Gunsight (display model)

GSA 200 Series
Part No. 3993/31360
Serial No. 002
Ferranti - Scotland

Perspex Model

The GSA (Gunsight Surface to Air) is a lead-computing sight ideally suited for 20-40mm guns. The lead angle is the angle at which the gun must be aimed ahead of the direction of travel of the target to ensure that the shells fired and the target reach that point simultaneously. The instrument also allows for the setting of target motion, target range and shell ballistic properties. When tracking a moving target the gyroscope assembly within the GSA Gunsight causes the reticle image to lag behind the movement of the gun barrel. Consequently by keeping the target aligned within the reticle image the gun barrel is moved ahead of the target by the required angular allowance.

The gyroscope comprises a mirror and rotor assembly balanced about a low inertia joint by a copper dome. A gyromotor drives the rotor assembly through gearing. The gyro rotor is free to deflect in both the horizontal and vertical planes about its suspension. The deflection from the symmetrical position is proportional to the rate (precession) of the rotor axis in space. The mirror at one end of the rotor reflects a reticule to the gunner showing the deflection as the required lead angle. The dome at the other end of the rotor rotates in a magnetic field produced by a current through a set of four "sensitivity" coils. The interaction between the induced eddy currents and field provides a torque which restrains the precessions of the gyro rotor in the direction of the target as the target is tracked. By making the current in the sensitivity coils a suitable function of range, the rotor deflection from its position of symmetry will give a sightline deflection equal to the lead angle requirement. The gyro has an extra pair of windings on its vertical axis which enable the rotor to be deflected to compensate for the shell ballistics.

A lens in the optical path focuses the reticle at infinity and this collimated reticule is reflected into the gunner's eye by a semi-reflecting combining glass through which the target may also be seen.

The GSA200 front view shows the Combining Glass, two lampholder assemblies, a dimmer control, two switches reading OFF/MAX/MIN/GYRO and OFF/FIXED and a sunscreen. The gunner has a choice of the GYRO reticle for air targets or the FIXED reticle for ground targets. This demonstrator unit has Perspex panels set into the sides such that the internal mechanism can be seen.