GB1605299A

GB1605299A - Radar systems

Info

Publication number: GB1605299A
Application number: GB2717055A
Authority: GB
Inventors: T J Rey
Original assignee: Duncan B S
Current assignee: Duncan B S
Priority date: 1955-09-23
Filing date: 1955-09-23
Publication date: 1988-07-13

Description

(54) IMPROVEMENTS IN OR RELATING TO RADAR SYSTEMS (71) 1, BEATE SUSANNE, DUNCAN of 32 Fellows Road, London N.W.3, a British Subject do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to an improved frequency or phase modulated radar system wherein all or part of the modulation waveform is random.

In the present art of frequency modulation radars, the transmitted wave is modulated in frequency by a periodic waveform, and part of it is mixed with the reflected wave or waves.

The mixture is detected after elimination of the components in and beyond the region of the carrier frequency, and the detected signal forms an autodyne beat. The range distance of the reflecting surface is then measured as the instantaneous or average frequency of the beat signal, or as the rate of its zero crossings as more fully discussed in the Transactions of the Institute of Radio Engineers, Volume ANE-1, No.2, June 1954. The use of radars with periodic frequency modulation is limited for a variety of reasons.For close range applications, the maximum frequency deviation exceeds the amount which high frequency generating valves are conveniently subjected to; the maximum deviation may be restricted by applying an artificial Doppler effect into (i.e. by phase modulating) the part of the transmitted signal which is mixed with the received signal, but this method reduces the sensitivity.

According to the present invention, acontinuous wave is radiated that is modulated in frequency or phase by a source of random signals; the reflected wave is mixed with a portion of the transmitted wave to form an autodyne beat signal that is random (at least in part), and of which the particular Fourier spectrum has a root mean square value of frequency that depends on the distance to the reflector.

In accordance with one aspect of the present invention, a more accurate method of range determination that is particularly well suited for scanning multiple targets is to provide local autodyne beats for comparison with the received autodyne signal; and if the two beats are identical, then a target is present at the range corresponding to the known range associated with the local autodyne beat signal.

Another aspect of the present invention is to detect or display only those targets which are moving and/or to indicate their speed.

An essential part of the present invention is the provision of means for generating the local autodyne beat signal for comparison with the received autodyne beat signal.

The objects, construction and operation of the present invention will become more readily apparent from the following description and accompanying drawing, in which: Figure 1 is a schematic diagram of one form of the present invention, Figure 2 is a schematic diagram of a preferred embodiment of the present invention that is applicable to the scanning of multiple targets, Figure 3 is a schematic diagram of one method to generate the local comparison beat signals, Figure 4 is exemplary of the type of Fourier frequency spectra associated with the present invention, and Figure 5 shows how the root mean square value of a particular Fourier frequency spectrum varies with target range.

With reference to Figure 1, a source 1 of oscillation of frequency WO is frequency or phase modulated by a source of random signals, such as noise generator 2 whose power and spectral distribution are controlled. The randomly modulated wave is radiated by antenna 3, which also serves to receive the reflected wave. However, for maximum range sensitivity, separate transmitting and receiving antennas could be used. The received wave is mixed with a portion of the transmitted wave in first detector 4; the difference term of the product of the two signals is an autodyne beat that is at least partly random.

As is shown by the mathematical theory that precedes the statement of claims, and as is shown in Figure 4, which is a plot of relative amplitude versus frequency, the Fourier spectra of this random autodyne beat signal comprises a family of curves, for example, a, b, and c, having root mean square values of frequency represented by OV, OX, and OZ, respectively. As further shown in Figure 5, these root mean square values of frequency are indicative of target range over a particular region Rl to R2.

The mean power of the random autodyne beat signal in a given frequency band depends both on the character of the modulation and on the target distance, and the frequency spectrum of the autodyne beat widens with increasing range. A given range is associated with a particular Fourier frequency spectrum, and this is indicated by obtaining a measure of the root mean square value of the frequency spectrum.

As shown in Figure 1, the random autodyne beat signal from first detector 4 is fed to an amplifier 5, whose action is similar to that of a band-pass filter having cutoffs at Wa and Wb.

The effect of amplifier 5 is to cut-off the high frequency portions of the frequency spectrum.

The resulting signal is fed into an amplitude limiter 6, which limits the amplitude of the filtered autodyne beat signal and eliminates the effect of target size upon the determination of range. The effect of limiter 6 is much the same as an amplifier with strong AVC. The normalized autodyne beat signal is then fed into band-pass filter 7, which has a narrower bandwidth than amplifier 5. The effect of filter 7 is to take a fixed and relatively narrow slice of the Fourier spectrum. The part of the spectrum falling within the bounds of filter 7 is an indication of the particular root mean square value of frequency as well as an indication of the particular frequency spectrum in question; hence, it is a measure of relative time delay, or, in other words, target range.

The output of filter 7 is fed to second detector 8, which may be a peak detector and/or an amplitude discriminator associated with an integrating capacitor. The output of second detector 8 is then fed to a suitable indicator 9 to record the range directly.

Another form of the present invention that provides greater accuracy and is particularly well-suited to the range scanning of multiple targets is exemplified by Figure 2. Here, a local autodyne beat signal that corresponds to a known range is compared with the received autodyne beat signal; and if the two beat signals are identical, then a target is present at the range corresponding to the known time delay of the standard or local autodyne beat signal. As before, a source 10 of oscillations Who is frequency or phase modulated by a source of random signals, such as noise generator 11. The randomly modulated wave is radiated by antenna 12, which also may serve to receive the reflected wave. The received wave is mixed with a portion of the transmitted wave in first detector 13 to form an autodyne beat signal that is at least partly random.After suitable amplification by amplifier 14, the received autodyne beat signal is mixed in mixer 15 with the local autodyne beat signal from source 16. The difference term of the product is obtained as the output of first filter 17. The output of first filter 17 is suitably amplified by amplifier 18 and limited by limiter 19 and is applied to second filter 20.

The bandwidth of the second filter 20 is less than the bandwidth of the first filter 17, but still exceeds the Doppler frequency. The output of the second filter 20 is applied to an amplitude discriminator or clipper 21. The clipping threshold partially determines the range resolution of the system. The output of amplitude discriminator 21 is a maximum, that is, a rectangular wave repeating at the Doppler frequency, if the time delays associated with the two autodyne beat signals are identical.

Although the two autodyne beat signals are at least in part random, they are nevertheless coherent, having originated with the same random noise generator. If the time delays of the received and the local autodyne beat signals are not identical, then the output of the second filter 20 is a mixture of a sinusoida at Doppler frequency and random noise within the frequency range of the second filter 20.

The random components are dominant if the difference in time delays is so great that the rms value of the random phase angle exceeds the rms value of the Doppler modulation (rev2).

The present invention may be applied to a variety of apparatus that are directly concerned with the measurement of distance or time delays, such as: a radio altimeter for indicating the height of an aircraft; a collision warning radar for aircraft, ships, or the like; as well as various search or tracking radars. Moreover, the present invention is particularly well-suited to handling multiple targets, since comparison beat 16 may readily take the form of a parallel bank of standard delay lines.

When the present invention is applied to a tracking radar, all or part of the delay line for the comparison beat 16 may be made variable.

For example, in the case of a lumped line, the series inductance and shunt capacitance are varied jointly so as to vary the time delay without affecting the characteristic impedance; other sections of the delay means may be fixed and provided with taps. The output of amplitude discriminator 21 then operates a device such as motor 22, which adjusts the amount of the local or standard delay until the amplitude is approximately a maximum. The corresponding time delay may then be displayed, and it corresponds to the range of the target being tracked.

When the radar is tracking, the cut-off frequency of the second filter 20 may be reduced to the highest Doppler frequency expected. The repetition rate of the rectangular wave at the output of the second filter 20 is the Doppler frequency and is thus proportional to the relative target speed. If this Doppler frequency is measured, an indication is thus obtained of the target speed.

Moreover, a conventional PPI display may be obtained in connection with a conventional scanning antenna by coupling the radial deflection voltage of the cathode ray tube with the adjustment of the local delay line. Target speed may then be indicated by modulating the tube grid with a voltage proportional to the Doppler frequency. It should be noted that in altering the delay, it may be necessary to modify the amount of post-delay amplification in order to compensate for any increase of attenuation with line length. Also, the echoes from stationary targets may be suppressed by adjusting amplitude discriminator 21 to a sufficiently high clipping level.

The resolution of the system may be enhanced by several methods that are wellknown in the art of radar techniques. For example, the amplification required after first detector 13 may be carried out at an intermediate frequency, say 30 megacycles, by introducing a suitable carrier into mixer 15.

Moreover, the transmitting and receiving antennas may be separate, and the transmitting and receiving sections may be isolated from one another except for the controlled amount of transmitter signal which is introduced into first detector 13. Finally, the modulation voltage of the transmitter may contain a sinusoidal component so that the random modulation spectrum is displaced on the frequency scale; this step is well-known in the design of proximity fuzes, and it reduces the effects of spurious amplitude modulation and microphonic noise.

Figure 3 illustrates one method of generating the standard or local autodyne beat signal that is used for comparison purposes. Part of the random modulation waveform (originating in noise generator 11 of Figure 2) is fed into delay line 23. The output is then amplified in amplifier 24 to equalise the attenuation incurred in delay line 23. (An attenuator from the source to subtractor 25 could also be used for equalisation). The output of amplifier 24 is fed into subtractor 25, in which the delayed random modulation waveform is subtracted from the random signal that is also fed into delay line 23, e.g. by phase inversion and superposition. The difference is fed into integrator 26, which would not be necessary if the radiated carrier was phase modulated instead of frequency modulated.The integrated difference is then fed to phase modulator 27, which phase modulates an auxiliary carrier 28. The standard or local autodyne beat signal may then be developed by mixing the unmodulated auxiliary carrier and the phase modulated auxiliary carrier in mixer 29. Since the frequency of the auxiliary carrier cancels out in mixer 29, it is seen that the auxiliary carrier 28 need not correspond to the radiated carrier, but may be at any convenient frequency. The entire apparatus shown in Figure 3 corresponds to the comparison source 16 shown in Figure 2.

Another method of forming the standard autodyne beat is to delay part of the transmitted wave in a local delay means and to mix the amplified delayed output with the input to the delay means. Since the high average frequency of the carrier may make the realisation of the delay means impracticable, it is preferred to mix part of the modulated carrier with a subsidiary sinusoidal carrier of a somewhat different mean frequency, and to use their difference term instead of the modulated carrier for generating the standard autodyne beat signal with the aid of a local delay means.

The present invention may be more clearly understood by considering the underlying mathematical theory, which is as follows: W Let a carrier wave of frequency be 2# modulated in frequency by a random voltage source, and let the instantaneous frequency deviation be (t). The time variation of the modulated oscillation may then be written

Similarly, the time variation of a received signal which has been delayed by the time T through reflection is

the beat resulting from the process of mixing these two signals is l/2 cos [Wot + H + to where (3) WD = Doppler frequency due to reflector motion relative to the transmitter, t1 = W,T,

The properties of random signals have been discussed by S.O.Rice, in the Bell System Technical Journal, October 1944 and January 1945. Following Rice and assuming 8 to be white Gaussian noise in the frequency band (W.WN) then

where Cn and n are random quantities. From equations (4) and (5),

Denoting the time average of a quantity x by x, then if the root mean square value of the random frequency deviation

the statistical properties of equation (6) are such that lim W = (#T) (8a) WNT < 0 lim 62 = 2Q2/WIWN (8b) W1T- > ce The spectrum of the beat (equation 3) may be represented by expi(WDt+#+#) = exp i#. exp i WDt. exp i # (9) The first term is D.C. and the second term is the Doppler frequency. The third term is D.C.

plus a band spectrum.

If 02 ~, the beat spectrum is essentially 12 pure Doppler. However, if 62 > iT2/12 (10) the beat spectrum is essentially random.

For small delays (WNT 1), (11) expi0 = 1 + i0 is D.C. with a small white portion in the band (Wl, WN), as may be seen by inspection of equation (6). The power contribution of 6 to the band (Wa,Wb) is

As the delay time T grows, the contribution of 62 to the power series for exp i0 contributes power into the band (O,WN WI), of amount #4T4/4; its contribution to the band (W ,Wp) is

if O#Wα < Wss, WIS < (WN - W1) (13) For large delays T such that # 1, 1, the beat spectrum has a Gaussian frequency distribution such that the power in the narrow band W,W+6W is

The variance is the r.m.s. instantaneous beat frequency # = 2#2 [1 - cos (W1 + WN) T/2 sin (WN - W1) T/21 (WN - W1) T/2 (15)

Hence if AW is a band contained inside the greater band DW, the ratio power in OW (16) power in AW increases monotonically with62, i,e. with delay time T up to T = r/(W1 + WN) (17) However, this restriction is avoided with the aid of a standard comparison signal cos 85 (18) where 05 is given by equation (4) on replacing the reflector delay time T by the local or standard delay Ts.

On mixing the signals described by equations (3) and (18), they yield the difference term cos (Wot + 8 - 8, + 'i'i - #s) (19) This contains the random argument

Clearly (6 - 6s)2 becomes zero and (19) becomes a pure Doppler signal as T- > Ts. One method of generating the comparison signal (18) is to subject the output of the random source 9 to a known delay Ts and to subtract this from the present signal; the difference is integrated to form the signal

On modulating an auxiliary carrier of # frequency 2# by the signal (21) with regard to its phase, the result is cos (ut + y + 05), where "is an arbitrary phase. On mixing this signal with the unmodulated carrier, the result is cos 85; this is the time dependence of the local autodyne beat delivered by the source 16.

WHAT I CLAIM IS:1. Apparatus for measuring the range of a distant object, comprising, a source of periodic carrier oscillations, a source of random oscillations for modulating said first source in frequency or phase, means to radiate the randomly modulated carrier wave, means to receive a reflected wave, means to fix said reflected wave with a portion of said radiated wave, whereby an autodyne beat signal is produced, said autodyne beat signal being at least in part random, and means concomitant therewith for identifying the particular Fourier frequency spectrum of said autodyne beat signal, said particular Fourier frequency

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

Denoting the time average of a quantity x by x, then if the root mean square value of the random frequency deviation

the statistical properties of equation (6) are such that lim W = (#T) (8a) WNT < 0 lim 62 = 2Q2/WIWN (8b) W1T- > ce The spectrum of the beat (equation 3) may be represented by expi(WDt+#+#) = exp i#. exp i WDt. exp i # (9) The first term is D.C. and the second term is the Doppler frequency. The third term is D.C.

plus a band spectrum.

If 02 ~, the beat spectrum is essentially 12 pure Doppler. However, if 62 > iT2/12 (10) the beat spectrum is essentially random.

For small delays (WNT 1), (11) expi0 = 1 + i0 is D.C. with a small white portion in the band (Wl, WN), as may be seen by inspection of equation (6). The power contribution of 6 to the band (Wa,Wb) is

As the delay time T grows, the contribution of 62 to the power series for exp i0 contributes power into the band (O,WN WI), of amount #4T4/4; its contribution to the band (W ,Wp) is

if O#Wα < Wss, WIS < (WN - W1) (13) For large delays T such that # 1, 1, the beat spectrum has a Gaussian frequency distribution such that the power in the narrow band W,W+6W is

The variance is the r.m.s. instantaneous beat frequency # = 2#2 [1 - cos (W1 + WN) T/2 sin (WN - W1) T/21 (WN - W1) T/2 (15)

Hence if AW is a band contained inside the greater band DW, the ratio power in OW (16) power in AW increases monotonically with62, i,e. with delay time T up to T = r/(W1 + WN) (17) However, this restriction is avoided with the aid of a standard comparison signal cos 85 (18) where 05 is given by equation (4) on replacing the reflector delay time T by the local or standard delay Ts.

On mixing the signals described by equations (3) and (18), they yield the difference term cos (Wot + 8 - 8, + 'i'i - #s) (19) This contains the random argument

Clearly (6 - 6s)2 becomes zero and (19) becomes a pure Doppler signal as T- > Ts. One method of generating the comparison signal (18) is to subject the output of the random source 9 to a known delay Ts and to subtract this from the present signal; the difference is integrated to form the signal

On modulating an auxiliary carrier of # frequency 2# by the signal (21) with regard to its phase, the result is cos (ut + y + 05), where "is an arbitrary phase.On mixing this signal with the unmodulated carrier, the result is cos 85; this is the time dependence of the local autodyne beat delivered by the source 16.

WHAT I CLAIM IS:1. Apparatus for measuring the range of a distant object, comprising, a source of periodic carrier oscillations, a source of random oscillations for modulating said first source in frequency or phase, means to radiate the randomly modulated carrier wave, means to receive a reflected wave, means to fix said reflected wave with a portion of said radiated wave, whereby an autodyne beat signal is produced, said autodyne beat signal being at least in part random, and means concomitant therewith for identifying the particular Fourier frequency spectrum of said autodyne beat signal, said particular Fourier frequency

spectrum being characteristic of a time delay and whose identification allows the range of the distant object to be obtained.
2. Apparatus according to Claim 1 wherein means are provided to determine the root mean square value of frequency of said particular Fourier frequency spectrum, whereby the range of the distant object is obtained.
3. Apparatus according to Claim 1 comprising a random beat amplifier tuned to a certain band of frequencies, a limiter associated with said amplifier for normalizing the wave form of the autodyne beat signal and eliminating the effect of target size, a filter associated with said limiter, said filter having a narrower bandwidth than said amplifier, and a detecting means associated with said filter, whereby the range of the distant object may be presented on a suitable indicator.
4. The arrangement claimed in Claim 3, wherein said detecting means is a peak detector.
5. The arrangement claimed in Claim 3, wherein said detecting means is an amplitude discriminator.
6. Apparatus for measuring the range of a distant object, comprising the steps of modulating the frequency or phase of a carrier wave with a source of random signals, radiating the randomly modulated carrier wave, receiving a reflected wave, mixing said reflected wave with a portion of said radiated wave, whereby an autodyne beat signal is produced that is at least partly random, and determining the particular Fourier frequency spectrum of said autodyne beat signal, whereby the distance to the object is obtained.
7. Apparatus according to Claim 6 wherein the particular root mean square value of frequency of the Fourier frequency spectrum associated with said autodyne beat signal is determined, whereby the distance to the object is obtained.
8. Apparatus for measuring the range of a distant object, comprising, a transmitter for radiating an electromagnetic wave, said wave being modulated in frequency or phase by a source of random signals, a receiver for receiving a wave reflected from a distant object, means to mix said reflected wave with a portion of said radiated wave, whereby a beat signal is produced, said beat signal having a particular Fourier frequency associated therewith, and means concomitant therewith for identifying said particular Fourier frequency spectrum of said beat signal, said particular Fourier frquency spectrum having a range associated therewith, whereby the range of a distant object is determined.
9. Apparatus according to Claim 8, comprising means concomitant therewith for determining the root mean square value of frequency of said particular Fourier frequency spectrum, whereby the range of the distant object is determined.
10. Apparatus for measuring the range of a distant object, comprising a source of periodic carrier oscillations, a source of random signals for modulating said first source in frequency or phase, means to radiate the randomly modulated carrier wave, means to receive a wave reflected from the distant object, means to mix said reflected wave with a portion of said radiated wave, whereby a first beat signal is produced, said first beat signal being at least partly random and being indicative of a certain range, said range depending on the range of the distant object, local means to generate a second beat signal, said second beat signal being responsive to said source of random signals and being indicative of a known range, said known range corresponding to a known delay line, and means concomitant therewith for comparing said first and second beat signals, whereby the range of the distant object is determined.
11. Apparatus as claimed in Claim 10, wherein said delay line for generating said second beat signal is made variable.
12. Apparatus as claimed in Claim 10 wherein said local means comprises means to generate a group of standard beat signals, whereby the range scanning of multiple targets is realised.
13. Apparatus as claimed in Claim 10, wherein said local means comprises variable means to generate a group of variable range beat signals, whereby the range scanning of multiple targets is realized.
14. Apparatus for tracking a distant object, comprising, means to radiate an electromagnetic wave, said wave being modulated in frequency or phase by a source of random signals, means to receive a wave reflected from the distant object, means to mix said reflected wave with a portion of said radiated wave, whereby a first beat signal is produced, said first beat signal being at least partly random, local means to generate a second beat signal, said second beat signal being responsive to said source of random signals, means to compare said first and second beat signals, and means responsive therewith for varying the length of the delay line for generating said second beat signal, said second beat signal thereby becoming substantially identical to said first beat signal, whereby the distant object is tracked.
15. Apparatus according to Claim 14 comprising means for varying said delay line of said second beat signal; said second beat signal thereby becoming substantially identical to said first beat signal, whereby the distant object is tracked.
16. An arrangement for securing information concerning a distant object, comprising, means to radiate an electromagnetic wave, said wave being modulated in frequency or phase by a source of random signals, means to receive a wave reflected from the distant object, means to mix said reflected wave with a portion of said radiated wave, whereby a first beat signal is produced, said first beat signal being indicative of the position in space of said distant object, locals means to generate a second beat signal, said second beat signal being responsive to said source of random signals, a mixer for mixing said first and second beat signals, a first filter associated with said mixer, said first filter having a certain bandwidth associated therewith, means to suitably amplify and limit the output of said first filter, a second filter associated with said amplifying and limiting means, said second filter having a certain bandwidth associated therewith, said bandwidth of said second filter exceeding the Doppler frequency but being narrower than the bandwidth of said first filter, and an amplitude discriminator associated with said second filter, said amplitude discriminator having a maximum output wherever said first beat signal corresponds to said second beat signal.
17. Apparatus as claimed in Claim 16, wherein said first beat signal is indicative of the range of the distant object, said second beat signal is indicative of a known range, whereby the range of the distant object is determined.
18. Apparatus as claimed in Claims 16 and 17, wherein all or part of said local delay line for generating said second beat signal is made variable.
19. Apparatus as claimed in Claims 16, 17 and 18, wherein said local delay line for generating said second beat signal is made responsive to the output of said amplitude discriminator, whereby the distant object is tracked in space.
20. Apparatus as claimed in Claims 16, 17, 18 and 19, having local means for generating a plurality of said second beat signals, whereby information may be secured on multiple targets.
21. Apparatus as claimed in Claims 16, 17, 18, 19, and 20, wherein said local means for generating said comparison beat signal comprises, a delay line responsive to said source of random signals, an amplifier associated with said delay line for compensating for any attenuation incurred therein, a subtractor responsive to said amplifier and said source of random signals for subtracting said delayed random signal from said random signal, an integrator associated with said subtractor, said integrator being used whenever the carrier wave is frequency modulated, an auxiliary source of oscillations, a phase modulator associated with said integrator and said auxiliary source of oscillations, said phase modulator having two inputs, said inputs comprising the output of said auxiliary source of oscillations and the output of said integrator, and a mixer concomitant therewith for mixing the output of said phase modulator with an output of said auxiliary source of oscillations, whereby a comparison beat signal is obtained.
22. Apparatus as claimed in Claims 16, 17, 18, 19, and 20, wherein said local means for generating said comparison beat signal comprises a delay means for delaying a first sample of the radiated wave, an amplifier associated with said delay means for compensating for any attenuation incurred therein, and a mixer for mixing the output of said amplifier with a second sample of the radiated wave, whereby a comparison beat signal is obtained.
23. Apparatus as claimed in Claim 1 including apparatus for comparing the received random beat signal with a standard beat signal, said standard beat signal having a known range associated therewith, whereby the range of a distant target is obtained.
24. Apparatus as claimed in Claim 1 including apparatus for automatically varying a standard beat signal so as to make it substantially identical to the received random beat signal, whereby a distant target may be tracked.
25. In combination with a tracking radar of the type described in Claims 14, 15, 16, and 19, means to measure the relative speed of the distant object, comprising, means to measure the Doppler frequency associated with the output of said second filter, said Doppler frequency being proportional to relative target speed, whereby the relative speed of the distant object is determined.
26. In combination with a tracking radar of the type described in Claims 14, 15, 16, 19 and 25, means to display the relative speed of the distant object, comprising, a cathode ray tube having a radial deflection responsive to the adjustment of said variable local delay means, and means to modulate said cathode ray tube with a voltage proportional to the Doppler frequency, whereby the relative speed of the distant object is displayed.