[bug] use clip instead of abs in singleaxis

docs/sphinx/source/whatsnew/v0.9.0.rst

@@ -189,6 +189,8 @@ Bug fixes
   (:issue:`1241`, :pull:`1242`)
 * Changed deprecated use of ``.astype()`` to ``.view()`` in :py:mod:`~pvlib.solarposition`.
   (:pull:`1256`, :issue:`1261`, :pull:`1262`)
+* Fix :py:func:`~pvlib.tracking.singleaxis` AOI wrong when sun behind module.
+  (:pull:`1273`, :issue:`1221`) 
 
 Testing
 ~~~~~~~

pvlib/tests/test_tracking.py

@@ -502,3 +502,18 @@ def test_slope_aware_backtracking():
     np.testing.assert_allclose(
         truetracking['tracker_theta'], expected_data['TrueTracking'],
         rtol=1e-3, atol=1e-3)
+
+
+def test_singleaxis_aoi_gh1221():
+    # vertical tracker
+    loc = pvlib.location.Location(40.1134, -88.3695)
+    dr = pd.date_range(
+        start='02-Jun-1998 00:00:00', end='02-Jun-1998 23:55:00', freq='5T',
+        tz='Etc/GMT+6')
+    sp = loc.get_solarposition(dr)
+    tr = pvlib.tracking.singleaxis(
+        sp['apparent_zenith'], sp['azimuth'], axis_tilt=90, axis_azimuth=180,
+        max_angle=0.001, backtrack=False)
+    fixed = pvlib.irradiance.aoi(90, 180, sp['apparent_zenith'], sp['azimuth'])
+    fixed[np.isnan(tr['aoi'])] = np.nan
+    assert np.allclose(tr['aoi'], fixed, equal_nan=True)

pvlib/tracking.py

@@ -458,7 +458,9 @@ def singleaxis(apparent_zenith, apparent_azimuth,
     sun_vec = np.array([xp, yp, zp])
 
     # calculate angle-of-incidence on panel
-    aoi = np.degrees(np.arccos(np.abs(np.sum(sun_vec*panel_norm, axis=0))))
+    # TODO: use irradiance.aoi
+    projection = np.clip(np.sum(sun_vec*panel_norm, axis=0), -1, 1)
+    aoi = np.degrees(np.arccos(projection))
 
     # Calculate panel tilt and azimuth in a coordinate system where the panel
     # tilt is the angle from horizontal, and the panel azimuth is the compass