referred to the bistatic HFSWR [16]. Figure 1 gives the wind directional distribution in relation to the ellipse normal direction. The radio wave propagating paths of the HFHSSWR are depicted by a solid bold line. Tx is one of the ellipse foci and can be treated as the transmitter in bistatic mode [16]. The cross-section first-order sea clutter for the HFHSSWR was written as [6] m =±1 ( ) S K = F ( K ) g (ϕ ) ( ) (2) The ratio of spectral power density of two Bragg waves has been previously used to estimate the wind direction and is expressed by σ 1 (ωd ) = 26 π 2 K 04 cos 4 (φ0 2 ) S ( mK ) δ (ω sponds to the approaching and receding Bragg wave. S K is the directional wave spectrum and is described by the product a nondirectional wave spectrum F(K) and a directional spreading function g(φ): d + m 2 K 0 g cos (φ0 2 ) (1) where ωd is the Doppler angular frequency, K0 is radar wave number, ϕ0 is the bistatic angle (Figure 1), K is the wave number, and ± corre- ( ) ( ) g (π + θ N − ϕ w ) σ (ω ) S − K B r= 1 B = = g (θ N − ϕ w ) σ 1 ( −ωB ) S K B (3) Figure 1. Depiction of the HFHSSWR geometry. R and T are receiver and transmitter, respectively. From [6], Tx and R are two foci of a ellipse, in which the characteristics of first-order sea cutter for HFHSSWR could be described. MARCH 2018 IEEE A&E SYSTEMS MAGAZINE 43