Does ardupilot cache messages?

ArduCopter Copter 4.5
1

I am using pymavlink for communication with our drone.
Our goal is to get the position data of the drone (GLOBAL_POSITION_INT, LOCAL_POSITION_NED). We just focus on LOCAL_POSITION_NED in this example.

After receiving the position message(s) I have some time consuming code which blocks the loop that collects the messages (it would not be a problem if drone messages would get lost while the loop is blocked).

However from the output of a test code i assume that the drone messages are somehow cached on the drone.

The code:

  • Sets the message Interval for " LOCAL_POSITION_NED" to 100 microseconds
  • in a loop receives messages from the drone
  • if the message is LOCAL_POSITION_NED:
    • Calculates the “Code time delta” using the python timestamp of the last
      LOCAL_POSITION_NED received and the current timestamp
    • Calculates the “Drone time delta” using the time_boot_ms field of the last
      LOCAL_POSITION_NED received and the current LOCAL_POSITION_NED
    • Sleeps for 78ms
  • Prints the deltas

The output shows that after approx. 79.0ms of python time delta between two messages the drone time delta is just 3-5 ms what lets me assume that the drone is caching the messages.

Also after some time (the output is shortened) a peak is visible where the drone “skips” approx. 7 seconds of position data (maybe when the cache is full)?

Can someone verify if the drone is caching data or am I missing something?
If the drone is caching data is it possible to disable the cache? (like I said, i would be ok with “loosing” data)

Thank you in advance.

connection = mavutil.mavlink_connection("com4", dialect='all', mavlink20=1) 
connection.recv_match(type='HEARTBEAT', blocking=True, timeout=10)

set_message_interval_and_wait_for_ack(connection, 32, 1_00)

time_since_ned = time.time()
first_tsb = 0
last_tsb = 0
for i in range(0,10000):  
    msg_ned = connection.recv_match(blocking=False)
    if msg_ned is not None and msg_ned.id == 32:
         ned_recv_time = time.time()
         ned_delta_ms = round((ned_recv_time - time_since_ned)*1000, 0)
         tsb = msg_ned.time_boot_ms
         tsb_delta = tsb-last_tsb
         print("Delta", ned_delta_ms, tsb_delta, tsb, msg_ned.id)
         time_since_ned = ned_recv_time
         if last_tsb == 0:
              first_tsb = tsb
         last_tsb = tsb
         time.sleep(78/1000)

connection.close()

Output:
Colums (from left to right)

  • ned_delta_ms: time delta between current and previous message according to time() delta in code
  • tsb_delta: time delta between current and previous message according to drone message time_boot_ms field
  • time_boot_ms: value as sent by the drone
  • msg.id (32==local_position_ned)

Delta 79.0 3 5436610 32
Delta 79.0 5 5436615 32
Delta 79.0 2 5436617 32
Delta 79.0 3 5436620 32
Delta 79.0 2 5436622 32
Delta 80.0 3 5436625 32
Delta 79.0 5 5436630 32
Delta 79.0 7370 5444000 32
Delta 80.0 5 5444005 32
Delta 86.0 2 5444007 32
Delta 79.0 3 5444010 32
Delta 79.0 2 5444012 32
Delta 79.0 3 5444015 32
Delta 79.0 5 5444020 32
Delta 79.0 2 5444022 32