def _at(self, time):
"""Propagate the KeplerOrbit to the given Time object
The Time object can contain one time, or an array of times
"""
pos, vel = propagate(
self.position_at_epoch.au,
self.velocity_at_epoch.au_per_d,
self.epoch.tt,
time.tt,
self.mu_au3_d2,
)
if self._rotation is not None:
pos = mxv(self._rotation, pos)
vel = mxv(self._rotation, vel)
return pos, vel, None, None
def test_velocity_in_ITRF_to_GCRS2():
# TODO: Get test working with these vectors too, showing it works
# with a non-zero velocity vector, but in that case the test will
# have to be fancier in how it corrects.
# r = np.array([(1, 0, 0), (1, 1 / DAY_S, 0)]).T
# v = np.array([(0, 1, 0), (0, 1, 0)]).T
ts = api.load.timescale()
t = ts.utc(2020, 7, 17, 8, 51, [0, 1])
r = np.array([(1, 0, 0), (1, 0, 0)]).T
v = np.array([(0, 0, 0), (0, 0, 0)]).T
r, v = ITRF_to_GCRS2(t, r, v, True)
# Rotate back to equinox-of-date before applying correction.
r = mxv(t.M, r)
v = mxv(t.M, v)
r0, r1 = r.T
v0 = v[:,0]
# Apply a correction: the instantaneous velocity does not in fact
# carry the position in a straight line, but in an arc around the
# origin; so use trigonometry to move the destination point to where
# linear motion would have carried it.
angvel = (t.gast[1] - t.gast[0]) / 24.0 * tau
r1 = mxv(rot_z(np.arctan(angvel) - angvel), r1)
r1 *= np.sqrt(1 + angvel*angvel)
actual_motion = r1 - r0
predicted_motion = v0 / DAY_S
relative_error = (length_of(actual_motion - predicted_motion)
/ length_of(actual_motion))
acceptable_error = 4e-12
assert relative_error < acceptable_error
