Logging - what is PM.PMT?

I’m puzzled what the PMT value means in the log. The docs say it’s “A number that increments each time a heart beat is received from the ground station”. How do I interpret the graph of this below? Is the y-axis the number of heart beats received per minute? For this flight the quad was hovering within 100’ for 30 minutes. Why would the PMT figure be stable for the first ~20 minutes then drop to a lower value and oscillate more for the last 10 minutes? This is a Pixhawk 2.1 running AC 3.5.4 using v2 Sik radios for telemetry. Would this suggest the rate of heart beats sent from the ground station is falling, or packets getting dropped, or what? Does the heart beat come from the radio firmware, or Mission Planner? Is this even something to worry about? I’ve looked at earlier logs for the same aircraft and ground gear and it doesn’t exhibit this falling value of PMT although those flights were shorter.

Not something I’m familiar with, but I dug around in the code to at least answer your first question.

PM.PMT is stored as PMTest1 in the code. PMTest1 is incremented every GCS heartbeat and is set = 0 every time perf_update() is called. The loop scheduler tells this function to repeat at 0.1 hertz. So PM.PMT is GCS heartbeats per 10 seconds. The spikes you see in the first half of your flight is probably just due to timing.

The heartbeat is sent by the GCS, which would be Mission Planner in your case. Not sure what would cause the oscillations in the last part of your flight. Signal strength might be my first guess, but you say the drone was only 100 feet away. Were you flying maneuvers? For example, yawing the drone may obscure the antenna at some angles, causing poor connection.

Also not sure if it’s worth worrying about. Probably not, unless a reliable GCS connection is important for your application. Even then, I’m not sure if this value is any indication of connection health.

Thank you very much for taking the time to dig this out of the code and explain it. Super useful. I’m still pondering what could cause the drop in heart beats and cyclic pattern towards the end of the flight, the aircraft was hovering in PosHold in no wind, and the GSC was stationary too. Maybe I was walking between the two radios.