This 10 cm x 16 cm x 37 cm metal black box houses the components of the ‘XR3’, a satellite navigation receiver made from 1987 by Polytechnic Electronics that worked with the United States Global Positioning System (GPS) satellites to determine a user’s latitude and longitude. The XR3 was an advancement on the Daventry GNSS Group’s experimental ‘XR1’ (ZBA9306) and was developed for military and surveying markets.

This instrument was made during a period of rapid change in available maritime navigation technologies. Together with the Navstar 603D Navigator, the XR3 was sold by Polytechnic Electronics as a ‘Hybrid Navigator’ that combined the features of Transit and Decca. Satellite navigation at this time did not replace Decca, but worked as a supplemental source of location data. Compared to the XR1, the XR3 had more advanced microprocessors that increased its tracking and fixing accuracy. It is about two-thirds of the size of the XR1 and in comparison was considered ‘compact and lightweight’.

This device could track up to 5 satellites at a time by “multiplexing” the signals, which meant it spent 4 – 5 milliseconds looking at each satellite. In turn, the XR3 could gather information from all the satellites within 20 milliseconds, and so keeping continuous track on them all. This was a major step forward compared to ‘slow sequencer’ satellite navigation receivers such as the XR1.