An aircraft ranging device

Te

15

26

4RAV

by

SCAlPAlI

1917

and

LIE N JO E. OLIBIES, PHILPPI E SCOU , U . AMR AM\

Graiuate, United. State N~aval Academy 19W5

Submitted in Partial Fualfillment of the, Requirement

for the degree of

in

from the

LSSE'GEliTrES RiZINTM' OF TECHNOLOGY 1986

Signature of Authors.

OC=i1OCAION BY TIE DE2ABIWMa OFi1O tALy 1"a.I1 lING Professor in Charge of Researcho

Chairman of Departmenta1

Committee on Graduate Stude ...

) +

TABILE OF COEMMeTS

Page

liroQduction...•...

I

Acanewledgment ... o .* .... ... ... 2

Sttemsent of the Problem ... ... 3

I Requrements of the Ideal Instrument... . 3

II Possible eMeas of Detection and Location...

4

By Sound

Waves

By

By Li&ht Waves*...•... 7

By Heat 1vess.*..*4..*** ... *...*... 7

III

Instruments

for Detecting Heat Wvea... ... lb

The Radimeter...-

O

...

The Bolometer ... ,,... l... 10

The Thermopile... ...

11

The Photo-electric Cell... 12

IV .The Problem f Rage... ... 13

Products of Combustion of Exhaust Gas ... 1Z The irror... 14

INTRODUCT ION

The subfect of locating: aireraft at night by means of the heat radiation of the exhaust was suggested to us by

Professors V. Bush and J. W. Barker. Realizing that one of the greatest problems of the Coast Artillery Corps at the present time is the location of aircraft at night, and

believ-ing that the present method based on sound does not fully

ftulfill the requirements ot a good aircraft, detector, we at once saw possibilities in the idea which warranted, its being

tried out.

&-1-In this work. not enough thanks can be given to

Pro-foesor J. W.. Barker whose constructive suggstions and untir-ing effbrts in our behalf were a great factor in makuntir-ing it

poasible to carry on the work in the face of mayr ~-iMtt lties.

Due thanks should also be given to Dr. G. L. Clark, Professor A. C. Hardy, Mr. F. V. Cunningham, Professor G. W. Pierce, Mr. T. i. Case, Dr. V. WV Coblentz and Professor H. B.

Phillips whose timely suggestions were of great aid.

Ve take. this occasion, to e~press our appreiation

to the Electrical Engineering Department of the Massachnsetts

Institute of ~acEbnology, The Mar Department, The, Bell

i

[-2

TER PROBIll

?bhe problem

is to devise

a means by which

a

plane

may be detected

and its

position located at night.

I

BEQUIREMENTS Or AN

UMIZUlT

UR DETECTIMG AND LO=APING

AICRBMA

-

AMI

XIG

In any definite problem there is always an ideal

which we should strive

to

attain. Human nature however, is

so inadequate that this ideal can

not always

be reached,

consequently,

the

solution

that can best appioach

the

optimm mst of necessity be the end to strive for. To our

mind

the ideal

instrment

for locating aircraft at

night

must fulfill the

following requirements:

It

must be

instantaneous in action, that

is,

the

disturbance caused by

the

plane

which

actuates the

detect-ing instrument should

not take any

appreciable

length of

time to travel

from

the plane to the

detector.

The signal

produced by

the

detecting instrument should

activate the

detecting tense

to a maximm

degree.

The

instrument

must

be

accurate in-its location of the plane position and capable

of following it.

The

range at which it is able to detect

iC

must be a maximum. It should be sensitiver to the disturbance crested by the plane only and insems4tve to all outside &isa-turbiMagentsf. The instrument meist be able to distinguish between planes of a group at all times so that there may be

a means by which the Battery Commander can "check" on the

plane being tracked. It should elimilmate the use of the,

seachlight so as to gain the advantage of surprise on the

enemye Finally, it mnst be portable and of rugged construction

in order to withstand service conditions, and mest have a

minimum of adjustments so that it may be operated by the

average enlisted man.

II

POSSIBIE MEHODS BY WHICH A PIAEL ALY BE LOCA~D AV, NIfIG

Assuming that we have a plane in the air at night,

the question is, what means have we available for locating its-position? First there is the disturbance in the air cteated by the propeller and the detonations of the exhaust which produce sound waves which may be used as a means for detection. Then there is the electric disturbance in the ether created by the ignition system of the engine which causes electric waves to be propagated through spaceand thus become an agent for detection. Also, there is the power

dissipated throtgh the exhaust in the form of" light or visible

rays

and

infsrared or

heat

rsqv

These

rays

may also be used

asE

a mUans

for detecttig

the-

pxaeeaee

of the plane.

Sound waves as a means for detecting airplanes at

night is the basis of the method now in use. It has the advan-tage of great volume, and simplicity and ruggedness of consg-truction. Against these however, may be urged the following disadvantages: Sound travels at the rate of 1100 feet per

second, consequently, it takes an appreciable time to reach the receiving apparatus so that the wave front actually receiv-ed startreceiv-edC at a point whti h is a - considerable distance from

the present position of the plane. This is called time lag , Iutrthermore, the velocity of the wind affects the speed of both the sound waves and the plane causing a fUrther

devia-tion in the located posidevia-tion. This is called wind lag. These deviations which are not susceptible of accurate prediction and correction make it impossible to accurately locate the plane. Moreover, dne to the fact that there is no means of knowing that two receiving apparatus are on the same plane of a group, firing data can be obtained only through the use of searchlights. This is a decided disadvantage as it reveals

It is a well known fact that the ignition system of

a gas engine acts as a spark gap transmitter producing radio waves which are propagated through space by the leads from the

distributot to the spark plugs acting as antenna. The action is as follows: The condenser across the breaker points in

con-junction with the breaker points and the primary of the

induct-ion coil produce radio frequency oscillatinduct-ions in the primary circuit. This action induces radio frequency oscillations in the secondary coil which is in series with the spark gap and the leads to the spark plug. In effect, we have a radio trans-mitting set which radiates energy through space as explained

above. Dr. G. L. Pickard states that during his experiments

on the polarization of short radio waves, he experienced inter-ference from passing airplanes. He estimated that the signals

from an airplane engine could be received at a maximum distance of one mile. By using the directional properties of coil

an-tennas it is possible to obtain the azimuth of the plane,

however, since its unidirectional property depends on the symmetry of the coil capacity to ground, the location of the

.plane in elevation would be difficult of attainment. This

method has the advantages of (1) the speed of the radio wave

which would eliminate time and wind lag, (2) ease of detection

under unfavorable weather conditions and (3) comparative

S-8-simplicity and ruggedness of constraction. The disavantages

are, the interference from other radio signals due to the

fact that the wave lengths emitted are in the ban lincluded between twenty and forty meters- and its inability to locate

the plane accurately.

Farthexmore, the greatest disa&vantage

is the facility of shielding the leads so that the energy emitted would- be such as to limit the range to a minimum

which would be eatirely inadequate as a means for defense. Vision used as a means for detecting adrla ea

is the method par excellance. If it were possible to use the method of vision at night., the problem would literally solve

itself.' There is a possibility of using this ifetho as the

exhaust actually emits a flame visible at night which is due

to the presence of incandescent carbon particles therein.

The range at which the flame is visible is limited, but by using a telescope it may be increased. However, this stream

of flame, being limited in length, may easily be obscrired

from viewr by shielding.

The beat waves emitted from the ezhbust of the

p;'ane would be a convenient means of detecting and locating

its position if a sufficiently .ensitiave device were available

for this purpose. These heat waves have the speed of light;

method based on. sound, would be eliminated. The wave- being electro-magnetic, it isen6Vtdeflected by the wind, consequent-_ ly, there is no wind lag. The above being true, the plane may be accurately- located and by employing twoe. detecting instrw-ments aefither end of a base line, f"ring data may be obtain-ed without the use of searchlights. Moreover, by the use-of selective detectors which make use, of the periodicity of impilse of the exhaust, a means is available to ascerta i Swhether or not the two instruments are on the same plane.

One disadvantage of this methad' lies in the fact tht there

is a certain amount. orfabsorption of the heat energy in the

atmosphere which would tend to reduce the range Alsoe, the exhaust may be shielded to acertain degree, but this shield" ing is limited by the loss of officiency of the engine. CoQn-sidering- the, fact that the defense is primarily against bomb-ing planes with heavy loads, the engine efficiency can not be greatly reduced without materially limiting their offensive

value.

Of the four methods described above, those by vision

and radio waves may at once be eliminated as proper shielding

'would reduce the range obtainable to such a degree that the defensive value would be negligible. The problem therefore reduces itself to a choice between sound and heat waves as a

-8.-means of detection. From a consideration of the previous

dis-cussion of

these two methods, the

follwoAng points may be

location. Moreover, the heat method has the advantage of selectivity mentioned above, whereas sound does net. Under

these considerations it would be possible, using the heat

Smethod to make an accurate track of the plane- and obtain i firing data without the use of searchlights, thus gaining

the advantage of surprise on the enemy. A comparison of the

maximum ranges of the two methods, which is of primary impor-tance, can not. at present be made. However, granting that

sound would give the greater range, the combination of the

two methods would still allow the searchlight to be eliminat-ed as the heat method would take i~ ass iged mission of

tracking the plane. For the above reasons and the fact that

the sound method has not been a complete success, the method

using the heat radiation from the exhaust has been selected

for the-solution of this problem.

-9

!

t

-III

INSMMlENMS WHICH MAY BE USED FOR DETECING EAT 1LVES

Heat energy, being invisible, requires the use of

instruments which transform this invisible radiation into

some other form of energy, the effects of which can then be

detected. Various instruments of this kind have been devised, most important of which are the radiometer, radiomicrometer,

bolometer, thermopile and photo6-electric cells.

The radiometer which is extensively used in

infra-red investigations, is a very sensitive instrument, but due

to its suseeptibility

t/

the slightest vibration, it

is neo.

cessary to isolate it f=rm all possible mechanical vibrations

during the progress of an investigation. This makes it so de-licate an instrument that its useif restiibted toPtbhe labora4

tory.

The raiomicrometer whih is in effect a moving coil galvanometer is aS sensitive as the radiometer, but it has the

same disa4dvantage in that it i too delicate an instrument to

be used under service conditions.

The bolometer is a more rugged instrument than the two previously mentioned, but it requires a very elaborate

installation and is difficult to keep in adjustment. It is not thearefore an entirely satisfactory instrument for field

-10-use.

The the thermoile is the saimplest type of instrument for detecting heat waves. It is zQdte rugged and may b-e made as se sitive as any of the aeabve instrumentsi It has already been employed for the detection of heat energy from the exi haust of an airplane. In 1919,~~ael 0. Hof man used a croase-hair thermopile the separate elements of which (vertical and

horizontal) were separate piles of thirty-two couplea each, electrically insulated from each other and connected to

separate galvanometers. The thermopiles were placed at the focus of a twenty-four inch mirror provided with slow screw movements in azinth'and elevation. With the plane developing fifty horsepower at a range of two thousand yards, an average galvanometer deflection of ten centimeters was otained. No

difficulty was experienced in picking up" and tracking the

plane. However the slighteat whiff of cloud drifting across

the field produced deflections in the galvanometer of the

on the angle of presentation of the exhaust rather than on the range.

The disadvantage of using delicate instrumenta- such

-11-as galvanometers when a thermopile is used may be overcome

if a tride valve and telephone were substituted _{for the}

galvanometer. By placing an interruptor in series with the

thermopile and primary of a step-up transformer, the

palsa-ing voltage induced in the secondary may be impressed on the grid of the, first tube of an amplifier , thus making it

posaib-ble to detect the thermopile current by means of a telephone. This arrangement would give the further advantage of greater

sensitivity. The chief disadvantage of the thermopiie, how-. ever, is its non-selectivity as it does not distinguish between the heat radiation from the exhaust of a plane and any other source in so far as concerns detection. If a ther&'

mopile could be constructed such that it would be sensitive

,

to a definite band of

wave lengths only, this disadvantage

would be minimized and the method would be of extreme value. It is an open question, however, Whether or not such a thermo-pile could be constructed.

The extreme selectivity of photo-electric cells g-ive them a decided advantage over any other type of instruw

met that may be used for detecting the heat radiation from

the exhaust of an airplane. If the cell used _{has a maximum}

sensitivity in the region where the energy distribution curve

of the exhaust gases is a maximum, W would have- an apparatus

,-l

that would be very sensitive to the heat radiation from the plane. In addition, any other source emitting rdiatio of wave lengths greater or less than those emanating from the exhaunt would not be detected by the cell. Moreover,, since the heat is&taiated in periodic trains corresponding in frequency to the periodicity of impulse of the exhaust, if the cell could recover its resistance between these periodic trains, the output of the cell would have the same frequency

as the number of impulses of the exhaust and the advantage,

previouly mentioned, of having a means to ascertain whether or not two instruments are on the same plane, would be gained.

IV

TM PRBsI= OP RAEGB

The products of combustion in the exhaust of an

car-bon dioxide and carcar-bon monoxide produce radiations which are readily absorbed by _{the -atmosphere. This abnormal dissipation}

reduces the range obtainable from their radiations to a

mini-mm. The incandescenat carbon particles produce most of the

visible flame emitted from the exhaust and at the same time

'are sources of black body radiation. The emission spectrum

-of

this

radiation which is

a maximum at about 1.28.

to l.- U

(U m

10-4cm.),

is supposed to be smooth and continpous, hence

order to obtain maximum range, for detection.

Due to the dissipation of energy in black body

radiation which is believed to be inversely as the square

of the distance, the range obtainable is limited. By

employ-ing a collecting apparatus for concentrating the rays, such

as arparabolic mirror, this limited range may be increased considerably. The collected rays or amount of energy

receiv-ed would then vary as the area.or, in other words, directly

as the square of the diameter of the mirror. The choice of dimensions of the mirror is governed by the sensitivity of

the detecting instrument placed at its focus and also the mission assigned. After the ultimate sensitivity of the

detector has been attained,, the only means of increasing the

range is by increasing the size of the mirror. On the other

hand, its portability must be taken into account. If it is to be used for fixed defense, the mirror may be made as

large as desired consistent with ease of operation, but if it is for field use, its portability must necessarily limit

its size. The material of the mirror must also be considered.

14-The surface mat have a minimum of absorption for the wave lengths. of the rays to be reflected. Measurements of the

reflectivity to infra-red r diation by various metals have aided the choice of suitable materials to be employed. Highly polished metallic mirrors give good reflection for wave

lengths in the neighborhood of 1. U. Platinum or silver electr -deposited on a glass surface, on account of their high resistance to the corrosive action of the atmosphere

and their high reflective: power, are also suitable for this

-^ purpose.

NOTE:

The remainder of this thesas, being of a confidential nature, is not included ii this:paper. It is, on file7

at the r

Department in Wshington, D. C.

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pl6Z

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