Data Structure:
(for the interface between OCU and urbie.
 not intended to represent the internal data structure 
 on urbie.)

  Time:   5-Byte, accurate up to 1/256 of a second (4msec)
  
  Robot State:
        Position (7 float), Orientation (6 float), velocity (6 float),
        articulation (1 float)

        More specifically,
        (pos_x, pos_y, pos_z, s_xx, s_yy, s_zz, s_xy,
         roll, pitch, yaw, s_roll, s_pitch, s_yaw,
         v_x, v_y, v_z, v_roll, v_pitch, v_yaw,
         articulation_angle)
        where s_?? is the covariance, v_?? is the velocity.

  Coordinate frame:
        
        Robot coordinate frame:

                         Front                 
                   
                    -----------
                    |    ^ Y  |
                    |    |    |
                    |    |    |
                    |    |--------->  X  
                    |         |
                    |         |
                    |         |
                    -----------

                         Back

                Z axis is up.

       Global coordinate frame:

               Y: North
               X: East
               Z: Up

       Rotation convention:
               rotation around Z axis: yaw
               "               X  "  : pitch
               "               Y  "  : roll

               The order of the rotation is
                  R(yaw) * R(pitch) * R(roll), assuming column vectors.
               (note that this roll-pitch-yaw representation of the 
rotation
                is suited for the users. Internally we may
                use quarternions instead for numerical stability).

   Image: (note that the image here is compressed before 
           being sent to OCU through Freewave)
        which_camera (int),
        window (4 int, in case only a rect portion of the image get 
transmitted.),
        resampling rate (int, incase the image is downsampled before 
xmittion.),
        time_stamping (5-byte time type),
        data_size (int, the actual size of the JPEG image in byte),
        data (variable length).

Periodic Tasks: 

     Robot State: heartbeat data xmitted though unreliable comm channel at
          ~ 10Hz.

Commands:

     SetTime ():
          parameters: time_type
          return:   time_type
          function: sync the OCU and robots in time.

     Teleop ();
          parameters: {speed, turn_speed, articulation angle).
          return: None, no acknowledgment.
          function: directly control the speed of the vehicle.
                    It is unclear though whether the speed are
                    relative or abosolute. And it is unclear where
                    the mapping from the three speed values to
                    actuators happens.


     TurnOnTansmitter ();
          parameter: which_cam (int)
          return: acknowledgement
          function: turn on the video transmitter, and feed video
                    from the sepcified the camera.

     TurnOffTrasmitter ();
          parameter: none
          return: acknowledgement
          function: turn off the video transmitter.

     InitFreewaveImage();
          parameters: which_cam (int), image_rate (int),
                         window (4 int), compression_rate (int),
                         resampling_factor (int).
          return: acknowledgement
          function: Initiate using the freewave to transmit
                    video.

     TerminateFreewaveImage();
          parameter: none
          return: acknowledgement
          function: terminate using freewave to transmitt video.