The Automatic Identification System is an electronic system enabling vessels to send localization messages. Those messages are used for several uses such as fleet control, traffic control or boarding prevention. Sent messages contain errors, falsifications and undergo spoofing due to the unsecured channel of transmission, and that weakens the whole system and the safety of navigation. This paper presents a detection method for this type of issues by studying physical characteristics of the signal. The power received being proportional to the distance from a transmitter to a receiver, it has thus been possible to verify the absence of abrupt changes, revealing the intrusion of a third party in exchanges. It was also possible to discriminate AIS transponders between them, i.e. ships, by extracting a fingerprint. This will lead to the determination of non-genuine messages and the highlighting of falsifiers, with the objectives to discover the fraudulent uses, point out the offenders, remove the falsified messages from the following studies and thus improve the effectiveness of the system.