Following is the reason: Class B transmitters, as ours, work within a process which is called SOTDMA. (self organized time division multiple access). This means the big vessels listen to the traffic and the information containing in all other telegrams and defines it’s own raster of transmission timing, which is then distributed to all other participants within the radio range. Our class B transmitter, as all the others too, is using a “listen-before-talk-method”. This means, that before transmitting, a ship has to observe the radio transmission timeslots, whether to be allowed to transmit, if the timeslot is not occupied, or whether to wait for a free time slot. A timeslot is very very short (app. 25ms). Therefore the starting time of a class B telegram must be very exact on time. This is only possible by a microsecond exact timebase. Normal GPS antennas do have an accuracy of 1 sec., sometimes a little bit better, but not as exact as necessary. With the GPS antenna which can be purchased from us the microcontroller inside the easyTRX is calculating the timebase in microseconds and therefore it is proofed that the telegram is transmitted exactly when it is allowed. With a normal GPS antenna this would not be possible.