Function:     irds_rd_bphf

Purpose:      To read BPHF data for ticks from an IRDS

Category:     IR

File:         irds_samples.c

Author:       P.R. Roelfsema

Use:          IRDS_RD_BPHF(    IRDS    ,    Input   character*(*)
                               SNIP    ,    Input   integer
                               TICK    ,    Input   integer
                               SRLON   ,    Output  double( >=NSATS )
                               ESRLON  ,    Output  double( >=NSATS )
                               SRLAT   ,    Output  double( >=NSATS )
                               ESRLAT  ,    Output  double( >=NSATS )
                               TWIST   ,    Output  double( >=NSATS )
                               LNGSUN  ,    Output  double
                               SUNRATE ,    Output  double
                               NSATS   ,    I/O	    integer
                               STATUS  )    Output  integer
                                       
              IRDS        Name of IRDS to read from.
              SNIP        Sequential snip number to read.
              TICK        Sequential tick of first sample to read.
              SRLON       Array to receive SRLON (radians)
              ESRLON      Array to receive error in SRLON (radians)
              SRLAT       Array to receive SRLAT (radians)
              ESRLAT      Array to receive error in SRLAT (radians)
              TWIST       Array to receive TWIST angles (radians)
              LNGSUN      Solar longitude (radians)
              SUNRATE     Rate of change in LNGSUN (radians/tick)
              NSATS       In: Number of satcal ticks to read.
	      		  out: Number of ticks found
              STATUS      Error return code:
                           1  - SRLON/SRLAT are intended positions
                           0  - no error.
                          -1  - IRDS does not exist
                          -2  - IRDS is not a legal irds
                          -3  - SNIP not in IRDS
                          -5  - TICK not in IRDS
                          -6  - gds read error

Note:

              The routine tries to return the best estimate of the position
            parameters. Thus it will first try to find the BPHF information 
            in the irds. If no BPHF data was added to the irds, the intended
            position parameters are returned (STATUS = 1). Since the intended 
            position  parameters have no error estimates the ESRLON and ESRLAT 
            arrays will contain zeros in this case. Also zeros will be 
            returned for TWIST and LNGSUN.

Updates:      Aug 20, 1990: PRR, Creation date
              Sep  5, 1990: PRR, added STATUS = 1.
              Sep 21, 1990: PRR, changed PSI/THETA to SRLON/SRLAT
	      Dec  6, 1990: DK, nsats is upon output the number of ticks found