Here you see how the german ship's radio officers handbook
listed 6 nautical NDBs in the mid-1960s - you will find a few
more details of the stations at each of their entries: *UK-298.8 *NO-298.8 *MN-298.8 *OB-298.8 *SM-298.8 *ÖG-298.8 .
You will find the name and the exact geographical position of the 6 stations which performed this network, working (at that time!) all on 298.8 kHz. There were 5 swedish stations, and 1 danish station in this net.
The second listing in this entry gives you the identification
in morse code ('Kennung' = callsign), the name again, approximate
coverage in nautical miles, the transmission times ('Sendefolge'),
and the general operation times ('Klarwettersendung').
'Sendefolge' means, that the transmission times of all station were circulating, '1' meant starting the transmission at minutes 0, 6, 12, 18, 24, 30, 36, 42, 48, and 54, all lasting exact 60 seconds = 1 minute. In this minute, you had the callsign in morse code, and a lasting tone.
'Sendefolge 2' then is minutes 1, 7, 13, 19, 25, 31, 37, 43, 49, and 55, all exactly for 1 minute. In general, this time scheme of 1 minute working and 5 minutes break was valid for all the next stations on 'Sendefolge' 3, 4, 5, and 6.
The meaning of 'Klarwettersendung' (clear sight transmission) was the time of transmission, when there was no nebula around. So in this case Hammerodde *MN-298.8 did only use the first and the second slot of an hour when there was clear sight, minutes 2 and 8. The other slots, minutes 14, 20, 26, 32, 38, 44, 50, and 56 were only used in nebulous times for transmission, at clear sight transmitter was off.
To avoid confusion, look at this list, made for sailors by
a navigation equipment company, here as an example a plan for
a boat trip from Germany to Sweden:
In this list you see several nautical NDBs, their callsigns, frequencies and coverage, and the exact times when they transmitted their signal for direction finding. And you can imagine, that - at least for the 6 stations mentioned in the beginning - you had the bearings of up to 6 known points without changing frequency within 6 minutes. This was surely enough to have a precise position of your boat.
This system was used worldwide, and until the early 1990's
in the baltic sea. Here is also an overlook to the other nautical
radio beacons in that area:
The above described chain 2097D is in the lower right area of this map, stretching from Sandhammaren to Hoburg.