Digital Selective Calling: GROUP CALL

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
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Digital Selective Calling: GROUP CALL

Postby jimh » Sat Mar 24, 2018 11:41 am

In digital selective calling (DSC) radios that conform to the CLASS-D specification the radio can make or respond to calls in several categories, namely:


DSC calls in the DISTRESS or ALL SHIPS category do not have addresses; the calls are implicitly addressed to all stations, ship and shore.

DSC calls of category INDIVIDUAL or GROUP have a specific address; for an individual vessel the address is the MMSI of that vessel; for a group call, the address is the group MMSI.

Use of digital seletive calling for individual ships is well understood and widely done. One vessel makes a DSC radio call addressed to just one other vessel by using the MMSI of just that one vessel.

Digital selective calling has provided the group call function for what it defines as "ships having a common interest." For recreational vessels, there is no specifically administered procedure for obtaining a group MMSI. A group MMSI can be self-designated. According the the USCG NavCen website:

USCG NavCen website wrote:Group ship station call identities for calling simultaneously more than one ship use the format 01M2I3D4X5X6X7X8X9, where the first figure is zero and X is any figure from 0 to 9. The MID represents only the territory or geographical area of the administration assigning the group ship station call identity and does not prevent group calls to fleets containing more than one ship nationality.

No process currently exists to assign non-federal group ship station identities. However, users having an MMSI assigned by FCC license, all of which have a trailing zero, may create a group identity by inserting a zero before the identity and removing the trailing zero (e.g. a user having an MMSI of 366123450 is allowed to use the group identity 036612345).

Creating a GROUP MMSI
A group of vessels with common interest can utilize DSC GROUP calls simply by adopting a particular group MMSI and entering that group MMSI into the group directory of their DSC radios; no special authorization or authority is necessary.

The group MMSI must follow the guidelines mentioned by the USCG;
  1. begin with a "0"
  2. use a MID (the maritime identification digit of the administrative region) from an MMSI of one of the group vessels
  3. end with the digits of one of the vessels in the group with an MMSI that ended in "0"

For example, if a USA-registered vessel has an FCC-issued MMSI of 367123450 (where 367 is the MID), then a valid group MMSI will be 036712345. If a dozen vessels agree to use that MMSI as a group MSSI, then any vessel in the group can call all vessels in the group with that group MMSI.

Joining a GROUP MMSI
A DSC radio becomes a "member" of any group when the operator of the DSC radio enters that particular group number into the radio's call directory in the group category. Once the MMSI is programmed into a DSC radio's group directory, that radio should respond to a group call it receives that is addressed to that group MMSI.

Entering a GROUP MMSI
The procedure to enter a group identity into a DSC radio's group directory will vary somewhat. Here it the process for entering a group identity into a Standard-Horizon GX1500 CLASS-D radio:

  1. From the normal operating condition of the radio:
  2. push and hold the CALL/MENU button; a new screen appears;
  3. scroll (using large rotary knob) to option DSC SETTINGS;
  4. push briefly ENT (ENTER) button; a new screen appears;
  5. scroll to option GROUP DIR;
  6. push briefly ENT button; a new screen appears;
  7. scroll to option ADD;
  8. push briefly ENT; a new screen appears with data entry cursor in field NAME;
  9. enter the group NAME by using the rotary knob to select characters and the ENT button to enter them;
  10. to end entry in NAME field, push and hold ENT button; entry shifts to MMSI field;
  11. first digit in MMSI for GROUP is set to zero;
  12. enter remainder of group MMSI by using the rotary knob to select digits and the ENT button to enter them;
  13. to end entry in GROUP MMSI field, push and hold ENT button;
  14. exit from menu using CLR button several times to return to normal operation.