Fix documentation on coordinate systems and transformations

doc/architecture/reference_points_coordinate_systems.adoc

@@ -5,7 +5,7 @@ endif::[]
 = Coordinate systems, reference points and coordinate transformation
 
 OSI uses DIN ISO 8855:2013-11 cite:[iso8855] for coordinate systems and transformations between coordinate systems.
-OSI uses three coordinate systems:
+OSI uses the following coordinate systems:
 
 Global coordinate system::
 Coordinate system for all entities that are part of ground truth.
@@ -14,9 +14,13 @@ The origin is the global reference point that is determined by the environment s
 This reference point may be derived from map data or other considerations.
 Global coordinates can be mapped to a geographic coordinate system via `osi3::GroundTruth::proj_string`.
 
+Host vehicle coordinate system::
+The host vehicle coordinate system's origin is defined to be at the center of the rear axle of the host vehicle.
+
 Sensor coordinate system::
 Coordinate system for all entities that are part of sensor data.
 The origin is the mounting position of the physical sensor or a virtual mounting position, depending on the OSI message.
+Both physical and virtual sensor coordinate systems are relative to the host vehicle coordinate system.
 
 Object coordinate system::
 Local object coordinate system.