c@ double precision function irco_sunlong( integer )
c
c#>             irco_sunlong.dc2
c
c Function	IRCO_SUNLONG
c 
c Purpose    calculate approximate ecliptic longitude of the sun from satcal
c 
c Author	850927 A.R.W.de Jonge
c
c Category	IRAS, WORLD COORDINATES
c
c File		irco.shl
c 
c Use doubleprecision function IRCO_SUNLONG( 
c	satcal )	I	integer
c
c 	satcal  	satcal clock time in ticks
c 
c Description
c	The routine calculates the approximate ecliptic longitude
c       of the sun from the time given in IRAS satellite clock ticks.
c 
c       The ecliptic longitude of the sun is in radian with reference 
c       to the ecliptic of date
c 
c       The accuracy is to about 30 arcsec.
c 
c Update	DK adapted for Gipsy; satcal is now an integer
c#<

      doubleprecision function IRCO_SUNLONG( satcal )

      integer	satcal

      doubleprecision	PI, CIRCLE, RAD, SUN0, SUN1
      doubleprecision	EARTH0, EARTH1, EQOC0, EQOC1
      parameter 	( PI = 3.141526535897932384 )
      parameter 	( CIRCLE = 2 * PI )
      parameter 	( RAD = PI / 180.0 )
      parameter 	( SUN0 = 279.10303475 * RAD )
      parameter 	( SUN1 =   0.98564735 * RAD )
      parameter 	( EARTH0 = 356.45501990 * RAD )
      parameter 	( EARTH1 =   0.98560026 * RAD )
      parameter 	( EQOC0  = 1.915476 * RAD )
      parameter 	( EQOC1  = 0.020010 * RAD )

      doubleprecision	et, eqoc, slong, anom

c convert satcal to ephemeris time in days since 0 jan 1983, 0hr ET
      et = ( satcal + 53 ) * 1.0000526 / 86400  +  26
c mean solar longitude (uniform movement)
      slong = SUN0 + SUN1 * et
c mean anomaly
      anom   = EARTH0 + EARTH1 * et
c equation of centre
      eqoc = EQOC0 * sin( anom ) + EQOC1 * sin( 2 * anom )
c add and normalise
      slong = slong + eqoc
      while slong .gt. circle
	 slong = slong - circle
      cwhile

      IRCO_SUNLONG = slong

      return
      end