JPH0549196B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a pulse Doppler type radar device that measures the distance to an aircraft target by frequency modulation.

[Conventional technology]

FIG. 4 is a configuration diagram showing a conventional radar device that measures the distance to an aircraft target using a frequency-modulated transmission wave with a high pulse repetition frequency.
In the figure, 1 is an excitation/receiver that generates a linear frequency modulated transmission wave, pulse-modulates it at a high pulse repetition frequency and sends it out, amplifies the received signal, and converts it into a digital signal after removing clutter; 2
3 is an antenna that amplifies the transmitted wave to high power and radiates it into space, and receives the reflected wave from the target. 3 is the above-mentioned excitation and
It is a signal processor that receives the digital signal from the receiver 1 and performs various signal processing to measure the distance to the target. 4 is a clutter removal filter, 5 is a Doppler filter, 6 is a signal detector, and 7 is a target. 10 is a distance measuring circuit; 12 is a display for displaying target distance, etc.;

Fig. 5 is a conceptual diagram in which a radar device is mounted on a flying object and searches for a target by scanning an antenna beam while moving.
5 is a radar device, 13 is a main beam, 14 is a side lobe, 15 is a target aircraft, and 16 is the ground.

FIG. 6 is a conceptual diagram for explaining that plane radio waves are incident on the blades of a jet engine compressor and undergo a Doppler shift in the negative direction. In the figure, 17 is the blade.

FIG. 7 is a frequency spectrum of a signal received by the radar device in the state shown in FIG. 5,
In the figure, 18 is the main beam clutter, 1
9 is a sidelobe clutter, 20 is a target Doppler signal, and 21 is a Doppler signal due to jet engine modulation.

FIG. 8 is a diagram showing temporal frequency changes of frequency-modulated transmitted waves and received signals when performing distance measurement. In the figure, 22 is the frequency change characteristic of the transmitted wave, and 23 is the target Doppler signal. Frequency change characteristic 24 is the frequency change characteristic of the Doppler signal due to jet engine modulation.

FIG. 9 is a diagram showing combinations when calculating distance using a plurality of Doppler frequency signals obtained in both FM phases.

Next, the operation will be explained.

A transmission wave from the excitation/receiver 1 in which constant frequency and linear frequency modulation are alternately repeated is pulse-modulated at a high pulse repetition frequency and sent to the antenna 2. The antenna 2 amplifies this transmitted wave to high power and radiates it into space with a predetermined phase shift from a number of antenna elements (not shown) to form an antenna beam. Then, it searches for a target while scanning the antenna beam electronically, and receives the received signal from the target along with unnecessary clutter. In FIG. 5, when the radar device 25 is flying at its own speed _VI and searches for the target aircraft 15 in a down-looking state, the main beam 13 and the side ropes 14 illuminate the ground 16, thereby eliminating unnecessary clutter. The main beam 13 receives a signal from the target aircraft 15. At this time, the signal from the target aircraft 15 is generated as a plurality of Doppler frequencies due to modulation caused by the rotation of the blades of the jet engine of the target aircraft 15. As shown in FIG. 6, the blade 17 rotates so that the radio wave irradiation surface moves away from the blade 17 in the direction of movement, so that the Doppler shift decreases. And blade 1
7 has a complex shape and differs in inclination and rotation speed, generating subharmonic modulated wave components. Due to the slight rotation of the blade, the radio waves that were irradiating _one point P
Since _two points P will be irradiated, the blade will move away even though the target aircraft 15 is approaching, and as a result, the relative speed with the target aircraft 15 will decrease.
Multiple Doppler frequencies lower than the Doppler frequency caused by _VR occur.

The Doppler frequency f ₁ of a target aircraft flying at a speed V _T is expressed by the following equation.

f ₁ = 2f ₀ /C ₀ (V _I +V _T ) cos θ ...(1) where f ₀ is the transmission frequency, C ₀ is the speed of light, and θ is the angle between the velocity spectrum V _I and the antenna beam.

Next, if the amount of frequency deviation due to jet engine modulation is f _JEM , then the multiple Doppler frequencies are expressed by the following equation.

f ₂ = f ₁ f _JEM = 2f ₀ /C ₀ (V _I +V _T )cosθ−f _JEM
……(2) f ₃ = f ₁ −2f _JEM = 2f ₀ /C ₀ (V _I +V _T )cosθ−2
f _JEM ……(3) f ₄ = f ₁ −3f _JEM = 2f ₀ /C ₀ (V _I +V _T )cosθ−3
f _JEM ……(4) 〓 f _k = f ₁ − (k−1) f _JEM = 2f ₀ /C ₀ (V _I +V _T
)cosθ-(k-1)f _JEM (5) The frequency spectrum of the clutter and signal shown above is shown in FIG.

It is determined by the relative speed to the target, and the true Doppler frequency 20 of the target given by equation (1) and the jet frequency given by equations (2) to (5) at lower frequencies are determined by the relative velocity of the target.
The received signal includes an engine modulation signal 21 and main beam clutter 18 and sidelobe clutter 19. The excitation/receiver 1 receives this received signal from the antenna 2, first amplifies it with low noise, converts it to an intermediate frequency using a local oscillation signal, removes clutter, and then detects the video band by phase detection. Obtain a Doppler signal and convert it to a digital signal.

Next, the digital signal is input to the signal processor 3, and the clutter is completely removed by the clutter removal filter 4 again. Thereafter, it is converted into a narrowband Doppler signal by a Doppler filter 5 using fast Fourier transform, and after amplitude detection of this output, it is detected as a signal by a signal detector 6 which compares the amplitude with noise. As shown in Figure 8, FM phase C with constant frequency, FM phase B with linear frequency modulation, and
A plurality of Doppler frequency signals are detected in each of the FM phases A. In FM phase C, f ₁ ,
When four Doppler frequency signals of frequencies f ₂ , f ₃ , and f ₄ are received, in FM phase B, signals of g ₁ , g ₂ , g ₃ , and g ₄ whose frequencies are shifted by a value corresponding to the distance of the target are received.
Four Doppler frequency signals with frequencies of are received. Similarly, in FM phase A, Doppler frequency signals of frequencies h ₁ , h ₂ , h ₃ , and h ₄ whose frequencies are shifted according to the distance of the target are received. With respect to the frequency change characteristic 22 of the transmitted wave, the frequency change characteristic 23 of the target Doppler signal has a frequency higher by f ₁ and is delayed by a time τ expressed by the following equation.

τ=2R/C ₀ ...(6) However, R is the target distance.

Similarly, the frequency change characteristic 24 of the Doppler signal due to jet engine modulation
With respect to the transmitted wave, the frequencies are higher by f ₂ , f ₃ , and f ₄ and are delayed by time τ.

If there is only one signal, the target distance can be calculated from the frequency f ₁ detected by FM phase C and the frequency g ₁ detected by FM phase B using the following equation.

f ₁ −g ₁ =F·τ=F·2R/C ₀ (7) where F· is the frequency modulation degree of FM phase B.

R=C ₀ /2F・(f ₁ −g ₁ ) ...(8) However, when there are two or more signals, four combinations of target distances can be obtained with two combinations of FM phase C and FM phase B. Sometimes it happens. Therefore, conventional radar equipment uses FM phase A with a different frequency modulation degree to
I was conducting an FM range to find the distance between two targets.

However, as shown in FIG. 8, when three or more Doppler frequency signals are present, it is difficult to determine the true distances of the three targets even if three FM phases are used. For this reason, the target number judgment circuit 7
When there is one Doppler frequency signal, FM phase C and FM phase B are used, and when there are two Doppler frequency signals, FM phase C, FM phase B, and FM phase A are used in the ranging circuit 10. The frequency of the target signal was sent to execute the calculation according to equation (8).
Further, when there are three or more, there are many combinations as shown in FIG. 9, and when there are N, there are N×N combinations. However, the frequency of FM phase B is
Since it is always smaller than FM phase C, R ₂₁ ,
R ₃₁ , R ₃₂ , R ₄₁ , R ₄₂ , and R ₄₃ are negative and are not subject to calculation. Actually, there are (N+1)×N÷2 distance calculation combinations. In this way, it was assumed that the true target distance could not be determined, and it was considered impossible to measure the distance.

[Problem to be solved by the invention]

In conventional radar devices, the target distance is calculated using the method described above, so even if there is one target and a plurality of Doppler frequency signals are detected by the jet engine modulation, it is impossible to measure the distance. A signal has been detected and
Moreover, making distance measurement impossible even when there is only one target has the problem of significantly degrading the performance of the radar device.

This invention was made to solve these drawbacks, and focuses on the fact that the differences between multiple Doppler frequencies due to jet engine modulation are almost equally spaced, so even if three or more signals are detected, , calculating the frequency difference between the multiple Doppler frequency signals in each of the two frequency modulation phases;
If the frequency differences match in the two frequency modulation phases, distance measurement is performed using the high frequency signal in each phase, and if they do not match, the signal with the maximum amplitude in each phase is used for distance measurement. The purpose is to calculate the true target distance by measuring distance using .

[Means to solve the problem]

In order to achieve such an objective, the radar device according to the present invention uses a means for determining the true distance by using different distance measurement calculation methods depending on the number of detected signals in the signal processor, and the target of the signal processor is A frequency difference calculation judgment circuit is provided at the output end of the number judgment circuit,
If the number of signals is 2, the frequency difference between the signals is calculated, and if they are equal, the distance measurement circuit uses the higher frequency signals of the two FM phases to perform distance measurement calculations. performs conventional distance measurement calculation using three FM phases, calculates the frequency difference between the signals even if the number of signals is three or more, and if they are equal, calculates the frequency difference between the two FM phases. A distance measurement circuit uses the signal to perform distance measurement calculations,
If they are different, the signal with the maximum amplitude is selected from each FM phase and used to perform distance measurement calculations, and this distance data is input to the distance judgment circuit and compared with the radar's maximum distance performance. If it is within this range, it is determined to be the true target distance, and if it is greater than that, it is determined to be a fake target and removed.

[Effect]

In this invention, after detecting a plurality of Doppler frequency signals, a jet engine modulation signal is determined based on the frequency difference between the signals, and if the frequency differences are the same, the highest frequency equal to the true Doppler frequency is selected. By performing distance measurement calculations using the signal with the maximum amplitude and, conversely, using the signal with the maximum amplitude when there is a difference, it is possible to prevent ranging from being impossible due to receiving multiple signals when there is only one target. At the same time, it becomes possible to accurately calculate the true target distance no matter what jet engine modulation frequency of the target, thereby preventing deterioration of detection performance of the radar.

〔Example〕

Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 1 shows the configuration of a radar device according to the present invention. In the figure, numerals 1 to 7, 10, and 12 are the same devices or parts as the conventional radar device described above. In this invention, when the target number determining circuit 7 determines that two or more signals are detected, the frequency difference between the plurality of signals from the signal detector 6 is calculated, and the frequency difference between the signals due to the jet engine modulation is determined. A frequency difference calculation/judgment circuit 10 determines when there are three or more signals and their frequency differences are unequal.
A maximum amplitude signal selection circuit 11 selects the signal with the maximum amplitude, and a distance determination circuit 11 determines whether the distance measurement calculation result is appropriate by comparing it with the detection performance of the radar.

FIG. 2 shows an example of distance measurement calculation using all combinations of four or three Doppler frequency signals detected in two frequency modulation phases, and the amplitude intensity of each signal on the frequency axis.

FIG. 3 shows a distance calculation flowchart from the target number judgment circuit to the distance judgment circuit of the signal processor in this invention.

Next, the operation of the radar device according to the present invention will be explained.

The transmitted wave is sent from the excitation/receiver 1 to the antenna 2, radiated from the antenna 2 toward the target aircraft 15, and received again by the antenna 2 together with the clutter, and then amplified, frequency converted, and The signal is subjected to clutter removal and phase detection, converted into a digital signal, and input to the signal processor 3. Then, in the signal processor 3, clutter is removed by a clutter removal filter 4, extracted as a narrow band Doppler frequency signal by a Doppler filter 5, and a signal detector 6
amplitude detection and signal detection from noise are performed and detected as a Doppler frequency. and,
The number of signals and the frequency of each signal detected in FM phase C and FM phase B are input to the target number judgment circuit 7, and when the number of signals in FM phases C and B is 1, respectively, FM phases C and B are detected. Perform distance measurement calculation using the signals of FM phase C and B, and when the number of signals of either FM phase C or B is 2 or both are 2,
Conventional radar equipment uses the signals of FM phases C, B, and A to perform distance measurement calculations, and when the number of signals is 3 or more in either FM phases C or B, distance measurement is impossible. Also in this invention, the operation of the target number determining circuit 7 is similar to that of the conventional radar device. In this invention, when the number of signals in either or both of FM phases C and B is 2, and after it is determined that the number of signals is 3 or more, the distance measurement calculation is different from the conventional one. To understand this, let's discuss the received jet engine modulation signal. These signals have lower frequencies than the true Doppler frequency signals determined by the relative velocity to the target, and their frequency intervals f _JEM are almost equally spaced. Also, FM phases C and B and FM phase C of Doppler frequency by jet engine modulation.
The frequency difference between and A is determined by the distance, and the values are almost the same at each frequency. Figure 2 shows this graphically, where f ₁ is the true Doppler frequency determined by the relative speed to the target, and f ₂ , f ₃ , and f ₄ are the Doppler frequencies due to jet engine modulation. It is the frequency. And g ₁ is the true Doppler frequency received during linear modulation (FM phase B), which is a frequency lower than f ₁ by (F·2R/C ₀ ) shown in equation (7).

Further, the frequency intervals of the signals f ₁ to f ₄ and g ₁ to _{g 4} are f _JEM and are equal. Therefore, the combination of distance calculations is more suitable for FM phase B and FM phase C than for FM phase C.
Considering that FM phase A is always lower and FM phase A is always higher than FM phase B, the frequency difference is negative (FM phases C and B and C and A) or positive (FM phases C and A). Since cases B and A) can be excluded, there are 10 possibilities (4+3+2+1) in the case of Figure 2 a. Figure 2b is
This figure shows that the signals generated by jet engine modulation may not always appear in the same number and at the same frequency as shown in Figure 2a, and if the level of the modulation component is low, it may be buried in noise and detected. This corresponds to when it is not possible. In this case, there are five combinations of distances. Among these distances, the true target distance that we want to find with this invention is
This is R ₀ calculated from f ₁ and g ₁ .

R ₀ =(f ₁ -g ₁ )C ₀ /2F (9) FIG. 2c is a diagram showing the frequency and amplitude of each signal in the case of FIG. 2b. Typically, the true target Doppler signals f ₁ and g ₁ are always detected and feature the largest amplitude. Therefore, by utilizing the characteristics that the jet engine modulation signals have equal frequency spacing and the amplitude of the true target Doppler signal is the largest, conventionally one target can be determined. This prevents the reception of signals equivalent to multiple targets, which makes ranging impossible and degrades detection performance, and allows accurate calculation of the true target distance. The operations from the target number determining circuit 7 to the distance determining circuit 11 shown in FIG. 1 will be explained with reference to FIG. 3.

The target number judgment circuit 7 determines the FM in the stepper.
The number of signals detected in phases C and B is 2 or more, or 1
If the value is 1, the distance measuring circuit 1 is determined by the combination of the signals from FM phases C and B.
Distance calculation is performed at 0. The result is sent to the distance judgment circuit 11 and compared with the radar maximum distance performance R _n in a step. If the value is less than or equal to R _n , the result is determined to be the true target distance in the step and is sent to the display. ,Is displayed. When R is larger than _n ,
It will be removed after it is determined to be a fake target by Step-O. In the stepper, the number of signals is 3 or more, or
If it is 2, select FM with the step key.
It is determined whether phases C and B are both 2, and if one is 2 and the other is 1 (NO), a two-target distance measurement calculation using FM phases C, B, and A is performed step by step. This calculation result is sent to the distance determination circuit 11 and subjected to similar processing. When both signals are 2, the signal is sent to the frequency difference calculation/judgment circuit 8, and Stepke calculates the frequency difference between the two signals at FM phases C, B, and A. The stepco then determines whether all these frequency differences are the same, and if they are different, the process advances to the stepcope and similar processing is performed. If they become the same, the jet
Since the signals are considered to be caused by engine modulation, in order to obtain the true target Doppler signals f ₁ and g ₁ as shown in Fig. 2a, the high-frequency signals of FM phases C and B are processed in the stepper, respectively. The distance measurement circuit 10 performs distance measurement calculations. This result is subjected to similar processing after the step. Next, if the target number is 3 or more in the stepper, it is sent to the frequency difference calculation judgment circuit 8.
In steps, the frequency difference between the respective signals in FM phases C and B is calculated. Then, in the step, these frequency differences are compared to see if they are all the same, and if they are the same, it is determined that there are multiple Doppler signals from one target due to jet engine modulation. Then, the highest frequency (true drap frequency) is selected and distance calculation is performed to obtain the true target distance. Next, if the frequency difference is different, the comparison is made again in step S to see if the number of signals is greater than k. The value of k is set to a number slightly greater than the number of expected jet engine modulation signals. If the number is greater than k, it is considered to be an interference wave from the outside, so the stepper system determines that it is an interference signal, and sends an interference signal marker and its angle information to the display 12 for display. If it is smaller than k, as shown in Figure 2b, it is a jet engine modulation signal, but the modulation component below the noise level is not detected, or there are different signals within the same beam of the antenna. This is a case where there are three or more targets with different speeds and distances, and a solution cannot be obtained in principle using the three-phase ranging method. Therefore, in the present invention, in the stepper, the FM phases C and B are
The signal with the maximum amplitude is selected from among the signals, and distance measurement calculations are performed step by step using this signal. With this method, as shown in Figure 2c, a true Doppler signal can be obtained and the true target distance can be obtained, and even if there are actually three targets, the target with the closest distance has a high threat level. Since the amplitude is the largest, it is possible to calculate the distance to this target. and,
This distance data is sent to the stepper and undergoes similar processing, and when it is determined that it is the true target distance, it is displayed on the display 12.

In this way, the correct true target distance will always be obtained, independent of the jet engine modulation frequency and target distance.

Although the above embodiment shows a radar device using an antenna that performs beam scanning electronically, the present invention is not limited to this, and can be applied to a radar device using any type of antenna or transmitter. Also, the signal processor can be configured by hardware or software.

〔Effect of the invention〕

As described above, the present invention has a simple configuration that utilizes most of the signal processor circuit of a conventional radar device and adds a frequency difference calculation judgment circuit, a maximum amplitude signal selection circuit, and a distance judgment circuit. Even when a large number of Doppler signals are detected due to the rotation of the aircraft target's jet engine blades, the target distance is always calculated without depending on the jet engine modulation frequency or target distance, thereby preventing deterioration of radar detection performance. It has the effect of preventing

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram showing a radar device according to an embodiment of the present invention, FIG. 2 is a diagram showing a combination for performing frequency modulation ranging in this invention, and FIG. 3 is a flowchart for distance calculation in this invention. Figure 4 is a schematic configuration diagram showing a conventional radar system, Figure 5 is a conceptual diagram of the radar system searching for a target aircraft while flying, and Figure 6 is a diagram showing how the radar system searches for a target aircraft while flying. A conceptual diagram for explaining the Doppler shift in the negative direction. Fig. 7 is a diagram showing the frequency spectrum of the signal received by the radar device shown in Fig. 5. Fig. 8 is a diagram showing the temporal change of the transmitted wave and the received wave. The diagram shown in FIG. 9 is a diagram showing combinations when distance calculation is performed using a plurality of Doppler frequency signals. In the figure, 1 is an excitation/receiver, 2 is an antenna,
3 is a signal processor, 4 is a clutter removal filter, 5
is a Doppler filter, 6 is a signal detector, 7 is a target number judgment circuit, 8 is a frequency difference calculation judgment circuit, 9 is a maximum amplitude signal selection circuit, 10 is a distance measuring circuit, 11 is a distance judgment circuit, 12 is a display , 13 is the main beam, 14 is the side rope, 15 is the target aircraft, 16 is the ground, 17 is the blade, 18 is the main beam clutter, 19 is the side rope clutter, 20 is the target Doppler signal, 21 is the jet engine.
Doppler signal by modulation, 22 is the frequency change characteristic of the transmitted wave, 23 is the frequency change characteristic of the target Doppler signal, 24 is the jet engine
Frequency change characteristics of Doppler signal due to modulation, 25 is a radar device, f ₁ , f ₂ , f ₃ , f ₄ , g ₁ ,
g ₂ , g ₃ , g ₄ , h ₁ , h ₂ , h ₃ , h ₄ are Doppler frequencies,
f _JEM is the jet engine modulation frequency, R ₀ is the true target range, R ₁ , R ₂ , R ₃ are the simulated ranges, F is the frequency modulation depth of linear frequency modulation, R _JEM is f1 _TEM PRF is the pulse repetition frequency, f ₀ is the frequency of the transmitted wave, R ₁₁
~ _R44 is the distance calculated by the combination of _f1 ~ _f4 and _g1 ~ _g4 , and A~R44 is each step of the distance calculation flow from the target number determining circuit to the distance determining circuit. In addition,
In each figure, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A transmitting wave of a constant frequency and a transmitting wave of linear frequency modulation are alternately generated for a predetermined period of time, and this transmitting wave is pulse-modulated with a predetermined pulse width and a predetermined pulse repetition frequency, and sent to the antenna, and a received signal is An excitation/receiver for amplification, an antenna for amplifying the transmitted wave from the excitation/receiver, radiating it into space, and receiving a reflected signal from the target, and clutter included in the received signal from the excitation/receiver. , the Doppler frequency from the target is detected by frequency analysis during the transmission period with a constant frequency and the transmission period with linear frequency modulation, and the distance to the target is calculated from the difference between the Doppler frequencies obtained in both periods. The signal processor is equipped with a signal processor that performs calculations and a display that displays target specifications, and the signal processor receives frequency modulation from the Doppler frequency signal from the target aircraft by the rotation of the engine, compressor, and blades of the target aircraft. , when multiple Doppler frequency signals are received in both the constant frequency transmission period and the linear frequency modulation transmission period, the distance measurement calculation process is changed depending on the number of received signals. In this case, calculate the frequency difference between the signals, and if the frequency differences are equal, use the higher frequency signal in the two FM phases, and if they are different, use the signals in the three FM phases to perform distance measurement calculations. , On the other hand, when the number of signals is three or more, the frequency difference between the signals is calculated, and if the frequency differences are equal, the two
A radar device characterized by having means for performing distance measurement calculations using a high frequency signal in the FM phase and, when different, using a signal with the maximum amplitude among the signals in each phase.