GNSS, Compass and Sensor Reliability

Automation listens to sensors before it listens to you

GPS hold, waypoint flight, return-to-home and many safety features depend on the aircraft's sensor picture. If that picture is wrong, the aircraft may confidently do the wrong thing.

The remote pilot's job is to cross-check the inputs before trusting the automation: position, heading, attitude, mode state, warnings and what the aircraft is actually doing in the sky.

A 10 second loop. Four input tiles appear one at a time: GNSS position, compass heading, IMU attitude and telemetry warnings and mode. Each sends a pulse down to a central check that asks whether the inputs agree. When every input agrees, each tile gets a green tick, the check turns green and the path to Automation OK lights up, with the pilot still ready to intervene. With a compass error or a GNSS error selected, that tile shows a wrong or jumping reading and a red cross, the check fails and the path to Recover lights up instead: hold, simplify and recover. The still frame shows the agreeing case with the decision made.

Read the How your drone knows where it is transcript

GNSS position depends on satellite geometry and clean signals

GNSS receivers estimate position from satellite signals. Accuracy depends on satellite geometry, receiver quality, antenna placement, signal strength, obstruction and the local environment.

A high satellite count is useful, but it is not the whole story. A receiver can still be affected by poor geometry, reflections, blockage or interference, especially near structures or terrain.

Left: a sky view looking up, with north at the top and the horizon as the outer ring. Satellites appear one by one and the satellite count rises. Right: the drone seen from above, inside a shaded area that shows how uncertain its GNSS position is. The size and shape of that area are worked out from where the satellites sit in the sky, but the scale is illustrative and no distances are given. Spread versus clustered: eight well-spread satellites give a small, round error area and good geometry. The same eight satellites then gather in one part of the sky. The count stays at eight, but the error area stretches into a long ellipse and the geometry is poor. Building edge: a building rises on the west side of the sky. The satellites behind it lose their signal, the count drops and the remaining satellites sit on one side, so the error area grows and stretches.

Multipath creates false confidence

Multipath occurs when satellite signals bounce off buildings, metal structures, vehicles, terrain or other reflective surfaces before reaching the receiver. The aircraft may receive both direct and reflected signals and calculate a position that is offset or unstable.

Remote pilots see this as drifting position hold, position jumps, poor mapping alignment, unexpected RTH behaviour or warnings that appear when flying near structures.

Three satellites at the top send signals straight down to a drone over open ground, and its position estimate, shown as a dashed ring, sits on the drone. The drone then flies close to a tall building. Now one satellite's signal also bounces off the building wall before reaching the drone. With both direct and reflected signals, the position estimate drifts and jumps away from where the drone really is, while the GNSS status badge still shows OK: false confidence. When the drone flies back away from the building, the reflection stops and the estimate rejoins the drone. Offsets are schematic.

Compass interference changes the aircraft's idea of heading

A compass gives heading information. It can be affected by magnetic fields from vehicles, reinforced concrete, steel structures, magnets, power infrastructure, payloads, batteries, cables and electrical equipment.

A poor compass environment can create toilet-bowling, yaw errors, incorrect RTH direction, unstable position hold or mode warnings. Calibration does not fix a bad launch location; sometimes the correct answer is to move.

A plan view of a launch site with a parked vehicle and a reinforced concrete pad. A drone is set down on the concrete beside the vehicle. A heading dial on the right shows two arrows: where the drone's nose really points, and where its compass says it points. Near the vehicle and concrete the two disagree. The drone is spun for a compass calibration, but afterwards the arrows still disagree: calibration does not fix a bad launch location. The drone is then carried away to open ground, and the compass heading and nose direction agree. The size of the error is schematic.

IMU and barometer errors affect attitude and height confidence

The IMU helps the aircraft understand attitude and motion. Barometric and height systems help it estimate vertical movement. Vibration, shock, temperature, poor calibration or sensor disagreement can all reduce confidence.

The pilot may see this as poor hover stability, drifting height, unusual vibration warnings, inconsistent speed or mode limitations. Treat those signs as evidence, not as background noise.

Cross-check before relying on GPS hold or RTH

Before relying on GPS hold, return-to-home, geofence or waypoint flight, check whether the sensor picture is coherent. The map position should match the real site. The heading should match the aircraft orientation. The active mode should match the pilot's expectation. Telemetry should be free of unresolved warnings.

If the inputs do not agree, keep the aircraft close, simplify the operation, use the approved manual or degraded-mode procedure, and recover while there is still margin.

• Check home point, satellite/GNSS quality, heading, mode and warning state before take-off.
• Do not launch from vehicles, reinforced concrete, metal covers or obvious magnetic interference sources.
• Avoid pushing deeper into a task when position, heading or telemetry confidence is degrading.

Bad inputs can make good automation unsafe

Automation may behave perfectly according to bad inputs. A return-to-home using a wrong home point, wrong height, poor heading or unstable position can create a new hazard instead of solving the original one.

The safe mindset is simple: when the sensor picture becomes doubtful, the operation has changed. Recover, hold, relocate or move to an approved degraded-mode response before the aircraft decides for you.

Practice Questions