Function:     irds_rd_detpos

Purpose:      To extract (projected) sky positions data from an IRDS

Category:     IR

File:         irds_dpos.c

Author:       Do Kester

Use:          IRDS_RD_DETPOS(  IRDS   ,    Input   character*(*)
                               SNIP   ,    Input   integer
                               SDET   ,    Input   integer
                               TICK   ,    Input   integer
			       COOR   ,    Input   integer
			       PROJ   ,    Input   integer
                               YLON   ,    Output  double( >=NDATA )
                               ZLAT   ,    Output  double( >=NDATA )
                               TWIST  ,    Output  double( >=NDATA )
                               NDATA  ,    In/Out  integer
                               STATUS )    Output  integer
                                       
              IRDS        Name of IRDS to read from.
              SNIP        Sequential snip number to read.
              SDET        Sequential detector number to read.
                          SDET = 0 corresponds to boresight,
                          SDET < 0 corresponds to the center of gravity
                          of band number SDET (see IRCC_BANDNR etc.)
              TICK        Sequential tick of first sample to read.
	      COOR	  coordinate system identification ( cf irco.dc2 )
			  0 is a valid number => no transformations	
              PROJ	  projection system number ( cf irco_prname.dc2 )
			  0 is a valid number => no projection
	      YLON        Array to receive LON coordinates (rad)
              ZLAT        Array to receive LAT coordinates (rad)
              TWIST       Array to receive TWIST angles (rad. ccw w.r.t. +ZLAT)
              NDATA       I - max number of samples to return
                          O - number of samples actually returned.
              STATUS      Error return code:
                           0  - no error.
                          -1  - IRDS does not exist
                          -2  - IRDS is not a legal irds
                          -3  - SNIP not in IRDS
                          -4  - SDET not in IRDS
                          -5  - TICK not in IRDS
                          -6  - gds read error
                          -7  - no coordinate info in header
                          -8  - bad SDET
			  -9  - not enough memory for boresight data
			  -10 - only one point bphf tick found
Description:
	The BPHF data are read for the satcal ticks in the scan. 
	The boresight positions are transformed to the proper detector
	positions, still in sunreferenced coordinates.
	Transformation and projection take place ( when requested 
	with coor and proj ) and the resulting positions are
	interpolated to the sample times. No conscious shortcuts or 
	approximations have been taken in these calculations. 

	The routine has been optimized for an inner loop over the 
	detectors and an outer loop over the snips.

	It is assumed that the ecliptic coordinate system has been precessed
	to 1983.5.
	The returned angles will be in radians

Bugs: 	Funny things will happen when the snip crosses an edge of the
	projection type.

Updates:      	30 Nov 1990: DK
		30 May 1991  DK, improved interpolation
		03 Jun 1991  DK, partial satcals allowed.
		07 Aug 1991  DK, free memory.