7.4 Compass, Altimeter, Airspeed & IMU

7.4 Compass, Altimeter, Airspeed & IMU

Quick Answer: RPAS “instruments” are mostly sensors + software. The magnetic compass needs clean magnetic fields and correct understanding of variation. Barometric altimeters convert pressure to height and can err with temperature and settings. Airspeed systems need good pressure ports (mainly fixed-wing). The IMU (gyros + accelerometers, often fused with magnetometers/GNSS) stabilizes the craft—calibrate before Advanced work. Geofences limit where software will allow flight; they do not replace airmanship.

Sensor literacy prevents the classic “it just flew away” narrative that is often really compass/IMU corruption, bad home points, or altitude misreads near people and airspace boundaries.

Magnetic compass principles

A magnetic sensing system (mechanical compass in manned trainers; magnetometer on RPAS) measures the horizontal component of the Earth’s magnetic field to estimate magnetic heading.

True north, magnetic north, and variation

• True north: geographic North Pole direction (meridians on VNC).
• Magnetic north: direction a perfect compass points (magnetic poles).
• Magnetic variation (declination): angular difference between true and magnetic north at a location. East or west variation is charted and changes slowly over years (isogonic updates).

Navigation math on charts uses variation to convert true ↔ magnetic. RPAS autopilots often work in mixed frames (GPS track true-ish, compass magnetic); calibration routines and map software hide some complexity—but exam questions still expect the definition of variation and that it is location-dependent.

Deviation and local interference

Deviation is error caused by local magnetic fields on the aircraft or nearby objects—steel structures, vehicles, magnets in cases, camera ND filters with steel rings, high-current wires, ground power units, and reinforced concrete.

Calibration importance

Manufacturers require compass calibration (and sometimes IMU calibration) after travel to a new region, after magnetic interference events, after airframe/payload changes, or when the app warns of inconsistency. Do it:

1. Outdoors, level ground, away from metal and vehicles.
2. Per the exact manufacturer pattern (rotate, flip, etc.).
3. Before flights that demand precise RTH paths, tight corridors, or Advanced airspace compliance.
4. Again if the aircraft yaws incorrectly, “toilet bowls,” or reports compass errors.

A calibrated compass in a bad magnetic site still fails—calibration is necessary, not sufficient. Choose launch points with magnetic hygiene in mind.

Altimeter principles and errors

How barometric altitude works

A pressure altimeter (or barometer feeding the FC) converts static pressure to altitude using a standard atmosphere model. Higher altitude → lower pressure → higher indicated altitude (with correct setting).

RPAS may display:

• Height above takeoff (relative baro AGL-ish from launch point)
• ASL / MSL fused with GNSS
• Terrain-relative estimates from maps/radar/stereo on some platforms

Know what your telemetry number means before you brief ATC-style altitudes or claim 122 m AGL compliance over rising terrain.

Error sources

Classical manned phrase “from high to low, look out below” (flying toward lower pressure or lower temperature without update) still teaches the right fear: you may be lower than you think. For RPAS, also invert the risk: over-reading height near a 122 m limit or near controlled airspace floors/ceilings creates compliance and collision problems.

Multirotor practical altitude control

Most small multirotors mix baro + IMU + GNSS + sometimes optical flow. Gusts and ground effect can make the altitude loop pump. Pilots should:

• Verify altitude source in the app before mission-critical height busts.
• Use conservative margins under hard ceilings (airspace or company limits).
• Recheck home point elevation when launching from rooftops vs landing below.

Airspeed indicator (ASI) principles and errors

An ASI measures the difference between pitot (dynamic) and static pressure to indicate airspeed. It is critical on fixed-wing and some VTOL RPAS for stall margin and performance; many pure multirotors emphasize groundspeed and attitude instead of a traditional pitot ASI.

Errors and limitations

Pre-flight on pitot-equipped RPA: covers removed, ports clear, no mud, heaters working if installed. In icing conditions (generally a no-go for small RPAS operations when structural ice is a risk), pitot ice is an early killer of air-data integrity.

IMU: gyros, accelerometers, fusion, failures

The Inertial Measurement Unit (IMU) typically includes:

• Gyroscopes: measure angular rates (roll/pitch/yaw rates).
• Accelerometers: measure specific force / acceleration including gravity reference for attitude.
• Often co-packaged magnetometers and fused with GNSS and baro in an AHRS / EKF (attitude and heading reference / Kalman filter).

Why the IMU is flight-critical

Stabilized flight depends on timely, consistent inertial data. If the FC believes the aircraft is level when it is banked, it will apply the wrong motor mix—oscillation, flip, or flyaway-looking trajectories can follow.

Errors and malfunctions

Mitigations: balanced props, sound soft mounts, manufacturer IMU calibration, avoid launching with damaged frames, and heed app health warnings. After any crash or major maintenance, treat IMU health as suspect until verified in a controlled hover.

Geofencing concepts

A geofence is a virtual boundary—horizontal polygon and/or altitude cap—enforced by firmware/GCS:

• Stay-in fences: mission or regulatory box; aircraft refuses commands that exit or auto-returns/terminates at the edge.
• Stay-out fences: no-fly databases (airports, prisons, stadiums, temporary restrictions) depending on manufacturer and region.
• Altitude fences: max height AGL/ASL limits.

Limits of geofencing

• Databases can be out of date relative to a fresh NOTAM or temporary restricted area.
• GPS loss may degrade fence enforcement or force mode changes.
• A fence is not authorization: NAV CANADA RPAS Flight Authorization and CARs compliance are legal layers above a consumer no-fly map.
• Over-reliance creates complacency (“the app would stop me”)—still plan charts, airspace class, and aerodrome distances yourself.

Use geofencing as a safety net and company procedure tool, especially for new pilots, but brief what happens at the boundary (hover, RTH, land, terminate) before Advanced missions.

Why calibration before Advanced airspace ops matters

Controlled airspace, aerodrome proximity, and near-people profiles leave little room for sensor-induced lateral wander or altitude lies. Before those flights:

1. Compass calibrated in a clean magnetic environment; no errors on status page.
2. IMU healthy; no vibration warnings; props balanced.
3. GNSS solid fix; home point correct; RTH altitude set and obstacle-checked.
4. Altitude source understood; mission heights include margin under legal/authorized caps.
5. Geofence / max height set intentionally—not leftover from yesterday’s indoor test.
6. Hover check confirms stable position hold before entering the high-risk volume.

If compass or IMU faults appear, do not “hope GPS will cover it” beside a Class C fence line. Land, fix, re-calibrate, and re-brief.

Sensor pre-flight card (exam-ready)

Bottom line: Variation explains true vs magnetic north; interference and calibration decide whether the compass is usable; altimeters and ASIs are pressure instruments with classic errors; IMUs are the inner-loop heart of stability; geofences help but do not replace planning. Calibrate and verify before Advanced airspace and people operations—sensor negligence is how technically “airworthy” airframes still create incidents.