Loss of GPS Mid-Grid Over Featureless Terrain
A 12 kg hexacopter is flying an automated photogrammetry grid at 80 m AGL over a large, uniform sand flat with few vertical features. The mission is 60% complete when the ground station shows the GPS status drop from 14 satellites to 4, then to no fix. The aircraft switches to attitude mode and begins to drift slowly with a 10 kt wind. The pilot is 300 m away with a clear line of sight. The aircraft is currently 450 m from the pilot, near the far edge of the grid, and the return-to-home function is configured to climb to 50 m and fly back to the launch point. The battery is at 55%.
1. What has likely happened, and why is the configured RTH not a reliable solution here?
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The GPS receiver has lost a usable fix, probably through loss of satellite geometry or signal quality over featureless terrain, and the flight controller has fallen back to attitude mode. In attitude mode the aircraft holds attitude, not position, so it drifts with the wind.
The RTH function depends on a position fix to navigate home. With no fix, RTH cannot compute a track and may either fail to engage, behave erratically, or rely on a stale home position. Climbing to 50 m also does nothing to solve the navigation problem and may increase drift exposure. Do not treat RTH as a safety net when the failure is loss of the navigation source itself.
2. As the pilot, what do you do in the first 30 seconds, and what is your recovery strategy?
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First 30 seconds: take manual control in attitude mode, stop the automated mission, and arrest the drift by flying into wind. Confirm orientation visually and by the aircraft's attitude response, and gain altitude only if it improves line of sight and clearance.
Recovery strategy:
- Fly the aircraft manually back toward the pilot, using visual reference and the wind direction, not the map display.
- Keep the aircraft upwind of your position so drift carries it toward you rather than away.
- If a fix returns, do not immediately re-engage the automated mission; verify position and home point first.
- If control is lost or orientation is uncertain, use the failsafe that does not depend on GPS (e.g., controlled descent in a clear area) and accept the loss rather than let it drift over people or property.
With 55% battery you have time, but the drift and the distance mean you should close the gap deliberately.
3. What operational and configuration lessons does this event expose?
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- Flying an automated grid over featureless terrain is a known GPS-degradation risk; a site survey should have flagged it and the mission should have been planned with manual fallback and closer stand-off.
- RTH was configured as if GPS would always be available; the failsafe set should include a GPS-independent option (e.g., hover-and-land or attitude-mode descent) appropriate to the site.
- The pilot was 300 m away with line of sight, which is workable, but 450 m at the far edge left little margin; keep the aircraft within a distance where manual attitude recovery is comfortable.
- Brief a lost-GPS procedure before flight and practise it, because it is a control problem, not a navigation problem.