diff --git a/pyModeS/decoder/adsb.py b/pyModeS/decoder/adsb.py
index b9821b2..bd8daa4 100644
--- a/pyModeS/decoder/adsb.py
+++ b/pyModeS/decoder/adsb.py
@@ -159,9 +159,10 @@ def velocity(msg, rtn_sources=False):
             ground track or heading (degree),
             rate of climb/descent (ft/min), speed type
             ('GS' for ground speed, 'AS' for airspeed),
-            direction source ('true_north' for ground track, 'mag_north' for
-            magnetic heading), rate of climb/descent source ('Baro' for
-            barometer, 'GNSS' for GNSS constellation).
+            direction source ('true_north' for ground track / true north
+            as refrence, 'mag_north' for magnetic north as reference),
+            rate of climb/descent source ('Baro' for barometer, 'GNSS'
+            for GNSS constellation).
             
             In the case of surface messages, None will be put in place
             for vertical rate and its respective sources.
diff --git a/pyModeS/decoder/bds/bds06.py b/pyModeS/decoder/bds/bds06.py
index 69cff54..2c6c346 100644
--- a/pyModeS/decoder/bds/bds06.py
+++ b/pyModeS/decoder/bds/bds06.py
@@ -154,7 +154,8 @@ def surface_velocity(msg, rtn_sources=False):
         (int, float, int, string, string, None): speed (kt),
             ground track (degree), None for rate of climb/descend (ft/min),
             and speed type ('GS' for ground speed), direction source
-            ('true_north' for ground track), None rate of climb/descent source.
+            ('true_north' for ground track / true north as reference),
+            None rate of climb/descent source.
     """
 
     if common.typecode(msg) < 5 or common.typecode(msg) > 8:
diff --git a/pyModeS/decoder/bds/bds09.py b/pyModeS/decoder/bds/bds09.py
index 8f5e386..3ccdae6 100644
--- a/pyModeS/decoder/bds/bds09.py
+++ b/pyModeS/decoder/bds/bds09.py
@@ -40,9 +40,10 @@ def airborne_velocity(msg, rtn_sources=False):
             ground track or heading (degree),
             rate of climb/descent (ft/min), speed type
             ('GS' for ground speed, 'AS' for airspeed),
-            direction source ('true_north' for ground track, 'mag_north' for
-            magnetic heading), rate of climb/descent source ('Baro' for
-            barometer, 'GNSS' for GNSS constellation)
+            direction source ('true_north' for ground track / true north
+            as refrence, 'mag_north' for magnetic north as reference),
+            rate of climb/descent source ('Baro' for barometer, 'GNSS'
+            for GNSS constellation).
     """
 
     if common.typecode(msg) != 19: