M51 was a one-way Morse code broadcast operated by the French Ministry of Defense; more specifically, the CNMO TSR in Favières, Eure-et-Loir (control center/RX) and Vernon, Eure (TX) near Paris. It was active from 1996 until 5 June 2015. The format has seen a revival from 11 September until mid-November 2018; however, it has been heard only on 3881 & 6825 kHz.

Format

M51 continuously sends 5-letter group messages, interspersed with a header of the following format:

Notes:

• BT is sent literally as BT, i.e. two separate letters.
• The serial number is between 01 and 90 inclusive, and rolls over.
• The month is represented by the first letter of its, presumably, French name (same as the first letters of English ones) - January/June/July, March/May and April/August are not distinguished.
• The time is local time in France - CET/CEST - and is usually off sync by a few seconds.
• For transmissions that begin Friday, Saturday or Sunday, the date is always incorrect, usually put a few days behind the correct date. However, for broadcasts on all other days, the date is the same as the broadcast date.

Broadcasting style (2015)

M51 was sent in transmissions of up to 21 hours. The usual time frame for the beginning of a transmission was between 12:00 and 16:00 UTC.

Every 10th message followed almost exactly hourly. Most messages were 100 groups long. The message occupying the closest time slot before H+00 were usually shorter, about 60 groups, but sometimes full-length messages were inserted, beaming the other messages +2 minutes off the hourly sync.