C$Procedure                         interp_drag_bias
C
      subroutine interp_drag_bias ( x, ydata, n, y )
C
C
C*******************************************************************************
C
C       Copyright (C) 1993, California Institute of Technology.  U.S.
C       Government Sponsorhip under NASA Contract NAS7-918 is
C       acknowledged.
C
C*******************************************************************************
C
C$ Log
C
C  Date         Name               Description
C  -----------------------------------------------------------------------------
C  30-AUG-1991  Bruce Shapiro      Creation of interp_drag_bias
C				   Built from interp
C
C$ Purpose
C
C  Linear interpolation into an array
C
C$ Input_Arguments
C
C  Name    Type   Dim        Units   Description
C  -----------------------------------------------------------------------------
C  X         DP    -   		-   x-value of desired point
C  Y         DP    -   		-   return value  y-value
C  Ydata     DP    -		-   y-data in world coordinates
C  N         I     1		-   number of data points
C
C$ Parameters                                       
C
C
C$ Declarations_of_Input_and_Output_Arguments
C

      INTEGER N
      DOUBLE PRECISION YDATA ( N )
      DOUBLE PRECISION X
      DOUBLE PRECISION Y
C
C$ Declarations_of_Local_Variables
C     
C  Name    Type   Dim        Units   Description
C  -----------------------------------------------------------------------------
C
C$ Namelists
C
C
C$ Data_Statements
C
C
C$ Method
C-&

      INTEGER I1
      DOUBLE PRECISION X1, X2, Y1, Y2

C     assume the data is distributed one point per day to the last point
C     with the first point being for day 0

      i1 = idint ( x ) + 1

C     now do a linear interpolation

      if ( i1 .lt. 1 ) then
         y1 = ydata ( 1 )
         y2 = ydata ( 2 )
         x1 = 0.0
         x2 = 1.0 
      else if ( i1 .GE. n ) THEN
         y1 = ydata ( N - 1 )
         y2 = ydata ( N )
         x1 = dble ( N - 2 )
         x2 = dble ( N - 1 )
      else 
         y1 = ydata ( I1)
         y2 = ydata ( I1 + 1)
         x1 = dble ( I1 - 1)
         x2 = dble ( I1 )
      end if

      if ( x2 .eq. x1 ) then
         y = y1
      else 
         y = y1 + ( x - x1 ) * (y2-y1) / (x2-x1)
      end if
     
      return
      end 

C      program tester
C      double precision x, y, xdata(12), ydata(12)
C      integer n
C
C      data n / 10 /
C      data xdata /2, 3, 5, 7, 11,
C     &		  13, 17, 19, 23, 29, 31, 37/
C      data ydata /100, 200, 300, 400, 500,
C     &            600, 700, 800, 900, 1000, 1001, 1002 /
C
C      write (6,*) xdata
C      write (6,*) '*****'
C      write (6,*) ydata
C      write (6,*) '*****'
C
C      x = 8.4
C      call interp_drag_bias ( x,  ydata, n, y )
C      write (6,*) x,' ', y
C      end