diff --git a/bilby/gw/detector/networks.py b/bilby/gw/detector/networks.py
index 4efd3d8db..b56306b52 100644
--- a/bilby/gw/detector/networks.py
+++ b/bilby/gw/detector/networks.py
@@ -358,6 +358,8 @@ def __init__(
         if isinstance(maximum_frequency, float) or isinstance(maximum_frequency, int):
             maximum_frequency = [maximum_frequency] * 3
 
+        brng = 90 - xarm_azimuth
+
         for ii in range(3):
             self.append(
                 Interferometer(
@@ -376,29 +378,23 @@ def __init__(
                 )
             )
 
-            xarm_azimuth += 240
-            yarm_azimuth += 240
-
-            latitude += (
-                np.arctan(
-                    length
-                    * np.sin(xarm_azimuth * np.pi / 180)
-                    * 1e3
-                    / utils.radius_of_earth
-                )
-                * 180
-                / np.pi
+            phi1 = np.radians(latitude)
+            phi2 = np.arcsin(
+                np.sin(phi1) * np.cos(length * 1e3 / utils.radius_of_earth) +
+                np.cos(phi1) * np.sin(length * 1e3 / utils.radius_of_earth) * np.cos(np.radians(brng))
             )
-            longitude += (
-                np.arctan(
-                    length
-                    * np.cos(xarm_azimuth * np.pi / 180)
-                    * 1e3
-                    / utils.radius_of_earth
-                )
-                * 180
-                / np.pi
+            latitude = np.degrees(phi2)
+
+            lam1 = np.radians(longitude)
+            lam2 = lam1 + np.arctan2(
+                np.sin(np.radians(brng)) * np.sin(length * 1e3 / utils.radius_of_earth) * np.cos(phi1),
+                np.cos(length * 1e3 / utils.radius_of_earth) - np.sin(phi1) * np.sin(phi2)
             )
+            longitude = np.degrees(lam2)
+
+            brng += 240
+            xarm_azimuth += 240
+            yarm_azimuth += 240
 
 
 def get_empty_interferometer(name):
diff --git a/test/gw/detector/networks_test.py b/test/gw/detector/networks_test.py
index 6f7b211d3..942bd882f 100644
--- a/test/gw/detector/networks_test.py
+++ b/test/gw/detector/networks_test.py
@@ -387,7 +387,7 @@ def c(a):
         for pair in list(combinations(self.triangular_ifo, 2)):
             delta_lat = np.radians(pair[1].latitude - pair[0].latitude)
             delta_long = np.radians(pair[1].longitude - pair[0].longitude)
-            pair_a = a(delta_lat, delta_long, pair[0].latitude, pair[1].latitude)
+            pair_a = a(delta_lat, delta_long, np.radians(pair[0].latitude), np.radians(pair[1].latitude))
             pair_c = c(pair_a)
             distance = bilby.core.utils.radius_of_earth * pair_c
             self.assertAlmostEqual(distance / 1000, pair[0].length, delta=1)