A racon is a radar transponder. It listens for radar pulses and answers them, so instead of
appearing as whatever natural echo it happens to return, the structure paints a coded mark on
the radar screen of every vessel nearby. The mark is a Morse character, programmed before
installation, extending outward from the structure's position - so a navigator does not merely
see something, they see which something it is. In poor visibility, in rain, at night, that
distinction is the whole value: MSM describe it as providing important information to mariners
under whatever meteorological conditions.
Being frequency-agile is what makes a modern racon work reliably. Marine radars do not all
transmit on the same frequency within the X and S bands, and an older racon had to sweep its
receiver across the band, meaning it only answered a given radar part of the time. A
frequency-agile design detects the interrogating frequency and responds on it, so it answers
all active radars in the vicinity rather than a statistical sample. The RBM4 responds on both X
and S bands with 1 W of transmitter power in each.
Two features address the standing problem with any transponder, which is answering things it
should not. An advanced side-lobe suppression system discriminates genuine pulses from the
side lobes of a radar's beam - without it, a racon triggered by side lobes paints a smeared or
duplicated mark and degrades the very display it exists to clarify. And an inhibition input
allows the emission to be suppressed under particular circumstances, which is how a racon is
taken off air for work nearby or during a survey without disconnecting it.
The length of the Morse response is set to match the radar display range in use, by
programming the duration of the answer according to the pulse length before installation. That
is a design decision made with the waterway in mind rather than a field adjustment, so it is
worth settling early. Receiver sensitivity is configurable, and configuration is done over an
RS-232 serial port or from a PC or an Android app on a tablet or phone - which for a device
mounted high on a structure is a meaningful convenience.
MSM design the unit to the IALA RO101 recommendation for marine radar beacons, ITU
Recommendation ITU-R M.824-4 covering the technical parameters of racons, and IMO standards.
It is rated IP68, protected against reverse polarity and transient overvoltages, accepts a
wide supply range, draws little power and is described as maintenance free. MSM's wider
offshore range carries ATEX and IECEx Category 2 and Category 3 certification; the exact
marking for the racon variant supplied is confirmed against your area classification at
quotation rather than assumed here.
