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G.D. SPEAKE OBE Technical Officer 60 Group RAF
Calibration of Radar Stations

Calibration of both CH and GCI stations was organised by a small team based at 60 Group Headquarters at Leighton Buzzard. They issued guidance notes, arranged courses, maintained central records and carried out other functions which  one would expect from a central organisation. For example, one such activitv was to prepare and circulate charts from which one could read off true height of aircraft above the curved earth surface from a measured range and angle of elevation.

However, the practical work of calibration was delegated to Wing Headquarters where specialist sections were set up for the purpose. (In the early part of the war they were composed largely of civilians but by late 1942 all had been absorbed into the RAF).

PREPARATION ~ As and when a station became due for calibration one, or sometimes two, members of the section would be assigned to the job. On arrival at the station one of the first tasks would be to ensure, with the cooperation of the resident technical staff and possibly also of a quarterly overhaul party. that it was in a fully operational state. For example it was vital to ensure that the signals from the two components of each of the crossed dipoles used for direction finding were in phase. One of the more daunting tasks facing the calibrator was to climb the receiver mast with an Avometer to check continuity of the feeders and to ensure that the reflector dipole, used to determine whether a target was in front or behind the station, was being brought into operation when needed!

CH - For azimuth calibration of East Coast stations autogiros were used, but since - unlike helicopters - they could not actually hover without some forward movement, they flew in as tight a circle as possible over a selected point and the measured bearings from the station were averaged and comapared with the known bearing of that point. For West Coast stations the autogiro was replaced by a Hornet Moth fixed-wing aircraft. (The Hornet to the better known Tiger Moth except that pilot and passenger sat side by side. A calibrator visiting an island often had a convenient means of transport available thereby).

The aircraft, which was equipped with a squegging oscillator tuned to the station frequency, was flown directly over the station on designated bearings, and was tracked by a theodolite until such time as it was no longer visible. The pilot was then called on the R/T and requested to return on the reciprocal bearing, again being tracked by the theodolite as soon as the aircraft came into view. A telephone connection between theodolite observer and operations room ensured that the theodolite readings and goniometer observations were correlated in time so that minor departures of aircraft from the planned course were not critical.

The advantage of this method over that whereby an aircraft circled over known landmarks was that it was only necessary to know the map coordinates of the station and one other landmark, such as a church, in order to set up the theodolite. Thereafter it was possible to take measurements on any desired bearing which was particularly important when the station was right on the shoreline and the main area of interest was out to sea.

Charts and/or tables showing measured vs true bearing were produced on site and, in the fixed stations, were used as the basis for setting up the Electrical Calculator employed as a plotting aid. On the transportable (ACH) stations the calibrator used this information to produce what was called a "distorted rose", which was a circular scale on which the bearing marks had been displaced from the usual position to the extent that an operator using them with goniometer readings and a measured range obtained a true plot. This was overlaid on the plotting table.

During the calibration exercise the people primarily involved were the operator at the tracking console, the theodolite observer, a person to note theodolite readings at regular intervals (it was, in general, impracticable for one person to be responsible for tracking the aircraft and to break off frequently to take a reading), a recorder in the operations room to note down both goniometer and theodolite readings, and an operator to maintain R/T contact with the pilot of the aircraft. The calibration officer normally acted either as goniometer operator or as theodolite tracker, depending on what were the skills of the other people present, and the other tasks were carried out by station personnel.

For height calibration, ie the correlation of a goniometer reading obtained by comparison of signals from two dipoles at different heights and the angle of elevation of a target giving rise to them, it was necessary to use an aircraft capable of flying at similar heights to those expected from potential enemy targets. Typically a Blenheim, equipped with a repeater which received a signal from the ground transmitter and retransmitted an enhanced one on the same frequency, was used. The calibrating officer normally contacted the local Met. office preparatory to a flight and asked what would be the altimeter reading for a prescribed height (say SOOO ft). He passed this information in code form to the pilot, who then carried out a series of flights on radial bearings from the station. To obtain information relating to higher angles of elevation a similar procedure would be carried out with the aircraft at say, 18000 feet.

Since the relation between the range and the angle of elevation of a target above a curved surface in an atmosphere which refracts radio waves is not a simple trigonometrical one charts, prepared by the Headquarters team at 60 Group, were used to derive angles of elevation from the known height and measured range; and the results were used to provide the necessary data for the station’s Electrical Calculator. On some sites, particularly those where the land sloped sharply towards the sea, the conversion between goniometer reading and angle of elevation varied sharply with azimuth and it was important that flights were taken on enough bearings to take that into account.

It will be apparent that calibration, either azimuth or height, was not intrinsically a lengthy procedure but it was almost always prolonged because the flying conditions had to be favourable, and in particular a Hornet Moth had to be in visible range for some miles from the station. (It is fair to say also that a pilot could not be expected to take the same risks with his aircraft that he might have done in carrying out an operational exercise against the enemy). It was not unusual, therefore, for a calibration officer to be on a station for two weeks or more before a satisfactory outcome was achieved, but throughout this period the station was maintained in a fully operational state.

GCI - The procedures for calibrating a GCI station were simpler than those for a CH because no appreciable azimuth errors were likely to occur, the stations being on flat sites inland. Moreover, even had such errors been present they would be equally applicable to the target and to the intercepting fighter. It was therefore only necessary to determine from flight results the relation between angle of elevation of a target and the ratio of the signals received from the two aerials at different mean heights which formed the basis of the system.

As with the CH system an aircraft was flown out at a constant height from a position over the station and simultaneous readings of range and ratio of signals were taken. The larger signal was assigned 10 units and at any given range the calibrator would assess the smaller to as close an accuracy as he regarded as practicable. He might, for example, call out 10 to 5% or 3% to 10. The results from outward and return runs would be plotted and on a good site an experienced calibrator would get a consistency between outward and inward observations of a few percent.

One significant difference between flights carried out for CH and GCI calibrations was that in the former case the aircraft was entirely under the pilot’s control but carrying out a series of operations agreed in advance, or during the flight, with the calibration team, whereas that at the GCI was subject to the instructions of a ground control officer. Another difference was that, while it was possible by theoretical means to get a reasonably close approximation to the results of a height calibration of a GCI, where the mean heights of the antennas were known and the site was level, it was virtually impossible to do so on a typical CH.

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