Re: Too many VHF aerials?

Brent Cameron

I haven’t used the Raymarine system with AIS but I believe that your AIS already has all the information to show the details on the targets but keeps them hidden to reduce clutter.  

AIS systems use a scheme called Time Division Multiple Access much like your cellphones do so they can all transmit and receive using only a very a few channels and not step all over each other.  

Class A systems use Self-Organizing TDMA where there are something like 4500 slots spread equally across two channels available over a minute and each transmitter ‘reserves a slot’ and all receivers need to keep a map of all of the other transmitters.  It takes two channels of receive to accomplish this.  In Class A systems, this transmission Is continuous (I.e. once every 2-10 seconds depending on vessel's speed while underway down to every 3 minutes for a ship at anchor, they update their slot information).  They use 4 channels (one transmit, two receive (duplicates to avoid bandwidth collision /noise issues), and finally a DSC channel.  They also require a highly synchronized time source (delivered from the connected GPS).  Now, to answer your question, they transmit in this “slot” the Vessel’s MMSI, Nav Status (at anchor, underway using engine, etc), rate of turn, SOG, Position, COG, and true heading….   That gets transmitted every time.  Once every 6 minutes or so it sends the Ship’s name, radio call sign, type, dimensions, destination and all the other stuff we see on our screens when we ask for the detail.  

Class B systems on the other hand are much simpler and can use either Carrier-Sense or Self Organizing TDMA (much less frequently).  Carrier-Sense works by listening for an empty slot where the signal is no bigger than the background noise and then it transmits on that but might use a different slot the next time.  Class B systems by default only transmit (at 2W) once every 30 seconds while underway and every three minutes when the SOG is less than 2 knots.  They only use three channels… one transmit and two DSC enabled receive channels.  Because they have much less bandwidth available, with this scheme, they have restricted the packets into only 4 different types of messages.  The first is a burst safely message that your system can send when a button is pushed.  The second is the standard position report info (sent every 30 seconds-3 minutes),  the third contains the rest of the position information (ship type, dimensions, etc.) but only is sent when it is polled by the coast guard, and the fourth is extended information sent every six minutes (but this is broken down into two separate messages sent 1 minute apart due to bandwidth issues).    The system was designed so that it’s messages operate a much less priority than Class A and are the first to get dropped (by distance away from you) as things get really busy.

Your life vest MOB AIS beacons actually use a third standard… Pre-Annouce TDMA in which they pick a random slot and transmit on that (because they want to keep the battery going long enough they can’t be listening for free slots or reserving slots with SO-TDMA.). They only send position and identifier.  

So, to answer your question Tom, when you click on it, Raymarine is showing you the detail that it already has gathered from the various sources over the course of the last 6 minutes or so.  Of course if it hasn’t yet got the extended detail, it might not show immediately.   That’s also why your screen hangs on to lost targets for some period of time as well…. It has to remember them for some timeout period set in your settings.  

The system is pretty cool but it does have a bunch of issues, not the least of which is that it isn’t secure and can easily be jammed and spoofed so we should always take it with a grain of salt.   Also, although ships over 300T are required to carry it (plus all passenger ships regardless of size), it isn’t required to be turned on for military vessels or fishing boats (outside of the EU). Apropos of nothing at all, Iranian Oil Tankers have been known to turn theirs off as well!  

As a final warning, I also know that commercial ships with their Class A systems often also have the option to turn off class B targets entirely to reduce “clutter” so don’t ever assume that they can see you even with the all the right antennas, setups, etc.   especially around busy harbours.  I can just imagine what the systems are gong through at ports like Singapore!  

The day is coming when all of this will be on satellite but the big problem has been that because of their elevation, satellites can see many more vessels than fit in the normal slots designed for line of sight visual range… so they are having to launch fleets of them with restricted antennas to scan only portions of the sky (or build satellites with dozens of receivers and antennas each focused on specific locations).   

As scary as all of this is,  they have similar issues with airplanes using the ADSB system - which the FAA is using to replace Air Traffic Control Radar eventually!    Sorry you asked?

Brent Cameron, Future Amel owner & Amel Owner's Registry Moderator

Brent Cameron

Future Amel Owner & Amel Owner Registry Moderator

Oro-Medonte, Ontario, Canada

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