Availability of planet-SSB and planet-obs vectors for ephemeris modeling

[Hazboun6]

In order to use Pint-based pulsars in enterprise along with the BayesEphem ephemeris model we need access to various solar system barycenter position vectors with respect to the Earth and various planets at each of the TOAs. This Issue directly relates to Issue 238 on the enterprise GitHub repo. @paulray has already submitted PR #901 to add the position vectors for Neptune. BayesEphem requires planet-ssb vectors, which can be calculated from the ssb-observatory and obs-planet vectors available in Pint. However we also need the earth-ssb vector for the ephemeris model, which can either be furnished directly or calculated from the obs-earth vector.

[Hazboun6]

There are use cases for other planet-ssb vectors, for instance PR 232 adds in a modeling for perturbations from all planets. The minimal use for getting BayesEphem working for the 15 year data set is now the Earth-obs or Earth-ssb vector.

[luojing1211]

I think these two functions are what you need.

PINT/src/pint/solar_system_ephemerides.py

Lines 203 to 276 in 8854638

	def objPosVel_wrt_SSB(objname, t, ephem, path=None, link=None):
	"""This function computes a solar system object position and velocity respect
	to solar system barycenter using astropy coordinates get_body_barycentric()
	method.
	The coordinate frame is that of the underlying solar system ephemeris, which
	has been the ICRF (J2000) since the DE4XX series.
	Parameters
	----------
	objname: str
	Solar system object name. Current support solar system bodies are listed in
	astropy.coordinates.solar_system_ephemeris.bodies attribution.
	t: Astropy.time.Time object
	Observation time in Astropy.time.Time object format.
	ephem: str
	The ephem to for computing solar system object position and velocity
	path : str, optional
	Local path to the ephemeris file.
	link : str, optional
	Location of path on the internet.
	Returns
	-------
	PosVel object with 3-vectors for the position and velocity of the object
	"""
	objname = objname.lower()
	load_kernel(ephem, path=path, link=link)
	pos, vel = coor.get_body_barycentric_posvel(objname, t)
	return PosVel(pos.xyz, vel.xyz.to(u.km / u.second), origin="ssb", obj=objname)
	def objPosVel(obj1, obj2, t, ephem, path=None, link=None):
	"""Compute the position and velocity for solar system obj2 referenced at obj1.
	This function uses astropy solar system Ephemerides module.
	Parameters
	----------
	obj1: str
	The name of reference solar system object
	obj2: str
	The name of target solar system object
	tdb: Astropy.time.Time object
	TDB time in Astropy.time.Time object format
	ephem: str
	The ephem to for computing solar system object position and velocity
	path : str, optional
	Local path to the ephemeris file.
	link : str, optional
	Location of path on the internet.
	Return
	------
	PosVel object.
	solar system obj1's position and velocity with respect to obj2 in the
	J2000 cartesian coordinate.
	"""
	if obj1.lower() == "ssb" and obj2.lower() != "ssb":
	obj2pv = objPosVel_wrt_SSB(obj2, t, ephem, path=path, link=link)
	return obj2pv
	elif obj2.lower() == "ssb" and obj1.lower() != "ssb":
	obj1pv = objPosVel_wrt_SSB(obj1, t, ephem, path=path, link=link)
	return -obj1pv
	elif obj2.lower() != "ssb" and obj1.lower() != "ssb":
	obj1pv = objPosVel_wrt_SSB(obj1, t, ephem, path=path, link=link)
	obj2pv = objPosVel_wrt_SSB(obj2, t, ephem, path=path, link=link)
	return obj2pv - obj1pv
	else:
	# user asked for velocity between ssb and ssb
	return PosVel(
	np.zeros((3, len(t))) * u.km, np.zeros((3, len(t))) * u.km / u.second
	)

[aarchiba]

I think the request is that TOAs objects have, or be able to have, pre-computed vectors. This is close enough to true for all the non-Earth planets - there is the observatory-planet vector and the SSB-observatory vector - but there isn't a precomputed column that says anything about the geocenter.