SETI at Harvard: META and BETA

Click here for an image of the META/BETA radio dish

Click here for an image of the META receiver

META (the Million-channel Extraterrestrial Assay) is a search program that has been in operation since 1985 at the Harvard/Smithsonian 26-meter steerable Cassegrain radio telescope at Harvard, Massachusetts. BETA (the Billion-channel Extraterrestrial Assay) is the follow-on search, expected to begin in 1995. META looks for narrowband radio signals near the 1420 MHz line of neutral hydrogen and its second harmonic using an 8.4 million channel Fourier spectrometer of 0.05 Hz resolution and 400 kHz instantaneous bandwidth. The observing frequency is corrected both for motions with respect to three astronomical inertial frames (heliocenter/LSR, galactic barycenter and cosmic microwave background), and for the effects of Earth's rotation, which provides a characteristic changing Doppler signature for narrowband signals of extraterrestrial origin. The search covers most of the northern sky (-30 to +60 degrees declination) in meridian transit mode, with each potential source passing through the antenna beam pattern in approximately 2 minutes, during which the three reference frames are covered once in each antenna polarization.

In an analysis of 5 years of META data, during which 60 trillion channels were searched, 37 candidate events were found exceeding the average detection threshold of 1.7x10-23 W/m2, none of which has been detected upon repeated reobservations.

    BETA will replace META in 1995, and features
  1. ) rapid and automatic re-observation of candidate events
  2. ) better discrimination of interference, and
  3. ) much greater frequency coverage.

    BETA uses the 26-meter dish with dual (east-west) feedhorns (and a third low-gain terrestrial discone) to feed a 240 million channel Fourier spectrometer (80 million channels of 0.5 Hz resolution and 40 MHz instantaneous bandwidth for each feed) whose outputs feed an array of programmable "feature recognizers." These recognizers sift through 250 Megabytes of spectral data every second, seeking distinctive spectral features that transit from the east to the west horn without appearing in the low-gain terrestrial antenna.

    BETA's contemporary hardware consists of HEMT low-noise frontends, an array of 63 quadrature mixer/digitizers with GPS phase-locked local oscillators, and an array of 63 4-million-channel complex FFT boards feeding a flexible state-machine based feature recognizer/correlator array resident in a set of Pentium motherboards; the latter communicate with a UNIX workstation via thin-wire Ethernet.

    BETA will search the full "waterhole" of 1.4-1.7 GHz as 8 hops of 40 MHz, each hop taking 2 seconds (16 seconds for a full cycle through the waterhole; thus each potential source is visited 8 times at each frequency hop, in each sky beam). A good candidate (seen first in east, then west, never terrestrial) triggers the antenna to leapfrog a few beamwidths to the west, inviting the source to perform an encore. If that ever happens, the antenna will break off its survey and go into sidereal tracking mode, repeatedly moving on and off the candidate source, archiving all integrated spectra.