Groups of maritime NDBs in the early 1950s

In the years until about the end of the 1980s, maritime NDBs often used to work on a shared frequency. This does not mean, that they all transmitted at the same time; instead, they had time slots of 2 minutes length, giving the radio officer the possibility to make reliable direction findings from several stations within a few minutes, without changing the frequency.

Watch here the details for the 2 norwegian lighthouse beacons Geitungane and Feiestein, located in the upper right of this chart, for their transmissions as by the early 1950s. All the stations connected here with a black line are transmitting on a shared frequency.

The 1951 edition of the nautical handbook for radio officers from the german hydrographic institute lists the transmission scheme of these 2 stations very detailed as follows:

the details are in german, and should be easy to understand. They give a registration number, name and callsign, and exact geographical position in the first line. Next lines are frequency and coverage, followed by the detailed description of the transmitted signals. Each transmission slot has a length of 2 minutes, and is repeated every 6 minutes. So while the one station is pausing, one of the other stations in the group may transmit its own scheme.

Feiestein (LMS) transmits for 2 minutes in the minutes 00 / 06 / 12 / 18 / 24 / 30 / 36 / 42 / 48 / 54
Geitungen (LMN) has its 2-minutes-slots in the minutes 02 / 08 / 14 / 20 / 26 / 32 / 38 / 44 / 50 / 56
the minutes 04 / 10 / 16 / 22 / 28 / 34 / 40 / 46 / 52 / 58 are free for a possible 3rd station

The above times are for nebula conditions, in times of clear sight they have reduced transmissions (it was a lighthouse transmitting!); Geitungen uses the minutes 02 / 08 and 32 / 38 for direction finding signals. Feiestein will work only on request via coast station LGK.

This was the situation in the early 1950s, later the scheme was amanded for up to 6 stations working for 1 minute each. There is another short explanation added to NDB Noord Hinder Lighthouse *NR for these kinds of operations.

Thanks to the german hydrographic institute for allowing me to use excerpts of historical nautic publications, they are (C) BSH