**5. Conclusions**

This paper proposes a frequency diversity algorithm to achieve passive azimuth estimation using two hydrophones. Compared with the traditional cross-correlation method, the algorithm has a high processing gain and can obtain a clear target trajectory. When the energy of the target signal is evenly distributed in the frequency domain, the processing gain of the algorithm using two hydrophones may even exceed the CBF processing gain of multiple array elements. In addition, in the theoretical derivation, the relationship between the resolution, the sensor interval and the processing bandwidth is analyzed. In the simulation and experiment, the feasibility of the algorithm and the advantages, compared with the cross-correlation method, are verified.

The algorithm proposed in this paper can obtain a clear target trajectory using the wideband signals received by only two hydrophones. This is significant for the application of target estimation in the field of buoy and AUV collaborative operations. The idea of using frequency domain information to virtualize array element domain information has important academic value for passive sonar azimuth estimation.

**Author Contributions:** Conceptualization: P.L. and X.Z.; data curation: X.Z.; formal analysis: P.L.; investigation: P.L. and W.Z.; methodology: P.L.; project administration: X.Z.; resources: X.Z.; supervision: P.L.; writing—original draft: P.L.; writing—review and editing: P.L. and W.Z.

**Funding:** This research was funded by the National Natural Science Foundation of China, grant number [61271443].

**Conflicts of Interest:** The authors declare no conflict of interest.
