Automatic identification system is a navigation aid system that allows vessels to exchange automatically positions, identity and other information. It improves the safety of the maritime traffic and helps to monitor it. However, this system can be manipulated to send falsified information and to mask illicit activities. While previous research has proposed strategies to detect these threats, none of them suggest a solution that considers the time-division multiple access (TDMA) communication protocol. In this work, the compliance of the sent messages with this protocol, specified by the system's standard, is checked for every ship to detect message falsifications. Furthermore, because the ships velocity affects TDMA protocol, a strategy based a Kalman filter is applied to track every ship and to assess the consistency of their velocity data sent. The proposed strategy was validated on real data and showed very promising results. Being computationally cheap, the method can be run in real time. Source codes of the method are open-source to foster research activities from both industry and academia in this field.