diff --git a/pyModeS/adsb.py b/pyModeS/adsb.py
index 84fd820..d5fa103 100644
--- a/pyModeS/adsb.py
+++ b/pyModeS/adsb.py
@@ -427,9 +427,9 @@ def surface_position_with_ref(msg, lat_ref, lon_ref):
     ni = _cprNL(lat) - i
 
     if ni > 0:
-        d_lon = 360.0 / ni
+        d_lon = 90.0 / ni
     else:
-        d_lon = 360.0
+        d_lon = 90.0
 
     m = util.floor(lon_ref / d_lon) \
         + util.floor(0.5 + ((lon_ref % d_lon) / d_lon) - cprlon)
diff --git a/tests/test_adsb.py b/tests/test_adsb.py
index 9430644..d64b4f1 100644
--- a/tests/test_adsb.py
+++ b/tests/test_adsb.py
@@ -32,9 +32,9 @@ def test_adsb_airborne_position_with_ref():
 
 
 def test_adsb_surface_position_with_ref():
-    pos = adsb.surface_position_with_ref("8FC8200A3AB8F5F893096B22B4A8",
+    pos = adsb.surface_position_with_ref("8FC8200A3AB8F5F893096B000000",
                                          -43.5, 172.5)
-    assert pos == (-43.48564, 175.87195)
+    assert pos == (-43.48564, 172.53942)
 
 
 def test_adsb_surface_position():