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Polling a VLD is the capability of remotely controlling it to some degree. This is a valuable tool in fisheries management as it permits the VMS operator to vary the frequency of his position information as a function of the behaviour and whereabouts of a vessel. Whilst in port, for example, the position of a vessel is useful only to confirm that it is still in port. This can be accomplished with a single, daily report. During operation in fishing grounds or, particularly, near sensitive areas, the VMS operator will require much higher frequency data.

In the world of open systems, this means downloading a unique identifier directly into the VLD so that the equipment will recognize that a polling command is being received from an authorized entity. When it receives a correctly formatted command packet which a header including an authorized polling identifier as well as its own identity, it obeys the command which follows.

In the case of a closed system, the process is somewhat simpler. The service provider offers the qualified user a menu of possibilities to be executed from the system base station and, having identified that user, normally through a system of passwords or callback, provides the service or services requested from the menu.

The basic commands required for VMS are to begin reporting, to report position immediately, to modify the reporting frequency or to stop reporting. A number of more advanced commands, relating to micro encoded messages and, perhaps, data gathered from on board sensors, could also be envisaged, but they fall outside our current terms of reference.

It is also beyond our terms of reference to establish formats or methods for establishing control over a terminal, either directly (open system) or through the base station of a closed system. The reason is that such a operation strikes at the very heart of system security. Were an unauthorized entity able to establish control over a VLD, one could imagine that a vessel’s direct competitor could observe its movements and use that information to poach on the fishing grounds of the first vessel. The service provider would be held legally responsible were that to happen.

From a point of view of normalization the objective is to establish a set of generic commands for the basic kinds of poll which will be recognized by any service provider, once the issuer of the command has gone through an identification process. Currently, with only three systems on offer, this is not a pressing problem.

A number of fisheries management organizations, particularly in the context of the European Union pilot project, have developed systems that can deal separately with all three systems. The real problem will develop in the next few years, when there may be several times that many systems. For this reason, it is useful to establish some recommended guidelines for polling commands.

In an effort to be as unintrusive as possible as far as current practice is concerned, the actual identifier for these commands will be derived from the existing Inmarsat designations. The reasoning behind this is that these designations are already used for several thousand fishing vessels and it is important to avoid putting the integrity of that operation in jeopardy by developing an approach that would require mass reprogramming. Furthermore, for the two existing closed systems, adding these designations to their list of commands is a relatively straightforward, mostly centralized process.

Table 12.1 Polling codes




Initiate position reporting


reporting interval expressed as hhmm where the maximum value is 2400 and the minimum value is 0015

Report position immediately


Program reporting interval


new interval expressed as hhmm where the maximum value is 2400 and the minimum value is 0015

Stop position reporting


In this scheme, the only difference between a command to report immediately and to program the reporting interval is the extension which defines that interval. Upon reception of a 04h command without extension, a VLD which is already programmed to report hourly, will immediately report its position and set the mechanism which times its reports to zero, thus once again reporting hourly counting from the time of the command.

A 04h command with extension functions in the same way, but with the extension defining the interval from the time the polling command is received. Using our extended message format, the command to reprogram the reporting of a terminal to intervals of three hours and 45 minutes would take the following form:

A command to stop position reporting would be expressed thus:
These commands have the advantage of being short and simple, but they can only be recognized when issued after the VMS system has negotiated the system’s proprietary access protocol.

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