INTERSTELLAR SCATTERING EFFECTS ON THE DETECTION OF NARROW-BAND SIGNALS

We have investigated the detection and decoding of narrow-band radio signals after propagation through the turbulent, ionized interstellar medium. For most lines of sight through the Galaxy, spectral broadening due to scattering less-than-or-similar-to 0.1 Hz at 1 GHz. A few lines of sight may have as much as 5 Hz of broadening. Spectral broadening is therefore unimportant for the detection of hypothesized signals from extraterrestrial intelligence. Intensity scintillations, however, are of considerable importance. They both help and hinder detection: signals too weak to be detected without the scattering medium may be modulated above the detection threshold while, conversely, signals above threshold can be modulated below. In strong scattering (distances greater-than-or-similar-to 100 pc at 1 GHz), we demonstrate that multiple observations of a given target comprise a strategy that is superior to single observations even when the total time per target is held fixed. Decoding information carrying signals may encounter difficulties due to intensity scintillations (due to the ionosphere and solar wind as well as the interstellar medium) and, for distant objects, from temporal broadening like that seen from pulsars.