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[aprssig] Marine tracking

WB4GQK at aol.com WB4GQK at aol.com
Thu Mar 24 01:24:31 UTC 2005


Hi Scott and all,

The marine "tracking" system is actually called AIS for Automatic 
Identification System. Now it's true that the marine DSC for Digital Selective Calling 
does provide GPS coordinates of the vessel that's doing the calling. If the 
vessel being called has their VHF or HF rig turned on then it will automatically 
identify itself to acknowledge that it's being called and will in turn transmit 
it's GPS position. At the same time it will provide a loud alarm to the 
vessel's crew that they are being called.  In the radio's TFT monitor there will be 
displayed the name of the calling vessel and it's GPS location. There is 
nothing plotted anywhere. Obviously if you are called on marine VHF the calling 
vessel will be within 30 miles of your location.

The HF DSC is one more big problem. Theoretically it should operate just like 
the VHF system does, however the big catch is if both vessels have their HF 
rigs turned on to the same marine BAND, such as the 8 mHz calling frequency 
then the system works. But if the calling vessel is set on the 4 MHz band and the 
called vessel is tuned to the 12MHz band then they don't make a connection. 
Now the GMDSS tries to get around this problem by using the ALE Automatic Link 
Establishment software that controls the HF marine SSB rig. There is actually 
a second receiving antenna and PC receiver that scans all the marine bands and 
when an emergency Mayday is transmitted all DSC equipped HF SSBs will 
automatically switch to the emergency channel and display the stricken vessel's GPS 
coordinates. Commercial vessels are being required to install this GMDSS 
software/hardware system. It's about a $1400 setup PLUS the cost of a marine SSB 
radio with DSC capability. Private boat owners like myself, don't want to fool 
with the added expense plus another antenna. My SSB does have DSC capability.

With all of the above equipment all you have is position information and no 
tracking. Now the AIS operation does provide full tracking identical to APRS, 
on marine navigation displays. There are at least a dozen manufacture's that 
build AIS systems. It is composed of a completely separate VHF TX/RX along with 
a separate antenna, that operates on marine VHF channels 87A and 87B only! 
161.975 MHz and 162.025 MHz. And there's no voice capability. These units will 
display vessel icons such as tankers, container ships, tugs, sailboats whatever, 
and it will provide course, speed and on the big commercial systems the ships 
navigator can display his INTENDED TRACK! The cheapest one of these systems I 
have found run $2000 plus the integration into your particular navigation 
software display system!

I have a jury rig operation where I pull only the LAT LON data fields from 
the AIS frequency transmissions and then using the mouse on my navigation screen 
I match the position info and click to drop a marker. A minute or so later do 
it again and get a line which displays the vessel's course and rough speed. 
Considering I have a vessel's position, track and some idea of his speed while 
he is still some 25 miles away it sure beats a small boat radar!

73 de Jim




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