*Communication* **Bistatic High-Frequency Radar Cross-Section of the Ocean Surface with Arbitrary Wave Heights**

**Murilo Teixeira Silva \*,†, Weimin Huang † and Eric W. Gill †**

Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John's, NL A1C 5S7, Canada; weimin@mun.ca (W.H.); ewgill@mun.ca (E.W.G.)

**\*** Correspondence: murilots@mun.ca

† These authors contributed equally to this work.

Received: 28 December 2019; Accepted: 15 February 2020; Published: 18 February 2020

**Abstract:** The scattering theory developed in the past decades for high-frequency radio oceanography has been restricted to surfaces with small heights and small slopes. In the present work, the scattering theory for bistatic high-frequency radars is extended to ocean surfaces with arbitrary wave heights. Based on recent theoretical developments in the scattering theory for ocean surfaces with arbitrary heights for monostatic radars, the electric field equations for bistatic high-frequency radars in high sea states are developed. This results in an additional term related to the first-order electric field, which is only present when the small-height approximation is removed. Then, the radar cross-section for the additional term is derived and simulated, and its impact on the total radar cross-section at different radar configurations, dominant wave directions, and sea states is assessed. The proposed term is shown to impact the total radar cross-section at high sea states, dependent on radar configuration and dominant wave direction. The present work can contribute to the remote sensing of targets on the ocean surface, as well as the determination of the dominant wave direction of the ocean surface at high sea states.

**Keywords:** bistatic radar; HF radar; electromagnetic scattering; radio oceanography
