diff --git a/pyModeS/decoder/bds/__init__.py b/pyModeS/decoder/bds/__init__.py
index f3ed2d2..7ace4c2 100644
--- a/pyModeS/decoder/bds/__init__.py
+++ b/pyModeS/decoder/bds/__init__.py
@@ -60,25 +60,34 @@ def is50or60(msg, spd_ref, trk_ref, alt_ref):
         vy = v * np.cos(np.radians(angle))
         return vx, vy
 
+    # message must be both BDS 50 and 60 before processing
     if not (bds50.is50(msg) and bds60.is60(msg)):
         return None
 
-    h50 = bds50.trk50(msg)
-    v50 = bds50.gs50(msg)
-
-    if h50 is None or v50 is None:
-        return "BDS50,BDS60"
-
+    # --- assuming BDS60 ---
     h60 = bds60.hdg60(msg)
     m60 = bds60.mach60(msg)
     i60 = bds60.ias60(msg)
 
+    # additional check now knowing the altitude
+    if (m60 is not None) and (i60 is not None):
+        ias_ = aero.mach2cas(m60, alt_ref * aero.ft) / aero.kts
+        if abs(i60 - ias_) > 20:
+            return "BDS50"
+
     if h60 is None or (m60 is None and i60 is None):
         return "BDS50,BDS60"
 
     m60 = np.nan if m60 is None else m60
     i60 = np.nan if i60 is None else i60
 
+    # --- assuming BDS50 ---
+    h50 = bds50.trk50(msg)
+    v50 = bds50.gs50(msg)
+
+    if h50 is None or v50 is None:
+        return "BDS50,BDS60"
+
     XY5 = vxy(v50 * aero.kts, h50)
     XY6m = vxy(aero.mach2tas(m60, alt_ref * aero.ft), h60)
     XY6i = vxy(aero.cas2tas(i60 * aero.kts, alt_ref * aero.ft), h60)
@@ -98,6 +107,7 @@ def is50or60(msg, spd_ref, trk_ref, alt_ref):
     try:
         dist = np.linalg.norm(X - Mu, axis=1)
         BDS = allbds[np.nanargmin(dist)]
+        print(dist, BDS)
     except ValueError:
         return "BDS50,BDS60"
 
diff --git a/pyModeS/decoder/bds/bds50.py b/pyModeS/decoder/bds/bds50.py
index a2e534a..e20bd38 100644
--- a/pyModeS/decoder/bds/bds50.py
+++ b/pyModeS/decoder/bds/bds50.py
@@ -40,7 +40,7 @@ def is50(msg):
         return False
 
     roll = roll50(msg)
-    if (roll is not None) and abs(roll) > 60:
+    if (roll is not None) and abs(roll) > 50:
         return False
 
     gs = gs50(msg)
diff --git a/pyModeS/decoder/bds/bds60.py b/pyModeS/decoder/bds/bds60.py
index b8d91cb..e5749cd 100644
--- a/pyModeS/decoder/bds/bds60.py
+++ b/pyModeS/decoder/bds/bds60.py
@@ -4,6 +4,7 @@
 # ------------------------------------------
 
 from pyModeS import common
+from pyModeS.extra import aero
 
 
 def is60(msg):
@@ -54,6 +55,13 @@ def is60(msg):
     if vr_ins is not None and abs(vr_ins) > 6000:
         return False
 
+    # additional check knowing altitude
+    if (mach is not None) and (ias is not None) and (common.df(msg) == 20):
+        alt = common.altcode(msg)
+        ias_ = aero.mach2cas(mach, alt * aero.ft) / aero.kts
+        if abs(ias - ias_) > 20:
+            return False
+
     return True
 
 
diff --git a/tests/sample_run_commb.py b/tests/sample_run_commb.py
index 369f4e7..1162875 100644
--- a/tests/sample_run_commb.py
+++ b/tests/sample_run_commb.py
@@ -46,7 +46,7 @@ def bds_info(BDS, m):
         )
 
     else:
-        info = None
+        info = []
 
     return info
 
@@ -87,5 +87,5 @@ def commb_decode_all(df, n=None):
 
 
 if __name__ == "__main__":
-    commb_decode_all(df=20, n=100)
-    commb_decode_all(df=21, n=100)
+    commb_decode_all(df=20, n=500)
+    commb_decode_all(df=21, n=500)
diff --git a/tests/test_bds_inference.py b/tests/test_bds_inference.py
index 5ef67f0..c5c849f 100644
--- a/tests/test_bds_inference.py
+++ b/tests/test_bds_inference.py
@@ -17,8 +17,8 @@ def test_bds_infer():
 
 def test_bds_is50or60():
     assert bds.is50or60("A0001838201584F23468207CDFA5", 0, 0, 0) == None
-    assert bds.is50or60("A0000000FFDA9517000464000000", 182, 237, 1250) == "BDS50"
-    assert bds.is50or60("A0000000919A5927E23444000000", 413, 54, 18700) == "BDS60"
+    assert bds.is50or60("A8001EBCFFFB23286004A73F6A5B", 320, 250, 14000) == "BDS50"
+    assert bds.is50or60("A8001EBCFE1B29287FDCA807BCFC", 320, 250, 14000) == "BDS50"
 
 
 def test_surface_position():