**6. Conclusions**

We proposed a long-term continuous tracking method for vessel targets with stereoscopic HFSWR based on an IMMEKF combined with an ELM to solve the problem of trajectory breaking in large-scale marine surveillance. The IMMEKF is applied for the vessel target tracking, meanwhile the ELM network is combined to judge whether the present track is associated with the former track segments. For fully embodying the characteristics of track segments, we selected the average velocity, average curvature, ratio of the arc length to the chord length, and the wavelet coefficient as the feature vectors to train and test the ELM. The tracking algorithm IMMEKF and the track segments associating scheme of the ELM work simultaneously and iteratively. Both the simulation and the field experiment results showed that the proposed method has better tracking performance than the conventional algorithms, with an average correct track segment association rate of over 91.2%. Further, field experiment data from stereoscopic onshore HFSWR located in Weihai showed that the ELM-based track association method had higher accuracy, with lower error and missing association probability due to effective features extracted from the track segments. The method can effectively solve the problem of track fracture caused by rapid maneuvering, strong clutter, target occlusion, long sampling intervals, low detection probabilities, and interference from other vessels near channels, providing a new way for long-term continuous tracking of vessel targets in complex environments. Moreover, the new method is efficient and easy to implement due to its simple structure, which allows the real-time tracking of vessel targets for stereoscopic HFSWR.

**Author Contributions:** Conceptualization: L.Z.; Methodology: D.M. and L.Z.; Software: D.M. and J.N.; Validation: J.N. and D.M.; Investigation: D.M.; Resources: Y.J.; Data Curation: Y.J.; Writing—Original Draft Preparation: L.Z. and D.M.; Project Administration: Y.J.; Writing—Review and Editing: Q.M.J.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project is sponsored by National Key R&D Program of China (No. 2017YFC1405202), and the National Natural Science Foundation of China (No. 51979256, No. 61671166, No. 41506114).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; the collection, analyses, or interpretation of data; the writing of the manuscript; or the decision to publish the results.
