diff --git a/README.rst b/README.rst
index 4e88878..fa14a34 100644
--- a/README.rst
+++ b/README.rst
@@ -1,6 +1,42 @@
 The Python ADS-B/Mode-S Decoder
 ===============================
 
+PyModeS is a Python library designed to decode Mode-S (including ADS-B) message. It can be imported to your python project or used as a standalone tool to view and save live traffic data.
+
+This is a project created by Junzi Sun, who works at `TU Delft <https://www.tudelft.nl/en/>`_, `Aerospace Engineering Faculty <https://www.tudelft.nl/en/ae/>`_, `CNS/ATM research group <http://cs.lr.tudelft.nl/atm/>`_. It is supported by many `contributors <https://github.com/junzis/pyModeS/graphs/contributors>`_ from different institutions.
+
+Introduction
+------------
+
+pyModeS supports the decoding of following types of messages:
+
+- DF4 / DF20: Altitude code
+- DF5 / DF21: Identity code (squawk code)
+
+- DF17 / DF18: Automatic Dependent Surveillance-Broadcast (ADS-B)
+
+  - TC=1-4  / BDS 0,8: Aircraft identification and category
+  - TC=5-8  / BDS 0,6: Surface position
+  - TC=9-18 / BDS 0,5: Airborne position
+  - TC=19   / BDS 0,9: Airborne velocity
+  - TC=28   / BDS 6,1: Airborne status [to be implemented]
+  - TC=29   / BDS 6,2: Target state and status information [to be implemented]
+  - TC=31   / BDS 6,5: Aircraft operational status [to be implemented]
+
+- DF20 / DF21: Mode-S Comm-B messages
+
+  - BDS 1,0: Data link capability report
+  - BDS 1,7: Common usage GICB capability report
+  - BDS 2,0: Aircraft identification
+  - BDS 3,0: ACAS active resolution advisory
+  - BDS 4,0: Selected vertical intention
+  - BDS 4,4: Meteorological routine air report (experimental)
+  - BDS 4,5: Meteorological hazard report (experimental)
+  - BDS 5,0: Track and turn report
+  - BDS 6,0: Heading and speed report
+
+
+
 If you find this project useful for your research, please considering cite this tool as::
 
   @article{sun2019pymodes,
@@ -14,40 +50,6 @@ If you find this project useful for your research, please considering cite this
 
 
 
-Introduction
----------------------
-PyModeS is a Python library designed to decode Mode-S (including ADS-B) message. It can be imported to your python project or be used as a standalone tool to view and save live traffic data.
-
-Messages with following Downlink Formats (DF) are supported:
-
-**DF17 / DF18: Automatic Dependent Surveillance-Broadcast (ADS-B)**
-
-- TC=1-4  / BDS 0,8: Aircraft identification and category
-- TC=5-8  / BDS 0,6: Surface position
-- TC=9-18 / BDS 0,5: Airborne position
-- TC=19   / BDS 0,9: Airborne velocity
-- TC=28   / BDS 6,1: Airborne status [to be implemented]
-- TC=29   / BDS 6,2: Target state and status information [to be implemented]
-- TC=31   / BDS 6,5: Aircraft operational status [to be implemented]
-
-
-**DF20 / DF21: Mode-S Comm-B replies**
-
-- BDS 1,0: Data link capability report
-- BDS 1,7: Common usage GICB capability report
-- BDS 2,0: Aircraft identification
-- BDS 3,0: ACAS active resolution advisory
-- BDS 4,0: Selected vertical intention
-- BDS 4,4: Meteorological routine air report (experimental)
-- BDS 4,5: Meteorological hazard report (experimental)
-- BDS 5,0: Track and turn report
-- BDS 6,0: Heading and speed report
-
-
-**DF4 / DF20: Altitude code**
-
-**DF5 / DF21: Identity code (squawk code)**
-
 
 Resources
 -----------
diff --git a/pyModeS/decoder/adsb.py b/pyModeS/decoder/adsb.py
index ff27e26..e74f6c2 100644
--- a/pyModeS/decoder/adsb.py
+++ b/pyModeS/decoder/adsb.py
@@ -43,15 +43,19 @@ def typecode(msg):
 
 
 def position(msg0, msg1, t0, t1, lat_ref=None, lon_ref=None):
-    """Decode position from a pair of even and odd position message.
+    """Decode surface or airborne position from a pair of even and odd
+    position messages.
 
-    Works with both airborne and surface position messages.
+    Note, that to decode surface position using the position message pair,
+    the reference position has to be provided.
 
     Args:
         msg0 (string): even message (28 hexdigits)
         msg1 (string): odd message (28 hexdigits)
         t0 (int): timestamps for the even message
         t1 (int): timestamps for the odd message
+        lat_ref (float): latitude of reference position
+        lon_ref (float): longitude of reference position
 
     Returns:
         (float, float): (latitude, longitude) of the aircraft
@@ -61,11 +65,10 @@ def position(msg0, msg1, t0, t1, lat_ref=None, lon_ref=None):
     tc1 = typecode(msg1)
 
     if 5 <= tc0 <= 8 and 5 <= tc1 <= 8:
-        if (not lat_ref) or (not lon_ref):
+        if lat_ref is None or lon_ref is None:
             raise RuntimeError(
-                "Surface position encountered, a reference \
-                               position lat/lon required. Location of \
-                               receiver can be used."
+                "Surface position encountered, a reference position"
+                " lat/lon required. Location of receiver can be used."
             )
         else:
             return surface_position(msg0, msg1, t0, t1, lat_ref, lon_ref)
@@ -79,7 +82,7 @@ def position(msg0, msg1, t0, t1, lat_ref=None, lon_ref=None):
         return airborne_position(msg0, msg1, t0, t1)
 
     else:
-        raise RuntimeError("incorrect or inconsistent message types")
+        raise RuntimeError("Incorrect or inconsistent message types")
 
 
 def position_with_ref(msg, lat_ref, lon_ref):