# Avionics Questions and Answers – Strap Down and Gimballed Inertial Navigation Systems

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This set of Avionics Multiple Choice Questions & Answers (MCQs) focuses on “Strap Down and Gimballed Inertial Navigation Systems”.

1. In which type of inertial navigation system are the gyro and accelerometer rigidly mounted to the body?
a) Gimballed system
b) Fixed system
c) Strapdown system
d) Void system

Explanation: In a strapdown navigator, Gyroscopes and accelerometers are rigidly mounted to a sensor assembly that is usually mounted to the vehicle on a set of strap mounts. The gyroscopes track the rotation of the body and compute the orientation of the body.

2. What the gyro act as in a strapdown system?
a) Angular rate sensors
b) Null sensors
c) Angle indicators
d) Orientation indicators

Explanation: In a strapdown system, the gyros do not act as null sensors as in a gimballed system but act as inertial angular rate sensors. Angle and orientation are computed from the measured angular rates by the algorithms that calculate heading and orientation.

3. Which one of the following is not a purpose of strapdown calculations?
a) Calculate vehicle’s attitude
b) Transform accelerometer measurements into navigational coordinates
d) Provide inputs to air data computer

Explanation: The purpose of strapdown computations are to calculate the vehicle’s attitude relative to the navigational coordinates using the gyro measurements, to transform the accelerometer measurements from vehicle axes into navigational coordinates, and to perform the dead-reckoning computations. Air data computer is independent of inertial navigation system.

4. Which of the following is a four element entity consisting of a scalar and a vector and is used to describe strapdown system rotation?
a) Directional cosines
b) Directional sines
c) Quaternion
d) Triternion

Explanation: Describing rotation motion in the strapdown system employ more than the minimum three parameters. A quaternion is a four element entity ting of a scalar part and a vector part. It defines the instantaneous axis of rotation.

5. What does the quaternion integration algorithm use to control error?
a) Linearization constraint
b) Normalization constraint
c) Differential constraint
d) Integral constraints

Explanation: Quaternion integration algorithms usually make use of the normalization constraint to control error growth in the computations. The sum of the squares of the quaternion elements are subtracted from unity to yield the normalization error.

6. What type of error occurs if the axis of rotation is moving in space?
a) Linearization errors
b) Control error
c) Coning error
d) Differential error

Explanation: Coning refers to a motion in which the axis of rotation is itself moving in space. In this type of motion, the axes of the body trace a cone in space.

7. Pre-processing algorithms are also called as _________
a) Coning algorithm
b) Error control algorithm
c) Differential algorithm
d) Resolution algorithm

Explanation: Pre-processing the gyro data at a higher rate than the quaternion iteration improves the approximation in the vector change in angle in order to follow the actual motion of the rotation axis closely. The pre-processing algorithm is also called as coning algorithms.

8. Why is the gyro and accelerometers shock mounted to the body?
a) Impact protection
b) Reduce drift error
c) Prevent voltage surges
d) Limit vibration errors

Explanation: The gyros and accelerometers are rigidly mounted on a block with temperature sensors. The block is usually shock mounted to the vehicle to control the bandwidth of the motion sensed at the instruments. Shock mounting limits vibration rectification errors within the errors themselves.

9. Sculling errors occur when rotation and acceleration are combined.
a) True
b) False

Explanation: In the presence of rotation and acceleration know as sculling, this approximation leads to errors referred to as sculling errors. It can be avoided by executing the velocity transformation at least four times faster than the anticipated frequency of vibration.

10. Which of the following is false with respect to gimballed platform?
a) Orients the accelerometers into a coordinate frame relative to Earth
b) Euler angles are not easily readable from the gimbals
c) Protect the instruments from large angular motion
d) Protection from vibration, temperature and magnetic effects.

Explanation: One of the main purposes of gimballed system is to provide a convenient readout of vehicle attitude by reading each of the gimbals angles separately, thereby yielding convenient Euler angle outputs.

11. When does a gimbal lock occur on a three axis platform?
a) Inner gimbal approaches 90°
b) Inner gimbal approaches 0°
c) Outer gimbal approaches 90°
d) Middle gimbal approaches 90°

Explanation: The condition in which the middle gimbal angle approaches 90° is called gimbal lock. In this condition, the innermost and outermost axis coincide, thus depriving the platform one of its degrees of freedom.

12. Which of the following factors affect the ability of a stable element to remain non-rotating relative to space?
a) Drift rate of the reference gyro
b) Accuracy of gyro torquer
c) Linear motion of the vehicle
d) Orthogonality of the components on the platform

Explanation: The ability of stable element to remain non-rotating relative to a space or Earth coordinate frame depends on drift rate of the reference gyros, accuracy of gyro torquers, linear motion of the vehicle, orthogonality of the components on the platform.

13. In any order of gimbal, the innermost gimbal has unlimited freedom.
a) True
b) False 