DENSHAM AND
ASSOCIATES PTY LTD
The GMDSS specialists
Densham and Associates offer a number of different configurations for GMDSS Coast Radio Stations, depending on the GMDSS Sea Area coverage required.
This is a simple Sea Area A1 station.
It uses a standard VHF land mobile transceiver operating on marine channel 70 (156.525 MHz), connected to an industrial specification PC running the TransOceana DSC system software.
The DSC system sends audio to modulate the transceiver, and receives audio from the transceiver to demodulate. The software converts the received audio to DSC messages and displays them. Similarly, the software allows generation of DSC messages, which are fed to the transceiver for broadcast.
The transceiver PTT is controlled by a RS232 data line from the PC.
More radios and more PCs may be added as required. Remote sites may also be linked into a centrally controlled network
This is a simple A2 station.
It uses a MF/HF transceiver operating on the 2 MHz GMDSS DSC channel (2187.5 kHz), connected to a PC as per the A1 station. As with the A1 station, more radios and PCs may be added as required. A1 and A2 stations may be combined and controlled from a single PC, or the PC may be duplicated for redundancy.
This an A2 station with a dedicated watchkeeping receiver. This configuration allows the MF/HF transceiver to be used for other tasks, as the dedicated receiver provides continuous watch on the DSC channel - the same as an A2 ship installation.
The DSC system also controls the frequency/mode of the transmitter, automatically changing it to 2187.5 kHz whenever a DSC call is to be transmitted.
The watchkeeping receiver and/or the transmitter may be located at separate sites if required, in order to reduce interference.
A3 systems use separate watchkeeping receivers for each of the DSC channels from 4-16 MHz. Coast Stations are not permitted to use scanning receivers.
This is a typical A3 receiver system. The receivers are fed off a single antenna, through an active antenna multicoupling system.
A receiver is provided for each of the DSC channels. There is also a spare receiver with all of the DSC channels programmed. This is patched in if any of the other receivers fail.
Recovered audio from the DSC receivers is fed to the DSC system. The DSC watchkeeping receivers are are not controlled by the DSC system, as they are fixed frequency.
A dedicated receiver is provided for Narrow Band Direct Printing (NBDP -radio telex). This is controlled by the DSC system.
Two radiotelephone (R/T) receivers are shown - the control heads for these are normally located in the operator console with the transmitter controller(s) and the DSC system. Alternatively, the R/T receivers can be controlled from the DSC system or a separate voice switch.
This system can be converted to an A2/A3 by simply adding another DSC receiver operating on 2187.5 kHz.
This is a typical A3 transmitter. As with the A2 version, this is remote controlled by the DSC system, which changes its frequency to any of the 4-16 MHz DSC channels as required.
An A3 station will normally have multiple transmitters, with one dedicated to DSC/NBDP, and the others used for R/T. The transmitters are often sited at a remote location, and controlled over a multiplexer and link. The control heads for the R/T transmitters are normally located in the operator console.
As discussed previously, one PC can control multiple systems - for example, a A2/3 station with multiple A1 VHF sites may be controlled from one operator console.
A brief report on one of our recent A2/3 installation may be found here.
Multiple PCs may be used to provide redundancy. The audio/control signals to/from the radios are split and fed to 2 separate PCs. This provides a hot spare arrangement.
Multiple PCs can also be connected to provide multiple operator control positions, or network monitoring by a supervisor.
The DSC PCs may be networked using either a local area network (LAN) or the Internet.
Internet control allows the system to be monitored and/or operated from anywhere an internet connection is available - literally, anywhere in the world.
The system can even be accessed using a laptop/netbook computer over a WiFi connection....the possibilities are endless.
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DENSHAM AND ASSOCIATES PTY
LTD
The GMDSS specialists
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