USRE21955E - Radiant energy distance determining - Google Patents

Description

Nov. 25, 1941. .1. G. CHAFFEE RADIANT ENERGY DISTANCE DETERMINING SYSTEM Original Filed Deo. 50, 1959 2 Sheets-Sheet l J. G. CHAFFEE Nov. 25, 1941.

RADIANT ENERGY DISTANCE DETERMNNG SYSTEM 30, 1939 2 SheeiS-Sheet 2 Original Filed Dc.

Ressued Nov. 25, 1.941 y UNITED STATES PATENT oFFlcE zur;

Joseph o. omnes, Hackensack, N. J., umm u Bell- Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York No. 311,723, December 30, 1939.

Application for reissue September 30, 1941, Serial No.

13 Claims.

This invention relates to an improved form of radiant energy distance determining system of the type which employs an exploratory wave, preferably a radio wave, the frequency Aof which is continuously varied between prescribed limits.

The illustrative embodiments to be described in 'detail hereinafter vare radio systems and apply the principles of the invention to the determination of the clearance of aircraft with respeci; to adjacent objects, particularly the surface of the earth above which the craft is`proceeding.

It is taught in the prior art, as for example in United States Patents 2,045,071 and 2,045,072, lssued June 2'3, 1936, to L. Espenschied, that the distance to a reflecting surface may be determined by directing toward the surface an exploratory wave the frequency of which is contin- 4 ucusly being varied between prescribed limits, re-

ceiving reections of the wave from the surface, beating the received reflected waves with energy directly received from the source of transmitted waves and determining the frequency of the beat note thus produced-to obtain an indication of the distance to the reilecting surface. Where, as is Ynormally required,` a relatively wide range of distances is to be measured without readjustments of the parameters of the system the arrangements of the prior art usually result in an inconveniently wide range of frequencies between the maximum and the minimum beat note frequencies. For example, in some systems of this vtype beat frequencies ranging between 50 and 50,000 cycles per second are produced. In reducing such systems to practice it has been found difficult to provide amplifying systems which will maintain the requisite properties with sufllcierit uniformity and gain over such a range of freduencies.

To avoid this and a number of other dimculties,

the systems of the present invention, as will appearinmore detail hereinafter, provide for hetj y erodyning the received reflected component of he exploratory frequency modulated wave with ond'wave of suitable frequency to produce a fziconven'ient intermediate frequency wave, the second waveL being frequency modulated in synchronism with and to the same extent as the exploratory waveV being set out. Within limits, which will be set out hereinafter, this will result in frequency modulation of the intermediate wave. the

extent of which will be directly proportional tcV the distance to be measured. p

'Ihe method of the invention includes the further step of amplitude limiting the frequency (ci. o- 1) Cil modulated intermediate frequencywave to effectively eliminate casual amplitude variations and thereby producing a distance indicating wave the sole significant'characteristic of which is the extent to which it is frequency modulated. The systems of the invention will therefore, obviously, be independent of variations in amplitudey such, for example, as those resulting from changes in altitude.

Accordingly, outstanding among the objects of the invention is the provision of altlmeter systems in which appropriate amplification of the reflected exploratory wave may be conveniently achieved.

Another object is the provision of an altimeter system in which very effective discrimination against noise interference may be readily provided. p

Another object is the provision of an altimeter system ln'which a single reliable characteristic of the distance indicating wave may be readily isolated and employed to actuate distance indieating means. l.

A further object is to provide an altlmeter system in which the distance indicating characteristlc of the distance indicating wave which is employed varies in asubstantially linear manner over the range of distances to be observed.

A further object is to provide an altlmeter system, employing a frequency modulated exploratory wave, the distance indications of which system are not affected by amplitude variations of the reflected exploratory wave.

Another object is the provision of a radio altimeter system in which the operating range may be conveniently changed.

Other objects will become apparent during the course of the following description and in the appended claims.

The principles underlying the present invention may b e briefly explained as follows:

If two radio frequency oscillators, differing in mean frequency by such an amount as to yield a suitable intermediate frequency wave when their outputs are impressed upon a detector. are frequency modulated exactly in phase and to an exactly similar extent, upon detection an unmodulated intermediate or difference frequency wave will be obtained exactly as though both oscillators had not been frequency modulated.

However, if the output of one oscillator traverses a path having a different time delay characteristic from the path'traversed by the output ofthe other oscillatorand the two outputs are then combined and detected. the resulting intertenna. At the output of the modulator one aideband resulting from the modulation processy is segregated, by means of an appropriate' bandto be measured) is relatively small, the relationship will be substantially linear over the range of distances to be measured. The frequency` modulated intermediate frequency is amplified, limited in amplitude to an extent suiilcient to remove all amplitude variations and impressed upon a conversion circuit.

' The function of the conversion circuit is to convert the frequency modulated wave into an amplitude modulated wave. A discussion of 'I'he Detection of Frequency Modulated Waves" is given in my article of that title published on pages 517 to 540, inclusive, of the Proceedings of the Institute of Radio Engineers, volume 23, of May, 1935.

The output of the conversion circuit is passed through a second detector and the low frequency signal resulting is supplied to an indicating circuit comprising, in one suitable form, a filter, a linear rectier, and a direct current meter.

'I'he pass-band of the illter may be made extremely narrow since it need pass only a single frequency, namely, that representing the frequency modulation of the exploratory wave. It may, therefore, Aafford very effective protection against noise interference, that is, interference resulting from stray or unwanted currents, which arise from aemultitude of heterogeneous causes as is welllmown in the art.

'I'he meter can be calibrated in terms of time delay, or preferably in terms of the distance to the reflecting surface, which is obviously directly related to thediil'erence in the length of the two paths traversed by the two oscillator outputs,

prior to their combination in the first detector.

The path of the output of one oscillator, in a system designed for use as a radio altimeten'obviously includes a radio link. That is, one oscillator energizes an antenna the wave from which is directed toward the earths surface. A component of the wave is received after reflection from the earths surface and is combined with the output of the other oscillator and detector, etc. as described above.

The metallic paths of-the two oscillator outputs are preferably adjusted to be of like electrical characteristics so that the differences obtaining between them at the first detector will result solely from the radio link. i

The indicatingV meter is preferably calibrated so that the distance to the'reilecting surface may be read directly in feet.

In balancing the metallic paths, as above mentioned, relative phase and amplitude adjustments can, obviously, be made by changing the relative phase andamplitude, or extent, of the frequency modulation of the two oscillators as an alternative to the method of introducing relay in the output of one of the oscillators.

As a modification, to avoid the necessity 'of maintaining two radio frequency oscillator frequencies with great precision, a single, frequency-modulated, radio frequency oscillator can be employed and part of its output can be' combined in a modulator with ailxed frequency wave -having the frequency desired for the intermediate frequency, while the remainder of its output n employed to energize the transmitting anpass filter, from the other products, and is used in place of an independent source of frequency l modulated waves.

A further modification of the method of the invention oil'ering still more advantages, as will become apparent hereinafter, comprises employing'a single, frequency-modulated, radio frequency oscillator, its output being combined in a modulator with a fixed frequency wave having a particular frequency determined by, but not identicalwith, the frequency desired for the intermediate frequency. At the output of the modulatontwo side-bands resulting from the modulation processand differing substantially in frequency, are segregated by appropriate bandypass filters, and one side-band is employed to 20- energize'the transmitting antenna while the other is introduced, through an appropriate phase adjusting means if such is found necessary, into the first detector for combination with reflected components of the transmitted frequency.

The outstanding virtue of such an arrangement is that the resulting intermediate frequency `diilers from the frequency of the fixed frequency beating oscillator. This results in a reduction of cross-talk difficulties and simplifies the shielding problems. A further advantage is that the metallic paths of the two side-bands selected are more nearly of like character, which facilitates the initial adjustment of the system for zero distance. In fact, if, as may readily be done, the two band-pass filters are designed to have similar delay characteristics it is possible to dispensey entirely withA thephase adjusting means.

Direct transmission between the transmitting and the receiving antennas should preferably be reduced to a minimum by making them highly directive; spacing them, in so far as it is practicable to do so, to avoid coupling between them and by lother similar means, well known to the art. If a suiilcient reduction in direct transmission from the transmitting antennav to the receiving antenna cannot be conveniently effected by simpler means, it may be balanced out by the well-known method of feeding "some energy -of the transmitted frequency directly to the relinear response over the range of interest for any particular system. Sucha'circult is included, for example, in Fig. 5 of my article entitled The Application of Negative Feedback to Frequency Modulation Systems, published in the Bell System Technical Journal. vol. XVIII, pages 404 to 437,.Ju1y 1939. Obviously, the feedback circuit should be omitted, and in the systems of this invention the beating oscillator of the above-mentioned Fig. 5 would also not be used. Other forms of conversion circuits are shown, by way of example, in United States Patents 1,715,561,

issued June 4, 1929, to F. Mohr, and 1,922,29il,A

issued August l5, 1933, to C. W. Hansell.

An additional reason for employing a balanced conversion circuit is that it willvfacilitate the application ofconventional automatic tuning arrangements to the systems of the invention. Such arrangements are not shown in the appended drawings as it is felt that they might introduce confusion, The application of such arrangements to the systems of the invention is believed *to be quite obvious. For example a tuning control voltage obtained in conventional manner from the balanced conversion circuit can obviously be carried back to either oscillator II or I2' of the system of Fig. l of the appended drawings. Means for obtaining this frequency control potential are indicated in the aforementioned Fig. of my paper referred to above.

Since a wide range of amplitudes of the reflected signals will normally be obtained where systems of the invention are employed to measure the altitude of aircraft over a wide range of altitudes above the surface of the earth, the incorporation of special features, to be described in detail hereinafter, will be desirable in connection with the amplitude limiting means employed.

The features of the invention will become more apparent in connection with the following detailed description of illustrative embodiments thereof, together with the accompanying drawings in which:

Fig. l shows in block schematic diagram form one embodiment of the invention;

Fig. 2 shows in block schematic diagram form a modification of the arrangement of Fig; l which requires only one frequency-modulated oscillator;

Fig. 3 shows a further modification in block schematic diagram form of the system of Fig. 1 in connection with which cross-talk, shielding and adjustment problems are simplified; and

Fig.4 4 shows one form of amplitude limiting means, provided with automatic means to control` the amplitude gains introduced by the first de tector and the intermediate frequency amplifier, which is of value for use with systems of the invention.

In more detail, in Fig. 1, frequency-modulated oscillators Il and I2 differ in mean frequency by an amount sufficient to produce a convenient intermediate frequency when their respective outputs are combined and detected.

Low frequency oscillator I4 is employed to frequency modulate both radio frequency oscillators II and I2 at a convenient rate. As will be subsequently explained in detail, the frequency of oscillator I4 determines the range of distances over which a linear relation obtains between dis-f tance and the indication afforded by the over-all system. Simply by changing the frequency of oscillator I4, therefore, the range of the system I2, respectively. These devices provide meansfor insuring an exact correspondence between the frequency variations imparted to oscillators II and I2', both with respect to phase and to magnitude. The arrangement indicated is that providing the greatest degree of flexibility. Obviously, if oscillators II and I2 have identical mod- Y ulation characteristics, devices IB to I9, inclusive, can be dispensed with. Switching means 20 and 24 and transmission line 22' are provided so that the output of oscillator I2 may be connected directly to the detector 34. whereby a zero distance adjustment for the system may be made by adjusting devices Il to I3, inclusive, until the output of the system falls to aero.

During the operation of the system, switching devices "and 24 connect the output of oscillator I2 to transmitting antenna III and the input of detector 34 to receiving antenna I2, respectively, as shown in Fig. 1. Reflecting shields 28 and 2l enhance'the directive properties or antennas 2l and 32, respectively. The primary purpose of these shields is to reduce direct transmission of energy from antenna 30 to antenna I2. The wave 29 radiated by antenna l0 strikes' a reflecting surface (in the case of aircraft altimeters this surface is, of course, the surface of the earth) and a reflection of the wave 29, namely, wave Il, returns to receiving antenna l2, whence it is led to first detector 34 where it is combined with the output of oscillator II and an intermediate frequency is produced. as explained above. the extent of the frequency modulation of which is dependent upon the difference in time delay existing between the path followed by the output of oscillator II and that followed by the portion of the transmitted wave, represented by reflected wave. BI.

The frequency-modulated, intermediate frequency is ampliedin intermediate frequency amplifier 36, passed through amplitude limiter 28 and converted to an amplitude modulated signal in conversion circuit 40. The signal thus obtained will have the frequency of oscillator I4 and its amplitude will, within the limits of operation which will be defined hereinafter, be substantially linearly proportional to the altitude or distance being measured by the system.

The amplitude modulated signal thus obtained is passed through second detector 42 and then through filter 4I which eliminates extraneous noises, except for those which perchance fall Within the relatively very narrow band required to pass the detected signal frequency of the system. As will be explained in more detail hereinafter the operating range of the system may be changed by simply adjusting oscillator I4 to a different frequency and adjusting filter 4I to pass the new frequency. Since filter 4I is required to pass only a single frequency, namely, the frequency to which oscillator I4 is adjusted, it can discriminate very sharply and effectively against all other frequencies and thus substantially eliminate interferenceby unwanted, stray. or noise currents. 'I'he output of filter 4I is rectitled by rectifier 43 and the rectified current is supplied to direct current meter 44 which is calibrated to read altitude, or distance, directly in feet or other convenient units.

In Figs. 1 to 3, inclusive, auxiliary connections 31 between limiter 38, detector 34 and amplier 376 are indicated. The nature and purpose of these connections will be described below in connection with Fig. 4.

In Fig. 2 a modification of the system of Fig. 1 is shown whereby a single, frequency-modulated, oscillator I2 will suffice. q

In this modified system, the signal introduced into first detector J4 for comparison with reected Wave 3l is obtained in modulator 46 by beating a portion of the output of oscillator I2l with the output of a second oscillator 48, having a xed frequency, to obtain the required heterodyne wave. Modulator 46 is preferably of the balanced type well known in the art. Band-pass lter 41 selects a side-band modulation product for combination in detector 34 with reflected wave Il.

41 introduce a small delay in the path between oscillator I2 and first detector 34. Consequently, with switches 20 and 24 in the horizontal position, the two high frequency waves delivered to the input to first detector I4 will not be modulated in exact synchronism unless a corresponding delay is introduced into the path including v switches 20, 24 and conductor 22. This is provided by variable delay network. Il included in the circuit as indicated. vThis device provides means for obtaining an initial or zero altitude" balance, and in this respect is analogous to devices I4 to I0, inclusive, in Fig. 1. The remaining parts of the .system of Fig. 2 are identical with the correspondingly designated parts of the system of Fig. 1 and consequently need not be described again in detail.

The system of Fig. 3 diil'ers from that of Fig. 2 principally in that for the system of Fig. 3 energy for radiation from antenna 30 is also obtained at the output of modulator 4B through a second band-pass filter .45, designed to select the opposite side-band from that selected by filter 41, that is, by way of example, if filter 41 selects the upper side-band (i. e. fi|fz, where f1 is the frequency oi' oscillator I2 and fz is that oi oscillator 44) then i'ilter should select the lower sideband (i. e. fi-fa) in which case the intermediate frequency resulting from the combination of the output of filter 41 with the reflected wave in first detector I4 will be 2h, where fz is (as above assumed) the frequency of oscillator 48. This is desirable since it reduces cross-talk and shielding problems which may arise in systems of the type illustratedby Fig. 2 in which. oscillator 4B has the same frequency as the mean value of the intermediate frequency of amplifier 3B. One or more variable delay networks, such as 49, may be introduced, if necessary, to'obtain a balance of the circuit'branches for the zero altitude, o'r distance, condition as described in connection with the systems of Figs. 1 and v2. The system of Fig. 3 can. however, conveniently be designed to provide substantially identical metallic paths for the two frequencies of thelsystem and at most only minor adjustment of the system should therefore be required.

The signal which is observed, to obtain an indication of the distance to the reflecting surface, for all systems of this invention has, at the output of the second detector, a frequency equal to that of the oscillator I4 of Figs. 1, 2 and 3 and an amplitude which is determined by the extent to which the intermediate frequency wave is frequency modulated, under any given set of conditions. Il the aircraft carrying the system isl on the ground the delay of the radio link is substantially zero, and, assuming that the phase characteristics of the metallic circuits are identical, the intermediate frequency wave will be of constant frequency. Consequently, the output of the receiver will be zero. As the aircraft rises in the air, delay proportional to altitude will be introduced into the radio link and the intermediate wave will, therefore, be frequency modulated to a degree which will depend upon the delay.

'I'he relationship between the effective modu- It is, of course, evident that the intermediate lation for the syste. and the distance of the aircraft above the renecting surface is as follows:

As=maximum frequency shift imparted to the transmitted wave. A..=maximum frequency shift experienced by the intermediate frequency wave. fm=signal frequency, that is, the frequency generated by the oscillator I4, oi' Figs. 1, 2 and 3. h=altitude of plane in meters. c=velocity of propagation of radio waves in meters per second.

The argument of the sine function in the above expression is numerically equal to one half of the phase delay experienced by the signal. If this quantity is less than about 0.1 radian the above equation becomes very closely Aw No# (2) so that the amplitude of the received signal is closely proportional to the altitude of the plane. If the output indicating means is made to have a linear response in terms of detected signal amplitude it will likewise indicate altitude in linear fashion.

If, for instance, a signal frequency, fm, of 1,000 cycles is employed the response will be essentially linear for altitudes from zero to about 2390 meters, or approximately ri850 feet which range is adequate for the navigation of most commercial aircraft while cruising. If this frequency is increased to 10,000 cycles the corresponding range will extend from zero to about 239 meters or '185 feet which range is convenient for use while landing or taking oii'. In either case the product of. fm and h is the same at the upper altitude limit given, so that equal values of AJ, and hence similar meters readings, will be obtained. It is thus seen, for example, that by altering the frequency of oscillator I4, of Figs. 1, 2 and 3, the sensitivity and range of the altimeter can be readily altered to suit the requirements of the moment. For this reason oscillator I4 is normally provided with one of the numerous means well known to the art for adjusting its frequency to any one of several appropriate values. Since greatest freedom from noise is realized by making the pass-band of filter 4I very narrow it is advantageous to provide it also with one of the likewise numerous means well known to the art, for adjusting its pass-band to pass only the particular frequency to which oscillator I4 has been adjusted. For convenience the frequency adjusting means of the filter should be mechanically coupled with that of the oscillator so that the intermediate frequency signals will be obtained over the wide range of reflected wave intensities which are experienced. In Fig. 4'it will be assumed for purposes of illustration that vacuum tube 5I is the first detector tube. vacuum tube 'Il is the intermediate frequency amplifier tube and vacuum tube l is the amplitude limiter tube. Vacuum tube ill isoperated at reduced plate and screen potentials so that itis readily overloaded by-the intermediate frequency wave applied to its grid. The associated circuits coupling these tubes together and to associated pieces of apparatus are conventional except for the circuit containing resistance $0, potentiall source ll, condenser Il -and coil il. By virtue of this circuitthe rectined voltage developed across resistor SII in the control grid circuit of amplitude limiter tube Il is impressed upon the control grid circuits of nrst detector tube ll and amplifier tube 1l as shown. The arrangement shown has the effect oi' reducing the wide variation in the values of intermediate frequency voltage which quency wave to indicate the distance of the remote object.

1i. An altim'eter for aircraft comprising on said craft, means for generating two radio frequency waves of frequency difference equal to an intermediate frequency means for synchronously modulating to equal degrees both of said radio frequency waves by a particular low frequency, means for radiating one of said radio frequency waves toward the surface of the earth beneath said aircraft, means for receiving reflections of said radiated wave, means for combining said received reflected wave with the other said radio frequency wave and detecting the combination to obtain a frequency modulated intermediate frequency wave, means for amplifying and am- Vplitude limiting said intermediate frequency would ordinarily be delivered to the limiter dura ing normal operation. and constitutes a form of automatic gain control for the combination ofy first detector and amplifier whereby the amplitude of the intermediate frequency wave reaching the conversion circuit lo can more readily be made substantially independent of the amplitude of the signals introduced into the first detector. This is, ashas been pointed out above, necessary to insure the satisfactory operation of systems of the invention over the wide ranges of amplitude of the received reflected wave en-v countered in practicing the invention over wide' altitude ranges.

is reflected back to said object from said surface,

receiving the reflectedwave on said object, producing on said 'object a heterodyne wave modulated in frequency synchronously with and to the same degree as the radiated wave, combining the received reflected` wave with the heterodyne wave to produce a different frequency wave, and utilizing the frequency variation of the said difference frequency wave to obtain an indication of the distance between said object and said surface.

2. The method in accordance with claim 1, the heterodyne wave being produced by modulating a portion ofthe generated carrier wave with a wave of fixed frequency intermediate the mean carrier frequency and the modulation frequency, and selecting a modulation product different in its mean frequency from. the generated carrier.l

3. In a radio range finding system of the type in which the distance of a remote object is de-l termined by .radiating a frequency modulated wave towards the object, receiving a wave reected therefrom, and determining the phase dif ference of the modulation of the transmitted and received waves, the method of operation which comprises producing a heterodyne wave diering in its mean frequency from the radiated wave and modulated in frequency synchronously with and to the same degree as the radiated wave, beating the received wave with the heterodyne wave to produce a different frequency wave, and utilizing the frequency variation of the said dierence frewave. and means for converting said frequency modulated intermediate frequency wave to an amplitude modulated wave and means for determining the extent of the modulation of said intermediate frequency wave whereby an indication of the distance between said craft and the surface of the earth beneath it is obtained.

5. An altimeter for aircraft as defined in claim .4, said means for determining the extent of the `modulation' of said intermediate frequency wave,

including a frequency to amplitude modulation converter, a second detector, a filter, a rectifier y quency amplifying means a rectified voltage de' 40' and a direct current meter.

6. An altimeter for aircraft as defined in claim 4,the said first-mentioned detecting means, the said intermediate frequency amplifying means and the said amplitude limiting means each including a thermionic vacuum tube device having a control grid circuit, and means for impressing uponlthe control grid circuits of said first-mentioned detecting `means and said intermediate freveloped in thecontrol grid circuit of said amplitude limiting means whereby the range of altitudes measurable by said altimeter is extended.

'1. The method of determining the distance n from an object to a reecting surface which com' prises generating on said object a radio frequency wave modulated in frequency at a relatively low rate, beating said wave with a fixed frequency equal to half the frequency of a desirable inter- ,mediate frequency wave, severally selecting an upper and a corresponding lower second order side-band frequency product from the resulting modulation products, directively radiating one of the selected side-band frequency waves so that it is reflected back to said object from said surface, receiving the reflected wave on said object, combining the received reflected wave with the other selected side-band frequency wave to produce a difference frequency wave,A and utilizing the frequency variation of the said difference frequency wave to obtain an indication oi the distance between said object and said surface.

8. In a radio altimeter for aircraft, means for generating a radio frequency wave, means for frequencymodulating said wave at a relatively low rate, means for generating a second wave having a frequency half that of a convenient intermediate frequency wave, means for beating said first wave with said second wave, means for severally selecting the upper and lower side-band frequencies of a particular order of the resulting modulation products, means for radiatingone of said side-band frequency waves so selected toward the surface of the earth, means for receiving reflections of said wave on said craft, means IQ! combinngsaid reflected wave with the other of said selected side-band frequency waves to obtain a difference frequency wave, and means .fori

determining the extent of the frequency modulation of said difference frequency wave to obtain an indication of the distance of said craft above the surface of the earth.

9. 'I'he altimeter of claim 8 and means for changing the frequency of the modulation of the rst said radio frequency wave to effect an appropriete change of satisfactory operating range' for said altimeter. l

10. An altimeter for aircraft comprising on said craft a first means forgenerating a radio frequency wave, a second means for frequency modulating the s'aid radio frequency wave at a relatively low frequency rate, a third means for modulating the resulting frequency modulated l radio frequency wave with a wave of fixed frequency intermediate the frequencies of said rst and said second-stated means, means for selecting from the products resulting from the operationofthe third-stated means an upper side-bandfrequency wave and a lower side-band frequency wave of like order of modulation, means for radiating one of said side-band frequency waves to strike the earth beneath said craft, means for receiving reflections from the earth of said radiated side-band frequency wave, means for combining the received reflected waves with the said other side-bandirequency wave and detecting the combined'sideband waves to obtain a. frequency modulated, intermediate frequency wave, means for amplitude limiting the resulting frequency modulated,intermediatefrequency wave and converting its frequency modulation into amplitude modulation, means for detecting the resulting amplitude modulation, means for selecting from the products produced by* the said second detecting means the frequency of the above said second means for modulating at a low frequency rate, said last-stated selective means excluding substantially all other frequencies and means for indicating the amplitude of said selected frequency including means for rectifying the selected frequency and means for indicating the magnitude of the resulting rectified current.

11. An altimeter for aircraft as in claim 10 the said second means being adjustable to function at any one of a plurality of low frequencies and the last-stated selective means being adjustable to severally select any one of the said plurality of low frequencies and to substantially exclude all other frequencies.

12. The method of determining the difference t in the time delay of two electrical paths which comprises the steps of generating a carrier wave modulated in frequency and transmitting it over one of said paths, generating a heterodyne wave I modulated in frequency synchronously with and to the same degree as said carrier wave and transmitting it over the other of said paths.

A combining the two said waves after transmission to produce a difference frequency wave and utilizing 'the frequency variation of the said difference freqeuncy wave to obtain an indication a of the difference in the time delay of the said two electrical paths.

q 13. The method of determining distance which 'comprises the steps of generating a radio wave modulated in frequency at a relatively low rate. transmitting said radio wave over the distance to be determined, receiving said wave, generating a heterodyne wave modulated'in frequency synchronously with and to the same degree as the generated radio, wave, combining the received wave with the heterodyne wave to produce Aa difference frequency wave and utilizing the frequency variation of the said difference frequency wave to determine the said distance.

JOSEPH G. CHAFFEE.

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