Loran C: Chains & Charts

Until the appearance of the now ubiquitous GPS, hyperbolic radio navigation was widely used for position fixing at sea.  These were called "hyperbolic" because the lines of constant time and phase difference between each transmit pair formed hyperbolas. Loran C was the most universal of these navigational aids. 

Three Loran C transmitter chains were used in USNS Michelson's survey operations during 1962-64. Master stations are labeled "M"; the phase locked secondary stations are designated "W", "X", "Y" and "Z". The "GRI" (Group Repitition Interval) was different for each Loran C chain.  All stations transmitted at 100 khz.  At the receiver the time base was set according to the GRI (or pulse repitition rate) of the chain selected for use.

A nautical chart of the Caribbean overprinted with Loran C lines of position.

A Loran C chart of the ocean west of San Diego.

A portion of a Loran C chart covering Boston and Cape Cod. 
Nantucket was the "X" station on the chain.

The Loran C User Handbook provides an introduction to the system along with comprehensive technical information on how it was deployed and operated. This 1992 edition was updated to include technical improvements prior to the time it was generally superceded by GPS.

Besides being the preferred radio navigator from roughly 1960 through the late 1990s, Loran C was widely used to synchronize the frequency division multiplexers employed in telephone transmission.