SUBROUTINE BACKGROUND 5,2

      IMPLICIT NONE
      include 'champ.cdk'
      include 'chobs.cdk'
 
      REAL mean 


      open (16,file='./files/cont.io',form='unformatted')
         DO NT=1,NTRAJC
           read(16) ((ZCONT(IW,JW,NT),IW=1,IKTX),JW=1,IKTY)
         ENDDO
      close(16)
      open (16,file='./files/bkg.io',form='unformatted')
         DO NT=1,NTRAJ
            read(16) ((Zb(IW,JW,NT),IW=1,IKTX),JW=1,IKTY)
         ENDDO
      close(16)

      CALL GAUSSIAN

C     Standard deviation of background error
C     --------------------------------------

      DO IW=1,IKTX
      DO JW = 1,IKTY
         ZO(IW,JW)= Zb(IW,JW,NSTOP) - ZCONT(IW,JW,NSTOP)
      END DO
      END DO
      CALL KR(ZO,ZR,KTX,KTY,IKTX,IKTY,N,N2,LWRK,
     .                  FF1,FF2,FF3,WORK,TRIGS,IFAX,ZI)

      mean = 0
      DO IW = 1,N
      DO JW = 1,N
         mean = mean + ZR(IW,JW)
      END DO 
      END DO 

      mean = mean/N**2

      DO IW = 1,N
      DO JW =1,N
         sigmab = sigmab + (ZR(IW,JW) - mean)**2
      END DO 
      END DO

      sigmab = sqrt(sigmab/N**2)

      print*,'== BACKGROUND == : sigma b =', sigmab
     
      RETURN
      END SUBROUTINE