SUBROUTINE radlow( rs, sza_v, az_v, sat_v, pt_v, outrad, outtcl,
     &                                                   nout, success)
c
c PROCEDURE NAME:       radlow
c PROCEDURE TYPE:       fortran subroutine
c VERSION DATE:         27.VI.97
c
c PURPOSE:
c   Use interpolation of the low-radiance table to generate a set
c   of (cloud optical thickness, radiance) pairs for subsequent use
c   in determining cloud optical thickness from the radiance.
c
c CALLING PARAMETERS:
c   Name        Type    i/o     units   Description
c   --------    ----    ---     -----   --------------
c   rs		R8      i       frac    Surface reflectivity
c   sza_v       R8      i       deg     Solar zenith angle
c   az_v        R8      i       deg     Solar-FOV-satellite azimuth angle
c   sat_v       R8      i       deg     Satellite zenith angle
c   pt_v	R8	i	atm	Terrain pressure
c   outrad      R8      o       I/F     radiances at reflectivity wavelength
c   outtcl      R8      o               corresponding cloud optical thicknesses
c   nout        I4      o               Number of elements in outrad & outtcl
c   success     L       o               Successful completion flag
c
c
c EXTERNAL ROUTINES REQUIRED:
c       lagrangeco      determine appropriate degree and coefficients
c                       for Lagrange interpolation.
c
c NOTES & CAVEATS:
c
c REVISION HISTORY:
c   27.VI.97 Documentation added (eac).
c
c============================================================================
      IMPLICIT NONE
      INTEGER*4 nsza_p,    naz_p,   nsat_p,   ntcl_p,   npt_p
      PARAMETER(nsza_p=11, naz_p=7, nsat_p=9, ntcl_p=5, npt_p=2)
c-----------------------------------------------------------------------
c Calling parameters
c
      REAL*8 rs, sza_v, az_v, sat_v, pt_v
      REAL*8 outrad(0:ntcl_l-1), outtcl(0:ntcl_l-1)
      INTEGER*4 nout
      LOGICAL success

c-----------------------------------------------------------------------
c Common blocks
c
      INTEGER*4 nsza_l, naz_l, nsat_l, ntcl_l, npt_l
      REAL*8 sza_l(0:nsza_p-1), az_l(0:naz_p-1), sat_l(0:nsat_p-1),
     &       tcl_l(0:ntcl_p-1), pt_l(0:npt_p-1),
     &  rad_l(0:nsza_p-1, 0:naz_p-1, 0:nsat_p-1, 0:ntcl_p-1, 0:npt_p-1),
     &  tprm_l(0:nsat_p-1, 0:ntcl_p-1, 0:npt_p-1),
     &  flxd_l(0:nsza_p-1, 0:ntcl_p-1, 0:npt_p-1),
     &  sbar_l(0:ntcl_p-1, 0:npt_p-1)

      COMMON /radlowcom/ sza_l, az_l, sat_l, tcl_l, pt_l,
     &                   rad_l, flxd_l, tprm_l, sbar_l, 
     &                   nsza_l, naz_l, nsat_l, ntcl_l, npt_l

c-----------------------------------------------------------------------
c Local variables
c
      REAL*8 szacoef(0:3), azcoef(0:3), satcoef(0:3), ptcoef(0:3)
      INTEGER*4 sza_0, szadeg, szain
      INTEGER*4 az_0, azdeg, azin
      INTEGER*4 sat_0, satdeg, satin
      INTEGER*4 pt_0, ptdeg, ptin

      REAL*8 sum
      INTEGER*4 itcl, isza, iaz, isat, ipt

d     write(6,*)'> radlow'

      nout= ntcl_l

      CALL lagrangeco(sza_v,sza_l,nsza_l,sza_0,szadeg,szacoef,szain)
      CALL lagrangeco(az_v, az_l, naz_l, az_0, azdeg, azcoef, azin)
      CALL lagrangeco(sat_v,sat_l,nsat_l,sat_0,satdeg,satcoef,satin)
      CALL lagrangeco(pt_v,pt_l,npt_l,pt_0,ptdeg,ptcoef,ptin)


      DO itcl=0, ntcl_l-1
        sum=0.d0
        DO isza= 0, szadeg
          DO iaz= 0, azdeg
            DO isat= 0, satdeg
              DO ipt= 0, ptdeg
d               write(6,*)itcl, isza, iaz, isat, ipt
                sum= sum + 
     &            ( 
     &             szacoef(isza)*azcoef(iaz)*satcoef(isat)*ptcoef(ipt) 
     &            ) * 
     &            ( 
     &             rad_l(isza+sza_0,iaz+az_0,isat+sat_0,itcl,ipt+pt_0) +  
     &              ( 
     &               rs*tprm_l(isat+sat_0,itcl,ipt+pt_0)* 
     &                  flxd_l(isza+sza_0,itcl,ipt+pt_0) 
     &              ) / 
     &              ( 
     &               1.D0 - rs*sbar_l(itcl,ipt+pt_0) 
     &              ) 
     &            )
              END DO  !ipt
            END DO    !isat
          END DO      !iaz
        END DO        !isza
        outrad(itcl)= sum
        outtcl(itcl)= tcl_l(itcl)
      END DO          !itcl

      success=.TRUE.

d     write(6,*)'< radlow'

      RETURN
      END