A fast and accurate means of measuring the total power is needed. In recent years, mosaicing has been used often and successfully at the VLA to image fields of view comparable to, or larger than, the primary beam of the antennas. To measure the visibilities for the smallest u-v separations required in mosaiced images, we need total power measurements with single antennas. These could be made with the VLA antennas by scanning rapidly across the region being imaged and adding the ``single-dish'' information to the interferometer data. This capability requires a stable total power system for each antenna and some on-line and off-line software development.
A second reason for a robust total power system is for observing transient phenomena which can significantly change the system temperature over short timescales. An outstanding example is that of solar flares, which can change the system temperature by an order of magnitude in a few seconds.