GPS degradation, compass error and a misrecorded home point
A 5.2 kg hexacopter is 14 minutes into a 20-minute mapping flight at 90 m AGL, 280 m from the pilot. Wind is 7 m/s with 10 m/s gusts. The battery reports 28% and 21.6 V under load; the low-voltage warning is set at 21.0 V. The GPS satellite count falls from 15 to 5, the flight mode changes from GPS to ATTI, and a compass error appears. The aircraft begins drifting laterally toward a row of trees. Return-to-home is configured at 100 m, and the failsafe is set to RTH. The tablet shows the home point 20 m from the launch pad because it was recorded before GPS had converged. The pilot is flying visually and has a clear open area behind them.
1. Interpret the indications and explain why the aircraft is drifting.
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The drop from 15 to 5 satellites means the flight controller can no longer rely on a stable position solution. In ATTI mode the aircraft does not hold position; it maintains attitude and altitude with limited automatic correction, so the 7 m/s wind and 10 m/s gusts push it laterally. The compass error adds a second heading problem: the controller may not trust its yaw estimate, which can degrade wind compensation and make automatic return unreliable. The battery is not yet at the warning threshold, but 28% under load with voltage sag leaves little margin for a prolonged recovery. The drift toward trees is therefore a predictable combination of wind, loss of position hold and heading uncertainty, not a random fault.
2. What immediate actions should the pilot take?
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The pilot should take manual control, stop the mission, and fly the aircraft away from the trees toward the open area behind the pilot. The priority is a controlled landing in a clear space, not completing the mapping line. The pilot should reduce altitude carefully, keep the aircraft in sight, use small inputs, and allow for wind drift rather than fighting it with large corrections. RTH should not be selected: the home point is 20 m off, the RTH altitude is 100 m, and GPS/compass degradation makes automatic navigation unreliable. If the aircraft reaches a safe open area, land promptly and disarm. The pilot should announce the abnormal condition to the observer, keep people clear, and not attempt to re-set the home point until the aircraft is stable and the cause is understood.
3. How should the RTH, failsafe and home-point settings be critiqued after the flight?
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The settings were unsafe for this site. An RTH altitude of 100 m is only 10 m above the flight altitude, and with a 20 m home-point error the aircraft could climb, fly to the wrong point and descend near obstacles. The RTH failsafe is also questionable when GPS and compass health are marginal; a hover or controlled land failsafe may be safer in a confined area, depending on the approved procedure. The home point must be recorded only after a stable GPS fix and confirmed on the ground station before launch. The operator should add a pre-flight check for satellite count, home-point position and compass health, and define an immediate manual-landing trigger for GPS loss near obstacles. The low-voltage warning also needs a landing reserve that accounts for wind, not just calm-air endurance.