LOng RAnge Navigation (LORAN)

We just learned that the Coast Guard will cease transmitting LORAN-C signals at midnight zulu February 8, 2010. This will bring to an end almost seventy years of development and operation by the US of a very valuable hyperbolic radio navigation system.

I personally never owned a LORAN-C set. I used LORAN-C in the navy, but never liked it. Maybe I disliked it because the set I used was a klunky converter to a LORAN-A set.

I liked LORAN-A. I was introduced to LORAN-A operation on a WWII-vintage DAS-3 LORAN-A receiver, during midshipman training cruise on USS Macon the summer of 1957. It took a little while to get the hang of it, but it gave the operator a lot of control. I used a DAS-4, a slightly improved model, on USS Cabildo and USS Higbee.

The system was intended to provide a fix by plotting time differences from two pairs of stations arranged as a chain, with the master station between two slave stations. The geometry of the arrangement would cause lines of position to cross each other at an angle that resulted in an acceptable fix. A single LORAN line could also be crossed with a sun line. That often came in handy.

LOng RAnge was a bit of a misnomer. The system was designed to use a ground wave radio path, and the maximum distance the signal would reach was about 900 miles during the day and maybe 1600 miles at night. That wasn't enough to reach across the wide Pacific.

A skilled operator could use the system over greater distances by measuring the time difference of sky waves - as long as they were one-hop waves off the E-layer of the ionosphere. If you were really skilled, when operating in the Western Pacific, it was often possible to match the ground wave of a master station with the sky wave of a slave station, apply a time difference correction from a special table, and plot an accurate line of position.

A key skill requirement was the ability to distinguish a sky wave from a ground wave. That took a bit of time and patience. Sky waves were less stable than ground waves, but they could appear fairly stable for a short period. If the operator inadvertently matched a ground wave with a sky wave, the resulting time difference could plot very far away from the ship's actual location. I believe that is what happened when USS Frank Knox ran aground on Pratas Reef in the South China Sea in 1965.

I never trusted LORAN-C because there were too many automatic features. I wanted to see the actual wave form to see what I was dealing with. And I preferred to plot the line of position myself on a piece of paper.