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RLG Operation

The RLG body is a solid glass block, with three narrow tubes drilled in it. A

mirror is placed at each corner, forming a triangular optical resonator path.

The tubes are filled with a heliumneon mixture at low pressure. A high

voltage (around 1kV) is applied between the cathode and the two anodes,

causing a discharge.The discharge provides enough energy to cause

regenerative lasing action in the gas, with light beams circulating aroundthe triangular resonator path. In fact, there are two lasers within the same

cavity - one with a clockwise (CW) beam, the other counterclockwise

(CCW).

When the gyro is at rest, the two beams have the same frequency.

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Now consider the block rotating in a CW direction. A photon in the CW

beam,starting at the bottom lefthand mirror finds, after one traverse of thecavity, that the mirror has moved slightly further away. Thus it sees a

slightly increased path length.

Similarly, a photon in the CCW beam finds a shorter path length. The

difference in path lengths causes a small difference in frequency By

making one of the mirrors partially transparent, samples of both beamscan be extracted and the

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Schuler Tuning

e earth is not flat. As we move, close to the surface, we need to keep tilting

tform(with respect to inertial space) to keep the axes of the N and E accelero

izontal. To do this, we can use the gyro torquers, and feed them with a signalortional to the N and E velocity. The angular torquing rate that we apply is equal to

, where v is the linear velocity from the first integrator, and R is the radius of the earth.

ce = a / R w here a is the acceleration sensed by the accelerometer.This is

ation of motion of a simple undamped pendulum of lengthR, which has a perio

illation of around 84 minutes, known as the Schuler period ( MSchuler,1923).

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What is Lock-in?

A complication arises at very low rotation rates.The mirrors are notperfect and produce miniscule amounts of backscatter, which couples

energy between the two beams. This coupling of energy between two

oscillators can cause the frequencies to lock together. To overcome this

'lock-in', a dither motor applies a very small oscillatory rotation to the

entire block.

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Strapdown Systems

Gimballed I.Ns. can be very reliable, accurate,and good value for money.

However, the gimbal arrangement is mechanically very complex. It

contains delicate sliprings; the motors dissipate power, thus the

instruments see a varying thermal

environment as the gimbals move about ; mechanical resonances are

unavoidable. They can also be expensive to maintain - if a gyro or

accelerometer needs to be replaced, the gimbal set has to be dismantled

and, after replacing the instrument

and rebuilding (in a surgicallyclean environment), there are lengthy

calibrations to be done.Testing inertial platforms is timeconsuming.