added "ecef2ned" function and tests

Author: aqreed

skaero/geometry/coordinates.py

@@ -93,6 +93,44 @@ def ned2ecef(v_ned, lat, lng):
     return v_ecef
 
 
+def ecef2ned(v_ecef, lat, lng):
+    """
+    Converts vector from geocentric coordinates (ECEF - Earth Centered,
+    Earth Fixed) at a given latitude and longitude to local geodetic horizon
+    reference frame (NED - North, East, Down).
+
+    Parameters
+    ----------
+    v_ecef: array-like
+        vector expressed in ECEF coordinates
+    lat : float
+        latitude in degrees
+    lng : float
+        longitude in degrees
+
+    Returns
+    -------
+    v_ned : array-like
+        vector expressed in NED coordinates
+    """
+    if abs(lat) > 90:
+        raise ValueError('latitude should be -90º <= latitude <= 90º')
+
+    if abs(lng) > 180:
+        raise ValueError('longitude should be -180º <= longitude <= 180º')
+
+    lat = deg2rad(lat)
+    lng = deg2rad(lng)
+
+    Lne = array([[-sin(lat) * cos(lng), -sin(lat) * sin(lng), cos(lat)],
+                 [-sin(lng), cos(lng), 0],
+                 [-cos(lat) * cos(lng), -cos(lat) * sin(lng), -sin(lat)]])
+
+    v_ned = Lne.dot(v_ecef)
+
+    return v_ned
+
+
 def body2ned(v_body, theta, phi, psi):
     """
     Converts vector from body reference frame to local geodetic horizon

tests/test_coordinates.py

@@ -162,6 +162,77 @@ def test_3(self):
                                     expected_value))
 
 
+class Test_ecef2ned(ut.TestCase):
+    """
+    Test function that transforms ecef-basis vectors to ned-basis
+    """
+    def test_latitude_wrong_input(self):
+        v_aux = array([1, 0, 0])
+        self.assertRaises(ValueError, ecef2ned, v_aux, 91.0, 0)
+        self.assertRaises(ValueError, ecef2ned, v_aux, -90.001, 0)
+        self.assertRaises(TypeError, ecef2ned, v_aux, 't', 0)
+
+    def test_longitude_wrong_input(self):
+        v_aux = array([1, 0, 0])
+        self.assertRaises(ValueError, ecef2ned, v_aux, 0, -190.1)
+        self.assertRaises(ValueError, ecef2ned, v_aux, 0, 180.1)
+        self.assertRaises(TypeError, ecef2ned, v_aux, 0, '0')
+
+    def test_1(self):
+        lat, lng = 0, 0
+
+        v_ecef = array([1, 0, 0])
+        expected_value = array([0, 0, -1])
+        self.assertTrue(np.allclose(ecef2ned(v_ecef, lat, lng),
+                                    expected_value))
+
+        v_ecef = array([0, 1, 0])
+        expected_value = array([0, 1, 0])
+        self.assertTrue(np.allclose(ecef2ned(v_ecef, lat, lng),
+                                    expected_value))
+
+        v_ecef = array([0, 0, 1])
+        expected_value = array([1, 0, 0])
+        self.assertTrue(np.allclose(ecef2ned(v_ecef, lat, lng),
+                                    expected_value))
+
+    def test_2(self):
+        lat, lng = 0, 90
+
+        v_ecef = array([1, 0, 0])
+        expected_value = array([0, -1, 0])
+        self.assertTrue(np.allclose(ecef2ned(v_ecef, lat, lng),
+                                    expected_value))
+
+        v_ecef = array([0, 1, 0])
+        expected_value = array([0, 0, -1])
+        self.assertTrue(np.allclose(ecef2ned(v_ecef, lat, lng),
+                                    expected_value))
+
+        v_ecef = array([0, 0, 1])
+        expected_value = array([1, 0, 0])
+        self.assertTrue(np.allclose(ecef2ned(v_ecef, lat, lng),
+                                    expected_value))
+
+    def test_3(self):
+        lat, lng = 90, 0
+
+        v_ecef = array([1, 0, 0])
+        expected_value = array([-1, 0, 0])
+        self.assertTrue(np.allclose(ecef2ned(v_ecef, lat, lng),
+                                    expected_value))
+
+        v_ecef = array([0, 1, 0])
+        expected_value = array([0, 1, 0])
+        self.assertTrue(np.allclose(ecef2ned(v_ecef, lat, lng),
+                                    expected_value))
+
+        v_ecef = array([0, 0, 1])
+        expected_value = array([0, 0, -1])
+        self.assertTrue(np.allclose(ecef2ned(v_ecef, lat, lng),
+                                    expected_value))
+
+
 class Test_body2ned(ut.TestCase):
     """
     Test function that transforms body-basis vectors to ned-basis