Attitude and Heading Reference Systems Questions and Answers

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the reason explains the assertion. Micro-electromechanical sensors are used in light aircraft attitude systems because they are small, cheap and rugged, being etched from silicon in volume, and that combination is what makes an affordable attitude and heading reference system possible at all in that market.

The larger bias is the accepted cost. It matters less for attitude than it would for navigation, because the gravity and magnetic references bound the error, whereas a position solution would have nothing to hold the same bias in check.

Answer: B

Correct answer: 1 and 2 only

Statement 1 is correct: pitch, roll and heading are exactly the outputs the system exists to provide. Statement 2 is correct: it does not provide position, because that would require integrating specific force twice, with the error growing as \( t^2 \) and nothing available to bound it.

Statement 3 is false. The air data computer supplies altitude, airspeed and Mach number from pressure and temperature measurements - aerodynamic quantities that no inertial sensor measures. The two systems are complementary, and both are fitted.

Answer: C

Correct answer: 1 and 2 only

Statement 1 is correct: three orthogonal rate gyros measure the angular rate vector in body axes. Statement 2 is correct: an accelerometer measures specific force, which is the sum of the aircraft's acceleration and the reaction to gravity, which is why a stationary vertical accelerometer reads about \( 9.81\ \mathrm{m/s^2} \).

Statement 3 is false, and it is the direct consequence of statement 2. Both contributions act identically on the proof mass, so the sensor cannot separate them. That ambiguity is what limits the accelerometers to unaccelerated flight as a vertical reference, and it is a physical limit rather than a shortcoming of any particular design.

Answer: D

Correct answer: 1 and 2 only

Statement 1 is correct: integrating a constant bias gives an attitude error \( bt \), linear in time. Statement 2 is correct: angle random walk comes from integrating white noise, whose independent increments accumulate as \( \sqrt{t} \).

Statement 3 is false and contradicts the first two, since linear and square-root growth are plainly different. The distinction is not academic: it is what allows a filter to estimate bias as a separate state, and it explains why bias dominates over long intervals while noise matters most over very short ones.

Answer: A

Correct answer: 1 and 2 only

Statement 1 is correct: two units allow a comparator to detect that the outputs disagree. Statement 2 is correct: three allow the failed unit to be identified as the one disagreeing with both others, and excluded.

Statement 3 is false. Averaging is worse than median voting, because a faulty unit's output drags the mean towards itself in proportion to how far it has strayed, whereas the median is unaffected by the size of the outlier. Averaging also conceals the fault instead of isolating it.

Answer: B

Correct answer: A-1, B-2, C-3, D-4

Each sensor is matched to the quantity it senses. The rate gyro triad (A) measures angular rate about three axes (1); the accelerometer triad (B) measures specific force along three axes (2); the magnetometer (C) measures the direction of the earth's magnetic field (3).

The pitot tube (D) is the odd one out, measuring total pressure and belonging to the air data system (4) rather than to the inertial unit at all. The distractors mainly swap the gyro and accelerometer roles, which is the pairing to check first - rate against force.

Answer: C

Correct answer: A-1, B-2, C-3, D-4

Each mechanism is matched to how its error accumulates. Gyro bias (A) integrates once into an attitude error linear with time (1). Angle random walk (B) comes from integrated white noise and grows with the square root of time (2).

An accelerometer bias in a position solution (C) is integrated twice and so grows with the square of time (3). Scale factor error (D) is an error in gain, so it grows with the motion experienced rather than with elapsed time (4) - it vanishes when the aircraft is still and is largest in a fast roll.

Answer: D

Correct answer: A-1, B-2, C-3, D-4

Each technique is matched to what it establishes. Levelling on the accelerometers (A) gives initial pitch and roll (1) from the sensed gravity vector. Gyro compassing (B) gives true heading from the horizontal component of the earth's rotation rate (2).

Reading the magnetometer (C) gives magnetic heading (3), to which variation must be applied for true heading. Median voting (D) is not an alignment technique at all but a redundancy one, excluding a failed unit among three (4) - the odd item out in an otherwise alignment-focused list.

Answer: A

Correct answer: A-1, B-2, C-3, D-4

Each term takes its standard meaning. Strapdown (A) means sensors fixed to the airframe with the transformation performed in software (1). A complementary filter (B) combines a high-passed gyro signal with a low-passed reference (2).

Coning (C) is the attitude drift caused by simultaneous oscillation in two rotational axes (3), and sculling (D) is its translational counterpart - spurious velocity arising when angular rate and acceleration oscillate in correlation (4). Coning corrupts attitude, sculling corrupts velocity, and both need compensation terms computed at high rate.

Answer: B

Correct answer: Position

Position is not an output. The system computes pitch, roll and heading, and stops there - which is exactly what distinguishes it from an inertial navigation system.

The boundary is set by error growth. Attitude requires integrating angular rate once and is bounded by the gravity and magnetic references, so ordinary sensors suffice. Position would require integrating specific force twice, so an accelerometer bias would give an error growing as \( t^2 \) - about \( 6.5 \) km in an hour for a bias of only \( 0.001\ \mathrm{m/s^2} \) - with nothing available to hold it in check.