Using a Rangefinder for Autolanding

If you have fitted a rangefinder to your aircraft then you can use it for much more accurate landing control. To allow the rangefinder to be used for landing you need to set the RNGFND_LANDING parameter to 1. The first rangefinder with RNGFND_LND_ORNT orientation found, will be used. Normally, this will be “down” (25) for most planes, but Tailsitter Quadplanes would normally use “back” (4) so that the rangefinder is pointing at the ground during a tailsitter landing with its nose vertical.

When using a rangefinder for landing the altitude given by the rangefinder is used only in the landing approach and to determine the flare point, and is designed to allow the aircraft to more accurately follow the glide slope and to flare at the right time.

Note

The effectiveness of a rangefinder can depend on the surface you are flying over, so it is a good idea to do some low passes in a flight mode such as FBWA first, then examine the logs to check that the rangefinder is working correctly.

Also note that if you have a longer range rangefinder then it is a very good idea to set the minimum range of the rangerfinder well above zero. For example, the PulsedLight Lidar has a typical range of over 40 meters, and when it gets false readings it tends to read ranges of less than 1 meter. And setting RNGFND1_MIN to 1.5 , if its the first system rangefinder, will discard any rangefinder readings below 1.5 meters, and will greatly improve the robustness of the Lidar for landing.

If the autopilot has a good rangefinder (such as LIDAR) then you can safely choose quite small numbers for LAND_FLARE_SEC and LAND_FLARE_ALT, and flare closer to the ground than with the default values. That will generally produce a better landing.

A value for LAND_FLARE_SEC of 1.5 and LAND_FLARE_ALT of 2 is a good place to start with a LiDAR.

Impact of Glide Slope Terrain variation

If using rangefinder during landing, the rangefinder readings, once engaged (ie when getting good readings below the RNGFND1_MAX altitude on approach), will continuosly adjust the glide slope target altitudes as it approaches. If the terrain is irregular, or there is a deep ditch before the runway threshold, using the rangefinder can actually be worse than just using the barometeric altitude.

In these cases, you may wish to lower the RNGFND1_MAX altitude to a value closer to the expected vehicle altitude at the end of the runway so that it engages and is used shortly before the flare.

Alternatively, if the terrain is sloping toward or away from the runway, you may use the TERRAIN_FOLLOW parameter (bit 1(all modes) or 3(AUTO mode only) set to provide additional corrections to the rangefinder readings for the difference between landing point terrain altitude and current approach altitude above terrain to help prevent landing overshoots and undershoots. Be sure to carefully test for your particular situation and be ready to abort the autolanding if it does not look safe.