For single dish observers, single sideband systems offer three main advantages: (a) rejection of image noise from the atmosphere and antenna; (b) improved calibration as the response to broadband calibration loads and the atmosphere is through only one sideband; (c) reduction of spectral confusion by image sideband lines. Millimeter-wave interferometers use phase switching techniques to separate the signal and image sideband responses. This eliminates the problems presented by item (c) above. The MMA is meant to operate in both interferometric and total power, single dish modes, however. The problems of spectral line confusion can be particularly onerous to many single dish Galactic spectroscopy projects. The motivation for SSB systems are substantial on these grounds alone.
The issue is less clear concerning atmospheric and antenna noise
effects and depends rather critically on the particular observing
frequency and water vapor conditions. In the 1.3 mm (230 GHz) band
under excellent sky conditions (1 mm PWV), an SSB system offers
50% improvement in observing speed over a DSB system.
This difference increases to greater than 85% at 345 GHz. Above 400
GHz, the difference in observing speed is over a factor of 2. If
atmospheric conditions deteriorate or if the antennas do not have such
low spillover losses as specified, the advantage of SSB operation is
larger.
Below 300 GHz, noise reduction alone is perhaps not a compelling case for SSB operation. Nevertheless, we conclude that because of single-dish mode observing and because of noise reduction above 300 GHz, SSB capability is worth substantial technical development effort.