Automatic Dependent Surveillance - Broadcast (ADS-B) is an essential communication protocol used in modern air traffic control. However, lacking security measures such as authentication and encryption, ADS-B messages can be forged and modified easily by malicious attackers. This paper addresses the problem of authenticating an ADS-B message by means of specific emitter identification (SEI), employing the unintentional modulation on pulse (UMOP). A complete identification system is presented, including data acquisition, feature extraction and classification. In order to create the feature vector for classification, the transition region in a pulse is first delimited and then used for extraction of UMOP features through Hilbert-Huang Transform. The performance of the method is tested by real signal from 30 ADS-B transmitters. Our proposed method is shown to achieve an recognition ratio of over 94 %, and performs better under low SNR compared with two previous techniques. It is demonstrated that the SEI technique proposed can be used as an additional tool to enhance the security of ADS-B protocol.
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