A technique has been described for measuring the crosspolarisation radiation patterns of linearly polarised, frequency reuse satellite ground station antennas, using a satellite as the remote source. This involves sampling of the imperfect satellite wave with a high polarisation purity ancillary antenna, and the application of a correction factor to the measured crosspolarisation levels. In practice, the ancillary antenna may have a small but finite crosspolarisation level, and error levels due to this are of interest. Error level expressions are given here, and some typical results are considered.