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Thread: GEE

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    Default GEE

    I'm currently reading "Black night for Bomber Command; The tragedy of 16 December 1943" by Richard Knott (Pub 2007). It is basically an account of the loss of a large number of aircraft whilst attempting to land in very poor conditions around midnight on 16/17 December 1943 - the aircaft having been to Berlin.

    It contains a number of references to aircraft either landing or positioning themselves at low level in the vicinity of an airfield whilst using GEE. Quote "Our altimeter setting was way off and we were descending through cloud on GEE ..... when I saw the tops of trees......... " (meaning the aircraft was practically at sea-level)

    In my ignorance I always assumed that GEE would be inactive below a certain altitude - at least I've never heard of it being used as a landing aid before. Am I mistaken and was this really standard practice?

    Brian

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    From memory, Gee used time receipt of signals received from ground stations in the UK. It was not usable at low altitudes over the continent because the relatively low frequencies used would require higher altitudes for decent reception at such long ranges. Low altitude reception (and accuracy) would improve the closer you got to the transmitting stations. I would expect that the main drawback to using Gee as an approach aid would be the time required for a navigator to produce usable results. You have time when cruising at high altitude, but not when approaching the ground. The old navigator joke of "turn left three minutes ago" can be funny, but "pull up three minutes ago" usually is not.

    Also from memory, Gee was the forerunner of postwar LORAN, which could be used as a crude approach aid when close to transmitters, if you had one of the new fangled instruments that continuously calculated and displayed your LORAN derived position.

    Added in edit: the quote "when I saw the tops of trees" means you are close to ground level, which may or may not be close to sea level. If your estimated ground position is off a bit, say beacause of the time required to convert Gee receptions into actual positions, you could be descending on to high ground when you expected to be descending beside it. Today the accident reports call this "controlled flight into terrain".
    Last edited by Bill Walker; 2nd February 2011 at 01:59.

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    Thank you Bill.

    Re your edit. For brevity I had not provided all the information and had omitted to say that in this instance the aircraft was approaching Bardney; there's precious little 'high' ground in the vicinity so I thought 'asl' was acceptable - my apologies for misleading you.

    Thank you for your advice, which is along the lines I was thinking. Interestingly the author makes several references to pilots not using SBA in the conditions and records that its use was neither mandatory nor were crews required to practice the technique. Seems strange to one largely ignorant of the fine details that pilots would opt to use a technique not designed for landing in poor conditions in favour of one designed exactly for that purpose.

    Another thought that crosses my mind is that radio waves can be bent by inversions - a marked increase in temperature over a very short height range - such as undoubtedly existed on the night in question. If that was happening then it would lead to positioning errors for an aircraft using GEE below the inversion.

    Just musing.

    Brian

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    Brian,
    If Ian The Radar is on the circuit then he's the guy to pronounce on the refraction and/or reflection of the various radio transmission frequencies. I think, however, that the higher frequencies (VHF, UHF, etc) are more prone to inversion problems (the 'ghosting' of French TV stations on the same frequency as UK stations under certain inversion conditions is an example). GEE (or AMES Type 7000, to give it its proper name) operated in the 20 - 85 Mc/s range. My experience is that Txs at the bottom end of that range are far more likely to be affected by ionospheric changes/variations than by inversions. But at the top end there may have been problems with inversions. Ian will decide!
    But if your brain does not explode with the "Peruvian Nose-Flute Music" then the answers are all clearly laid out at: http://www.radarpages.co.uk/mob/navaids/gee/gee1.htm. Simple really!!
    HTH
    Peter Davies
    Last edited by Resmoroh; 2nd February 2011 at 12:50. Reason: spelling
    Meteorology is a science; good meteorology is an art!
    We might not know - but we might know who does!

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    I always thought that Bardney was set in somewhat rolling countryside (Lincolnshire Wolds ?), certainly not a flat part of Lincolnshire.

    Ian

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    I scrolled round Bardney using Streetmap (http://www.streetmap.co.uk/map.srf?x=514500&y=371500&z=120&sv=austacre+wood&s t=3&tl=Map+of+Austacre+Wood,+Lincolnshire+[Forest/Wood]&searchp=ids.srf&mapp=map.srf) and was hard put to find any heights above 100 ft asl - most of the area around the airfield appeared to be of the order of 30 ft asl.

    Brian

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