Merge pull request #7548 from meeseeksmachine/auto-backport-of-pr-754…

…5-on-5.1

Backport PR #7545 on branch 5.1 (Fixed the GEI->HME transformation to preserve the length unit of the coordinate)

changelog/7545.bugfix.rst

@@ -0,0 +1 @@
+Fixed a bug with any coordinate transformation starting in `~sunpy.coordinates.GeocentricEarthEquatorial` (GEI) returning output with AU as the length unit, rather than preserving the length unit of the initial coordinate.

sunpy/coordinates/_transformations.py

@@ -1072,7 +1072,7 @@ def gei_to_hme(geicoord, hmeframe):
     earth_object_int = geicoord.cartesian.transform(rot_matrix)
 
     # Find the Sun-object vector in the intermediate frame
-    sun_object_int = sun_earth_int + earth_object_int
+    sun_object_int = earth_object_int + sun_earth_int  # add in this order to preserve the original units
     int_coord = int_frame.realize_frame(sun_object_int)
 
     # Convert to the final frame through HCRS

sunpy/coordinates/tests/test_transformations.py

@@ -1055,6 +1055,36 @@ def test_rsun_preservation():
             assert_quantity_allclose(out_coord.rsun, args_out['rsun'])
 
 
+_framepairs = [
+    ('hcrs', 'heliographic_stonyhurst'),
+    ('heliographic_stonyhurst', 'heliographic_carrington'),
+    ('heliographic_stonyhurst', 'heliocentricinertial'),
+    ('heliographic_stonyhurst', 'heliocentric'),
+    ('heliocentric', 'helioprojective'),
+    ('heliocentricmeanecliptic', 'heliocentricearthecliptic'),
+    ('heliocentricearthecliptic', 'geocentricsolarecliptic'),
+    ('heliocentricmeanecliptic', 'geocentricearthequatorial'),
+    ('itrs', 'geomagnetic'),
+    ('geomagnetic', 'solarmagnetic'),
+    ('solarmagnetic', 'geocentricsolarmagnetospheric'),
+]
+
+
+@pytest.mark.parametrize(("frame1", "frame2"), _framepairs)
+@pytest.mark.parametrize("unit", [u.m, u.AU])
+def test_unit_preservation(frame1, frame2, unit):
+    coord = SkyCoord(CartesianRepresentation(0, 0, 0) * unit,
+                     frame=frame1, obstime="2001-01-01", observer="earth")
+
+    # Transform one direction and verify the unit is preserved
+    result1 = coord.transform_to(frame2)
+    assert result1.cartesian.xyz.unit == unit
+
+    # Transform back and verify the unit is preserved
+    result2 = result1.transform_to(frame1)
+    assert result2.cartesian.xyz.unit == unit
+
+
 def test_propagate_with_solar_surface():
     # Test propagating the meridian by 6 days of solar rotation
     meridian = SkyCoord(0*u.deg, np.arange(0, 90, 10)*u.deg, 1*u.AU,