Magnetic disturbance: why a drone’s compass gets lost next to steel

Most drones use a magnetometer (compass) to measure the Earth’s field and determine which way the aircraft is facing (heading). Steel members and rebar distort the Earth’s field, and magnetised steel or current-carrying equipment create fields of their own. The compass then reports the wrong heading. When GNSS position and heading disagree, the aircraft corrects in the wrong direction and can drift in spirals (the “toilet-bowl” effect). If the take-off spot has steel plates or rebar below it, the heading may be wrong before the aircraft even leaves the ground.

The approach is either to protect the compass or to find heading without it. Protection means mounting the sensor away from motors and wiring and calibrating away from steel. Independence means dual-antenna GNSS, which derives heading from the position difference between two antennas, or visual or LiDAR heading estimation, which looks at the surroundings. Some aircraft automatically switch to a non-magnetic mode when they detect interference.

Magnetic disturbance is almost guaranteed around steel structures: bridges (especially steel bridges), transmission towers, telecom towers, vessels and columns in process plants, plant piping racks, storage tanks, and the hulls and decks of ships and offshore structures. It also occurs when taking off from reinforced-concrete roofs or steel-plated construction sites.

• Choose the take-off spot: avoid reinforced slabs, steel plates, manhole covers and vehicles. Use a non-magnetic (e.g. plastic) landing pad raised off the ground if needed.
• Check heading before take-off: confirm that the aircraft’s orientation on the app map matches where the nose actually points.
• Prefer aircraft with non-magnetic heading: for repeated work near steel, choose aircraft with dual-antenna GNSS or visual/LiDAR heading estimation.
• On a warning, move away first: increase distance from the structure and see whether heading recovers. If not, move to safety in attitude mode.
• Train attitude-mode recovery: keep the skill to disable position hold and fly back by hand when a heading error makes it misbehave.
• Log where it happened: recording where disturbance occurred lets you change how that section is flown next time.

Few countries have rules aimed at magnetic disturbance itself. But whether flying in Japan under a permission or approval or in the US under Part 107, keeping the aircraft in a condition to fly safely is the operator’s responsibility. Continuing to fly with a known heading fault and causing an accident leads to liability. Following the manufacturer’s guidance on where to calibrate and how to take off is the practical foundation of safety.

Related: strong electromagnetic fields, GNSS-denied, height, world regulations.

• Inside steel and without satellites: in tanks or under steel bridge decks, where the field is distorted and GNSS is unavailable, only vision and LiDAR remain. Add darkness or dust and it is still hard.
• Disturbance is not known in advance: on the same structure, magnetisation of members and equipment operating state change the effect. Mapping it before flight is not yet common practice.
• Differences between aircraft: whether non-magnetic heading exists, and how the aircraft reacts on detecting interference, varies by model. Site procedures still have to be rewritten for each aircraft.

Inspection aircraft treat the compass as just one of several heading cues, automatically down-weight it when disturbance is sensed, and hold heading with cameras, LiDAR and GNSS. Each structure’s 3D map carries the “disturbed zones” logged on previous visits, so the route is planned around them from the start. Aircraft spiralling under steel bridge decks have become a story from the past. That is the editors’ outlook.