frs.f

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      subroutine frs(xx,xr,xi,rfsea)
c
c     compute the reflectivity of a flat ocean
c
c**********************************************************************
c
      implicit real*8 (a-h,o-z)
      xzx=-1.0d-8
      cost=dcos(xx*3.1415926d0/180.d0)
c     calculate the elements of fresnel matrix
      sinsq = (1.0d0-cost**2)
      sinsq2 = sinsq**2
      a     = (xr**2-xi**2 -1.0d0 + cost**2)
      b     = (-2.0d0 * xr * xi)
      r     = dsqrt(a**2 + b**2)
      tmr    = cost**2*r
      xp    = dsqrt(2.0d0*r + 2.0d0*a)
      xmm   =      (2.0d0*r - 2.0d0*a)
      if(xmm .lt. 0.0d0 .and. xmm .gt. xzx) xmm=-xmm
      xm=dsqrt(xmm)
      dnmr1  = (sinsq2 + tmr + cost*sinsq*xp)
      qrmu   = (sinsq2 - tmr)/dnmr1
      qimu   = (cost*sinsq*xm)/dnmr1
      dnmr2  = (cost**2 + r + cost*xp)
      rrr    = (cost**2 - r)/dnmr2
      rri    = (cost*xm)/dnmr2
      rer    = qrmu*rrr - qimu*rri
      rei    = qimu*rrr + qrmu*rri
      r11    = rer**2 + rei**2
      r22    = rrr**2 + rri**2
      r33    = rer*rrr + rei*rri
      r34    = rri*rer - rei*rrr
      rfsea=0.5d0*(r11+r22)
      return
      end
c**********************************************************************