subroutine cnstnts
c
c***********************************************************************
c
c     cnstnts    september, 2001   charlie wellemeyer    ssai
c
c     purpose
c        initializes various constant parameters used in the retrieval
c
c     calling sequence
c        call cnstsnts
c
c     common areas
c        absoft, contrl, lpoly, prfprm, pterp, satnams
c
c     calling routine      start
c
c     subroutines called   coffs
c
c     modifications
c
c        Sept 2004:
c        Add a switch between n14 fix mode and zigzag mode for absorption
c        crossection paramters for the temperature correction to dndx
c        LK Huang
c
c***********************************************************************
c
      implicit none
c
      real*4 xdenom
      integer i, j, jmax, k, l, ic
c
      integer     n, nc, ns, nitr, nref
      parameter   (n = 81, nc = 10, ns = 8, nitr = 8, nref = 1690)
      real        p_top, p_bottom, h, r
                              !pressure at top & bottom of atmosphere
                              !  (atmos), constant scale height (cm),
                              !  radius of Earth (cm)
      parameter   (p_top = 1.0e-4, p_bottom = 1.0, h = 7.996e5,
     1                  r = 6.378e8)
      real        delz, cons
c
      include 'absoft.com'
      include 'contrl.com'
      include 'lpoly.com'
      include 'pterp.com'
      include 'prfprm.com'
      include 'satnams.h'
c
c  define total ozone parameters
c
c  load appropriate absorption temperature coefficients into arrays
c
        do i=1,8
          c0(i) = 0.0
          c1(i) = 0.0
          c2(i) = 0.0
        enddo
      if(n07()) then
          do i=1,8
            c0(i) = c0n07(i)
            c1(i) = c1n07(i)
            c2(i) = c2n07(i)
          enddo
      else if(n09()) then
          do i=1,8
            c0(i) = c0n09(i)
            c1(i) = c1n09(i)
            c2(i) = c2n09(i)
          enddo
      else if(n11()) then
          do i=1,8
            c0(i) = c0n11(i)
            c1(i) = c1n11(i)
            c2(i) = c2n11(i)
          enddo
      else if(n14()) then
         if(wlenth(2) .ge. 273.46 .and. wlenth(2) .le. 273.56) then
          do i=1,8
            c0(i) = c0n14(i)
            c1(i) = c1n14(i)
            c2(i) = c2n14(i)
          enddo
	 else
          do i=1,8
            c0(i) = c0n14a(i)
            c1(i) = c1n14a(i)
            c2(i) = c2n14a(i)
          enddo
	 endif
      else if(n16()) then
          do i=1,8
            c0(i) = c0n16(i)
            c1(i) = c1n16(i)
            c2(i) = c2n16(i)
          enddo
      else if(n17()) then
          do i=1,8
            c0(i) = c0n17(i)
            c1(i) = c1n17(i)
            c2(i) = c2n17(i)
          enddo
      else if(n18()) then
          do i=1,8
            c0(i) = c0n18(i)
            c1(i) = c1n18(i)
            c2(i) = c2n18(i)
          enddo
      else if(n19()) then
          if(abs(wlenth(12)-343) .lt. 0.6) then
           do i=1,8
            c0(i) = c0n19n(i)
            c1(i) = c1n19n(i)
            c2(i) = c2n19n(i)
           enddo
          else if(abs(wlenth(12)-335) .lt. 0.6) then
           do i=1,8
            c0(i) = c0n19m(i)
            c1(i) = c1n19m(i)
            c2(i) = c2n19m(i)
           enddo
          else
           do i=1,8
            c0(i) = c0n19(i)
            c1(i) = c1n19(i)
            c2(i) = c2n19(i)
           enddo
          endif
      else if(ssbuv()) then
          do i=1,8
            c0(i) = c0ssb(i)
            c1(i) = c1ssb(i)
            c2(i) = c2ssb(i)
          enddo
      else if(n04()) then     
          do i=1,8
            c0(i) = c0n04(i)
            c1(i) = c1n04(i)
            c2(i) = c2n04(i)
          enddo
      endif
c
c**** compute denominators for l-coeffs
c
      do 500 j=1,7
      jmax=j+3
      l=0
      do 500 k=j,jmax
      l=l+1
      xdenom=1.0
      do 400 i=j,jmax
      if(i.eq.k) go to 400
      xdenom=(xzlog(k)-xzlog(i))*xdenom
  400 continue
      densol(l,j)=xdenom
  500 continue
c
c**** compute denominators for l-coeffs for pressure interpolation
c
      do 750 k=1,4
      xdenom=1.0
      do 725 i=1,4
      if(i.eq.k) go to 725
      xdenom=(logpr(k)-logpr(i))*xdenom
  725 continue
      denprs(k)=xdenom
  750 continue
c
c  define profile algorithm parameters
c
c
c           Specify z & p_lvl
c
      delz = h*log(p_bottom/p_top)/(n - 1)
      do i = 1,81
         z(i) = delz*(i - 1)
         p_lvl(i) = p_bottom*exp(-z(i)/h)
      end do
c
c           Compute optical coefficients (assume center wavelengths do
c           not change)
c
      do ic=1,nc
        wvl_c(ic) = wlenth(ic)
      enddo
      delwvl = 1.132          !this is an input number
c
      call coffs(wvl_c, delwvl, a0, a1, a2, b, p0, p2, w, nc, ns)
c
c           o  Define cov_e
c
         cons = -100/log(10.0)
         do i = 1,nc
            cov_e(i,i) = (cons*sigmae)**2
                              !sigmae is the fractional error in
                              !  radiance/flux
            if(iopts(i+10)) cov_e(i,i) = cov_e(i,i) * 10.0 ** 6.0
            do k = 1,i-1
               cov_e(i,k) = 0.0
               cov_e(k,i) = 0.0
            end do
         end do
         do i = 1,nc
            cov_e0(i) = cov_e(i,i)
                              !save diagonal elements of cov_e
         end do
c
c           o  define rad_m0, krngr_mu & krncl_mu
c
         do i = 1,nc
            rad_m0(i) = 0.0
         end do

         do i = 1,10
            do j = 1,11
               krngr_mu(i,j) = 0.0
               krncl_mu(i,j) = 0.0
            end do
         end do
c
      return
c
      end