Hyperfix is a land-based, short-range, mediumwave, navigation system formerly marketed by Racal (now Thales). It was very popular with offshore oil drilling operators, and also used by a few navies. Frequencies are between about 1600 and 2500 kHz. Listeners usually note that it sounds like CW Morse code, sometimes mistaking the signal for the Russian "cluster beacons." But while both usually transmit straight CW (A1A) emission, Hyperfix is not in Morse, and the timings are slightly different. Also, both have the capability to transmit data in more complex modulations.
The sample here has been heavily filtered to remove most of the noise, revealing the complex nature of the Hyperfix signals. What under most conditions sounds like two repeating dits at slightly different pitches is revealed to be at least seven different pips, some overlapping, in three general clusters of very closely spaced frequencies. These precisely calibrated cycles come from one "master" transmitter and anywhere from 3 to 18 precisely timed "slaves," acting together in a navigation "chain." The receiver obtains positions by comparing these signals, usually in a "hyperbolic" mode which gives the system its name.
Hyperfix is rapidly being replaced by differential GPS. The once-loud chain in Los Angeles appears to have been shut down a year or two ago.
All plots made with GRAM.EXE.