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FAQ (Frequently Asked Questions)

The Chain Home radar system

Since heightfinding relied on measurement of the ratio of signal strengths received by lobes created by the ground interference pattern, the ground in front of the antennae should, ideally have been flat out to a range of about one mile. A flat site, when account was taken of all other essential requirements, (see appendix 2) clearly was not achievable for all locations around the coast of UK, and compromises had to be made.

After installation, phasing and alignment of the instrumentation on the ground, a large number of calibration flights were carried out using an auto-gyro making circular and radial tracks at fixed heights. The deviations from the actual to the indicated heights were carefully noted and correction factors calculated for all azimuthal angles within the operational sector centred on the station's 'line-of-shoot.'

These correction factors were 'hard-wired' into the calculator ('fruit machine') by linking the contacts of banks of Post Office type 'Uniselectors' in a manner which took account of the azimuth at which the height was being measured and making the appropriate correction. The calculator therefore received, in addition to the elevation angle from the gonio and range from the A scan of the CRT, the azimuth noted from the original D/F of the aircraft.

 At higher elevation angles (angles above 8o), there was some ambiguity because of confusion with the multiple upper lobes of the 215 feet dipole stack, but these could be quickly resolved by a skilled operator noting the results of a series of readings. In fact, almost all aircraft of interest ('hostiles') were first observed at long ranges and therefore low angles of elevation, so this problem did not often arise.

For aircraft in the first elevation gap, heightfinding was achieved using the 95 feet (angle = 5.9o) dipole stack and a third stack at 45 feet (angle = 12.5o).

All the measurements were taken using the same goniometer, the feeders from the various antennae elements being selected by the operator, using buttons operating a remote, motorized set of change-over switches.

Counting of aircraft in close formation (raid strength) relied on the skill of the operator who, when experienced, was able to make an assessment by observing the 'beat' rate of the composite echoes. To assist in this assessment, the transmitted pulse could be momentarily shortened to 6 microseconds (from 20 microseconds) by a push button on the console, the shorter pulse improving the range resolution by about 3:1.

It should be mentioned at this point that the great success of CH was due in no small measure to the incredible acquired skill of experienced operators, particularly the WAAFS (Women's Auxiliary Air Force). Signals at extreme ranges, well below 'noise' level, were detected and tracked.

The mechanism by which this was achieved is still not fully understood but believed to be due to an unconscious form of pattern recognition within the noise structure, somewhat analogous to the 'cocktail party' effect. Also, unlike scanning (searchlight) radars, CH, being a 'floodlit' system, provided up-date at p.r.f. rate with a corresponding integration gain when using a CRT with a long persistence phosphor. Figure 6 illustrates typical performance achieved by experienced operators on an average size bomber such as the Heinkel 111.

 (This article is taken from "The GEC Journal of Research", Vol. 3 No.2 1985 pages 73-83 and has been reproduced with the kind permission of the Editor. The copyright of the material remains with the owner.)


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