msg: Improve vehicle_local_position documentation

Clean up formatting, improve consistency of descriptions and ensure units are defined

msg/vehicle_local_position.msg

@@ -1,36 +1,36 @@
 # Fused local position in NED.
 
-uint64 timestamp		# Time of this estimate, in microseconds since system start
-bool xy_valid			# true if x and y are valid
-bool z_valid			# true if z is valid
-bool v_xy_valid		# true if vy and vy are valid
-bool v_z_valid			# true if vz is valid
+uint64 timestamp			# Time of this estimate since system start, (microseconds)
+bool xy_valid				# true if x and y are valid
+bool z_valid				# true if z is valid
+bool v_xy_valid				# true if vy and vy are valid
+bool v_z_valid				# true if vz is valid
 
 # Position in local NED frame
-float32 x				# X position in meters in NED earth-fixed frame
-float32 y				# X position in meters in NED earth-fixed frame
-float32 z				# Z position in meters in NED earth-fixed frame (negative altitude)
+float32 x				# North position in NED earth-fixed frame, (metres)
+float32 y				# East position in NED earth-fixed frame, (metres)
+float32 z				# Down position (negative altitude) in NED earth-fixed frame, (metres)
 
 # Velocity in NED frame
-float32 vx 				# Ground X Speed (Latitude), m/s in NED
-float32 vy				# Ground Y Speed (Longitude), m/s in NED
-float32 vz				# Ground Z Speed (Altitude), m/s	in NED
+float32 vx 				# North velocity in NED earth-fixed frame, (metres/sec)
+float32 vy				# East velocity in NED earth-fixed frame, (metres/sec)
+float32 vz				# Down velocity in NED earth-fixed frame, (metres/sec)
 
 # Heading
-float32 yaw
+float32 yaw				# Euler yaw angle relative to NED earth-fixed frame, -PI..+PI,  (radians) 
 
-# Reference position in GPS / WGS84 frame
-bool xy_global			# true if position (x, y) is valid and has valid global reference (ref_lat, ref_lon)
-bool z_global			# true if z is valid and has valid global reference (ref_alt)
-uint64 ref_timestamp	# Time when reference position was set
-float64 ref_lat		# Reference point latitude in degrees
-float64 ref_lon		# Reference point longitude in degrees
-float32 ref_alt			# Reference altitude AMSL in meters, MUST be set to current (not at reference point!) ground level
+# Position of reference point (local NED frame origin) in global (GPS / WGS84) frame
+bool xy_global				# true if position (x, y) is valid and has valid global reference (ref_lat, ref_lon)
+bool z_global				# true if z is valid and has valid global reference (ref_alt)
+uint64 ref_timestamp			# Time when reference position was set since system start, (microseconds)
+float64 ref_lat				# Reference point latitude, (degrees)
+float64 ref_lon				# Reference point longitude, (degrees)
+float32 ref_alt				# Reference altitude AMSL, MUST be set to current (not at reference point!) ground level, (metres)
 
 # Distance to surface
-float32 dist_bottom		# Distance to bottom surface (ground)
-float32 dist_bottom_rate		# Distance to bottom surface (ground) change rate
-uint64 surface_bottom_timestamp		# Time when new bottom surface found
-bool dist_bottom_valid	# true if distance to bottom surface is valid
-float32 eph
-float32 epv
+float32 dist_bottom			# Distance from from bottom surface to ground, (metres)
+float32 dist_bottom_rate		# Rate of change of distance from bottom surface to ground, (metres/sec)
+uint64 surface_bottom_timestamp		# Time when new bottom surface found since system start, (microseconds)
+bool dist_bottom_valid			# true if distance to bottom surface is valid
+float32 eph				# Standard deviation of horizontal position error, (metres)
+float32 epv				# Standard deviation of vertical position error, (metres)