diff --git a/pyModeS/decoder/adsb.py b/pyModeS/decoder/adsb.py
index 9253ee5..b9821b2 100644
--- a/pyModeS/decoder/adsb.py
+++ b/pyModeS/decoder/adsb.py
@@ -159,7 +159,7 @@ 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 ('gnd_trk' for ground track, 'mag_hdg' for
+            direction source ('true_north' for ground track, 'mag_north' for
             magnetic heading), rate of climb/descent source ('Baro' for
             barometer, 'GNSS' for GNSS constellation).
             
diff --git a/pyModeS/decoder/bds/bds06.py b/pyModeS/decoder/bds/bds06.py
index d387696..69cff54 100644
--- a/pyModeS/decoder/bds/bds06.py
+++ b/pyModeS/decoder/bds/bds06.py
@@ -154,7 +154,7 @@ 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
-            ('gnd_trk' for ground track), None rate of climb/descent source.
+            ('true_north' for ground track), None rate of climb/descent source.
     """
 
     if common.typecode(msg) < 5 or common.typecode(msg) > 8:
@@ -188,6 +188,6 @@ def surface_velocity(msg, rtn_sources=False):
         spd = round(spd, 2)
 
     if rtn_sources:
-        return spd, trk, 0, 'GS', 'gnd_trk', None
+        return spd, trk, 0, 'GS', 'true_north', None
     else:
         return spd, trk, 0, 'GS'
diff --git a/pyModeS/decoder/bds/bds09.py b/pyModeS/decoder/bds/bds09.py
index 50ec8e2..8f5e386 100644
--- a/pyModeS/decoder/bds/bds09.py
+++ b/pyModeS/decoder/bds/bds09.py
@@ -40,7 +40,7 @@ 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 ('gnd_trk' for ground track, 'mag_hdg' for
+            direction source ('true_north' for ground track, 'mag_north' for
             magnetic heading), rate of climb/descent source ('Baro' for
             barometer, 'GNSS' for GNSS constellation)
     """
@@ -74,7 +74,7 @@ def airborne_velocity(msg, rtn_sources=False):
 
         tag = 'GS'
         trk_or_hdg = round(trk, 2)
-        dir_type = 'gnd_trk'
+        dir_type = 'true_north'
 
     else:
         if mb[13] == '0':
@@ -93,7 +93,7 @@ def airborne_velocity(msg, rtn_sources=False):
         else:
             tag = 'TAS'
         
-        dir_type = 'mag_hdg'
+        dir_type = 'mag_north'
 
     vr_source = 'GNSS' if mb[35]=='0' else 'Baro'
     vr_sign = -1 if mb[36]=='1' else 1