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(NB This text is an extract from the late Harry Cole's unpublished work "The History of the Marconi Radar Company" 1997.)

The technical, operational and commercial advantages gained by the Marconi Company's exploitation of the 50cm wavelength is well known in the ATC Radar field. One reason for its success is all too often forgotten, probably because of its simplicity. For decades beyond the '50s, radar displays were operated in real time; the radar signals were written on the tube's phosphor as they appeared in the system and lasted (at low light level) for as long as the tube's afterglow was visible — about 20 to 30 seconds in the very low ambient lighting of ATC Operations rooms.

The original 50cm radars, S232 and S264, had beamwidths and pulse durations which gave the radar signals a relatively large size on the 12-inch diameter tube screen. For example, in the S232, with a beamwidth of four degrees and pulse duration of two microseconds, the aircraft signals when seen on the short-range scale of the display had physical dimensions of about a centimetre in azimuth and four millimetres in range at the tube's edge. This changed itself to a four millimetre square at mid range.

The MTI was capable of removing ground clutter from the screen and thus the aircraft signals appeared clearly. So large and clear were they that it gave the radar a unique character. As one stood at the threshold of an operations room door and looked through the gloom at the displays some ten meters away, one was shown the aircraft — they did not have to be looked for as in many other types of radar. This clarity was of great benefit to Controllers who often had to do things that caused them to look away from the tubes face - peripheral vision allowed them still to see the signals because of their size.

These radars and their successors, S650 and S670, developed as part of the S600 series were sold to 15 different countries over the period 1957 to 1976. A total of 57 installations was achieved.

But why did they become, eventually, unmarketable when their performance was so good? Here, politics and technology combined to play their part. In the early days of their history, these radars had sole use of the 24MHz band allotted at 600MHz as, by international agreement, 'Navigational Aids'. The fact that they used a self-coherent system through the use of power amplifiers for transmission gave the MTI exceptional (for those days) quality. Their 10cm rivals had great trouble countering weather clutter; in fact, apart from circular polarization techniques, there were only crude ways of enhancing the target to weather clutter ratio. Things were made slightly more difficult in 1965 when the international regulations split the 24MHz band into three lots of 8MHz with users able to share the 8MHz sub-bands with communica­tions facilities.

Although there were 23cm radars emerging at this time, they were without benefit of digital signal processing and thus could still not match the 50 cm MTI performance even with the improved analogue dual cancellation techniques available to all radars at that time. In fact, many viewed a 23cm radar as giving the 'worst of both worlds' — it still had a much larger antenna than its 10cm rival and had not sufficient weather immunity to match that of 50cm. Although the target visibility on displays was improving by the use of plot extraction, such processors were still quite crude and, without associated SSR, crossing tracks would become confused and ambiguous.

From the '70s, solid-state and digital engineering grew apace and soon became applied to MTI and Plot Extraction processes. These, together with improved stability of transmitters, produced MTI performance in 23cm systems which equalled that of 50cm equipments in pre-digital days. The concurrent spread of the use of SSR in conjunction with primary radars, plus the invention of better tracking algorithms, meant that radar signal display need no longer be limited to the real-time domain, and high-brightness target visibility was obtained.

In 1980, at the Company's test site at Bushy Hill, I saw the first example of a 23cm ATC radar, using digital MTI and other processes which gave as good a 'real-time' picture as the 50cm equivalent — at long last.

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