RTL is set to fly above the terrain, and has no way to measure it

What ArduPilot says:

• PreArm: no rangefinder
• PreArm: no terrain data
• PreArm: rangefinder not in firmware
• PreArm: RTL_ALT>RNGFND_MAX_CM
• PreArm: RTL_ALT_M above RNGFND_MAX

The full line reads “RTL_ALT_TYPE is above-terrain but …”. With RTL_ALT_TYPE 1 the copter returns at RTL_ALT above the ground under it, not above home, so it needs the ground's height: from a downward rangefinder (when WPNAV_RFND_USE is 1) or from the terrain database (TERRAIN_ENABLE 1). “no terrain data”: neither is available. “no rangefinder”: none is set up facing down. “RTL_ALT>RNGFND_MAX_CM”: RTL_ALT is beyond the rangefinder's reach (RNGFND1_MAX_CM).

What to do, most likely first

1. RTL_ALT_TYPE was set to 1 by a parameter file, and you do not fly over hills.

   FixSet RTL_ALT_TYPE to 0: RTL then climbs to RTL_ALT above home, which needs no terrain data.

2. You want terrain following, and the terrain database is off or has no data.

   FixSet TERRAIN_ENABLE to 1, keep an SD card in the board, and connect the ground station once at the flying site so it can send the terrain tiles.

3. A rangefinder is meant to do it, but none faces down, or RTL_ALT is higher than it can measure.

   CheckRTL_ALT and RNGFND1_MAX_CM are both in centimetres: compare them.

   FixSet the rangefinder's RNGFNDn_ORIENT to 25 (Down) and reboot. If the return height must stay above its range, set WPNAV_RFND_USE to 0 and use the terrain database instead.

Parameters to look at

Where to do it:Setup & Tuning GuideFailsafe Explainer