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    Radar and Naval Gunnery, was: Index corr., Octant as dipmeter
    From: Trevor Kenchington
    Date: 2004 Nov 23, 23:30 -0400

    This is really getting a long way away from non-electronic navigation.
    It is also a topic on which I have only limited book knowledge, when
    there are people around with intimate technical understanding. Still ...
    Alex wrote:
    > Yes. But radar is limited by the horizon
    > in the same way our vision is.
    I am not aware of any naval gunnery that has attempted over-the-horizon
    fire, so the limitations of the radar horizon aren't really limitations
    to gunnery at all. (Jared mentioned over-the-horizon radar but that uses
    very low frequencies which refract around the curve of the Earth: Good
    for detecting ICBM launches but not much use for controlling a gun.)
    Maybe Alex meant that radar is limited by the skyline. Bombardment of
    land targets, by land-based artillery and from the sea, very much can
    extend beyond the skyline. That is "indirect fire" and produced the
    challenges for the ships off Belgium during 1914-18.
    > It only helps when  it is dark or the weather is bad.
    I think the huge advantage of gunnery radars in the 1940 was that they could 
    give accurate ranges. Bearings to targets were easy enough to obtain visually 
    but finding the range depended on observing the fall of shot and correcting. 
    The German's did far better than the Royal Navy because the former used 
    stereoscopic range finders while the British continued to rely on split-image 
    approaches that are inherently less precise. That allowed German ships to 
    land crippling hits while the British were still trying to find the range. 
    The development of fire-control radars reversed the advantage, as the U.S. 
    Navy demonstrated with devastating effect in the Pacific -- once they learnt 
    how to use the new tool.
    > So, to my understanding, in WWII they could only
    > shoot at the targets in the open sea which were
    > "geometrically" visible. That is not behind the horizon.
    > Unlike in the land artillery or in the interesting case
    > described by Trevor, where they shot from "closed position".
    > I find the example of "navigation" described by Trevor very
    > amazing. One thing I don't understand in this:
    > at what distance you can hear and detect an underwater explosion?
    > Can you indeed hear in England an explosion near Belgian coast?
    As to the first: It depends on how big the explosion is, how sensitive
    your listening devices are and what the temperature and salinity
    structures in the ocean are like. At the extreme, the U.S. Navy has
    recently been blocked (by environmentalist law suit) from an experiment
    that would have had them generating underwater sound in the southern
    Indian Ocean and detecting it in the North Atlantic. The
    recently-retired SOSUS system, with hydrophone arrays deployed on the
    seabed, could monitor ships moving thousands of miles away by listening
    to the sound of engines and propellers.
    I do not know at what range the hydrophones of 1918 could detect a depth
    charge exploding. Given Buxton's account of the operations of the
    monitors, I assume that the circa 100 miles from Belgium to England
    posed no special problems. The more demanding task may have been
    measuring the time difference between the sound reaching on hydrophone
    and the time at which it reached another. I'd guess that used telegraph
    lines and an oscilloscope. (Similar systems were used on land by 1917 to
    locate German artillery batteries to guide counter-battery fire.)
    Alex later wrote:
    > For finding the distance, we used two devices: an optical
    > range-finder with base approx. 2.5 meters, and a radar.
    > The radar was substantially less precise in both distance
    > and direction finding, so it was used only at night or
    > when the clouds obscured the sky.
    That is interesting. Russian optical rangefinders must have been
    exceptional equipment -- or else the radar very poor. Could it have been
    that the optical gear was better suited to picking out an individual
    target and swiftly getting an estimated range, rather than that it
    provided a highly-precise range? The problem with the early naval radars
    was often in making good use of the data, rather than data precision,
    which should be much, much higher than that of any optical rangefinder.
    Finally, Fred Hebard wrote:
    > I was thinking of the Bismark destroying the Hood.  That may have been
    > at night, however.
    Admiral Holland's intent had been to commence the action at 0200 GMT,
    which in the Denmark Strait in May was immediately after sunset.
    "Bismarck" would have been silhouetted against the afterglow, while
    "Hood" and "Prince of Wales" would only have been visible from
    their mussel flashes. In practice, the Germans altered course somewhat
    westward (following the ice edge, not aware of the approaching British)
    and by the time the two made contact it was nearly 0600 GMT.
    Mullenheim-Rechberg (in the after gunnery control aboard
    "Bismarck") later reported that the eastern horizon was already light.
    Certainly, the ships were visible from one another before the firing
    In any case, that battle took place early in 1941. I don't think that
    any of the ships involved yet had gunnery-control radars, though all had
    search radars and the new set aboard "Suffolk" played a major part in
    the events.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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