C
C  FK
C  FK IS A FUNCTION THAT USES LINEAR INTERPOLATION TO FIND  FK,
C  GIVEN  ZK  AND THE ARRAYS  Z  AND  F. FOR EXAMPLE, IN ROUTINE
C  DERIVS,  BVF = FK(ALT,BVFZ,BVFF)  WILL BE USED TO GET  BVF  AT  ALT
C  FROM THE ARRAYS  BVFZ  AND  BVFF.
      REAL FUNCTION FK(ZK,Z,F,NZ,IZTOP)
      REAL Z(*), F(*)
C
      COMMON /IFLAGB/IFLAG
C
C  MAKE SURE Z(IZTOP) .GE. ZK
    5 CONTINUE
      IF(ZK.GT.Z(IZTOP)) THEN
         IZTOP = IZTOP + 1
         IF(IZTOP.GT.NZ) THEN
c            WRITE(2,9) ZK,Z(NZ)
    9       FORMAT(' ZK TOO HIGH. ZK,Z(NZ) = ',2(1PE15.5))
            IZTOP = NZ
            FK = F(NZ)
            IFLAG = 1
            RETURN
CCC           CLOSE(2)
CCC           CLOSE(3)
CCC           STOP
         ENDIF
         GO TO 5
      ENDIF
C
      IF(ZK.EQ.Z(IZTOP)) THEN
         FK = F(IZTOP)
         RETURN
      ENDIF
C
C  FIND IZK SUCH THAT  Z(IZK) .GT. ZK .GE. Z(IZK-1)
      DO 100 IZ=IZTOP,2,-1
      IZM1 = IZ - 1
      IF(Z(IZ).GT.ZK .AND. ZK.GE.Z(IZM1)) THEN
         ZRATIO = (ZK - Z(IZM1)) / (Z(IZ) - Z(IZM1))
         FK = F(IZM1) + ZRATIO * (F(IZ) - F(IZM1))
         IZTOP = IZM1 + 1
         RETURN
      ENDIF
  100 CONTINUE
C
C  ZK TOO LOW
!     WRITE(2,109) ZK,Z(1)
c      WRITE(6,109) ZK,Z(1)
  109 FORMAT(' ZK TOO LOW. ZK,Z(1) = ',2(1PE15.5))
      IZTOP = 2
      FK = F(1)
      IFLAG = 1
      RETURN
CCC      CLOSE(2)
CCC      CLOSE(3)
CCC      STOP
C
      END