AIS is used all over the world. My question is how AIS devices share information with other ships and land-based systems? The answer is AIS will organize all of these signals and get the timing just right.
How is AIS information broadcasted?
AIS devices send out information about the ship automatically and regularly, typically every few seconds.
AIS transponders on nearby ships or on land (for example, a vessel traffic services system) receive these signals. The information is displayed either on radar or on a chart plotter, showing ships’ positions and call sign information.
When other ships and maritime traffic services know these key details, they can use them to ensure ships are following a safe course and to prevent collisions. So ship personnel don’t need to physically see where other vessels are located in order to avoid them.
AIS combines technologies. It integrates a standardized VHF transponder with a positioning system, such as a long-range navigation system or global positioning system (GPS) receiver, with other electronic navigation sensors, such as a gyrocompass or rate-of-turn indicator.
Using time slots
The preceding section mentions how AIS communications happen regularly and automatically. Here, you can find out more about how that works.
TDMA
AIS communications are based on a time division multiple access (TDMA) system, which permits each device to communicate during a given time period, or time slot. So the signals aren’t being transmitted continuously, but only during their own time slots.
TDMA is the original communications system that cellular telephones used to use, except that cell phones had continuous communication with the cell tower — imagine trying to have a conversation if they hadn’t! But ships aren’t always within range of a tower, so TDMA was not the complete answer.
Each ship must have its own time slots to broadcast within. If they don’t, ships within close range of each other would constantly be competing for the same time slots when sending their position reports, and the AIS messages would be lost because of the resulting interference.
SOTDMA
Self-organizing TDMA (or SOTDMA) is the system that ensures AIS communications happen in an orderly way. It’s like making sure you make a reservation before going out to the best restaurant so that you don’t end up hungry.
Before an AIS transmitter is able to transmit, it first listens to the AIS frequencies to build a picture (a time slot map) that indicates the slot locations of other nearby AIS transmitters. With this information, an AIS transmitter knows that when it does transmit, it won’t interfere with another AIS transmitter.
Furthermore, with SOTDMA, the AIS device has to announce ahead of time which time slots it will use, and it can’t use the slots that have been reserved by another device. Think first come, first served — SOTDMA plays fair.
So by listening first, the AIS transmitter knows where other transmitters are and which slots they’re reserving next. To make this fair, the slot selection is random, and each selection has a random timeout (the length of time the AIS transmitter will be permitted to continue to use the time slot).
In this way, every device that receives the “preannouncement” notes that those time slots have been taken, and then knows to choose different time slots to transmit its signals. SOTDMA is a very polite technology indeed! This system allows the AIS devices to organize transmissions around each other in both an efficient and a non-interfering basis. Again, thinking back to cellular wireless communications, the limited range of these AIS signals over the ocean creates a cluster, or cell, of ships with AIS transmitters within communications range (so all ships can hear and be heard by other vessels). Within such a cell, these organized prearranged time slots and timeouts create an AIS communications cell. Figure 1-1 illustrates a communications cell made up of time slots. Depending on the ship’s speed and where it is going, the SOTDMA protocol will adjust the number of reporting time slots required, making time slots from as little as two seconds for fast-moving ships to up to three minutes when at anchor.
As vessels move, they may need to change their time slot assignments. So when an AIS device changes its time slot, it preannounces both the new time and the timeout for that location. Quite clever!