56 Strapdown systems and RLG

Question 1

Strapdown system basics

Answer 1

no gimbals are used
accelerometers are not isolated from the motion of pitch, roll and yaw
sensors are literally strapped to aircraft
sensitive axes of the gyros and accelerometers coincide with the vehicule body axes – by sensing rotation about 3 orthogonal axes the orientation of the platform is always known
computer is doing the work of the gimbal system

Question 2

Advantages Strapdown and RLG
Reduction in?
Increases in?

Answer 2

Significant reductions in

1. System hardware and weight
2. Power consumption
3. Maintenance
4. Cost

Significant increases in

1. Reliability
2. Service life

Question 3

Ring laser gyro (RLG) basic

Answer 3

Theory of operation
angular momentum, radius of arc and rotor mass are no longer a consideration
Light is used to measure angular rates of motion

Question 4

RLG errors

Answer 4

1. Differential path length errors
   Temperature differences between CERVIT cavities of even a fraction of a degree Celsius creates a time/phase difference
   Eliminated by using one CERVIT cavity for both lasers
2. Lock in
   Non perfect optics within the cavity and mirrors will cause a certain amount of back scattering
   Back-scattered energy will tend to reinforce the beam energy traveling in opposite direction
   At low rates of rotation, frequencies are combined, producing a dead zone
   This would lead to drift rates of 40 to 50 degrees per hour if left undetected

Question 5

Lock in elimination

Answer 5

Mechanical Bias (dithering)
Vibration of the RLG
Any error created by the dithering motion is averaged out and eliminated

Question 6

Advantages of INS integration/hybridization

Answer 6

Much improved accuracy

Higher mission completion rate as sensors may be used independently

Question 7

Disadvantages of INS integration/hybridization

Answer 7

Loss of complete self-containment

Loss of covertness, dependence on external radiation

Question 8

Hybridisation INS (4 ways)

Answer 8

Ground referenced hybrid system in which a ground based fixing system is integrated with the INS
GPS – INS hybrid
Doppler – INS hybrid
Celestial – INS hybrid in which the platform azimuth gyro is monitored by means of an automatic star tracker

Question 9

What is detected when the two opposing laser beams in a RLG are recombined after an angular displacement of the gyro?

Answer 9

A phase difference in the two beams as a result of differing path lengths. This is due to angular rotation of the cavity and is caused by movement of the gyro

Question 10

What is the largest source of errors in a RLG?

Answer 10

The quality of the mirrors in the optical cavity