There needs to be an AIS receiver to pick up the ship's transponder, which is why most of the free ship tracking sites only work near the coast. This seems unu

There needs to be an AIS receiver to pick up the ship's transponder, which is why most of the free ship tracking sites only work near the coast. This seems unusual if you're used to ADSB tracking sites, since the line of sight goes much further due to elevation. There's also more receivers feeding them.