SCS PTC-IIex, Руководство пользователя

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5 PTC-Firmware
system so that reliable time sharing of the one frequency between the various coast
stations is possible. It is basically possible to decode NAVTEX using any AMTOR
modem, however, it has proved in practice, that just reading the transmissions has a
number of disadvantages, and is therefore not of great value.
1. In the flood of messages sent, without pre-selection and a form of buffer memory, it is
very likely that the "interesting" messages will be lost.
2. The messages are sent more than once, the newer ones at least every 4 hours, the older
ones at longer intervals. As SITOR-B is very prone to errors when signals are weak,
the receiver should ensure that only the best copy of the message to date is stored, and
available for the Radio Officer or Navigator. This is totally impossible by just
"reading the mail".
3. As NAVTEX works on long waves, the reception is generally better at night than
during the day. When sunny, the electrical energy requirements of small ships can be
met by solar panels. At night this is not possible. The noise of a generator is also not
exactly customised to enhance the sleep of the crew. The power requirements at night
should thus be kept as low as possible. A continuously running laptop or other
computer is a relatively large load for the energy supply of a small ship. A NAVTEX
controller should therefore be able to operate without any additional computer, and
use little electrical energy itself.
The NAVTEX processor of the PTC-IIex solves many of the disadvantages mentioned
above. It enables:
•
Automatic, selective reading of NAVTEX transmissions. Either the code for the type
of message or the regional code can be selected.
•
Automatic processing of transmissions received more than once. Only the best copy is
held. Old data is automatically deleted.
•
A fast check on the type of received messages.
•
A memory buffer is available without using an external computer, as the NAVTEX
processor uses the PTC-IIex internal mailbox as a message store. It automatically lays
down, if not already there, a subdirectory called NAVTEX in the PTC-box. Data
written into the mailbox by the NAVTEX processor can be accessed via PACTOR or
Packet-Radio.
5.8.2 The NAVTEX System in Detail
As mentioned above, NAVTEX messages are sent in plain language using SITOR-B
coding. To mark the beginning, end, and type of message, NAVTEX uses a simple
convention.
•
Every message begins with the characters ZCZC, followed by a space. Then follows
the four figure message identifier plus a carriage return. The actual message now
follows.
•
Every message finishes with NNNN. (If these end characters are mutilated, the
NAVTEX-processor finishes writing the message at the latest after loss of receive
synchronisation).
The message identifier is constructed as follows:
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