A brief overview on the basics of ship tracking, including the limitations of when a ship can be tracked online.
Online vessel tracking relies on the use of Automatic Identification System (AIS) data. AIS data transmitted by a ship includes at a minimum: a unique identification for the ship, position, course and speed. Most modern vessels even today still use VHF, line-of-sight equipment, to transmit and receive this AIS data. This line-of-sight limitation will require even the tallest of vessels to be within 80-100 miles of shore in order for the data to be received. Smaller ships and older ships with weaker transmitters will transmit significantly less than 80-100 miles.
Newer Satellite-based AIS (S-AIS) relay services are available but at a very high cost. You will not get S-AIS satellite tracking data free on the internet. The land-based VHF relaying is what all free online ship tracking uses. With this limitation you will not typically get many updates while ships are crossing large open bodies of water. Tracking updates will once again be received when the ship is closer to a shoreline and within AIS land-based reception range.
There are also a couple ways land-based AIS data can be relayed, which in turn can extend the range of land-based ship tracking. Land-based AIS data at times can be relayed through data buoys, oil-platforms and ship-to-ship through AIS transceivers installed on each. These relays can occur when one end of the relay is within a land-based receivers range while the other end is not. The buoy and platform AIS transceiver installations are typically found in areas of high shipping traffic to help with routing and collision avoidance.
There are a few reasons why the ship you are trying to track is not showing on the tracking map or has not updated its’ position in a long time: