Loran C Receivers

Sperry's WPN-3 Loran C Receiver
Michelson was equipped with two WPN-3 Loran C radio navigation  receivers. These supplied line of position information to a digital computer, which calculated the ship's geographic location. Sperry, as prime navigational contractor, built the receivers and sent along a seagoing field engineer to help keep them running. I attended a navy school on the WPN-3 in Virginia during summer 1962.

These were electromechanical monsters, designed and built just before the advent of solid state electronics and digital signal processing. The WPN-3 was another device only the government would buy. While there were some semiconductor devices in the design, most of the signal handling and processing was done by vacuum tube circuits. Tubes were largely of the subminiature variety, with wire leads rather than pins. Foil wrappers were used as shields around the tubes. Removing, testing or replacing any of them was a precision operation, taking care not to cross the leads or short out the foil.

Each group, or chain, of Loran C transmitting stations operated at a different pulse repetition rate. Thus, the receivers had to maintain a time base equal to that of the pulse rate of the received signals. To do this our WPN-3s used several of the strangest vacuum tubes ever built, the magnetron beam switching tube, a/k/a "trochotron". These had one cathode and ten anodes in a tube surrounded by a cylindrical  permanent magnet. Each tube divided its input by ten. It was digital circuitry, sort of, base ten.

Advertising for the Burroughs Beam Switching Tube. Haydu was a subset of Burroughs, which later merged with Sperry to form Unisys. Burroughs' original business was making cash registers.

Loran C works by measuring the time and phase difference between pairs of received radio signals. WPN-3s read this out on little counter dials on the front of a gearbox module full of little motors, gears, sychros and servos along with analog to digital conversion "synchro resolver" devices feeding the NAVDAC computer. This was the only base two digital part of the equipment, which was mostly modular.

A portion of a Loran C chart showing overlayed diagonal lines of position. Chart is of Penobscot Bay, Maine.

Before getting underway we customarily zeroed out any errors by feeding both Lorans a 100 khz test source and adjusting electromechanical components (resolvers) in the gearbox to show all zeros on the counter dials and on the computer printout. This was called "cycle zero".

Cleaning the base insulators on the four whip receiving antennas was also on the pre-flight checklist. These were up above the 04 level just forward of the ship's stack. The ceramic insulators got dirty quickly from both stack residue and salty sea spray.

While the WPN-3 receivers tended to require intensive care, they produced acceptable results. Better receivers came later, long after my tenure aboard as Loran C surgeon in Michelson's survey control center.

My short semi-technical article on "hyperbolic" radio navigation is  here.

Loran C is now gone, or nearly gone, as is its technical antecedent Decca Navigator, replaced by the far more accurate and ubiquitous GPS.

Coverage area of the Northwest European Loran C system. Greatest accuracy was in the dark shaded areas.


An advertisement for Sperry's Loran C receiver from 1963.