Subroutine:     IRDS_EXTEND

Purpose:        extends an existing IRDS with one snip

File:   	irds_basic.c

Author:         Do Kester

Use:    CALL IRDS_EXTEND( IRDS,     I     character*(*) 
                          SOP,      I     integer
                          OBS,      I     integer
                          ATT,      I     integer
                          SCANTYPE, I     character*(*)
                          SCANCAL,  I     integer
                          SCANDUR,  I     integer
                          SNIPCAL,  I     integer
                          SNIPDUR,  I     integer
                          PSI,      I     real
                          PSIRATE,  I     real
                          THETA,    I     real
                          STATUS )  O     integer

        IRDS    name of (existing) irds to extend
        SOP     sop number of the extending snip
        OBS	observation number of snip
        ATT     attitude block number of snip
        SCANTYPE
        SCANCAL satcal at the beginning of the original scan
        SCANDUR duration in ticks of the original scan
        SNIPCAL ticks since scancal to begin of snip
        SNIPDUR duration in ticks of the snip
        PSI     intended psi at scancal
        PSIRATE intended rate in psi during scan
        THETA   intended theta of scan
        STATUS   0 : OK. 
                -1 : cannot find irds
                -2 : snipdur is too long
                -3 : other error while extending

Description:    
        The existing irds is extended with one snip. The obligatory
        header items are filled in at the snip level. The number of
        snips at the top level is increased by one. 
        The pixels are undefined.

Externals:      

Updates:        23/02/90 DK, document creation
                18/03/92 FL, OBS and SCANTYPE added, PLATNR and NPLATES removed