diff --git a/README.rst b/README.rst
index 2db2e20..194b0f6 100644
--- a/README.rst
+++ b/README.rst
@@ -60,7 +60,7 @@ Core functions for ADS-B decoding:
 
     pms.adsb.icao(msg)
     pms.adsb.callsign(msg)
-    pms.adsb.position(msg_odd, msg_even, t_odd, t_even)
+    pms.adsb.position(msg_even, msg_odd, t_even, t_odd)
     pms.adsb.altitude(msg)
     pms.adsb.velocity(msg)
     pms.adsb.speed_heading(msg)
diff --git a/pyModeS/adsb.py b/pyModeS/adsb.py
index 34b6ede..90a1d0b 100644
--- a/pyModeS/adsb.py
+++ b/pyModeS/adsb.py
@@ -222,11 +222,99 @@ def airborne_position(msg0, msg1, t0, t1):
     return round(lat, 5), round(lon, 5)
 
 
+def position_with_ref(msg, lat_ref, lon_ref):
+    if 5 <= typecode(msg) <= 8:
+        return airborne_position_with_ref(msg, lat_ref, lon_ref)
+
+    elif 9 <= typecode(msg) <= 18:
+        return surface_position_with_ref(msg, lat_ref, lon_ref)
+
+    else:
+        raise RuntimeError("incorrect or inconsistant message types")
+
+
+def airborne_position_with_ref(msg, lat_ref, lon_ref):
+    """Decode airborn position with one message,
+    knowing previous reference location
+    Args:
+        msg (string): even message (28 bytes hexadecimal string)
+        lat_ref: previous known latitude
+        lon_ref: previous known longitude
+    Returns:
+        (float, float): (latitude, longitude) of the aircraft
+    """
+
+    i = oe_flag(msg)
+    d_lat = 360.0/59 if i else 360.0/60
+
+    msgbin = util.hex2bin(msg)
+    cprlat = util.bin2int(msgbin[54:71]) / 131072.0
+    cprlon = util.bin2int(msgbin[71:88]) / 131072.0
+
+    j = util.floor(lat_ref / d_lat) \
+        + util.floor(0.5 + ((lat_ref % d_lat) / d_lat) - cprlat)
+
+    lat = d_lat * (j + cprlat)
+
+    ni = _cprNL(lat) - i
+
+    if ni > 0:
+        d_lon = 360.0 / ni
+    else:
+        d_lon = 360.0
+
+    m = util.floor(lon_ref / d_lon) \
+        + util.floor(0.5 + ((lon_ref % d_lon) / d_lon) - cprlon)
+
+    lon = d_lon * (m + cprlon)
+
+    return round(lat, 5), round(lon, 5)
+
+
 def surface_position(msg0, msg1, t0, t1):
-    # TODO: implement surface positon decoding
+    # TODO: implement surface positon
     raise RuntimeError('suface position decoding to be implemented soon...')
 
 
+def surface_position_with_ref(msg, lat_ref, lon_ref):
+    """Decode surface position with one message,
+    knowing reference nearby location, such as previously calculated location,
+    ground station, or airport location, etc.
+    Args:
+        msg (string): even message (28 bytes hexadecimal string)
+        lat_ref: previous known latitude
+        lon_ref: previous known longitude
+    Returns:
+        (float, float): (latitude, longitude) of the aircraft
+    """
+
+    i = oe_flag(msg)
+    d_lat = 90.0/59 if i else 90.0/60
+
+    msgbin = util.hex2bin(msg)
+    cprlat = util.bin2int(msgbin[54:71]) / 131072.0
+    cprlon = util.bin2int(msgbin[71:88]) / 131072.0
+
+    j = util.floor(lat_ref / d_lat) \
+        + util.floor(0.5 + ((lat_ref % d_lat) / d_lat) - cprlat)
+
+    lat = d_lat * (j + cprlat)
+
+    ni = _cprNL(lat) - i
+
+    if ni > 0:
+        d_lon = 360.0 / ni
+    else:
+        d_lon = 360.0
+
+    m = util.floor(lon_ref / d_lon) \
+        + util.floor(0.5 + ((lon_ref % d_lon) / d_lon) - cprlon)
+
+    lon = d_lon * (m + cprlon)
+
+    return round(lat, 5), round(lon, 5)
+
+
 def _cprNL(lat):
     if lat == 0:
         return 59
@@ -356,8 +444,8 @@ def velocity(msg):
         tag = 'AS'
 
     vr_sign = util.bin2int(msgbin[68])
-    vr = util.bin2int(msgbin[68:77])             # vertical rate
-    rocd = -1*vr if vr_sign else vr         # rate of climb/descend
+    vr = util.bin2int(msgbin[69:78])            # vertical rate
+    rocd = -1*vr if vr_sign else vr             # rate of climb/descend
 
     return int(spd), round(hdg, 1), int(rocd), tag
 
diff --git a/tests/run.py b/tests/run.py
index 60fa629..f8db80c 100644
--- a/tests/run.py
+++ b/tests/run.py
@@ -39,13 +39,28 @@ def test_adsb_callsign():
     assert adsb.callsign("8D406B902015A678D4D220AA4BDA") == "EZY85MH_"
 
 
-def test_adsb_position():
-    pos = adsb.position("8D40058B58C901375147EFD09357",
-                        "8D40058B58C904A87F402D3B8C59",
-                        1446332400, 1446332405)
+def test_adsb_airborne_position():
+    pos = adsb.airborne_position("8D40058B58C901375147EFD09357",
+                                 "8D40058B58C904A87F402D3B8C59",
+                                 1446332400, 1446332405)
     assert pos == (49.81755, 6.08442)
 
 
+def test_adsb_airborne_position_with_ref():
+    pos = adsb.airborne_position_with_ref("8D40058B58C901375147EFD09357",
+                                          49.0, 6.0)
+    assert pos == (49.82410, 6.06785)
+    pos = adsb.airborne_position_with_ref("8D40058B58C904A87F402D3B8C59",
+                                          49.0, 6.0)
+    assert pos == (49.81755, 6.08442)
+
+
+def test_adsb_surface_position_with_ref():
+    pos = adsb.surface_position_with_ref("8FC8200A3AB8F5F893096B22B4A8",
+                                         -43.5, 172.5)
+    assert pos == (-43.48564, 175.87195)
+
+
 def test_adsb_alt():
     assert adsb.altitude("8D40058B58C901375147EFD09357") == 39000
 
@@ -53,8 +68,8 @@ def test_adsb_alt():
 def test_adsb_velocity():
     vgs = adsb.velocity("8D485020994409940838175B284F")
     vas = adsb.velocity("8DA05F219B06B6AF189400CBC33F")
-    assert vgs == (159, 182.9, -263, 'GS')
-    assert vas == (376, 244.0, -274, 'AS')
+    assert vgs == (159, 182.9, -14, 'GS')
+    assert vas == (376, 244.0, -37, 'AS')
 
 
 def test_nic():
@@ -183,5 +198,9 @@ def ehs_decode_all(n=None):
             print ts, m, icao, vBDS
 
 if __name__ == '__main__':
-    adsb_decode_all(100)
-    ehs_decode_all(100)
+    # adsb_decode_all(100)
+    # ehs_decode_all(100)
+
+
+    adsb.airborne_position_with_ref("8D40058B58C901375147EFD09357",
+                                          49.0, 6.0)
\ No newline at end of file