Brezk-íslenzk viðskipti - 01.12.1946, Blaðsíða 16

The Decca System of
Navigation
British Invention will
help in Denmark’s
Hydrographic survey
Dial-tyþe Indicators of Decca Greenland.
Navigation (by courtesy of
“ The Shipbuilder and Marine
Engine-Builder ”).
LAST July, the English chain of marine
navigational stations, established by the
Decca Navigator Co„ Ltd., of London, was
declared officially operational by the Ad-
miralty and Ministry of Transport. This
group of Decca stations is located in the
South of England and comprises a master
station at Buntingford, Herts., with “ slave ”
stations at Stokeholy Cross, near Norwich,
and East Hoathley, near Lewes, Sussex; a
third “ slave ” station is nearing completion
at Wormleighton, Warwickshire.
In the Decca system, the principles of
“ hyperbolic navigation," evolved during
the war, more especially as an aid to air
navigation, find their expression in apparatus
which can be easily accommodated in a
relatively small space on board ship.
These are the principles involved. A
well-known geometric property of the
hyperbola is that the distances of any point
on it from the two foci of the curve differ by
a constant, whose value depends, however,
on the particular curve of the confocal family
to which the hyperbola belongs. (Two or
more hyperbolas having the same foci are
said to be confocal.)
IF, then, the time interval between the
receipt of synchronised electromagnetic
pulses from transmitters at the foci is
determined, the known speed of the electro-
magnetic signals determines the difference
between the distances of the receiver from
the two transmitters (í.e., the difference in
focal distance). The receiver is thus located
on a particular curve of the confocal group.
Two suitably disposed confocal systems
superimposed on a navigational chart con-
stitute a “ grid,” whereby a “ fix ” may be
obtained independently of sextant and
chronometer observations. If signals from
two pairs of stations are employed simul-
taneously, a precise “ fix ” is obtained at
the intersection of the two “ grid ” curves
concerned. The speed with which the
signals travel (186,000 miles per second)
affords virtually instantaneous indications,
so that the movement of the ship can be
continuously plotted on the chart.
The master station and the three “slave ”
stations each have a “ grid ” system—one
for each master and “ slave ” combination.
To facilitate identification, each combination
is characterised by a different colour; and,
in the actual charts, the three families of
curves are printed in colour, namely,
Stokeholy Cross, red, East Hoathley, green,
and Wormleighton, purple.
Transmissions from the chain of stations
are maintained throughout the 24 hours of
each day, thus affording an excellent navi-
gational aid to ships and aircraft equipped
with Decca receiving apparatus. The area
covered by the initial service now in opera-
tion extends to a minimum distance of 300
miles from London. In daylight hours,
transmissions from the stations can be
utilised for navigational purposes upto more
than 1,000 miles from the stations.
THE Decca receiving equipment—in con-
trast tothe complex and bulky apparatus
first employed during the war—is especially
simple and convenient, the necessary
data for a navigational “ fix ” being dis-
played directly on three dial-type indicators,
whose readings are correlated with the
“ grid ” superimposed on the chart in
use. The indications, it is learned, are
accurate within a matter of yards.
Indeed, it is understood, the system is of
such accuracy that, in coastal waters and
estuaries, it will constitute a valuable aid to
pilotage. As will be appreciated, however,
the ship set is not designed for the reception
of reflected signals, as is the normal radar
equipment; in busy or congested waters,
therefore, it cannot supersede visual navi-
gation nor replace the assistance afforded
by the plan-position indicator, which
displays at a glance the position of other
objects with respect to the ship. Never-
theless, the precise information of the ship’s
position afforded by the Decca equipment
greatly simplifies the whole problem of
navigation, particularly in that it enables
maximum attention to be focused on the
avoidance of collision, removing, as it does,
all anxiety and uncertainty as to the exact
position of the ship.
The principal feature of the stations—both
master and slave—is the aerial array,
mounted on a steel tower, 325 ft. in height.
At the master station there are, besides the
aerial tower, a transmitter building and a
reserve-power building, the “slave ” stations
being similar, except that the transmitter
building houses automatic phase-control
equipment, whereby the “ slave ” signals
are locked with those from the master to
secure the very accurate synchronisation of
the transmissions on which the system
depends.
Extreme precautions have been taken at
all the stations to ensure that failure in the
mains supply of electricity, or in the radio
equipment itself, shall not immobilise or
dislocate the service.
The immediate purpose ofthetransmitting
stations is to provide a means whereby
extensive trials of the system may be con-
ducted by governmental and service authori-
ties, with the object of securing further data
as to its efficiency and general performance,
having regard to its possible adoption as a
standard navigational aid.
The “ chain ” has, in fact, been established
by the Decca Company for the experimental
use of the 'Royal Navy during survey and
minesweeping work in the Thames Estuary
and parts of the North Sea, and the Ministry
of Transport have taken advantage of its
existence to test the usefulness of the system
to the mercantile marine.
OR this purpose, the Admiralty have
arranged to provide charts for the areas
around the Thames Estuary and extending
to the coast of Denmark, and to the Straits
of Dover in the south, while the Decca
Company are making receivers available to a
certain number of merchant ships trading in
those areas. The ships’ officers will be invited
to comment on the system. Concurrently
with these operational trials, scientific tests
are being carried out to determine the
accuracy of the system at different distances
from the transmitting stations and the areas
in which the system can be reiied upon by
day and by night, respective'y.
The company are investigating the
possibility of modifying the system so as to
provide what is known as “ lane identifica-
tion,” which will enable a vessel to deter-
mine its position on first entering the area
of coverage of any chain of stations. In an
announcement, the Ministry of Transport
(Continued on þage 21)
----ENSKU KUNNÁTTA ÞIN----------------------------
Vankunnáta á fungumálum vill oft verða tálmun milli þjóða.
Margir íslendingar geta ýmist talað eða lesið ensku. Áðrir
munu eflaust vilja auka kunnáttu sína í þessu tungumáli.
Innihald B.Í.V. mun þessvegna verða á tveim tungum.
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