RTL wanted a height above ground and is using a height above home

What ArduPilot says:

• RTL: no terrain data, using alt-above-home
• RTL: rangefinder unhealthy, using alt-above-home
• ERR NAVIGATION-6 (Subsys 22, ECode 6)

RTL is set to return at a height above the ground (RTL_ALT_TYPE set to terrain), but when it started the ground height was not available: no terrain source at all, a rangefinder without a good reading, or no terrain data for this place. This return is flown at the return height above home, which does not follow hills. In the log it is “ERR NAVIGATION-6” (no source, or the rangefinder) or “ERR TERRAIN-2” (terrain data missing).

What to do, most likely first

1. RTL began higher than the rangefinder can see, so there was no reading to start from.

   CheckThe message is “RTL: rangefinder unhealthy, using alt-above-home”; in the log RFND.Dist is at its maximum or RFND.Stat is not good.

   FixA rangefinder can only guide a return flown inside its range. For flights above it, set RTL_ALT_TYPE to 0 and choose a return height above home that clears every obstacle.

2. Terrain data for the copter's position or for home is not on the SD card.

   CheckThe message is “RTL: no terrain data, using alt-above-home” and the log has “ERR TERRAIN-2”.

   FixLoad terrain for the flying area through the ground station before take-off.

3. No terrain source exists: no downward rangefinder is used and TERRAIN_ENABLE is 0.

   FixSet up a source, or set RTL_ALT_TYPE to 0 so RTL plainly returns at a height above home.

Parameters to look at

Where to do it:Failsafe ExplainerSetup & Tuning Guide