ArduSub timeouts when rangefinder value > 2.5m

I have noticed that RFND.Dist values jump between something reasonable and 0 around when the ROV is > ~2.5m away from the seafloor.

When the ROV is close to the seafloor, the driver is sending DISTANCE_SENSOR messages at ~9Hz. Above ~2.5m the rate drops to ~4.5 Hz, so the gap between messages increases to ~220ms. This is easy to spot in the logs.


What is surprising is that the RNFD.Dist values show periodic 0 values when this happens, which AFAICT only happens if the 500ms timeout is hit.

https://github.com/ArduPilot/ardupilot/blob/ecd9694e47678384b083823c09899d4232c4cfa1/libraries/AP_RangeFinder/AP_RangeFinder_MAVLink.cpp#L79

Example RFND graph:


If the DISTANCE_SENSOR messages are arriving every ~250ms, this should never happen. I suspect that some DISTANCE_SENSOR messages are delayed by up to ~250ms, so a 500ms gap appears every ~7.5s. I do not have enough data to pinpoint the source of this delay.

Am I off-track here? Have others seen this behavior?

* Device: A50
* These graphs are a run with from ArduSub 4.6 DEV but I see the same behavior on many other logs from ArduSub 4.5 beta 1

Start with the linked AP_RangeFinder_MAVLink.cpp timeout logic and compare its 500ms threshold with the attached RFND.Dist and message-rate graphs. Trace DISTANCE_SENSOR timing in the affected ArduSub logs to determine where the delay occurs; done means identifying the cause of the periodic zero values and confirming the behavior across the stated ArduSub versions.

Written by the indexing model from the issue text.