Trisponder was a short-range position fixing system manufactured by Del Norte Technology Inc of USA. It operated at X-band microwave frequencies around 9.5 GHz. By the use of relatively high peak power pulsed transmitters (1000 watts) along with very narrow pulses it gave a range of up to 80 km and could achieve a range accuracy of 3 metres or better under good conditions. The system comprised a of fixed/stationary transponder units and a maximum of 4 vessel-mounted transponder (beacons) with a control-interrogator unit (Display Measuring Unit –DMU) which interfaced to one or more display units. All units operated from 24 V DC and despite the high peak RF output power, the transmitter-receiver units only consumed an average 17 watts of power. Each mobile could operate on two fixed beacons.

Decca Trisponder R03 Distance Measuring Unit, 4-channel, by Del Norte Technology. Top of case has BCD output, power socket, transponder socket, and an open/close air pressure equalizer. Object front has two LCD screens labelled Channel 1 and Channel 2, each screen has a red alarm bulb next to it. Top left knob is for Channel 1, and has options 1, 2, 3, 4. Top middle knob is for intensity. Top right knob is for channel 2 and has options 1, 2, 3, 4. Bottom left of front has 2 inputs labelled 1 and 3, and 2 range/calibrate crosshair screws labelled range/calibrate. Bottom left knob is mode with base, normal and rapid options for either master (left side) or time share (right side). Bottom right knob is function with power off, display off/on or auto range options. function.