In conditions of poor visibility it is common practice to plot radar measurements of the range and relative bearing of other ships with respect to own ship and to deduce the relative tracks and passing distances. Radar equipment is available which can to a greater or lesser degree relieve the navigator of the tedium of plotting, and which could be developed into aids of considerable sophistication within the limits set by their cost. However, errors in measurement limit the precision with which relative tracks and passing distances can be computed. An alternative to radar plotting would be a data link between ships to broadcast measurements of course and speed on demand; a radar transponder would transmit the required information in coded form.

This suggestion is highly controversial, but it seems worthwhile to derive the mathematical relationship between errors in the data and the ensuing error in the calculated passing distance and to compare the errors using radar data and transponder data. The value of the comparison is however limited because likely errors in the data are not accurately known. The analysis of the radar plotting method is probably of interest in itself because the deficiencies of the method are derived with some rigour.