C+
C NAME:
C	HOSOrbit
C PURPOSE:
C	Calculates the position of a Helios spacecraft at a given time
C CATEGORY:
C	Celestial mechanics
C CALLING SEQUENCE:
	subroutine HOSOrbit(iSc,iYr,Doy,rSun,rLng,VR,VT)
C INPUTS:
C	iSc	integer		spacecraft (1=Helios A; 2=Helios B)
C	iYr	integer		year and
C	Doy	real		day of year, where s/c location is required
C OUTPUTS:
C	rSun	real		spacecraft distance to Sun in AU
C	rLng	real		spacecraft ecliptic longitude (Equinox 1975.0 ???)
C	VR	real		radial velocity (AU/day)
C	VT	real		tangential velocity (AU/day)
C CALLS:
C	Julian, KeplerOrbit
C PROCEDURE:
C	The spacecraft orbit is assumed to lie in the ecliptic
C	See O. Montenbruck, Practical Ephemeris Calculations, p. 42-45
C	Perihelion passages for Helios A:
C
C	year       doy           JD               ecl long perih
C
C	1975      74.38411   2442486.8841             257.8521    !
C	1975     264.5102       2677.0102                .8493    !
C	1976      89.6610       2867.1610                .8570    !
C	1976     279.7965       3057.2965                .8478    !
C	1977     103.9367       3247.4367                .8448    !
C	1977     294.0973       3437.5973                .8496    !
C	1978     119.2591       3627.7591                .8517    !
C	1978     309.4239       3817.9239                .8518    !
C	1979     134.5879       4008.0879                .8530    !
C	1979
C	1980     149.9095       4388.4095                .8465    !
C	1980     340.2279       4578.7279                .8542    !
C	1981     164.978        4768.729              257.978
C	1981
C	1982
C	1982
C	1983
C	1983
C
C	Perihelion passages for Helios B:
C
C	1976     108.1035    2442885.6035             294.2079     !
C	1976     293.7552       3071.2552             294.2007     !
C	1977     113.5434       3257.0434             294.2332     !
C	1977     299.3565       3442.8565             294.2353     !
C	1978     120.2137       3628.7137             294.2470     !
C	1978     306.0831       3814.5831             294.2528     !
C	1979     126.9678       4000.4678             294.2577     !
C	1979     312.8478       4186.3478             294.2553     !
C MODIFICATION HISTORY:
C	1990, Paul Hick (UCSD)
C	1997, Paul Hick (UCSD), replaced explicit orbit calculation by
C		call to KeplerOrbit
C-
	    integer	iSc
	    integer	iYr
	    real	Doy
	    real	rSun
	    real	rLng
	    real	VR
	    real	VT

	real*8		JD,JD0
	real		aa(2)		/.6471445,.6372573/
	real		ee(2)		/.521757,.543814/
	parameter	(IA=12,IB=8)
	integer		NPER(2)		/IA,IB/
	real*8		JDPER(IA+IB)
     &		/2486.8841,2677.0120,2867.1610,3057.2965,3247.4367,3437.5973,3627.7591,
     &		3817.9239,4008.0879,4388.4095,4578.7279,4768.729,
     &		2885.6035,3071.2552,3257.0434,3442.8565,3628.7137,3814.5831,4000.4678,
     &		4186.3478/
	real		PerLng(IA+IB)
     &		/ 257.8521, 257.8493, 257.8570, 257.8478, 257.8448, 257.8496, 257.8517,
     &		257.8518, 257.8530, 257.8465, 257.8542, 257.978,
     &		294.2079, 294.2007, 294.2332, 294.2353, 294.2470, 294.2528, 294.2577,
     &		294.2553/

	real	OrbitElement(6)		/6*0./
	real	Position(3)
	real	Velocity(5)

	call Julian(0,iYr,Doy,JD,JD0)	!Convert input time to Julian days
	JD = JD-2440000

	J   = (iSc-1)*IA
	JD0 = JDPER(J+1)		! Find closest perihelion (in time)
	PER = PerLng(J+1)
	do I=2,NPER(iSc)
	    if (dabs(JD-JDPER(J+I)) .lt. dabs(JD-JD0)) then
		JD0 = JDPER (J+I)	! Time of perihelion passage in Julian days
		PER = PerLng(J+I)	! Ecliptic longitude of perihelion
	    end if
	end do

	OrbitElement(1) = aa(iSc)
	OrbitElement(2) = ee(iSc)
	OrbitElement(5) = PER
	OrbitElement(6) = PER
	call KeplerOrbit(JD-JD0,1.,0.,OrbitElement,Position,Velocity)

	rSun = Position(3)
	rLng = Position(1)

	VR = Velocity(5)
	VT = Velocity(4)

	return
	end