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AIS Class A and Class B Comparison
|Author||Topic: AIS Class A and Class B Comparison|
posted 12-07-2009 09:46 AM ET (US)
A comparison of AIS Class A Transponders versus Class B Transponders:
The Class-A devices operate at higher (+8dB) power and thus will have a greater range. Since they're carried aboard larger vessels, their antennas will also be higher, adding to their range compared to Class-B devices.
COMMUNICATION ACCESS SCHEME
The Class-A devices share time for transmitting in a cooperative manner so as to avoid interfering with each other as much as possible. Class-B devices operate in a manner so as to never interfere with any Class-A devices. They do this by delaying their transmission start in any transmit window until they have listened for the presence of any Class-A device that might be using that transmit window.
EXTERNAL INPUTS REQUIRED
The Class-A devices require several external sensor be connected to them to provide required information for sending to other vessels. Class-B devices may operate independent of other devices using their own GPS receiver.
POSITION MESSAGES and INTERVALS
Data about vessel position is sent at varying intervals, with the frequency of reporting increasing with increasing vessel speed and size. Class-B devices report their position less frequently as they are inherently smaller in size.
Class-A devices transmit static information about the vessel in addition to position and speed. Class-B devices just transmit position and speed data.
posted 12-07-2009 03:20 PM ET (US)
I think the only thing missing in your nice concise guide is data etc transmited at anchor and it's slightly missleading in so far Class B still sends quite a lot of info, just not all that Class A does.
The US Coast Guard have a several of good guides on AIS and some alert notices etc:
A good quote from the USCG:
"The new AIS Class B devices broadcast and receive virtually the same vessel identification and other information as Class A devices, and they have the same ability to acquire and display targets not visible to radar (around the bend, in sea clutter, or during foul weather)."
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