at a serious disadvantage if attacked by enemy aircraft, as their
duties tie them to a comparatively small area at a fairly low height.
To defend each of such machines by an aerial escort would absorb
too great a number of fighting aircraft, and so the duty falls most
frequently on the anti aircraft artillery and such machines as are
allotted for air defence work provided that the latter can be directed
to the spot in sufficient time to provide the protection required.
IV. The Defence in General.
It will now be realized that air defence is required both in the
actual theatre of active operations in the face of the enemy, and
in areas far to the rear of the fighting line, so long as the enemy has
machines capable of reaching those distant points and returning
again from them. Bombing attacks may be met anywhere, i.e.
both in the forward area of ground operations the " Front "-
and also in store depots, bases, ports, and large cities far removed
from them. Low-flying machines with bombs or machine-guns
may be encountered far in rear of the " fighting line," but prin-
cipally in or near it and over the communications imme-
diately behind it; so that, as a broad general rule, the nearer
the " line " the greater will be the proportion of low-flying
targets, and vice versa. Torpedo-carrying machines will be met
with over the sea; and photography machines anywhere be-
tween the " line " and points far in rear of it on the lines of
communications.
In order to place defending aeroplanes in positions favourable
for engaging their targets, it is necessary to obtain information of
the attack in sufficient time. This leads to two great essentials in
any scheme of air defence, namely: (a) intelligence, and (6)
communications.
(a) Intelligence can be treated under three headings:
(1) during peace, and before the beginning of an attack in war;
(2) during an attack; (3) immediately after an attack.
Intelligence before the beginning of an attack includes
information obtained during peace of all the resources of a
possible enemy; his preparations and probable intentions; with
the numbers, details and performances of his machines both
civil and military. On such information will the whole scheme
of air defence of a country and its forces in the field depend. In
peace such information can be collected, compiled, and as-
similated in a careful and comparatively slow manner. But
directly a state of war arises, speed in the collection and trans-
mission of that intelligence to those whom it most concerns, i.e.
the executive in the air defence services, becomes the prominent
factor. The authority responsible for the collection of that
information has to add comparatively suddenly to his ordinary
peace-time duties that of rapidly tracing the movements of both
hostile and friendly aircraft, as by no other method can an
officer check information sent to him by his observers. Only on
the efficiency of the preparations made for the use of telephone,
telegraph, and other signals can he hope to issue the warnings
which will be required by the population to enable them to take
cover during a raid. The state of war may even be heralded by
the air attack itself, and there may only be a matter of a few
hours for the transition from " intelligence duties during peace
and before an attack " to " intelligence during an attack."
It will be best to consider a concrete example, which will show
perhaps more than anything else the necessity for speed.
Take an imaginary city with an average radius of 12 m., with
its centre situated 30 m. west of the sea. One night a ship 60 m.
east of that city reports a number of aeroplanes as having been
heard passing high overhead, going west at an estimated ground
speed of lop m. per hour. The message, which is probably sent
" in clear," is picked up by some coastguard station, which sends it
to the local senior naval officer and so to the military garrison com-
mander near at hand. These officers, after digesting the report,
and confirming it if possible, send it on through their respective
headquarters to the central organ of the system. Thence it goes
to the railways, to the police, and to air defence headquarters, who
give the alarm to the railroad men, to the civil population, and to the
squadrons, guns, and lights, etc., of the defences, respectively.
The defending squadrons will probably be situated from 15 to
20 m. from the centre of the city, i.e. about 40 to 45 m. from the
source of the report. At the squadron aerodromes the pilots, who
are waiting ready to start up the machines, " taxi " over the aero-
drome, and then " take off and begin to climb to predetermined
heights, as the real height of the attack cannot be known at the
moment.
A little time-table will show the time probably left to them to get
up to, say, 10,000 feet.
minutes
Time taken by attack to travel 40 to 45 m., say . 27
Ship to shore ... .... 5
Coastguard to local H.Q 2
Local H.Q. to main H.Q 2
Main H.Q. to Air Defence H.Q. . . .2
Air Defence H.Q. to units i
Starting up machines, " taxi-ing " and taking off . 5
Total (say) 17
Leaving the machines to get their heights in . . 10
A single report of this nature would suffice to send out an alarm
far and wide, and turn the defence posts over a vast area into
seething points of activity ; whilst there might be nothing whatever
to show that those machines were hostile, or that if hostile they were
going to attack the city in question. The initial probability was
that they were hostile; and as they happened to be going west at a
point 70 odd miles east of the city, the time required to get the de-
fending aeroplanes into position would leave no option but to
assume that the attack was coming to that city. Yet the attack in
this instance might easily turn aside as soon as the coastline was
made, in order to proceed to some other objective; there was no
certain indication beforehand of the real one, and there may never be.
The foregoing example shows that the observer system of a defen-
sive organization for a big " vulnerable point " must extend to a
radius of from 70 to 100 m. from the probable main objective of
hostile attack by air if the executive is to have sufficient time to
get its defences into a state of readiness for action, and the civilian
population and railways properly warned of the approaching danger.
As soon as the attack enters the area in which anti-aircraft
posts exist, each of such posts within sight or earshot of the
attack becomes a potential source of information. It remains
then for the commander of the air defences to organize a system
of speedy intelligence within his own command, which can be
supplemented by reports collected from police and railways,
which may or may not assist in checking the reports received
from the defence posts themselves. This system continues its
work until such time as the attack withdraws to a point outside
its boundaries, when intelligence is again required from outside
sources until it is certain that the engagement is over.
Directly after the attack it becomes of importance immediately
to check the commander's ideas of the battle, to supplement them
with local details of what actually happened, and to compile as
complete an account as possible, showing: Nature and numbers
of aircraft employed on each side ; routes followed by attack and
defence; casualties to personnel and material; number and nature
of bombs dropped; expenditure of ammunition; size, speed, and
manoeuvres of enemy machines; new features of machines, if
any; efficacy of communications; weather conditions, etc. This
report is of high importance and may enable a commander, if it
is compiled and issued rapidly, to dispose his forces afresh in
sufficient time should features in the attack show this to be
necessary.
In this connexion, it is important to note certain peculiarities
of air-defence information. A report on the position of aircraft
in movement is incorrect the instant after the observation is
made, unless the time of the observation is given. The value of
the report decreases with every moment that elapses after the
observation. To be of value at all it must specify whether the
aircraft was seen or only heard; if the former, whether friendly or
hostile; and the time of the observation. To be of real value, it
should contain data as to the direction of flight, the number and
type of the machines and their height. One of the outstanding
curiosities of the air raids over England was the remarkable
inaccuracy of the reports rendered by eyewitnesses which were
received at the various headquarters. To men who have been in
the services the hypothesis that the man " on the spot " knows
"what is going on and therefore knows best what should be done,
will be familiar. The history of anti-aircraft operations during
the war abounds with instances showing the fallacy of that
hypothesis.
During the aeroplane raid of June 1917, over Sheerness, Graves-
end, Wrotham, and Folkestone, two independent reports were
received of an airship approaching London in broad daylight from
Page:EB1922 - Volume 30.djvu/125
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AIR DEFENCE
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