
In the \NCS\ we distinguish the following 4 ``Switching Modes'':

\total

\swb

\sww

\swf\ (only with heterodyne receivers)

The corresponding commands are:
\begin{verbatim}
SWTOTAL
SWBEAM
SWWOBBLER
SWFREQUENCY
\end{verbatim}

\swb, \sww, and \swf\ are realized by a system
with hardware synchronization signals that allow a precise and
fast switching {\em within} subscans.  

\total\ simply means that none of the other 3 switching modes
is active.

The system switches through a regular cycle with several
(1, 2, or 4) switching phases.

The 4 switching modes are mutually exclusive, i.e., at any time
the system uses only one of them.

During the transitions between phases,
e.g., while the Wobbler is moving between its positions,
no data are taken during the short ``blanking'' time.

The switching mode and its parameters should normally be set before
choosing an observing mode, because for some observing modes details of
the setup depend on the switching mode.

