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  AIS Class A and Class B Comparison

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Author Topic:   AIS Class A and Class B Comparison
jimh posted 12-07-2009 09:46 AM ET (US)   Profile for jimh   Send Email to jimh  
A comparison of AIS Class A Transponders versus Class B Transponders:

TRANSMIT POWER
A = 12.5-watts
B = 2-watts

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
A = Self-organizing Time Division Multiple Access (SO-TDMA)
B = Carrier Sensing Time Division Multiple Access (CS-TDMA) with carrier sensing polite to Class-A transmitters

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
A = GPS, Heading, Rate of Turn
B = none (can use internal GPS)

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
A = every ten seconds when under 14-knots
B = every 30 seconds when under 14-knot

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.

STATIC MESSAGES
A = vessel IMO number, ETA, Destination, Rate of Turn, Draft, navigation status
B = none

Class-A devices transmit static information about the vessel in addition to position and speed. Class-B devices just transmit position and speed data.

SJUAE posted 12-07-2009 03:20 PM ET (US)     Profile for SJUAE    
Jim

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:

http://www.navcen.uscg.gov/enav/ais/default.htm

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)."

Regards
Steve

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