Flaws, part 2...
The DSC mess
The GMDSS DSC system, particularly on HF, is plagued by an
appallingly high false alert rate - in excess of 99.5%.
This situation has reached critical levels, to the point that there
is talk of discontinuing DSC...
The root of the problem is that there is no real central operational
and administrative authority to oversee the DSC system - the
operation of DSC systems on vessels is effectively managed/regulated
by individual Flag States.
The Inmarsat system used to suffer from a false alert rate
approaching that of DSC - Inmarsat instituted a strong and well
managed program to combat the problem, with the result that the
Inmarsat false alert rate has dropped to negligible levels.
The situation has reached the point that DSC has become an
impediment to safety at sea.
Equipment complexity
Much of the DSC equipment available today is far too complex to
operate.
In addition, the operating routines vary significantly from brand to
brand, such that a user trained on one type of equipment will have
considerable difficulty operating another.
DSC controller operation is complicated by the inclusion of many
(unnecessary) commercial functions - DSC controllers can be used to
make automatic telephone calls via suitably equipped HF Coast
Stations. Unfortunately, there are very few, if any, Coast Stations
that offer this service.
Many controllers are equipped with small display screens - this
makes operation very difficult. There was a move some time ago
at IMO* to mandate a minimum DSC display screen size of 4 lines by
64 characters per line.
This recommendation was modified to require that a minimum of 160
characters be displayed in two or more lines. These
modifications have taken effect for all new equipment fitted to
ships as from 1 January 2000.
There is nothing in the IMO performance standards to prevent
manufacturers using a Personal Computer type system for DSC, i.e.:
one with a large display screeen - however none have been produced,
probably because of cost.
The modifications also mandated connection of a GPS and also the
provision of an alarm to indicate that no position data is received
from the GPS or, in the case of manual input, the position
information is over 4 hours old. Any position information not
updated for more than 23½ hours is also required to be erased from
the system.
Some manufacturers are producing simple to use DSC systems, with
large display screens.
It is hoped that this trend will spread.
*instigated by the author
A new DSC standard is here
After seven years of effort, ITU and IEC have at long last finished
the final standards defining a simplified DSC radio, with a
standardized software interface, free of most of the unnecessary
alarms and strange operation that has plagued the system since its
inception.
The new standard is designed to radically simplify the man-machine
interface, and automate most functions.
Revision 13 (the latest)
may be found on the ITU
web site.
False alerts
Equipment complexity is leading to an unprecedented number of false
alerts being transmitted.
The majority of these are caused by human error, however some have
been known to be deliberately sent out of ignorance by system
installers - they program in a bogus MMSI, and just push the
distress button to see if the system works....!
Such alerts are relatively easy to spot - the position is often
suspect (12.34s 123.45e, for example), and the MMSI is often equally
dubious (123456789, 111111111, 222222222, etc).
Can the system be too effective ?
HF DSC's effectiveness and world-wide range is leading to its
downfall.
Alerts are often received from stations quite literally on the other
side of the world. It is not uncommon to receive a 12 MHz DSC alert
from the North Sea in the South Pacific....
Whilst this is quite interesting from a technical perspective, it is
a complete waste of time from a SAR viewpoint.
Compare the DSC system with Inmarsat - with Inmarsat, alerts are
directed straight to the RCC - surrounding ships are only alerted
via a carefully controlled re-broadcast from the RCC.
Although HF DSC is designed for ship-shore alerting, it operates in
a broadcast mode - all ships monitoring the frequency receive the
call, and all are therefore able to relay it......
Users are too enthusiastic...
The ITU operational guidelines for HF DSC require that, basically, a
vessel receiving a HF DSC alert is to immediately set watch on the
corresponding R/T or NBDP channel for the band in which the DSC
alert was received and await communications from the vessel in
distress.
However, this is not happening - vessels are relaying HF DSC alerts
indiscriminately.
Ironically, the original distress alert being relayed is, in itself,
probably false...
This is leading to serious congestion of the DSC channels - to the
point that the original alert has become so corrupted that it
appears a Coast Station is in distress !!
Unfortunately, DSC is becoming an end in itself, rather than a
simple alerting mechanism.
Many ships fail to adequately consider the information in a received
alert before relaying - an alert from a ship with an obviously false
MMSI (such as 123456789) should not be relayed.
Perusal of the international MID list reveals that the following MID
numbers are not allocated:
000 123 222 333 444 777 888 999
It is very difficult for a ship's operator to change his/her DSC
system's MMSI at sea, so one can safety assume that MMSI's with the
above MID's must be bogus.
Users receiving a DSC distress alert from a vessel with a MMSI
starting with any of these MID's can therefore IGNORE the alert - it
can not be genuine.
Misplaced enthusiasm does not stop at the shoreline either.... Many
Coast Radio Stations are also guilty of jumping in too quickly and
relaying or acknowledging alerts that are clearly out of their area.
Continue to part 3 for some solutions.
DUNSTAN AND ASSOCIATES
The GMDSS specialists