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The British dealt with range measurement first, when it was realised that range was the key to producing a better fuse setting. This led to the Height/Range Finder (HRF), the first model being the Barr & Stroud UB2, a 2-metre optical coincident rangefinder mounted on a tripod. It measured the distance to the target and the elevation angle, which together gave the height of the aircraft. These were complex instruments and various other methods were also used. The HRF was soon joined by the Height/Fuse Indicator (HFI), this was marked with elevation angles and height lines overlaid with fuse length curves, using the height reported by the HRF operator, the necessary fuse length could be read off.

What was the first model of HRF (Height Range Finder) used by the British?
Barr & Stroud UB2