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A proximity fuse is a fuse that is designed to detonate an explosive automatically when close enough to the target to destroy it. By sending out
radio waves that are reflected by
the target and comparing the frequency of the outgoing waves to the incoming waves, a radio proximity fuse uses the Doppler effect to determine its proximity to a target. When the incoming waves sharply decrease in frequency, the target is at its closest
possible position.
Before their invention, detonation had to be induced either by direct contact, time since launch, or height. All of these have
significant disadvantages. Getting direct contact with a relatively small moving target is hard (even ignoring the effect of
wind); to set a time- or height-triggered fuse one must measure the height of the target (or even predict the height of the
target at the time one will be able to get a missile in its neighbourhood).
With a proximity fuse, all one has to worry about is getting a shell or missile on a trajectory that, at some time, will pass close by the target (that still is a
significant problem).
Use of timing to produce air bursts against ground targets requires observers to provide information for adjusting the timing.
This is not practical in all situations and is slow in any event. Proximity fuses remove these problems.
The proximity fuse was developed through a US and British collaboration during World War II. Vannevar Bush, head of the US Office of Scientific Research and Development (OSRD) during this
war, credits it with three significant effects. It was important in defense from Japanese Kamikaze attacks in the Pacific. It was an important part of the radar-controlled anti-aircraft batteries that
finally neutralized the German V1 bomb attacks on England. Third, it was released for use in
land warfare just in time for use in the Battle of the Bulge,
where it decimated German divisions caught in the open. The Germans felt safe from timed fire because the weather prevented
accurate observation. Bush cites an estimated 7 times increase in the effect of artillery with this innovation.
See: Pieces of the Action by Vannevar Bush, William Morrow and Co., inc. 1970
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