The GGS IID began to replace the GM II in Naval and RAF aircraft in May 1944. The conversion in Seafire aircraft was complete by June. A simpler version was also in use in American-manufactured 20 mm AA guns by this time.
The new sight had a ‘moving’ ring as well as a small fixed ring, side by side. Provided that the pilot kept the central blob of the moving ring on his target, the gyro mechanism in the sight made the blob lag behind the target as it appeared to cross the firing line, to lay off the correct aim ahead.
The aim ahead or deflection angle, would obviously vary according to the crossing speed of the target. However the gyro, spinning at constant rpm on its universal ‘Hook’s’ joint, would take no account of target speed unless its ‘tilt’ could be altered to take account of range. A rangefinder was therefore incorporated into the sight. Nowadays this is done by radar, but in 1944 it had to be done by an input from the pilot, mechanically, by a twistgrip on the throttle lever. Range is difficult to judge by sight unless some size-comparison from known objects is available. Knowing his target’s wingspan beforehand and setting this on a graduated dial, the pilot turned the twist grip so that eighty graticules forming the ‘moving’ ring in his sight ‘nipped’ the target’s wings, while he kept the central blob steady on the cockpit — his aiming point. This movement simultaneously altered the tilt of the gyro, so altering the blob’s lag to the correct angle. Needless to say, the gyro mechanism could not cope with large deflection angles and it toppled easily. The pilot’s sight then went blank while it recovered, and recourse was hurriedly made to the fixed ringsight. This occurred to me in a dogfight situation when I was jumped by Fw 190s with my No 2 while bombardment spotting over Normandy. I was not able to spend enough time tracking the target for the gyro to settle down properly and I used the fixed ring. However, it was excellent for long ranges when smooth tracking was possible and when extreme accuracy was required, and for rocket firing and dive bombing. It also allowed for skid errors of all kinds — or wind errors for ground attack — in the firing and target aircraft, and it avoided line-of-flight errors. It computed the actual flight paths of the target and firing aircraft and not the apparent flight paths. It also had the advantage that it allowed for bullet (gravity) drop and it allowed for a reduction in bullet speed at longer ranges. By setting ‘R/P’ on the sight selector, it also allowed for the large gravity drop when firing rockets. It was very useful for training, for it showed us all — in a variety of conditions — by just how much we should have been missing had we trusted our own estimation of the amount of deflection required. Pilots consistently underdeflected in action because they seldom had a training target capable of a realistic combat speed on the few occasions when live bullets were used. Drogue speeds were never more than half the speed of most real-life targets, as Churchill remarked upon in 1939. No doubt the same lack of realism applies today in practice missile firing. By the time we used the GGS in the Seafire XV and XVII with its heavier five bladed propeller, the sight was essential to allow for the huge gyroscopic yaw effects in a turning fight at full engine rpm.
The GGS sight occupied four times the volume of the GMII which it replaced. In the Spitfire/Seafire cockpit — and in the Firefly and Beaufighter cockpit too — the sight was eight inches from the pilot’s forehead. It represented a serious crash hazard when arresting into a barrier, or when ditching or making a forced landing ashore. Seldom if ever, did the pilot escape head injury and on occasion was knocked out and thus stood no chance of getting out from a fire or a ditching. As the space between the sight and the cockpit exit was narrower by several inches than a parachute plus dinghy, baling out snags were always reported by those lucky enough to make it. With a heavy Franks’ suit as well, the chances were minimal. (See Appendix 11 (d) — Baling out of Seafire III.)