ITU-R M.1371-5 has defined a standard for one-way reporting of location information over VHF AIS channel 1. Two carrier frequencies were allocated for AIS: 161.975MHz and 162.025MHz, each having a channel bandwidth of 25KHz. VDES provides data exchange between ship-to- ship and ship-to-shore via the use of more channels with higher channel bandwidth up to 100KHz 5.
The recent progress for New Space that leverages on the disruptive technologies for small satellite operating in 600km orbit opens up an opportunity to explore the use of small satellite to achieve timely data access from space 6. Small satellite can offer a low cost solution with lower entry barrier for AIS and VDES satellites forming a constellation to connect ship-to-ship, ship-to-shore for a global coverage. This is an effective means to extend the AIS and VDES to areas outside of coastal coverage.
In 1, access to channels by each ship is accomplished through a variant of the conventional time division multiple access (TDMA) technique but without having a central controller to coordinate the multiple access to the channel. For each channel, the frame length is one minute in duration and each frame consists of 2,250 time slots which the AIS messages will be transmitted. All AIS transmitter are synchronised to the Coordinated Universal Time (UTC) such that the start of each time slot is universally known. In essence, whenever a message is sent from a ship, the transmitter announces the next intended time slots that it will be transmitting. This information is contained within the message sent. Thus, message collisions are largely mitigated within the areas of reception for terrestrial transmission.
However, due to the large footprint of satellite, the message collision over the same time slot may occur from AIS and VDES satellite uplink signal because multiple ships which are out of their reception areas may transmit messages over the same time slots 7. The implication of a message collision is that most, if not all, of the messages involved in the collision will be lost. Solutions to recover these collided messages have been studied in the literature to de-collide the interference using the interference cancellation method with Doppler diversity and Crossed Dipoles Antennas 8 9. A bank of parallel receivers operating at different frequency offsets is used to enhance the signals corresponding with the considered frequency offset while attenuating the others. In the event that the Doppler shifts of two or more signals are not separable by the receiver bank, then interference cancellation will be invoked. This interference cancellation leads to high computational power which is not acceptable for small satellites due to its power budget constraints 8.