The dead-reckoning script: flying home without GPS

What ArduPilot says:

• DR: could not get home or vehicle location
• DR: enabled
• DR: failed to change to Guided_NoGPS mode
• DR: fly home yaw:… pit:… cr:……
• DR: leveling vehicle
• DR: GPS or EKF bad

Lines that begin “DR:” come from the Lua script copter-deadreckon-home, not from the firmware. It watches the GPS (satellites under DR_GPS_SAT_MIN, speed accuracy worse than DR_GPS_SACC_MAX) and the EKF failsafe. “DR: waiting for dist” and “DR: enabled” are normal: it is ready once the copter is DR_ENABLE_DIST metres from home. “DR: GPS or EKF bad” is the event: in Auto, Guided, RTL, Circle, Land, Brake or SmartRTL it switches to Guided_NoGPS, levels for 5 s, then leans DR_FLY_ANGLE degrees towards where home was (“DR: flying home”). When GPS and EKF are good for 3 s, or after DR_FLY_TIMEOUT seconds, it switches to DR_NEXT_MODE.

What to do, most likely first

1. The GPS really became poor: jamming, spoofing, or a blocked view of the sky.

   CheckIn the log, GPS.NSats and GPS.SAcc in the seconds before “DR: GPS or EKF bad”, against DR_GPS_SAT_MIN and DR_GPS_SACC_MAX.

   FixThe script is doing its job: it flies a heading, not a position, so wind carries it off the line home. Take over in AltHold or Stabilize as soon as you see the copter and bring it in by hand (“DR: pilot retook control”).

2. The thresholds are tighter than this GPS manages on a normal day, so it triggers on good flights.

   CheckLook at GPS.SAcc and GPS.NSats over a whole good flight: the speed accuracy should stay well under DR_GPS_SACC_MAX (0.8 by default) and the satellites above DR_GPS_SAT_MIN (6).

   FixSet the two thresholds with a margin from what your logs show. DR_GPS_TRIGG_SEC is meant to be how long the GPS must stay bad, but in ArduPilot's own copy of the script it is compared with milliseconds, so it acts at once whatever you set.

3. “DR: failed to change to Guided_NoGPS mode”, “DR: failed to set attitude target” or “DR: failed to change to mode …”: the copter refused what the script asked.

   CheckA “Mode change to … failed: …” line beside it gives ArduPilot's reason. DR_NEXT_MODE 6 (RTL) is refused for as long as there is no position.

   FixThe script keeps trying and stays in Guided_NoGPS meanwhile. Fly it home by hand, and afterwards choose a DR_NEXT_MODE that can start without a position if you expect the GPS not to come back.

The script needs SCR_ENABLE 1 and about 80 kB of script memory (SCR_HEAP_SIZE). DR_FLY_TIMEOUT 0 means it flies on the heading until the battery failsafe or you stop it. The firmware has a dead reckoning of its own (“Dead Reckoning started”), which is another thing.

Parameters to look at

Where to do it:Lua Applet CatalogFailsafe Explainer