GPS Satellite Navigation Receiver made by Navstar Systems in 1990. The XR4 GPS Receiver was made by Navstar Systems in 1990 and was aimed for use on ocean-going leisure, fishing and commercial vessels. Developed by a research and development team based in Daventry, UK, the unit was derived from the work started with the GPS receiver XR1 (see ZBA9306). The XR4 has 2 channel, 8 satellite tracking capability with an 8 state Kalman filter. Part of the design included the next generation of timekeeping- a compensated XO reference oscillator (see ZBA9320). At this time, Navstar Systems was a world leader in Marine Electronics.

As a commercial product, the XR4 was designed to be simple to understand and use but offered sophisticated features. It won a Design Centre award for its mechanical outline and user interface, and was considered the most cost effective and technically refined civilian GPS navigation receiver available at the time. For example, in the 1990s, the crew of the Royal Yacht Britannia utilised an XR4 GPS Receiver as part of the ship’s navigational system.