code: public Position decodePosition(double time, SurfacePositionV0Msg msg) {
  		if (last_pos == null)
  			return null;
  
  		return decodePosition(time, msg, last_pos);
  	}
docs: Shortcut for using the last known position for reference; no reasonableness check on distance to receiver
  @param time time of applicability/reception of position report (seconds)
  @param msg surface position message
  @return WGS84 coordinates with latitude and longitude in dec degrees, and altitude in meters. altitude might be null if unavailable
  On error, the returned position is null. Check the .isReasonable() flag before using the position.