  This memo describes the software engineering and technical details of the
design and implementation of the wvrgcal program and associated libraries. This
program performs off-line correction of atmospheric phase fluctuations in ALMA
observations, using the 183 GHz Water Vapour Radiometers (WVRs) installed on
the ALMA 12 m dishes. The memo can be used as a guide for detailed study of the
source code of the program for purposes of further development or maintenance.
