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Fishpond is the name which has been given to the second Indicator originally fitted to H2S Mark II and since fitted to all Marks of H2S to enable some form of warning of the approach of other aircraft to be obtained without the need for a separate and complete system and without interfering in any way with the normal operation of the H2S.
As the aerial system of the H2S equipment rotates and illuminates the ground out to range of up to 30 or 40 miles around the aircraft, the air in the lower hemisphere is illuminated in much the same way as it would be by an all round looking AI system. In the absence of any aircraft in the vicinity of the bomber no echoes are received between the time of the transmitter pulse being radiated and the return from the ground immediately below the aircraft, and if a simple 'A' scope presentation were used this part of the time base would be free of signals. Should there be an aircraft below the aircraft and at a slant range of less than the height of the aircraft, an echo would be received and would be painted on this clear part of the time base. It would have been possible to present this information about the presence of aircraft in the vicinity of the bomber on an 'A' scope, but this would have given no information of the azimuth of the aircraft relative to the bomber, yet this information is obtainable from the position of the H2S Scanner when the echoes are received. It is obvious, therefore, that some form of intensity modulated display should be used, either range azimuth or PPI. In the interests of simplicity a PPI was adopted and this was designed to display the time interval between the T pulse and the first ground return. The actual choice of the name Fishpond comes from the appearance of this display; the ground returns show up as a bright fringe around the edge of the PPI and any aircraft illuminated by H2S radiation show as 'fish' in this pond.
In the first place it was expected that Fishpond would be an interim measure and would eventually be displaced by a system giving greater coverage and having a presentation much easier to interpret. This influenced the design of Fishpond in that modifications to the H2S system could not be tolerated and the Fishpond itself had to be as simple as possible. There were therefore at the disposal of the designer of Fishpond the following signals from the main equipment - the linear sawtooth resolved into two components in quadrature as used in the main equipment for the H2S PPI, a low impedance source of video signals and the necessary A.C. and D.C. voltages. From those the Fishpond was designed by taking the sawtooth waveforms, and by means of a pair of amplifiers, amplifying them so that whereas on the main equipment the tube displayed 10, 30 or 50 miles: on the Fishpond indicator the range was invariably 5 miles. Maintaining a fixed range on Fishpond While the waveform input varied to suit the changing range requirements of the H2S PPI was achieved by having three preset gain levels for the amplifiers and arranging that as the operator of the main equipment changed the scan range of the main PPI, the requisite Fishpond gain was selected by a relay switch. Had the amplifiers in Fishpond faithfully reproduced the input waveforms the five miles displayed would have been the 50 microsecs. of time base starting at the mid-point of the forward sweep, but as described in a previous article, the H2S operator phases back the transmitter pulse so that it precedes the mid-point of the scan waveform by an amount corresponding to half the aircraft's height.
At the normal operating height of 20,000 ft. this would be approximately 2 miles or 20 microsecs. Thus, the 50 microsecs. displayed on the Fishpond would not include the 20 microsecs. immediately after the transmitter pulse, and could therefore have had no value as a warning device. However, since the input levels. required to give the Fishpond scan varied between 20 volts when the H2S was on the 10 mile range and 4v. when on the longest range and the actual input was 50v., squaring of the peaks was inevitable, and by suitably choosing the bias voltages on the amplifier the distortion was made to offset the balance point of the waveform and cause the point which should have been the pivot point of the display, to describe a circle with a radius equivalent to approximately 2½ miles. Under these conditions the transmitter pulse described a circle of about ½" diameter and thus, in addition to providing the necessary display of the time base immediately after the transmitter pulse, also allowed increased accuracy of azimuth determination of close in targets due to the target appearing as an arc some distance from the centre instead of as a point immediately at the centre.
The relative phasing of the signals was, however, much more difficult to achieve. The video signals from the main equipment were branched into two parts, one for the PPI and the other for the height tube. These were similar in amplitude but the signals for the former were pedestalled on some 10v. of square wave so that signals on the flyback were not painted on the PPI. A bright up square wave was necessary for the Fishpond Indicator, it being an intensity modulated device, but that already provided with the signals to the main PPI was phased wrongly since it started at a point on the sawtooth corresponding to the centre of the PPI, and as stated above was some two miles beyond the transmitter pulse. It would have been comparatively easy to take the signals to the height tube and mix with these a square wave correctly phased relative to the Fishpond scan, but an upper limit on, the size of the Fishpond unit had been set and this prohibited the inclusion of such a piece of circuitry. It was therefore decided to dispense with a bright up square wave and rely upon the absence of signals on the flyback stroke to keep the picture clean.
Although the signals available from the main equipment either as supplied to the PPI or the height tube had mixed with them a marker calibrated so that it was suitable for range measurement, this was not available to the Fishpond operator since it had to be controlled from the navigator's station for use on the main equipment, and also being used by him would invariably be set at ranges greater than the first ground return and would therefore not appear on the second Indicator. A simple circuit was therefore introduced which gave calibrated pips at approximately one mile intervals, and by using these the operator estimated the range of the oncoming aircraft. In H2S Mark IV, which uses a sine pot., and therefore allows the Fishpond scan to be produced independently of the main Indicator and to be adjusted to obtain an accurate scan rate, these markers were dispensed with and a perspex bezzle engraved with circles at one mile intervals was fitted to the face of the Indicator.
Thus within the limits set it was possible to produce an additional unit presenting in FPI form the main H2S time base from a time 5 microsecs. before the transmitter pulse to some 45/50 microsecs. after, displaying on this PPI echoes from objects at ranges less than the height, and also the first ground returns as a fringe round the perimeter of the tube. In this form Fishpond was introduced into Bomber Command and achieved some success. The new unit was criticised on several points, the most important being:—
When the possibility of introducing Fishpond was first suggested, the need for an additional observer was raised and it was decided that an additional member of the crew could not be carried but that the new unit should be installed in the Wireless Operator's compartment, and when other duties permitted the Wireless Operator should act as Fishpond observer.
Earlier it was said that the H2S Scanner illuminated the hemisphere beneath the aircraft. This, however, need not necessarily be so for the H2S performance since, being interested only in the ground out to some 30 or 40 miles, were the Scanner designed for maximum possible efficiency, there would be no radiation at angles nearer the horizontal than would be necessary for this range. At 20,000 ft. this is approximately 8°; therefore given optimum performance of the H2S Scanner, an aircraft subtending an angle at the bomber of less than 8° with the horizontal would not be illuminated by the H2S radiation, and would therefore not be detected on the Fishpond. Owing to the limited vertical aperture available to the designer of the Scanner it was inevitable that some radiation would be lost in directions above that required for painting at maximum range, and on X band, and more especially on S band, this radiation was sufficient for an aircraft to be detected when on the same level as the bomber at a range of approximately 2 miles. On the higher frequencies, for instance K band, where, having available the same physical depth of Scanner and therefore by comparison with the S band equipment a factor of 8 increase in aperture in terms of wavelength, the performance approaches more nearly to the ideal, Fishpond detection would be limited to angles between the vertical and some 100 from the horizontal. Thus, losing not only coverage in the upward hemisphere but also in the most important part of the lower hemisphere, it was considered that with equipments on K band, such as H2S Mark VI, it would be pointless to fit a Fishpond Indicator.
It will be realised that although the Fishpond picture gives slant range of the picture from the bomber and also the azimuthal direction, it cannot give any indication of relative heights. This was a drawback quickly overcome by the operational crews, who by doing a slight turn to port or starboard could see whether the oncoming aircraft was at the same level or appreciably below themselves by noticing whether the Fishpond return moved by an amount corresponding to the turn (as it would if it were on the same level), or remained more or less stationary.
On several occasions the possibility of presenting IFF on
the Fishpond tube so that the operator could readily distinguish
between other bombers and enemy fighters has been discussed,
but since with the present concentrations of aircraft in the
main stream there are always several on the Fishpond tube, the
interrogation would lead inevitably to a clutter of IFF signals.
It has eventually been decided against IFF, and the identification is done usually by the bomber taking evasive action and
noting whether the aircraft under observation attempts to
follow him or is content to proceed on a constant course.
When it became apparent that Fishpond would be more or less a permanent installation and not the interim equipment originally intended, it was decided that further work should be done and where necessary extra units introduced to clear as many of these faults as possible. Accordingly a unit which became known as the independent bright up unit was designed and this cleared two of the major troubles. There were three reasons for the poor, minimum range:-
The new unit enabled a square wave phased with respect to the transmitter pulse to be added to the height tube signals so that when impressed upon the grid of the PPI tube only the signals of the writing stroke of the time base were painted, and also by varying the amplitude of this square wave some measure of gain control by the Fishpond operator was possible. Modifications were introduced into the main equipment so that the paralysis of the IF amplifier was reduced as much as possible, and to overcome the spurious signals caused by the long lengths of cable, a Filter Unit which allowed frequencies up, to 2 megacycles to pass, but severely attenuated signals of higher frequency, especially those in the region of the IF frequency of the Amplifier, was introduced immediately after the pulse output of the Modulator. The introduction of these two units enabled the minimum range to be reduced from as much as 1000 yards down to approximately 200 yards. It was not possible to do anything about the criticism that the Fishpond display was orientated with respect to magnetic north, for although it was obvious that the Fishpond operator would have to transfer what he saw into directions relative to the aircraft such as port bow or starboard quarter, the orientation was introduced at the magslip on the Scanner, and to do otherwise a second magslip especially for Fishpond would need to be introduced. Even at this stage this was considered too big a modification to be introduced on to the Scanner owing to the consequent reduction of the number of Scanners produced.
The complaint that the transmitter pulse and the earlier part of the time base was lost at the centre of the tube was largely removed by the introduction into the main equipment of a scan corrected Indicator where it was not necessary to phase the transmitter pulse relative to the sawtooth to obtain some correction and the phasing could be preset to suit Fishpond.
On later Marks of H2S, for instance Mark IV, which from the beginning was designed to incorporate a Fishpond Indicator, further steps were taken to overcome some of the limitations of the Fishpond fitted to H2S Mark II. The most important of these was to introduce a variable gain law into the IF Amplifier so that in the time interval between the transmitter pulse and the first height return the gain was at a preset maximum while the gain over the rest of the repetition interval between the first ground return and the next transmitter pulse was under the control of the H2S operator. It was arranged that the maximum gain should be set such that the first ground return showed as a signal approximately two and a half times noise, this being sufficient to ensure reliable operation of the height finding circuits in the Mark IV, and also sufficient amplitude signals from approaching aircraft to be painted on the Fishpond tube. In the latest Marks of H2S it has been decided that the value a Fishpond is sufficient to warrant the addition of an extra magslip, and this will then give a picture cleared of most of the outstanding faults of the original unit.
The operational use of Fishpond has suffered due to the lack of training of operators but it is now being used successfully. Of the aircraft which are identified visually some 50% approach from the upper hemisphere and therefore are out of Fishpond cover, but,of the remainder, except for instances when the Wireless Operator was not manning the tube, there have been very few cases where the aircraft was known to be in Fishpond cover but was not detected by the Fishpond operator.
Apart from the use of,Fishpond as a warning set against enemy aircraft, its greatest use has possibly been in warning the air crew of a possible collision, and many instances have been reported where collisions have been successfully avoided by acting upon information from Fishpond. In future aircraft the fact that Fishpond is limited to the lower hemisphere will become less and less important because AGLT will be fitted, and even with the manually operated turret of AGLT Mark I it is possible for the whole rear hemisphere to be covered, the upper half by manual search with the AGLT turret and the lower part by Fishpond. Actually as AGLT Mark I is introduced Fishpond becomes more important because to cover the whole of the AGLT field of view continuously by manually scanning with the turret would impose too great a strain on the rear gunner, but by relying on Fishpond for warning of aircraft in the lower hemisphere the rear gunner has a much smaller area over to search and can do it efficiently.
Page last updated on 13th February 2018 by Colin Hinson.