Back in 2012 Iran was caught hacking the signals used by all large ocean going ships. This is AIS (Automated Identification System) which is essentially an automatic radio beacon (transponder) that, when it receives a signal from a nearby AIS equipped ship, responds with the ship's identity, course, and speed. This is meant to enable AIS ships to avoid collisions with each other. AIS is also used to send ships important traffic and weather information. Iran was sending false AIS signals to assist its smuggling operations. Iran is under a large number of international trade sanctions (because of the Iranian nuclear weapons program and other misbehavior). In the last few years security researchers have found even more ways to hack AIS and is calling for changes in the AIS software to make more difficult to spoof (send false signals).
AIS is one of two ship tracking systems required, by law, for most ocean going ships. INMARSAT (International Maritime Satellite) is a more elaborate and longer range system. It enables shipping companies the ability to keep track of their vessels, no matter where they are on the planet. INMARSAT uses a system of satellites, which transmit AIS-like signals to anywhere on the oceans. It only costs a few cents to send an INMARSAT signal to one of your ships and a few cents more to receive a reply. Shipping companies have found the INMARSAT a useful business tool as well as a safety feature.
Apparently Iran has been working for some time to come up with ways to confuse the international ship tracking system. These “maritime security technologies” were developed as a safety feature and have proved valuable in other way by providing the positions of ships caught in storms or taken by pirates. All ships now use GPS coordinates to record location. Iran often just has two ships trade INMARSAT IDs while they are near each other, leaving the U.S., or anyone else checking INMARSAT data, unable to track ships that have been switched.
These two systems are now required by law (international agreements) for all sea going vessels greater than 300-tons. The technology has worked, and the U.S. Navy has found them particularly useful in counter-terror operations. Coast Guards the world over have also found the systems a big help. But apparently pirates in some areas have gained access to the systems (via bribes or theft) and a large number of pirate attacks appear to have been helped by technology meant just to safeguard ships at sea. Iran, and other nations involved in smuggling, learned how to have INMARSAT send a false signal, concealing where the ship actually is. This can work for a while but a nation with lots of recon satellites, warships, and cooperation from most of the world’s shipping can get around this.
The Iranians are aware of the satellites and other means of double-checking transponder data and is constantly coming up with schemes to confound the American snoopers. Since Iran has been under sanctions for decades for being an outlaw state, threatening more punishment is seen as a challenge not a threat.