Tied Beams

Running in parallel with the cross-correlation is a digital array beamformer, which forms 32 independent single-pixel voltage beams for the full array. These beams are linear combinations of the complex voltage samples in each 10 KHz channel. The complex factors that enter as the coefficients of each linear term must include the effects of instrumental gain in the look direction, a frequency-dependent ionospheric phase term, and a geometric phase factor based on look direction and antenna tile position. The various phase factors must be well-enough determined, and specified in both time and frequency, to avoid significant decorrelation (phase error < 10°). Full time and frequency information are kept in the output of the digital beamformer.