JPS6082988A - Pulse radar device - Google Patents

Abstract

PURPOSE:To improve an MTI (moving target indicating function) characteristic by performing a frequency modulation to a pulse transmitting wave of a radar, extracting a demodulating wave of a modulating signal from a reflected receiving wave by said transmitting wave, and inputting it to the MTI. CONSTITUTION:An FM modulator 23 is provided between a coherent oscillator 10 and a mixer 7, so that a frequency of a transmitting coherent signal is FM- modulated by a frequency of a modulated wave oscillator 22. An output of the FM-modulated oscillator 10 passes through the mixer 7 and reaches a klystron 3, in which said output becomes a transmitting high frequency pulse, and it is transmitted from an antenna 1. This receiving wave passes through a transmitting and receiving switch 2 from the antenna 1, it is converted to an intermediate frequency at a mixer 5, amplified by an intermediate amplifier 8 and inputted to a pulse form demodulator 40. A demodulated wave fetched by receiving a Doppler deviation in said demodulator is inputted to an MTI50 through a phase detector 9. As for the phase detector 9, an output signal of the modulated wave oscillator 22 is used as a reference signal.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] This invention provides a moving target detection function (MovingTarg
etIndication: Hereinafter referred to as MTI)
The purpose is to dramatically improve I performance.

[Prior art]

Conventionally, there has been a device of this type as shown in FIG.

This is an example of a coherent MTI radar, and in the figure, 1 is a radar antenna, 2 is a transmitting/receiving switch, 3 is a transmitting klystron amplifier (a TWT amplifier may be used instead), and 4 is a talistron amplifier 3 5 is a mixer (Mixer) that converts the high frequency signal received by the radar into an intermediate frequency.
, 6 is a stable local oscillator (Stable Local 0
7 is a mixer for converting an oscillation signal from a coherent oscillator 10 (described later) into a transmission high frequency signal, 8 is an intermediate frequency amplifier, and 9 is a phase detector (Phase) for detecting a Doppler frequency shift signal.
5 intensive Detector), 10 is a coherent oscillator used as a primary oscillator of the radar, and 11 is a trigger oscillator that steps down the output of the coherent oscillator 10 to create a trigger signal at the transmission repetition frequency of the radar. be.

In addition, 12 is an ultrasonic oscillator (Carrier Wave0sci) that oscillates ultrasonic waves into the MTI delay line I4, which is a delay element that uses ultrasonic waves.
13 is a circuit that delays the output signal of the phase detector 9 by one period of the repetition period of the ultrasonic goo output from the ultrasonic oscillator 12 and outputs it. 5 is an amplifier for recovering the amplitude attenuation in the delay line 14, and I6 is a detector (De
tector ), and the radar signal is extracted as a video signal at this output. 17 is an attenuator for providing attenuation equivalent to the delay line 14.
or), 18. I9 are similar to the amplifying device 5 and the detector 16, respectively. 2o is a subtractor that performs subtraction between a delayed signal and a non-delayed signal to extract a difference.

Next, the operation will be explained.

If the transmission frequency of the radar is ft, and when the transmitted wave hits a moving target and is received as a reflected wave again, it has undergone a Doppler frequency shift fd, then the frequency of the reflected wave is expressed as ft±fd.

The Doppler frequency shift fd is expressed by the following equation.

2Vr 2νr-ft f d = -= - λ vr: Speed of moving target λ: Transmission wavelength C: Speed of light liquid This Doppler frequency shift fd is taken out as the output of the phase detector 9 in FIG. The principle is as follows.

That is, the frequency of the reference signal from the coherent oscillator 10 is fc, and the frequency of the received signal from the intermediate frequency amplifier 8 is fc.
c±fd, and each of them is expressed as a continuous sine wave signal Vcoho+Vif for simplicity, then Vcoho=Asin 2 yrf(tVif=Bsi
n (2yrfct±φd)φd=2πfdt A, B are amplitudes t is expressed as time. Therefore, the output Vpsd of the phase detector 9 is Vpsd =
Vcoho −Vif = (Asin 2nfct) (Bsin (27L
'fct±φd=−(cos φd−cos(4πf
ct +φd). In the above equation, the second term, the component including the intermediate frequency fc, is removed by a filter after the detection circuit, and only the first term (AB/2)·cosφd component is extracted from the phase detector 9. Here, φd=
Since 2nfdt, (AB/2) ・ cosφd
is a video signal whose amplitude changes depending on the value of the Doppler frequency shift fd.

When this video signal is input into a conventional MTI composed of the components 12 to 20 described above, only the change in amplitude due to the value of the Doppler frequency deviation fd is extracted. Therefore, only the moving target signal is output, and the fixed target signal is eliminated.

FIG. 2 shows an example of frequency response characteristics in a single cancellation method using one delay line in a normal MTI.

The maximum value of the output amplitude is normalized to 1. In the figure, T represents the pulse transmission repetition period of the radar, and the delay time of the delay line 14 is also T.

)) It is a well-known fact that in this type of MTI, there is a Doppler frequency at which the target signal is also canceled, which corresponds to the blind speed at every integer multiple of l/T.

Conventional pulse radar equipment is configured as described above, and if the reflected signal of a fixed target is an ideal one without any deviation in the Tomplar frequency, the reflected signal of the fixed target will be completely erased and the signal will move. Only the reflected signal M of the target is detected. However, in reality, reflected signals from fixed targets such as land and mountains have a spectrum that is wide on the frequency axis, as shown by diagonal lines in FIG. Such a Lasotar spectrum S could not be sufficiently eliminated using the MTI response characteristics as shown in FIG.

As a conventional method to solve this problem, a multiple cancellation system is constructed using multiple delay lines, a negative feedback technique is adopted, and an MTI response characteristic shaped as shown in Fig. 4 is used to fix the Some were designed to improve the target extinction ratio.

However, even with such a method, the effect was small considering the complexity of the MTI configuration.

[Summary of the invention]

The present invention was made in order to eliminate the drawbacks of the conventional ones as described above, and it applies frequency modulation to the transmitted wave of the radar, extracts the demodulated wave of the modulated signal from the received wave reflected by the transmitted wave, and By inputting MTI, MT
The object of the present invention is to provide a pulse radar device whose I characteristics can be dramatically improved compared to conventional ones.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 5, the same reference numerals as in FIG. 1 indicate the same parts. 22 is a modulated wave oscillator for FM, 23 is a frequency modulator,
Reference numeral 40 denotes a pulse-type FM demodulator, and reference numeral 50 collectively shows the configuration (moving target detection means) having the MT1 function of the components 12 to 20 in FIG.

Next, the operation will be explained.

In the device of this embodiment, a frequency modulator 23 is newly provided between the coherent oscillator 10 and the mixer 7, and the frequency of the coherent signal for transmission is frequency modulated (hereinafter referred to as FM) by the frequency of the modulated wave oscillator 22. That's what I do. Then, the frequency-modulated output of the coherent oscillator 10 passes through the mixer 7 and reaches the talistron 3, where it becomes a transmission high-frequency pulse and is transmitted through the antenna 1 into space. When the carrier frequency of the transmitted high-frequency pulse is fc, the modulation frequency is fm, and the maximum frequency deviation is Δf, the modulation index mf is given by m f - Δf / f m . If the relationship between the modulation frequency fm and the maximum frequency deviation Δf is determined so that the modulation index mf is a low value of about 0.3 to 0.4, the second and higher sidebands of the FM wave will become considerably smaller. , it is only necessary to consider the first sideband. Therefore, the frequency spectrum becomes as shown in FIG.

Figure 6 conceptually shows the spread of the spectrum of each component due to pulse modulation in addition to FM, and this spread is
This is determined by the pulse modulation waveform. The voltage e (tl) of this transmitted high-frequency pulse can be expressed mathematically as follows.

−cos(ωo t +mf sin Pt) ・=
+11 However, EO: Voltage amplitude = T/τ T: Transmission period τ: Transmission pulse width ωo = 2πfc, fc is carrier frequency mf = Δω/
P = Δf/fm Δω−2πΔf, Δf is the maximum frequency deviation P = 2πfm,
fm is the modulation frequency In the above equation, the term in square brackets is a term representing the waveform of pulse modulation, and the cos term is a term representing frequency modulation.

The voltage e'(tl) when this pulsed FM modulated wave undergoes a Doppler frequency shift is expressed as follows.

e'1tl- However, a: Value ωd-(4
π/λ0)・Vr, λ0 is the wavelength of the carrier wave.

Vr is the axial velocity of the target relative to the radar Pd = (4π
/λp)・Vr, λp is the wavelength of the modulated wave.

If we conceptually represent the spectrum of the pulsed FM wave according to the same equation (2) above as Vr, as shown by the dotted line in Figure 7, it deviates from the spectrum in Figure 6 by the de Noppler shift numerator d. It takes shape. This figure shows a case in which the frequency is shifted toward high (,
d', f d''.

Then, such pulse FM waves are received by a radar.

The received wave is sent from the antenna to the mixer 5 via the transmitter/receiver switcher 2.
The signal is converted to an intermediate frequency by the intermediate frequency amplifier 8, and is then amplified by the intermediate frequency amplifier 8 and input to the pulse type FM demodulator 40. This demodulator 40 is constructed in accordance with the spectra shown in FIGS. 6 and 7, and is shown in FIG.

In FIG. 8, 40a is an input terminal, 42 is an IF signal distributor, 43 is a bandpass filter for upper sideband, 44 is a bandpass filter for carrier wave, 45 is a bandpass filter for lower sideband, and 46 is a bandpass filter for each bandpass. Filter 43.44.
, an IF signal adder that adds the IF multiplied signals that have passed through 45;
47 is an amplitude limiter for making the amplitude of the received wave constant before it is demodulated by the frequency discriminator 48, 49 is a demodulated wave amplifier that amplifies only the frequency band of the demodulated signal, and 40b is an output terminal.

In the pulse-type FM demodulator 40, it is necessary to improve the signal-to-noise ratio by using a bandpass filter that passes only the frequency component of the pulse before performing frequency discrimination. As shown in the figure, each band filter 43 to 45 may extract only each component. Here, in a region where the modulation index is low, about 0.3 to 0.4, other sideband components are sufficiently small, so even if they are ignored, waveform distortion is small and the purpose of this embodiment is not hindered.

It will be explained below that when the pulse FM wave is demodulated by the demodulator 40 having such a configuration, the modulated wave and its Doppler frequency component are extracted.

What is shown in FIG. 9 is the input/output characteristic F of the frequency discriminator 48.
It is d. The horizontal axis shows the input angular frequency, and the vertical axis shows the output voltage ■. For the sake of simplicity, people.

The output wave will be explained as a continuous wave (cw).

The instantaneous angular frequency shift of the FM wave subjected to Doppler shift is (
From formula 2), the instantaneous angular frequency is d φ t −ωo±ωd+Δ(LICO3(P+:Pd) t −
It is expressed as (3).

Here, the change f in the input signal angular frequency of the frequency discriminator 48
If the characteristic Fd for in is a straight line within the maximum angular frequency deviation width ±Δω as shown in FIG.
It can be expressed as k (ω-ωO). Therefore (3)
The voltage ■ of the output wave fout with respect to the input wave fin expressed by the formula is V=k (ω-ω0) = k(ωO±ωd + Δωcos (P+Pd) t −
ω0°-± and ωd+ are Δωcos (P±Pd). However, k is a constant. Here, Δω is a constant value, so if this is set as Vc, then ωd+Vccos (P±Pd) t for ω0°-±.

Here, if the modulation angular frequency P is taken as (P for the Doppler angular frequency deviation ωd of the general transmission wave) ωd, then ωd in the first term of the above equation becomes DC-like. The output voltage waveform of the frequency discriminator 48 is illustrated in FIG. 10, and the kωd component is easily removed by the demodulation wave amplifier 49 at the subsequent stage. Therefore, the component of Vccos (P+Pd)t is taken out to the output terminal 40b. This means that the original modulation angular frequency P and its Doppler angular frequency deviation component Pd have been extracted. Note that the above condition P'S>
Satisfying ωd is very easy in this embodiment.

Next, the effects of this embodiment will be explained from the definitions of ωd and Pd used in equation (2) above.

Here, since and, Pd λ o fm ωd λp fc . This shows that the Tombler frequency shift is directly proportional to the carrier frequency fc and the modulation frequency fm.

The demodulated wave extracted by the pulse demodulator 40 and subjected to Doppler shift passes through the phase detector 9 to the MT
Enter I 50. Here, the phase detector 9 uses the output signal of the modulation oscillator 22 instead of the output signal from the coherent oscillator 10' as a reference signal. Further, the above MTI 50 is the same as the conventional one except that it operates in the demodulated signal band. Note that if the frequency of the demodulated wave is the frequency of the ultrasonic wave required by the delay line 14, the ultrasonic oscillator 12 and modulator 13 in FIG. 1 are unnecessary.

Then, the value of the Doppler frequency deviation of the demodulated wave entering MT I 50 is determined by the modulation frequency fm as shown in the following two series, and this modulation frequency fm is equal to the carrier frequency f
Since it is considerably lower than c, the clutter spectrum S in FIG. 3 is compressed by the ratio fm/fc, and becomes almost a line spectrum as shown in FIG. On the other hand, the Doppler frequency deviation of the moving target signal is also compressed by the same ratio, but in the case of a moving target, since it is moving, it is inherently smaller than the clutter as shown in Figure 2. It has a very large Doppler frequency shift. Therefore, assuming the maximum velocity Vrmax of the target to be captured by the radar in the radar axis direction, the Tombler frequency deviation of the target signal at this time is the 11th
If the modulation frequency fm is selected to such an extent that it does not become smaller than 1/T in the figure, the clutter spectrum can be reduced to frn7'f without impairing the detection characteristics of the moving target signal.
It can be compressed with a ratio of c.

Therefore, the fixed target cancellation ratio of MT T 50 is f m /
This means that the fC ratio has been improved. At this time, the detection range R of the signal corresponding to the speed of the moving target is only between 0 and 1/T, as shown in FIG.

Here, the modulation frequency fm is the maximum axial velocity V of the moving target
The following relationship exists with respect to rmax.

In other words, if fmd is the Do-7 Plas frequency deviation that the modulation frequency fm undergoes, fmd should be 1/T when direction maximum speed 3
If Matsuha is used and T = 5 m s is substituted for a trial calculation, f m =30.2 Ml1z. Therefore, for a pulsed radar using a frequency f c -3000 MHz as a carrier wave, the clutter spectrum is f c -3000 MHz.
m 30MHz 1 r C3000MFIz 100 . In addition, the detection characteristics of the target are Vr
max = 3 Man-eight, 1 / T = 200
Since the maximum point of the Doppler frequency deviation will be at Hz, no loss will occur in the target detection characteristics.

Also in the case of this embodiment, as shown in FIG.
If you shape the MTI characteristics using multiple delay lines,
The clutter spectrum compression effect and target signal detection characteristics will be further improved. Note that the modulation frequency of 30 MIIz in the above embodiment is a frequency that can be easily implemented.

In the above embodiment, a case was described in which frequency modulation was applied to the transmission pulse, but even if amplitude modulation or phase modulation is used, the same effect as in the above embodiment can be obtained in principle.

〔Effect of the invention〕

As described above, according to the present invention, the pulse transmission wave of the pulse radar device is subjected to frequency modulation and transmitted, the demodulated wave of the modulated signal is extracted from the received pulse signal, and the demodulated wave is Since the moving target detection function is activated, the clutter spectrum can be compressed without impairing the moving target detection characteristics, and the fixed target cancellation ratio can be significantly improved compared to the conventional one. There is an effect that can be done.

[Brief explanation of drawings]

Figure 1 is a diagram showing the configuration of a conventional pulse radar device.
FIG. 2 is a diagram showing the response characteristics of a conventional MTI, FIG. 3 is a diagram showing a clutter spectrum in a conventional MTI, and FIG. 4 is a diagram showing a conventional shaped MTI response characteristic.
FIG. 5 is a diagram showing the configuration of a pulse radar device according to an embodiment of the present invention, FIG. 6 is a diagram showing the spectrum of pulsed FM waves in the device, and FIG. 7 is a diagram showing the spectrum of pulsed FM waves in the device. FIG. 8 is a diagram showing an example of the configuration of the pulsed FM demodulator of the device, and FIG. 9 is a diagram showing the input/output characteristics of the frequency discriminator in the FM demodulator. 10 shows the output voltage waveform, and FIG. 11 shows the pulse voltage waveform according to an embodiment of the present invention.
FIG. 3 is a diagram showing the effect of compressing the clutter spectrum of a radar device. 23... Frequency modulator (transmission wave frequency modulation means), 4
0... Pulse type FM demodulator (modulation signal demodulation means), 5
0...MTI (moving target detection means). In the drawings, the same reference numerals indicate the same or equivalent parts. Agent: Masuo Oiwa Figure 6, Figure 7, Frequently Asked Questions, Figure 9, Figure 10, Figure 11.

Claims (1)

[Claims] (1) a transmission wave frequency modulation means that performs frequency modulation on a pulse transmission wave having a predetermined carrier frequency; a modulation signal demodulation means that demodulates a modulated signal of the received pulse signal;
A pulse radar device comprising: moving target detection means for receiving a demodulated wave of the modulated signal and detecting a reflected wave of the moving target.