Attitude and Heading Reference Systems Questions and Answers

Answer: C

Correct answer: Three is the fewest that can identify the failed unit

The table shows the progression. One unit gives nothing to compare against. Two can detect a disagreement but cannot say which is at fault, so both must be treated as suspect and the function is effectively lost. Three allow the odd one out to be identified - it is the unit that disagrees with both others - and excluded by median voting, so the system continues on the two that agree.

Three is therefore the minimum for fail-operational behaviour. Two units certainly can detect a failure, four are not prohibited, and averaging would make the surviving output worse rather than better.

Answer: D

Correct answer: Accepting larger bias for size and cost

The micro-electromechanical row trades accuracy for size and cost. Its bias is the largest of the four technologies listed, but the sensors are tiny, inexpensive and made in volume, which is what puts a full attitude and heading reference system within reach of light aircraft and unmanned vehicles.

The trade is not ruggedness - having no moving parts, these devices are the most rugged of the four, not the least. Nor is size given up: they are the smallest. What is given up is bias performance, which is why they are not used where navigation-grade accuracy is required.

Answer: A

Correct answer: Both require the aircraft to be stationary

Levelling and bias estimation share the same precondition, as the table's failure column shows: both fail if the aircraft moves. Levelling assumes the only specific force present is the reaction to gravity, so any motion tilts the sensed vector and corrupts the initial pitch and roll. Bias estimation assumes the true rates and accelerations are zero, so that whatever the sensors report is offset - an assumption movement destroys.

This is why alignment is performed at the stand rather than while taxiing. Neither task uses the magnetometer, neither is optional in a full alignment, and neither involves satellite reception.

Answer: B

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. The system stops short of position precisely because obtaining it would require integrating specific force twice, so an accelerometer bias produces a position error growing as \( \tfrac{1}{2}bt^2 \).

Nothing available to the unit can bound that growth: gravity bounds attitude, and the magnetic field bounds heading, but there is no comparable reference for position. An inertial navigation system attacks the same problem with navigation-grade sensors and an external aiding source, which is what makes position tractable there and not here.

Answer: C

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

Both statements are true and the second explains the first. The sensed specific force is the vector sum of the aircraft's acceleration and the reaction to gravity, so under acceleration the apparent vertical tilts by \( \arctan(a/g) \) - over eleven degrees for only \( 2\ \mathrm{m/s^2} \).

Because that tilt is indistinguishable from a genuine attitude change, a filter must reduce its trust in the accelerometers whenever acceleration is detected, leaning instead on the gyros, which are accurate over the short term. A Kalman filter does this by inflating the measurement noise on the accelerometer update.

Answer: D

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. Gravity is vertical, so it provides no reference at all in the horizontal plane - rotating an aircraft about its vertical axis changes no accelerometer reading.

Three rate gyros can measure changes of heading but not absolute heading, and integration drifts without something to bound it. A magnetic sensor supplies that absolute reference by measuring the direction of the earth's field. The only alternative is gyro compassing on the horizontal component of earth rate, which needs gyros far better than those in a typical unit.

Answer: A

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

Both statements are true and the second is the reason for the first. Levelling determines initial pitch and roll by assuming the only specific force present is the reaction to gravity, so that the sensed vector defines the local vertical.

Any motion of the aircraft adds specific force of its own and tilts that vector, so the assumption fails and the initial attitude is wrong. The same applies to the initial bias estimates, which assume the true rates are zero. This is why alignment is carried out at the stand and why a system cannot align while taxiing.

Answer: B

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. With two units a comparator can establish that the outputs disagree, but nothing in the data identifies the culprit, so both must be treated as unreliable and the critical function is lost.

A third unit resolves the ambiguity: the failed unit is the one that disagrees with both of the others, and median voting excludes it while operation continues on the two that agree. Three is therefore the fewest that gives fail-operational behaviour, which is why it is the usual minimum where attitude is critical.

Answer: C

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

Both statements are true and the second explains the first. Coning arises when the airframe oscillates about two axes at once, typically at vibration frequencies, and because finite rotations do not commute this produces a systematic drift that a slow integration loop cannot see.

The compensation must therefore be evaluated at a rate well above those frequencies - commonly several hundred hertz or more - even though the attitude output and the filter that blends in the reference sensors need update only tens of times a second. Splitting the computation this way keeps the error-critical inner loop fast and the outer loop cheap.

Answer: D

Correct answer: A is false but R is true

The assertion is false. Gyro bias produces attitude error growing linearly with time, whereas angle random walk produces error growing as the square root of time - quite different laws.

The reason is true and is exactly why they differ: a bias is a constant offset, which integrates to \( bt \), while random walk comes from integrating white noise, whose independent increments accumulate as \( \sqrt{t} \). Over four hours a gyro with \( 0.1 \) degrees per root hour contributes \( 0.2 \) degrees of random walk, while a bias of \( 0.5 \) degrees per hour contributes \( 2 \) degrees - which is why bias dominates the long-term error.