posted 05-27-2015 10:05 AM ET (US)        

http://continuouswave.com/ubb/Forum6/HTML/003681.html

for some details about the satellites in use and the private or government operators of the data collection systems.

A recent technical development has permitted enhanced reception in space by satellites of signals from AIS Class-B transponders. Previously the reception of signals in space by satellites from Class-B transponders was hampered by the lower transmitting power of those transponders and by the less frequent transmission of data from those transponders. In areas with high density of the stronger Class-A signals, the weaker Class-B signals were generally lost in the noise. The development that permits improved reception of Class-B signals is called ABSEA for Advanced Class-B Satellite Enablde AIS.

ABSEA is a technology developed by SRT Marine Technology, which is a major manufacturer of both devices for AIS use and the microchips used in those devices. The ABSEA technology will enhance the reception of signals by orbiting satellites collecting AIS data, specifically those satellites maintained by exactEarth, one of the major satellite AIS data collection services.

SRT Marine Technology have published a white paper that explains the ABSEA technology in very general concepts. The paper is also a good reference for the basics of AIS and satellite reception of AIS signals. See