Terrain failsafe: the height above ground is no longer known

What ArduPilot says:

• Failsafe: Terrain …
• Failsafe: Terrain data missing
• ERR FAILSAFE_TERRAIN (Subsys 23)

The copter was flying at heights measured above the ground (terrain following in Auto, Guided or RTL) and the ground height was unavailable for 5 s, or a command that needs it could not start. It switches to RTL flown at a height above home instead (already in RTL: “Restarting RTL - Terrain data missing”); if RTL cannot start it lands after a 4 s pause; on the ground it disarms. A height above home takes no account of hills: watch the copter and be ready to take over. “ERR FAILSAFE_TERRAIN-1” marks the event, code 0 the return of the ground height.

What to do, most likely first

1. The rangefinder lost the ground: the copter climbed above its range, or flew over water, tall grass or a steep slope.

   CheckIn the log compare RFND.Dist with the height flown in the seconds before the message.

   FixPlan terrain-following flights well inside the rangefinder's working range. When a downward rangefinder is fitted and its use in missions is on (the default), it alone is the terrain source: the terrain database is not used as a backup.

2. The terrain database had no data for this place: the ground station never sent it, or there is no SD card to store it.

   CheckThe ground station's terrain status shows tiles still pending.

   FixBefore the flight connect a ground station that has internet access and wait until the terrain for the whole route is loaded; keep an SD card in the flight controller and TERRAIN_ENABLE at 1.

3. The mission or RTL asks for heights above terrain, but no terrain source is set up at all.

   FixEither set up a source (a downward rangefinder, or the terrain database) or plan with heights relative to home.

Parameters to look at

Where to do it:Failsafe ExplainerMission Energy CheckSetup & Tuning Guide