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from_state_vector: Fix some angular values returning 2*pi instead of 0 #46

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from_state_vector: Fix some angular values returning 2*pi instead of 0 #46

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2eaa444
from_state_vector: Fixed arg_pe incorrect when it should be > 180°.
mgiuca Sep 16, 2025
b52ac6c
from_state_vector: Fix some angular values returning 2*pi instead of 0.
mgiuca Sep 16, 2025
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12 changes: 7 additions & 5 deletions src/orbital/utilities.py
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Original file line number Diff line number Diff line change
Expand Up @@ -284,12 +284,14 @@ def elements_from_state_vector(r, v, mu):
# Argument of periapsis is the angle between
# eccentricity vector and its x component.
arg_pe = acos(ev.x / norm(ev))
if ev.y < 0:
arg_pe = mod(-arg_pe, 2 * pi)
else:
# Right ascension of ascending node is the angle
# between the node vector and its x component.
raan = acos(n.x / norm(n))
if n.y < 0:
raan = 2 * pi - raan
raan = mod(-raan, 2 * pi)

# Argument of periapsis is angle between
# node and eccentricity vectors.
Expand All @@ -301,23 +303,23 @@ def elements_from_state_vector(r, v, mu):
# vector and its x component.
f = acos(r.x / norm(r))
if v.x > 0:
f = 2 * pi - f
f = mod(-f, 2 * pi)
else:
# True anomaly is angle between node
# vector and position vector.
f = acos(dot(n, r) / (norm(n) * norm(r)))
if dot(n, v) > 0:
f = 2 * pi - f
f = mod(-f, 2 * pi)
else:
if ev.z < 0:
arg_pe = 2 * pi - arg_pe
arg_pe = mod(-arg_pe, 2 * pi)

# True anomaly is angle between eccentricity
# vector and position vector.
f = acos(dot(ev, r) / (norm(ev) * norm(r)))

if dot(r, v) < 0:
f = 2 * pi - f
f = mod(-f, 2 * pi)

return OrbitalElements(a=a, e=e, i=i, raan=raan, arg_pe=arg_pe, f=f)

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25 changes: 8 additions & 17 deletions tests/test_orbital.py
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Original file line number Diff line number Diff line change
Expand Up @@ -761,15 +761,13 @@ def test_from_state_vector_elliptical_arg_pe_gt_180(self):
V = Velocity(16703.9010129, 0, 0)

orbit = KeplerianElements.from_state_vector(R, V, body=earth)
# XXX These do not match (they are 180° out, due to arg_pe).
# numpy.testing.assert_almost_equal(orbit.r, R)
# numpy.testing.assert_almost_equal(orbit.v, V)
numpy.testing.assert_almost_equal(orbit.r, R)
numpy.testing.assert_almost_equal(orbit.v, V)
self.assertAlmostEqual(orbit.a, 10000000.0, places=3)
self.assertAlmostEqual(orbit.e, 0.75)
self.assertAlmostEqual(orbit.i, 0.0)
self.assertAlmostEqual(orbit.raan, 0.0)
# XXX This is incorrectly calculated as 90°.
# self.assertAlmostEqual(orbit.arg_pe, radians(270.0))
self.assertAlmostEqual(orbit.arg_pe, radians(270.0))
self.assertAlmostEqual(orbit.M0, 0.0)

self.assertAlmostEqual(orbit.ref_epoch, J2000)
Expand Down Expand Up @@ -846,7 +844,7 @@ def test_from_state_vector_roundtrips(self):
# Elliptical flat (e=0.75), prograde.
(Position(2500000, 0, 0), Velocity(0, 16703.901013, 0)),
# Elliptical flat (e=0.75), arg_pe > 180°.
# (Position(0, -2500000, 0), Velocity(16703.901013, 0, 0)),
(Position(0, -2500000, 0), Velocity(16703.901013, 0, 0)),
# Elliptical flat (e=0.75), retrograde.
# (Position(2500000, 0, 0), Velocity(0, -16703.901013, 0)),
# Elliptical flat (e=0.75), f > 180°.
Expand Down Expand Up @@ -1077,19 +1075,12 @@ def test_set_v_arg_pe_gt_180(self):
# arg_pe = 270°.
V = Velocity(-10000, 0, 0)

def set_v(value):
orbit.v = value

# XXX The 'r and v changed' detection logic is triggered in this case,
# causing a RuntimeError to be raised. If this was not raised, the
# following asserts would be wildly off.
self.assertRaises(RuntimeError, set_v, V)
# numpy.testing.assert_almost_equal(orbit.r, R)
# numpy.testing.assert_almost_equal(orbit.v, V)
orbit.v = V
numpy.testing.assert_almost_equal(orbit.r, R)
numpy.testing.assert_almost_equal(orbit.v, V)
# arg_pe should have rotated around 180°, and M0 to match (so r is in
# the same spot as it was before).
# XXX This is incorrectly calculated as 90°.
# self.assertAlmostEqual(orbit.arg_pe, radians(270.0))
self.assertAlmostEqual(orbit.arg_pe, radians(270.0))
self.assertAlmostEqual(orbit.M0, radians(180.0))

def test_set_n(self):
Expand Down
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