Like any other observing mode, frequency switching has its own set of special problems. Most importantly, this mode is sensitive to atmospheric line features which are efficiently canceled in other observing modes. The principal trouble maker for many observations is mesospheric CO, but other minor atmospheric constituents, like ozone, may also play a role. At 2.6 (1.3) mm, these CO lines were found to be about 0.5 (4) K strong and about 0.7 kms wide, not unlike emission from a typical galactic dark cloud. These features can interfere either directly with the target line if the Doppler shift of the target is near zero, or interfere in other less obvious ways (see Fig. 3 for a simple example) We strongly recommend that the prospective observer checks such interference which can usually be avoided by chosing a suitable observing season. The Technical Report gives recipes on how to do this. In addition, it is planned to include in ASTRO a command which provides further help.
Another important boundary condition is that the frequency throw must be (considerably) larger than the velocity width of the target line if line cancellation is to be avoided. Since the maximum practical throw is currently about 45 kms , many potentially interesting FSw targets remain out of reach, like CO sources in the galactic plane, many circumstellar envelopes, or extragalactic lines.
A different problem arises if sources with a strong continuum are observed. On such sources, the spectroscopic baseline ripple is of more complex nature, since more than one scattering path tends to be involved. The simple trick of canceling the ripple by setting equal to the ripple period does therefore not work as efficiently. But not much experience could be acquired yet on this matter.