Robert Watson-Watt (1892-1973)

An instantaneous direct-reading radiogoniometer



then ensures that very little correction is necessary in
most cases, whilst the correction even of large divergences
can be effected.

                        III. PROPERTIES.

In its application to signal work the system has many
interesting and attractive properties which are believed
to offer considerable advantages over earlier systems,
and a brief enumeration of some of these may be

Some outstanding advantages have already been
described in sufficient detail to show that the device
will provide an automatic visual and direct-reading
radiogoniometer, capable of operation by the navigator
on the bridge or in the chart-room, without requiring a
knowledge of morse, and that it will deal with signal
trains of exceedingly short duration.

Attention has not yet, however, been called to one
of its other outstanding advantages. The difficulties of
direction-finding in cases of even slight jamming are
well known. The cathode-ray direction-finder alone has
the valuable property that it will give correct bear-
ings, simultaneously, even in the extreme case of two
or more signals of the same frequency and the same
field strengths arriving from different azimuths during
the same period. That this is so will be seen from the
fact that the ionic beam gives instantaneous response
to every impulse, so that if a marking impulse arrives
from one transmitter while the other is spacing, the beam
will trace out a line having the bearing of the former
transmitter, and conversely. There are three types of
pattern which may be obtained in practice. It the
stations are working independently at hand speed, then
the marking periods in both cases are interrupted by
comparatively long spacing gaps, during which many
marking impulses from the other station will arrive.
The predominant features of the image on the fluorescent
screen will then be two bright lines, each indicating the
correct bearing and amplitude of the corresponding
signal, and standing out perfectly clearly from a back-
ground of faint fluorescence. If the stations are working
independently at high speeds with very brief spacing
periods, then the pattern becomes a parallelogram full
of fluorescence the clearly defined sides of which are
respectively parallel to the two required bearings, and
of lengths proportional to the two signal strengths.
The faint background referred to in the first case is also
a Parallelogram of this type. The clear definition of the
edges of the figure is of course due to the fact that each
point on them is a stationary point on the high-frequency
path of the indicating spot, and has therefore a relatively
long exposure, giving correspondingly greater brilliance
to fluorescence. Lastly, in a case which is never likely
to be met in practice, if the two transmissions are not
independent but fed from the same source, then the
pattern becomes a generalized Lissajou figure, from
which the two bearings may be inferred. Excluding this
case, however, it is seen that in the worst practical case
of jamming, two identical stations sending identical text,
on identical frequencies and with identical field strengths,
the two bearings are still read directly and easily.
Increase in the number of the jamming stations merely
increases the complexity of the image. With three
high-speed stations, beyond which we have not had an
available jamming mixture on which to test, the three
bearings are still easily read simultaneously.

The next application of the special properties of this
device which calls for mention is that to the wide general
class of “bad minima” so troublesome in the older
systems. The cathode-ray direction-finder, with its
freedom from inertia effects and its discrimination in
amplitude, will throw light on the particular cause of
bad minimum which happens to be operative in any
given case, and will in very many cases allow a determina-
tion to be made, with a direct measure of its probable
error, in cases where otherwise no measurement at all
could be made or relied on. Thus the flickering bearing
due to intermittent contacts in metallic rigging will

[NLS note: a graphic appears here – see image of page]
FIG. 4.—Azimuthal distribution of atmospherics and
           thunderstorms on the 10th May, 1925.