signals, the strongest observed astronomical sky sources can be recovered by spatial filtering. We further focus on the subspace structure of the observed covariance matrices, and relate it to system properies such as crosstalk, and to the radio environment such as multipath effects.