page_title: British Aerospace Nimrod AEW3
section_title: Avionics
hierarchical_section_title: British Aerospace Nimrod AEW3 / Issues / Avionics
caption_attribution_description: English: Hawker Siddeley Nimrod AEW3 serial number XZ286 at the 1980 Farnborough Air Show
context_page_description: The British Aerospace Nimrod AEW3 was a planned airborne early warning aircraft intended as to provide airborne radar cover for the air defence of the United Kingdom by the Royal Air Force. The project was designed to use the existing Nimrod airframe, in use with the RAF as a maritime patrol aircraft, combined with a brand new radar system and avionics package developed by Marconi Avionics which later became BAE Systems Avionics.
  The Nimrod AEW project proved to be hugely complex and expensive for the British government, as a result of the difficulties of producing brand new radar and computer systems and integrating them successfully into the Nimrod airframe. Despite close to a decade's work, the project was eventually cancelled, with the RAF instead purchasing new build Boeing E-3 Sentry aircraft to fulfil the AEW requirement.
context_section_description: The choice of computer to integrate the various sensor systems was wrong, as the GEC 4080M was simply too slow and too underpowered to perform the tasks required of it: by the time of project cancellation, the mission system mean time between failure was around 2 hours, yet it took around 2 and a half hours to load all the mission data via a tape system. What mission performance there was largely due to the Cossor IFF interrogator which complemented the radar system: with the addition of IFF data, the system could successfully track aircraft carrying IFF transponders, but when the IFF was switched off, radar tracks would rapidly be lost. This meant that the system would successfully track civil and 'friendly' military aircraft, but would not reliably detect Warsaw Pact aircraft which did not carry a compatible IFF system – detection of which was the whole point of the project. The mission system electronic racks were earthed to different points on the airframe, which led to differences in earth potential and the introduction of short-lived, random track information which added to the computer overload. Finally, the advanced design of the radar proved difficult – the FASS method to gain full 360° radar coverage was problematic, involving as it did the scanner in the nose making a left to right sweep, with the signal then immediately passed to the scanner in the tail, which would sweep right to left. However, getting the two scanners to synchronise proved difficult, resulting in poor all-round surveillance capability. The system also split incoming raw radar information into upper and lower beams, each of which was then further split into in-phase and quadrature-phase channels. Each of these 4 channels contained identical individual elements (such as a spectrum analyser), which in theory should have been entirely interchangeable between locations. JTU trials showed that in fact the system would only work with a particular device in a particular place in the system: putting the same device in one of the other 3 channels would not give a serviceable system. The consequence of this was that the JTU trials aircraft would fly loaded with spare electronic devices so that when system failure occurred, there was a better chance of finding a particular combination of system elements which would work. This would not have been a sustainable practice had the aircraft entered service. The reason for this issue was never resolved: the JTU suspicion was that tolerances in transmitting information through each channel were too loose, so that as the processed information emerged from each channel to be correlated back into a coherent picture, such correlation was in fact impossible since each channel was offering up a different 'time slot' to the others.
caption: Even getting the radar scanners mounted on the Nimrod's nose and tail to synchronise proved problematic
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