  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 dual-polarization 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.
