real*8 function vmarea ( betap, model, yawmode )

C
C*******************************************************************************
C
C       Copyright (C) 1993, California Institute of Technology.  U.S.
C       Government Sponsorhip under NASA Contract NAS7-918 is
C       acknowledged.
C
C*******************************************************************************
C
C	Calculate the VMA Area of the Satellite
C
C	Revision Log
C
C	4/21/94		Correct typo VMARAEA for VMAREA in 'CONSTANT'
C			area section
C
C	12/6/94         add second vma tab for fixed yaw values
C
	double precision betap,vmatab(3,-90:90), betalo,betahi,alo,ahi,
     *			RESULT,x1,x2,xtemp, careas(2), breakpts(2),
     *			vmatab_FY(3,-90:90), careas_FY(2), 
     *			breakpts_FY(2)
	character model*4
	integer nvmatabs
	common /vma/vmatab, careas, breakpts, vmatab_FY,
     &          careas_FY, breakpts_FY, nvmatabs  

      common  /dragblock/  dragmodel, dragarea, atmos, atden
      character*10 dragmodel, atmos
      double precision dragarea, atden
	character*2 yawmode

        logical first
	data first/.true./

        IF (DRAGMODEL .EQ. 'CONSTANT') THEN
		VMAREA = DRAGAREA
	        if (first) then
		   write(8,*) 'Using Constant Satellite Area.'
	           first = .false.
	        end if
		RETURN
	ELSE IF (DRAGMODEL .NE. 'VMA') THEN
		WRITE(8,*) 'INVALID DRAGMODEL = ', DRAGMODEL
		STOP '>>>>> ERROR EXIT.'
	END IF
        if (first) then
	   write(8,*) 'Using VMA Model for Satellite Area.'
           first = .false.
        end if

	if (model.eq.'DRAG') then
	  iarea = 2
	else if (model(1:3).eq.'SRP') then
          iarea = 3
        else
          write(8,*) 'Invalid VMA model = ',model
	  write(8,*) 'Valid models = DRAG, SRP'
	  stop '?? VMA Error Exit.'
	end if
C
C	interpolate between two successive values.  Find bounding values
C	in table.

	if ( model .eq. 'DRAG') then
	   if ((nvmatabs.eq.1) .or. (yawmode.eq.'YS')) then
	      if ( dabs(betap) .le. breakpts(1) ) then
  	         result = careas(1)
	         goto 9999 
	      else if ( dabs(betap) .ge. breakpts(2) ) then
	         result = careas(2)
	         goto 9999 
	      end if
	   else
	      if ( dabs(betap) .le. breakpts_FY(1) ) then
  	         result = careas_FY(1)
	         goto 9999 
	      else if ( dabs(betap) .ge. breakpts_FY(2) ) then
	         result = careas_FY(2)
	         goto 9999 
	      end if
	   end if
	end if

	jlow = -100

	
C        do j=-90,89
C	  x1 = vmatab(1,j)
C          x2 = vmatab(1,j+1)
C          if (x2.lt.x1) then
C	    xtemp = x1
C	    x1 = x2
C            x2 = xtemp
C	  end if
C
C          if ( (x1.le.betap).and.(betap.le.x2)) then
C	    jlow = j
C	    jhi = j+1
C	  end if
C
C	end do

	jlow = int(betap)
	jhi = jlow+1

	if ( (jlow .lt. -90).or.(jhi.gt.90)) then
	   write(8,*) 'Invalid Betap = ',betap
           stop 'Invalid Betap in VMA model.'
        end if
 
C
C	determine beta primes and areas for interpolation into table
C       


	IF ( (nvmatabs .eq. 1) .or. (YAWMODE .eq. 'YS') ) then
	   betalo = vmatab(1,jlow)
	   betahi = vmatab(1,jhi)	

	   alow = vmatab(iarea, jlow)
           ahi  = vmatab(iarea, jhi)
	else 
	   betalo = vmatab_FY(1,jlow)
	   betahi = vmatab_FY(1,jhi)	

	   alow = vmatab_FY(iarea, jlow)
           ahi  = vmatab_FY(iarea, jhi)
	end if

C
C	linearly interpolate for the area
C
	RESULT = alow + (ahi-alow)*(betap-betalo)/(betahi-betalo)


 9999   continue

	VMAREA = RESULT
	return
	end