The relative gains of the WLM boxes must be accurately normalized in order to apply phase corrections to the astronomical data. That is, each WLM box must be on the same temperature scale. These relative gains are determined using the source changes during the course of a track. The right hand side of the figure above shows the WLM data for one antenna. The source changes are clearly seen. To calibrate the WLM gains, the change (or "derivative") in the WLM temperature from one integration to the next is plotted against the average change seen for all WLM boxes. An example of these plots are shown on the left hand side of the figure. The slope of this line establishes the relative gains of the WLM boxes.