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George wrote, in the midst of an authoritative message:

> For the range of altitudes that concern us in navigation (say 5 or 10
> degrees to 90 degrees) it doesn't matter at all HOW the temperature and
> pressure vary along the light path down through the atmosphere. It's just
> the end-value that matters.


Last time we discussed this topic, it was agreed that that was true if
the isoclines of air density were all approximately horizontal. What if
a celestial body is observed through a weather front? (Hard to do with
the clouds of a warm front but not necessarily impossible with a cold
front, where the isoclines are steeper.) Is the resulting error
necessarily always negligible?


George later added:

> Long before the days of satnav and radar, big naval guns
> were, I think, capable of bombarding a shore target that was out of sight
> over the horizon. (I know little about naval gunnery, so someone correct me
> please if that's wrong). The shore target coordinates may have been
> obtained from maps, but to lay the guns, the ship's position was needed, as
> precisely as possible. Any tool, such as a dipmeter, which enhanced the
> precision of an astro position, would have been well worth its keep.


Shore bombardment at such a range that the ship's position could not be
fixed by coastal methods was not developed until the 1914-18 war, when
Royal Navy monitors operated off the Belgian coast, bombarding the
German side of the seaward end of the Western Front.

Ian Buxton's excellent technical history of the monitors ("Big Gun
Monitors", World Ship Society 1978) contains some information on the
techniques developed. He does not mention celestial methods. One
alternative that he does record was to have a destroyer deploy an
anchored buoy and drop two depth charges. Hydrophone stations on the
English coast would detect the sound waves and determine the position of
the buoy, to within a few yards, by the time delay -- presumably using
the same hyperbolic geometry that we are now familiar with from
electronic navigation systems. The monitors would be informed of the
buoy's position by radio signal and would then position themselves
relative to it. (Buxton does not say how but these were ships with
rangefinders with baselines up to 15 feet. That must make visual
position-fixing a whole lot easier.)

With the ship's position determined, the gun layers adopted principles
developed by land artillery for indirect fire, in which they used an
aiming point at a known angle from the bearing of the target. Off
the Belgian coast, the monitors resorted to having a second ship further
out. Knowing the positions of both ships and the target, it was possible
to turn the gunnery director towards the second ship and have the guns
point for the target.

None of that would allow the sort of precision bombardment expected in
recent wars against Iraq. However, the monitors were content to land
their first shells anywhere near their targets, after which observers in
radio-equipped aircraft walked the gunners up to the target (which the
German's would be busy trying to hide with smokescreens before the ships
could find their range).

The 1939-45 war saw refinements of this system but no major changes,
except perhaps for the use of radar in fixing the ships' positions --
though that would have been of little help off Belgium in 1918 as that
coast likely provided no better radar targets than it did visible
objects to provide bearings. Maybe the U.S. Navy did something more
sophisticated in the Pacific. I wonder what advances were used off Korea
(where destroyers regularly shot up coastal targets but perhaps nobody
tried indirect bombardment from the sea) and Vietnam (where "New Jersey"
  used her 16-inch guns in the shore-bombardment role)? I imagine that
the latter saw electronic-navigation systems for precision position
fixing but it might have been difficult to set up a Loran chain off an
enemy-held coastline in the early 1950s.


Trevor Kenchington


--
Trevor J. Kenchington PhD                         Gadus@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

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