Attitude and Heading Reference Systems Questions and Answers

Answer: A

It solves the orientation problem and stops there, which is what distinguishes it from a full inertial navigation system. Position and velocity require integrating acceleration over time, and that is precisely where the errors grow without bound.

Answer: B

Removing the gimbals removes the bearings, the motors and their wear, and puts the burden on computation instead. It is why strapdown became practical only once processors were fast enough to update the attitude matrix at high rate.

Answer: C

Correct answer: Specific force along three orthogonal axes

An accelerometer measures specific force - the non-gravitational force per unit mass acting on its proof mass - resolved along its sensitive axis. Three of them mounted orthogonally give the specific force vector in body axes.

The distinction from acceleration matters: a stationary accelerometer with its axis vertical reads about \( 9.81\ \mathrm{m/s^2} \), not zero, because the case is pushing the proof mass upward against gravity. That is why the triad can serve as a vertical reference at rest but is ambiguous under acceleration. Angular rate is the gyros' business and magnetic field the magnetometer's.

Answer: D

Correct answer: Angular rate about three orthogonal axes

A rate gyro measures angular rate - degrees or radians per second - about its input axis, and three mounted orthogonally give the angular velocity vector in body axes. Attitude is then obtained by integrating that vector, which is what the strapdown attitude algorithm does.

Angular displacement is not measured directly; it is the integral, and this is precisely why gyro errors accumulate. A constant bias integrates into an attitude error growing linearly with time, which is what the accelerometer and magnetometer references exist to bound.

Answer: A

Correct answer: A reference to the local vertical

In level unaccelerated flight the only specific force acting is the reaction to gravity, so the accelerometer triad senses a vector of magnitude \( g \) pointing along the local vertical. Resolving that vector in body axes gives pitch and roll directly - the levelling function.

The reference is good only while the aircraft is unaccelerated, because an accelerometer cannot distinguish acceleration from gravity: both appear as specific force. That is why the vertical reference is used as a slow, long-term correction to the gyro-derived attitude rather than as an instantaneous measurement. Heading, rate of turn and airspeed come from other sensors entirely.

Answer: B

The measured specific force tilts with the applied acceleration, so a level aircraft accelerating appears to be pitched up. It is the same physics that produces the acceleration error in a mechanical attitude indicator.

Answer: C

Correct answer: A magnetometer or flux valve

Heading comes from a magnetic sensor - a three-axis magnetometer or, in older installations, a flux valve. The sensor measures the direction of the earth's magnetic field, whose horizontal component defines magnetic north, and applying the local variation gives true heading.

A magnetic reference is needed because neither the gyros nor the accelerometers can supply one on their own. Gravity is vertical and so says nothing about direction in the horizontal plane, and the gyros only measure change of heading, which drifts without an absolute reference. The air data computer supplies altitude and airspeed, and the radio altimeter height above terrain.

Answer: D

Correct answer: Keep it clear of airframe disturbance

A flux valve is mounted in a wingtip or fin to get it as far as possible from the aircraft's own magnetic influences - ferrous structure, electrical cabling, motors and avionics - all of which distort the field it is trying to measure. The remaining error is compensated by a swing that determines the hard-iron and soft-iron corrections.

Vibration and temperature are worse in an extremity, not better, and the cable run is longer, not shorter. Magnetic cleanliness is worth those penalties because a distorted field produces a heading error that no amount of filtering can distinguish from the truth.

Answer: A

Correct answer: Combining sensors so each covers the other's weakness

Sensor fusion here means combining sensors whose error characteristics are complementary. The gyros are accurate over short intervals but drift, while the accelerometers and magnetometer are noisy and disturbed by manoeuvring but have no long-term drift. Blending them gives an attitude solution that is both smooth and bounded.

This is not the same as redundancy, which uses identical sensors so that one can fail. Nor is it equal averaging, which would let each sensor's weakness through undiminished, or selection of a single best sensor, which would discard exactly the information the others uniquely provide.

Answer: B

The gyro supplies the fast changes and the accelerometer the slow reference, and the two filters sum to unity across the frequency range. It is the simplest expression of the fusion idea and the conceptual ancestor of the Kalman filter.