This is looking good! Let's call the new library "AP_SurfaceDistance" because we will likely use it in Plane as well.

maybe a SURF log message? with instance number

Key things to test are:

• Behaviour with rangefinder out of range
• Behaviour with rangefinder enable/disable on RC switch

Renamed to AP_SurfaceDistance and added logging. For logging I have added a status bitmask which should make it easier to track various fault conditions.

This is looking good! Let's call the new library "AP_SurfaceDistance" because we will likely use it in Plane as well.

Definitely want this in Plane!

This is great work! I would love to see sub use this in a future PR.

I have flight tested this code (git hash: 426c948) All appears to working as before.

Here is a break down of the test:

log file here: https://drive.google.com/file/d/1myPkiXPjzz0Ay9KEJwMSnXHhpa4FlaGW/view?usp=sharing

Made those changes and tested logging IRL in the office.
Only had a downward facing LiDaR fitted so only the first instance of SURF is populated with data in this test.

Apologies, I cannot make the call this evening, but this PR has been through a few reviews now and I am hopeful that it is good to go.
As a reminder, since last review the following changes have been made:

• Rename RANGEFINDER_HEALTH_MAX to RANGEFINDER_HEALTH_MIN
• Logging has been changed to report in meters and fixed-up data types. This has been tested. See plot above.
• Semaphore added to avoid accessing last_healthy_ms out of sequence with millis() in data_stale() helper.