Maritime Mobile Service Identity

What is the Global Maritime Distress and Safety System?

The Global Maritime Distress and Safety System, or GMDSS, is a set of digital signaling protocols
used for both routine and distress hailing. Aside from the convenience of automated hailing and
reduced congestion on calling channels, the MMSI is the key to enhanced emergency message traffic
possible with GMDSS.

Digital distress calls contain the MMSI, latitude, longitude, and time of fix.
This way, rescue authorities know

  • who you are
  • where you are
  • when you are there.

This saves valuable time in validating the emergency and deploying search and rescue assets.
High seas rescues that used to take days now take hours or even minutes thanks to the system.

AIS data contain your MMSI and certain other fix, course, and speed data to identify your vessel to all others
in the local AIS coverage area. In these instances, your personal record information is not used,
just your MMSI number.

The information you provide upon registration is used to identify your vessel and its characteristics
if you call for emergency assistance. These data reside on a secure site maintained by USPS,
and within the
Operations Systems Center of the United States Coast Guard.
Individual record data are never released nor used for any other purpose.


Sea Areas

GMDSS operates world-wide using layers of several separate, overlapping protocols.
Different frequencies have different propagation characteristics that result in different ranges for useful communications.
To take best advantage of each frequency band, four communications zones are identified under the GMDSS.


Sea Area A1

This zone relies on short-range communications. The primary signaling method is VHF radio. VHF is a line-of-sight communications system where range depends on antenna height; the common useful range for this system is 25 nautical miles. Digital broadcasts are more robust and can often be discriminated at distances of 50 miles, but the system is only rated for 25 miles as a conservative measure. Sea Area A1 is thus defined as inshore waters and near-coastal waters extending 25 nautical miles from shore.


Sea Area A2

This zone relies on long-distance communications such as medium frequency (MF) and high frequency (HF) communications systems. MF offers coverage over vast distances (usually up to about 100 nautical miles) and HF can provide worldwide coverage depending on atmospheric conditions. Sea Area A2 is defined as those areas beyond Sea Area A1 out to 100 nautical miles.

Note that VHF traffic is still usable when blue-water voyaging. Distress hailing on VHF is recommended in any location, because there may be a vessel close by that can render aid.


Sea Area A3

This zone consists of high seas areas beyond A1 and A2, within 70 degress north latitude and 70 degress south latitude. This is the typical coverage pattern for the INMARSAT satellite communications network and is essentially worldwide. VHF, MF, and HF signaling protocols are also encouraged in distress situations when voyaging in Sea Area A3.


Sea Area A4

Sea Area A4 consists of those regions not routinely covered by the other Sea Areas. These are the polar regions where no coverage is assured. The use of VHF, MF, and HF is recommended, but the definition does not specify assured coverage by any of these signaling methods. 

Use of EPIRBs in the GMDSS

Emergency Position Indicating Radio Beacons (EPIRBs) utilize 406 MHz signals which are line-of-sight in nature.
COSPAS-SARSAT satellite system, however, provides true global coverage, which means that
EPIRB emissions can be processed from any point on the Earth.
EPIRBs are your best choice for distress signaling in any Sea Area including Sea Area A4.