  We have developed Water Vapour Radiometers (WVRs) for the Australia Telescope
Compact Array (ATCA) that are capable of determining signal path length
fluctuations by virtue of measuring small temperature fluctuations in the
atmosphere using the 22.2 GHz water vapour line for each of the six antennae.
By measuring the line of sight variations of the water vapour, the induced path
excess and thus the phase delay can be estimated and corrections can then be
applied during data reduction. This reduces decorrelation of the source signal.
We demonstrate how this recovers the telescope's efficiency as well as how this
improves the telescope's ability to use longer baselines at higher frequencies,
thereby resulting in higher spatial resolution. A description of the WVR
hardware design, their calibration and water vapour retrieval mechanism is
given.
