Equipment tests and checks
Inmarsat-B equipment should be provided with a Distress Test facility -
allowing you to test your distress alarm without making a distress-priority
telephone/ telex connection. You should consult your equipment manual
to see how this test if performed. This is not, at the time of writing, one
ofthe regular compulsory tests required on GMDSS vessels but is there
to help you check your equipment for your own needs. Power and data
cable connections should be checked, occasionally, to ensure that they
are secure. The cables should not normally be bent at acute angles, and
should not be compressed. Your equipment user manual will detail any
operator tests and checks required for your particular installation.
Inmarsat-A/B High Speed Data (HSD) Service
Inmarsat-A and Inmarsat-B equipment can be fitted with a high
speed data modem - allowing transfer of data (including video images)
at speeds of 56 or 64kbit/ second on a single HSD channel. Two-channel
operation at up to 128kbit/second is also available.
Call Encryption - Inmarsat-B/lnmarsat-M
Some Inmarsat-B installations can be interfaced with SecureTelephone Units (STU) STU-II or STU III - enabling encrypted voice
and data communications to take place between the vessel and shore
subscribers. Similar facilities are available on some Inmarsat-M
installations. Your equipment supplier can provide more information on
call encryption, and which LES{s) support such transmissions.
Inmarsat-M and Mini-M services
Inmarsat-M and Mini-M are digital services, like Inmarsat-B, but
using much smaller antennas. The Inmarsat-M antenna is typically about
650mm high/wide and weighs around 45-90lb (20-40kg); a Mini-M antenna
is even smaller, weighing-in at around 9-30lb (4-14kg) depending on model.
Inmarsat-M coverage is within the global beam of the four Inmarsat
satellites shown in figure 8.8{a). Mini-M uses spot beam antennas on the
satellites. The beams are mainly directed at the main landmasses, giving
limited coverage at sea in the southern oceans. Mini-M coverage is
indicated by the shaded area in figure 8.8{a) and, for individual satellites,
in figure 8.8{b).
Users should note that the maps indicate Inmarsat's expectations
of coverage but does not represent a guarantee of service. The
availability of service at the edge of coverage areas fluctuates depending
upon a variety of conditions.MARINE COMMUNICATIONS 171