Re: ESAIL I'm confused
My understanding is that AIS works on VHF radio - hence being strictly line of sight.
Yes, you are correct - hence the advantage of using satellites. AIS uses two frequencies, 161.975 MHz and 162.025 MHz (marine channels 87 and 88), using short burst data transmissions at essentially random intervals so there is an acceptably low percentage of radio collisions even when a lot of ships are in radio range.
A satellite has line of sight to a very large area of ocean, and so a constellation of satellites can receive the AIS transmissions from ships over a vast area (almost the whole World), and relay the data back to ships or ground stations on the surface. Similar to how an EPIRB is VHF (and so line of sight), but is picked up by satellites and relayed to rescue services even though the EPIRB signal is out of range of any surface receiver.
The average port cannot receive AIS signals (or radar returns) from ships further away than 50 miles or so (line of sight), but satellite relays would provide ports with a similar tool that air traffic controllers enjoy with aircraft transponders (SSR), and so have greater advance warning and be able to plan how to handle the traffic better.