6.1 GNSS, Compass, IMU and Geofencing

Sensors That Keep Your RPA Airborne

A remotely piloted aircraft (RPA) depends on a small suite of onboard sensors to know where it is, which way it points, and how it is oriented. Before every flight, a remote pilot must confirm these sensors are healthy, calibrated, and free from interference. This section covers the four sensor systems that most affect flight safety: GNSS (position), the compass/magnetometer (heading), the IMU (attitude), and geofencing (virtual boundaries enforced by the flight controller).

GNSS: Position, Satellites and HDOP

Global Navigation Satellite System (GNSS) is the umbrella term for satellite positioning constellations such as GPS (US), GLONASS (Russia), and Galileo (EU). The RPA's receiver uses signals from these satellites to compute its horizontal and vertical position. Two numbers matter most on the ground station before takeoff:

• Satellite count — a 3D fix generally requires at least 6–7 satellites; more satellites give a more robust fix. A 2D fix (fewer satellites) lacks altitude accuracy and is not safe for automated flight.
• HDOP (Horizontal Dilution of Precision) — a unitless measure of geometric satellite quality. Lower is better: an HDOP below about 1.5 is good; values above 2.0 indicate poor geometry and larger position error.

Check both numbers during your pre-flight. A strong satellite count with a low HDOP confirms the flight controller can safely compute position, hold a hover, and execute Return-to-Home (RTH) if the command-and-control (C2) link is lost.

GPS Loss Behaviour

If GNSS signals drop out in flight — due to interference, jamming, or poor sky visibility near structures — the flight controller cannot maintain a stable position hold. Typical behaviours include:

• The aircraft drifts with wind instead of holding station.
• Some systems fall back to attitude mode (also called ATTI), where the autopilot holds level attitude and altitude but the pilot must manually correct position.
• RTH may become unavailable or unreliable.

Anticipate this: know your aircraft's GPS-loss mode before you fly, keep it within visual line of sight, and be ready to take manual control. Do not launch if the fix is weak.

Compass/Magnetometer: Heading and Interference

The compass (magnetometer) provides heading — the direction the nose is pointing. It works by sensing Earth's magnetic field, which means it is vulnerable to local magnetic interference. Common interference sources near a drone include:

• Ferrous metal in the airframe, launch surface, or a vehicle roof.
• Electromagnetic fields from motors, ESCs, and power cables.
• Reinforced concrete with rebar, and underground services.
• Powerlines and metallic structures.

Compass error is dangerous because the flight controller uses heading to compute position-hold corrections and RTH bearing. A mismatch between true heading and sensed heading can cause the aircraft to fly in the wrong direction on RTH — the classic flyaway scenario.

Compass calibration teaches the flight controller the local magnetic environment. Calibrate:

• After changing airframe components, motors, or battery type.
• After travelling more than a few hundred kilometres from the last calibration site.
• If the ground station reports a compass error or large interference value.

Always perform the compass interference check on the launch site before takeoff: rotate the aircraft through 360° and confirm the heading indication stays within tolerance and the interference value is below the manufacturer's limit.

IMU: Attitude, Vibration and Calibration

The Inertial Measurement Unit (IMU) combines accelerometers (which sense linear acceleration and gravity) and gyroscopes (which sense rotation rate) to determine the aircraft's attitude — pitch, roll, and yaw. The flight controller uses IMU data to keep the aircraft level and to respond to pilot inputs.

Vibration is the IMU's enemy. Excessive vibration from unbalanced props, worn motors, or loose mounts contaminates the accelerometer readings and can cause:

• Drift in level hold.
• "Toilet-bowl" oscillations in position hold.
• In severe cases, the controller misinterprets vibration as attitude and over-corrects, destabilising the aircraft.

IMU calibration (a level calibration on a flat, stable surface) zeroes the accelerometers and gyros. Calibrate when the ground station prompts it, after firmware updates, or if the attitude indicator shows offset when the aircraft is level. Balance props and isolate the IMU board per the manufacturer's design to keep vibration within limits.

Geofencing: A Safety Layer, Not a Legal Substitute

Geofencing is a software feature that creates virtual boundaries the flight controller will not cross. Typical geofence parameters include:

• A maximum height above ground level (AGL) — for example, 400 ft AGL.
• A maximum radius from the recorded home point.
• No-fly zones around controlled airspace, airports, or restricted areas.

Geofencing is a valuable safety net: if the pilot is distracted or the C2 link drops, the aircraft stops at the boundary rather than straying into controlled airspace. However, geofencing is not a substitute for legal compliance. The remote pilot remains responsible for obeying CASR Part 101, observing airspace restrictions, and holding any approvals required. Reasons you must not rely on geofencing alone:

• Some systems allow the geofence to be overridden — know whether yours does.
• Database no-fly zones can be out of date; always check current airspace and NOTAMs.
• A geofence height limit may not match the legal ceiling for your operation.

Treat geofencing as a backup safety layer, set it deliberately before takeoff, and never assume it replaces your own airspace and height checks.

Pre-Flight Sensor Checks (Summary)

Before takeoff, confirm the sensor stack is ready:

1. GNSS — satellite count sufficient for a 3D fix; HDOP low (about 1.5 or better).
2. Compass — calibrated recently; on-site interference check passed.
3. IMU — calibrated; no vibration warning; attitude indication level on the ground.
4. Geofence — set to the correct max height and radius for the operation; no-fly databases current.
5. Firmware — flight controller and battery firmware up to date.