(technically; a blind bomb-aiming system) was the direct development
from AI (airborne interception) and PPI and gave total independence
to the a bomber crew for it meant that for the very first
time, a screen 'map' of the ground below could be displayed
in the aeroplane at all times. Furthermore, because the whole
system was airborne, range was no longer a limiting factor.
It was the greater precision of image that could be produced
using 10 cm wavelengths that meant that the outlines of major
ground features became sufficiently clear to allow map overlays
to be used. Targets could be located with precision even under
poor weather conditions.
used involved a spiral scanning procedure [developed by A.
Hodgkin] and a downward facing scanner mounted in a blister
below the bomber. An image of a town 50 miles away could now
be displayed on the PPI screen.
Halifax bomber fitted with H2S arrived at Hurn airfield (for
flight testing) on 27th March 1942, just as TRE were moving
Lindemann and the RAF chiefs knew that
what was really needed was two systems; one to get the
bombers accurately to their targets and then a highly accurate
visual system to ensure that the bomb-aiming was accurate
regardless of weather conditions. The matter was discussed
at TRE Worth at one of A.P.Rowe's 'Sunday Soviets (26th October,
1941). P.I.Dee was there and he remembered some unusual results
that had been obtained with some early AI equipment developed
at Leeson House nearby. Some rushed experiments [in a Blenheim]
whilst flying over the Army camps on Salisbury Plain confirmed
that this equipment could pick out buildings on the ground
and display them on a PPI screen. Rowe was sufficiently impressed
when P.I.Dee showed him the filmed results that he instructed
him to organize a new research team immediately