A Novel Detector for SETI on Radio Telescope Arrays

The advent of SETI detection processing on interferometric telescopes presents challenges for signal processing. Beamforming the outputs of many dishes or aperture arrays is particularly difficult, requiring a very large number of beams to fill the primary field of view (FOV). Meanwhile the array gain is limited because of array sparseness. Incoherent beamforming (IBF) is the conventional alternative. IBF covers the primary FOV at a relatively low computational cost but has limited sensitivity. In this paper, an alternative to beamforming based on the observed covariance matrix is proposed. A method called Covariance Off-Diagonal Sum (CODS) computes the root-mean-square of the covariance matrix elements above the main diagonal, and is found to offer 3 dB (2x) improved sensitivity over IBF while still covering the primary FOV. The computational cost is approximately $N_{AP}/2$ times that of IBF, where $N_{AP}$ is the number of apertures (dishes or aperture array stations), but could be orders of magnitude below beamforming over the full FOV. An Eigenvalue detector is also considered. We hypothesize that CODS will be less sensitive to RFI compared to IBF. The utility of CODS is not limited to narrowband signals in SETI, but could be useful for rapid detection of pulsars and fast radio bursts as well.