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Old 18th Apr 2017, 12:43
  #1173 (permalink)  
Join Date: Dec 2013
Location: Norfolk
Age: 63
Posts: 3

Radar operates at the speed of light, 300,000 kilometres per second, or 300 metres every millionth of a second. Modern electronic components have reduced the size and increased the power of radar systems, but there is a physical limit to how much functionality can be built into a single system. Low power motion detector systems can operate at ranges of a few feet, but a system capable of picking up targets at a range of 40 miles will have its' performance compromised at very close ranges due to the laws of physics. In order to obtain unhindered close range performance, the use of two identical physically coupled but electrically isolated scanning dishes would be required. The sheer size of such an antenna system would preclude fitting in most aircraft.

I am as certain as I can be that the crew did not see Black Rock on the radar display and that Black Rock was certainly not presented on the display or recognised as presenting imminent danger. I am also sure that such an experienced crew were lulled into a false sense of security by perhaps not appreciating the ultimate physical limits of the radar system due to weather and possibly a latent deterioration in system performance due to the ageing of components.

A flawed approach pattern, lack of detailed mapping information, radar returns that were "unreliable" at close range and a loss of situational awareness in dodgy weather all combined to create the circumstances for this tragedy to happen. The classic holes in the swiss cheese lining up analogy of many aviation accidents.


Yes, 75 metres is a theoretical minimum range with a half microsecond transmit/receive switchover. You are quite correct that quarter wave stubs of waveguide are used to provide almost perfect isolation between the transmit and receive sections and that no electronic components are necessary to provide this isolation, just careful design and adherence to mechanical tolerances. But radar waveguide alters in size due to temperature changes and/or mechanical damage when knocked so some part of the transmitter energy can end up being fed towards the receiver circuits as the transmit pulse is fired off. To cater for this, there is a very high speed schottky diode mounted across the waveguide of the receiver section designed to short out if receive signals exceed a small fraction of a volt. It is this component that ages and needs to be replaced at regular intervals. When I worked for Marconi, every radar engineer carried half a dozen replacements in their tool kit. It was standard practice to replace them on every visit for servicing or fault finding.

Last edited by G0ULI; 18th Apr 2017 at 16:35. Reason: Corrected 300,000,000 to 300,000!
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