RFI mitigation for tied-array configuration with phase-only adaptive beamforming
P. Fridman (ASTRON) fridman@astron.nl

There are several specific features of large aperture synthesis radio interferometers (ASRI) like (WSRT, VLA, GMRT) which make a straightforward application of RFI mitigation adaptive beam-forming difficult and different from classic phased arrays: 1) ASRI are the highly sparsed arrays; 2) they are the correlation arrays, not additive; 3) direction-of-arrival (DOA) of a signal of interest (SOI) is a known and time dependant vector; 4)there is an auxiliary tied-array (TA) facility which is used during VLBI and pulsar observations. This mode is similar to the conventional phase-array: coherent adding of signals from array elements. This mode will also be a basic one for LOFAR stations. 5) there is no computer-controlled amplitude weighting before correlator or tied-array adder in the existing ASRI backend hardware; 6) noiselike radio astronomy SOI are usually much weaker than system noise (antenna + receiver) and RFI. An RFI mitigation beamformer should satisfy the following two requirements:
a) SOI must be protected, that is the response of the array is maintained constant for a prescribed direction, no matter what values are assigned to the weights;
b) the effects of RFI should be minimized.
Phase-only weights can be found as a solution of the corresponding system of nonlinear equations. The construction of the weight vector requires the knowledge of RFI's DOA, which may be known beforehand, or could be obtained from the observed correlation matrix. But it is necessary to have a special correlator for this purpose to follow all rapid scintillations of RFI, which are usually averaged by the main radio interferometer correlator. So, in principle, the system of nonlinear equations can be solved, and the phase corrections can be introduced into the phase control system. A more simple method of calculating the phase corrections in the tied-array mode is proposed. The tied-array total power detector (TPD) output is used as a cost function in a multi-variable optimization problem. To overcome the multimodality problem for the required small phase corrections a global maximum can be found using evolutionary programming (EP) method. The output of TPD is continuously measured and used to supply the EP algorithm with new data (cost function samples) which monitor performance of the tied-array with respect to RFI. The EP algorithm uses these data to calculate new phases which are introduced into the phase control subsystem after each iteration and a new value of the TPD output signal is used for the next step. Computer simulation results are given to illustrate the effectiveness of this method of RFI mitigation for different array configurations.