The Observer Corps
The Observer Corps was originally inspired by the Admiralty in 1914, who tasked the local police force with spotting and reporting the movements of enemy aircraft overhead. Later, in 1917 the Corps was reorganised following the findings of General Smuts' Committee for Air Organisation and became an army responsibility. The job was delegated to soldiers of low medical status who's skill at aircraft recognition was, at best, rudimentary. This system failed and during the twenties and early thirties the task reverted to the civil police .
Reorganised and funded as a volunteer group in 1935, the Observer Corps was fundamental to the "Dowding System" and in 1940 was afforded extra money at the insistence of Dowding himself. Armed with binoculars, a field telephone, an aircraft recognition booklet and a crude but effective sighting apparatus, small groups in exposed, sand-bagged emplacements were active day and night throughout the war.
As with radar there was great difficulty in accurately calculating the altitude of aircraft. This could be improved to some extent by triangulating sighting between two or more posts. The weakness of the Chain Home system was that once aircraft has passed inland they could no longer be tracked by the seaward facing radar. It was then that Observer Corps volunteers continued to track the progress, strength and direction of enemy aircraft inland and report by telephone to Operations Rooms at all levels. There was no feedback system to inform the volunteers of friendly fighters or bombers in the area and so reported sightings of aircraft which had not been plotted by radar were filed in "Lost Property Offices" and forwarded to Operations rooms for interpretation.
In foul weather or times of poor visibility aircraft were tracked inland by the sound of their engines alone. It is somewhat surprising that the Corps was never involved in verifying claims as this would have greatly improved the accuracy of loss assessment.
Still without a full appreciation of the linked defence system, tactics were employed by Kesselring, to obscure the bomber's objectives by splitting the formations as they reached the coast to confuse the watchers below. The high altitude "Freijagd" use of fighters also tended to put a strain on the system, but Park's counter-measures of concentrating on the bombers and using "spotter" squadrons helped to solve an otherwise difficult problem.
The vital importance of this volunteer force in the Defence of Britain should not be underrated. While the Dowding System could operate without radar, it could not operate without the Observer Corps which attained a total strength of 32,000, of which 4,300 were women. In recognition of their services the prefix "Royal" was appended in 1941