5 PTC-Firmware
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Files that have been input via PR are signified in the status display (ST), the LIst- and
CHeck- outputs by being marked with an X (AX.25), e.g: .... NX DL2FAK ...
The mail LED starts to blink when a file is written via PR whose filename is the same as
the MYCALL of the PTC-IIIusb for channel 0.
Read
Even during a file read operation, it is possible to give further commands to the PTC
mailbox. These commands are stored in a buffer, and only acted upon when the relevant
file read operation is complete. An empty input (only <Return>), interrupts the file read
operation.
User
Lists the presently active links, similar to the CStatus command in the pac:-menu. A
presently active PACTOR link is displayed in channel 0 of the list and in addition marked
with the comment PACTOR. Every active link with the PTC mailbox is displayed with the
callsign of the connected station, together with the appropriate digipeater list.
PR links that are not connected to the PR-Box are displayed as a so called NON-BBS-
CONNECT, without however, the callsign of the opposite station.
5.8 The NAVTEX-Processor
5.8.1 NAVTEX General
The NAVTEX service, introduced a number of years ago as part of the GMDSS (Global
Marine and Distress Safety System), is a maritime news service, broadcasting weather,
navigational and safety information to shipping. This gives the impression that the system
uses the most modern technology. In fact, underneath the impressive sounding name, is
nothing other than a network of marine coast stations that
broadcast plain language messages using the SITOR-B system, at specific times. This
system is known to amateurs as AMTOR mode B or FEC. It uses the usual 170 Hz FSK
modulation system, which is also an old and wide spread system in HF digital radio. For
NAVTEX, only one MF channel on the frequency of 512 kHz has been allocated. The
transmission range is approximately 800 km. A range limitation is an essential part of the
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
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