Using Multiple Beams to Distinguish Radio Frequency Interference from SETI Signals

The Allen Telescope Array is a multi-user instrument and will perform simultaneous radio astronomy and radio SETI (search for extra-terrestrial intelligence) observations. It is a multi-beam instrument, with 16 independently steerable dualpolarization beams at 4 different tunings. Given 4 beams at one tuning, it is possible to distinguish RFI from true ETI signals by pointing the beams in different directions. Any signal that appears in more than one beam can be identified as RFI and ignored during SETI. We discuss the effectiveness of this approach for RFI rejection using realistic simulations of the fully populated 350 element configuration of the ATA as well as the interim 32 element configuration. Over a 5 minute integration period, we find RFI rejection ratios exceeding 50 dB over most of the sky. Introduction Radio frequency interference (RFI) is a growing problem for all radio astronomy applications, but is especially problematic in the search for extraterrestrial intelligence (SETI). A key element of any RFI mitigation strategy is to discriminate RFI from naturally occurring signals, or in the case SETI, from artificial signals originating outside our solar system. Once the RFI is identified, corrective action can be taken. One approach keeps an ongoing database of RFI signals as they are identified, and abandons frequency ranges where RFI is recently or persistently observed. In this paper we examine a method of RFI discrimination that is especially appropriate for SETI observations and based on correlating the signals arriving from multiple single-pixel beams of an interferometer telescope. Two or more beams are pointed in different directions on the sky. If a signal appears in more than one beam it must be RFI since identical signals could never appear from two different stars. To quantify this method’s effectiveness, we must examine the beam sidelobe pattern through which the RFI enters the telescope. As a concrete example we consider the sidelobe pattern synthetic beams at the Allen Telescope Array (ATA). The Allen Telescope Array (ATA) is a new radio interferometer under construction at the Hat Creek Radio Observatory in Northern California. Each ATA interferometer element is a 20’ diameter offset Gregorian telescope, and is operable over 0.5-11.2 GHz. The ATA will be constructed in three stages comprising 32, 206, and 350 elements at each stage. We simulate the synthetic beam patterns of the 350 element ATA (ATA-350) and the 32 element ATA (ATA-32) and examine the sidelobes through which RFI may enter the synthetic beam. From the statistical distribution of sidelobe levels, we estimate the probability that an RFI signal will enter strongly into one beam while being weaker or absent in all others. This is the probability of a “false positive,” or that a bit of RFI masquerades as an ETI signal. We find that multi-beam discrimination is an effective way to identify RFI. Once identified, RFI can be eliminated from subsequent follow-up protocols, streamlining the ETI search thereby increasing search speed. Description of the Calculations Most of the calculations were performed using the proposed ATA-350 configuration although a few were made with the adopted ATA-32 configuration. We simulate observations where multiple synthetic beams are formed within the primary beam of a single antenna. The RFI is assumed to enter in a sidelobe of the primary beam because we avoid pointing at known RFI and because the primary beam (FWHM of 3.50.35° between 1-10 GHz) represents only a small fraction of the sky. Although the primary sidelobe pattern varies from high to low on a scale of half the primary beam width, we shall assume that this multiplicative factor does not change the statistical behavior of the synthetic beam sidelobe level (i.e. we assume the sidelobe statistics are the same as for an isotropic antenna). This seems reasonable especially considering that all synthetic beams are within one beam width of one another. Figure 1 displays the antenna layout for ATA-350 (left) and a synthetic beam pattern calculation (right). The white dot at the center of the beam pattern is the synthetic beam peak, and the blue circle indicates the half-power point of the antenna primary beam. Outside a few beam widths of the synthetic maximum, the sidelobe levels are quite uniform in statistical distribution. We find this to be quantitatively true in all our calculations. In the calculations that follow, beam patterns are calculated on a square grid with ~4 million points over an angular range that does not include the synthetic beam maximum. A histogram of sidelobe power is accumulated, which when normalized to the number of grid points, gives an estimate of the probability density P(s) of finding a sidelobe with a specific level s. Such a histogram is displayed in fig. 2. This is the distribution of levels in a “snapshot” observation. Using these data we may calculate the probability that RFI will appear as a “false positive” ETI signal by using the following trick. We place the synthetic beam maximum on the RFI and the observation beams are placed in the far out sidelobe region. This is justified by the inversion symmetry of the beam pattern: for a beam placed on a source, the sidelobe power for the RFI is the same as the sidelobe power on the source when the beam is placed on the RFI. We then compare the sidelobe levels at the positions of the different beams and set a rejection threshold of N dB for a false positive event. If one beam has a sidelobe level N dB higher than all the others, this is a false positive. For M observation beams, the probability of false positive PM (a.k.a. rejection ratio) is calculated from: ∫ ∫ − ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ′ ′ = 1