Heading and position from inertial sensing

Why must a directional reference be aligned?

A stable reference can still have the wrong initial direction.

Directional gyro and remote magnetic correction

A directional gyro maintains a stable short-term heading reference but does not independently know magnetic north. A free-running instrument is aligned to a trustworthy magnetic reference in the prescribed steady condition, then checked and corrected as it drifts. Heading drift can include real mechanical effects, Earth rotation and transport. Its smooth movement is a useful quality, not a guarantee of an accurate north reference.

A remote-reading compass system combines a magnetic detector, gyro, correction or precession circuitry and displays. The magnetic detector is placed to reduce aircraft interference; the gyro stabilises short-term indications, while slaving slowly corrects the reference. A free mode changes how correction is applied. A detected disagreement can therefore involve the magnetic source, gyro, synchronisation, control or display rather than one universal compass fault.

Solid-state attitude and heading

An AHRS uses solid-state angular-rate sensors, accelerometers and typically magnetic sensing to estimate attitude and heading. The sensors measure rotation and specific force; the computer integrates and combines them with models and available references. It does not require a spinning mechanical rotor to provide gyroscopic information. Electrical supply and data validity become central dependencies even though traditional bearing and vacuum-system problems disappear.

A wrong magnetic reference, disturbed acceleration input or failed rate sensor can affect different estimated quantities. The system’s filtering and validation define what remains available. Compare source flags and independent instruments rather than assuming an electronic display cannot drift. During an initialisation or reversion, follow the specified attitude and motion conditions. The laboratory separates an initial heading offset from accumulating drift so a correction to one is not mistaken for a correction to both.

Why does a small acceleration bias create growing position error?

Position is obtained by integrating motion, so errors accumulate through the same mathematics.

From acceleration to velocity to position

An inertial system measures angular motion and specific force, transforms them into a navigation frame, compensates for gravity and the rotating Earth, then integrates to obtain velocity and position. In a strapdown unit the sensors are fixed to the aircraft and the transformations are computed. A mechanically stabilised platform performs part of that orientation physically. Both depend on correct initial conditions and suitable corrections.

For a simple uncorrected straight-line example, constant acceleration bias b creates velocity error bt and position error ½bt². Double elapsed time gives four times position error, not merely twice, in this deliberately simplified case. Real navigation errors include alignment, sensor, Earth-model and processing effects, so no single quadratic formula describes the complete operational drift of every IRS.

Alignment and combined navigation

Alignment establishes the local vertical and heading reference, using the entered position and sensed gravity/Earth rotation as appropriate. The required stationary conditions, time and available modes are installation-dependent. Incorrect latitude or movement can prevent a valid alignment. An attitude-only reversion does not necessarily retain navigation position, and a powered-off aligned unit may not be recoverable to full navigation in flight.

An FMS may combine inertial position with GNSS and radio measurements rather than displaying an untouched stand-alone inertial solution. Updating the combined FMS position does not mean that every individual inertial unit has been realigned or repaired. After flight, residual groundspeed and drift information can help assess individual units. In the laboratory, change bias and elapsed time independently, then distinguish the increasing error from a fixed initial-position offset. An external update is a third, separate event.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.