JPH0136077B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improving the performance of a radar device that tracks targets located at low elevation angles.

First, a conventional tracking radar device will be explained with reference to the drawings. Figure 1 is a configuration diagram of a conventional tracking radar device. In Figure 1, 1 is a stabilized local oscillator (STALO).
2 is a coherent oscillator, 3 is a phase detector, 4 is a gate circuit, 5 is a pulse modulator, 6 is a distributor, 7 is an amplifier, 8 is a circulator, 9 is a mixer, 10 is an intermediate frequency (Intermediate frequency) Frequency: IF) amplifier (hereinafter referred to as IF)
It's called an amplifier. ), 11 is a 90° phase shifter, 12 is an antenna, and 13 is a computer.

Now, a coherent oscillator (COHO; COHerent)
IF (Intermediate) output from Osillator) 2
The continuous wave of RF (Radio Frequency) is mixed by a mixer 9 with the continuous wave of RF (Radio Frequency) output from a stabilized local oscillator (STALO) 1. The output of this mixer 9 is pulse-modulated by a pulse modulator 5 and reaches a distributor 6. At this time, it is assumed that the distributor 6 distributes N signals. The outputs of the distributors 6 are each amplified by an amplifier 7, and then passed through a circulator 8 to an antenna 12.
It is radiated as radio waves. The radiated radio waves are reflected by the target and reach the antenna 12. At this time, assuming that the target exists at a low elevation angle altitude, the reflected wave from the target T, in addition to the component that directly reaches the antenna 12, is further reflected from the sea surface or the ground as shown in Figures 2 and 3. Components that reach the antenna 12 are added, resulting in a so-called multipath effect. The signal is then input to the mixer 9 via the circulator 8 again. Incidentally, in the mixer 9, the received signal and the signal from the stabilizing local oscillator 1 are mixed, converted into IF, and then input to the IF amplifier 10. The output of the IF amplifier 10 is distributed to two phase detectors 3, one of which is mixed with the direct output from the coherent oscillator 2, and the output is mixed with the direct output from the coherent oscillator 2. After passing through the gate circuit 4, the output is divided into two phase detectors 3. The amplitude and phase components are extracted to obtain the real part I ₁ of the complex signal V ₁ . In the other phase detector, the output of the coherent oscillator 2 is mixed with the output of the IF amplifier 10 via a 90° phase shifter 11, and the output is mixed with the output of the IF amplifier 10 via the gate circuit 4, which outputs the amplitude of the received signal from the target,
The phase component is extracted and the imaginary part Q ₁ of the complex signal V ₁ is obtained. Similarly, #(N-1) antenna 1
2, the real part I _{N -1} , imaginary part Q _{N -1} of the complex signal V _{N -1} ,
#N antenna 12 has the real part I _N of the complex signal V _N ,
The imaginary part Q _N is obtained respectively.

These complex signals V ₁ , V _{2 .} . . , V _N are processed by the computer 1.
Enter 3. The computer 13 uses these complex signals to calculate the antenna 12 which has undergone multipath effects.
Calculation is performed to estimate the direction of arrival of the direct wave W ₁ from among the waves arriving at the wave W 1 .

By the way, in this calculation, it is assumed that the number n of directions of arrival of radio waves to the antenna 12 is known.
The above complex signals V ₁ , V ₂ , ...V _N and the complex outputs of the antenna obtained when the arrival directions of n radio waves are virtually arbitrarily selected V ₁ ', V ₂ ', ... V _N ' Find the direction of arrival of the radio waves that minimizes the difference between Among these directions of arrival, the direction with the largest elevation angle was taken as the direction of arrival of the direct wave.

According to the above method, the sea level O
Or, if the ground is almost flat, the reflected wave will be a specular reflection W ₂ and the number of directions of arrival of the radio wave will be a direct wave W ₁
Since it is clear that there will be two pieces in combination with this, it was possible to estimate the angle extremely accurately for a low-altitude target.

However, as shown in Figure 3, when the sea surface O or the ground is uneven compared to the transmission wavelength, the target T
In addition to the direct wave W ₁ and specular reflection wave W ₂ from each image T′, a diffuse reflection component W ₃ is added,
This corresponds to an increase in the number of images T' due to these diffused reflections. Therefore, since the number of images T' due to diffuse reflection is generally unknown, a large error occurs in the estimated elevation angle of the target.

These phenomena were determined by computer simulation, and the angular error (=|estimated elevation angle of the target - true elevation angle of the target|) with respect to the true elevation angle of the target was determined as shown in FIGS. 4 and 5.

If Figure 4 corresponds to Figure 2, Figure 5 corresponds to Figure 3.
An example of computer simulation results in a case corresponding to the figure is shown. In both FIGS. 4 and 5, the number of arrayed antennas is 5, and the signal (direct wave from the target) to receiver noise ratio is 30 _dB . As can be seen from these figures, when the sea surface or the ground is close to a flat surface, it is possible to track the target very accurately, but when there is diffuse reflection, the angular error increases significantly, making accurate tracking impossible. Ta. However, the vertical and horizontal axes in FIGS. 4 and 5 are normalized by the _3dB width of the antenna beam that would be formed when N antennas 12 are excited in the same phase.

In order to solve these drawbacks, the present invention removes the influence of diffuse reflection by switching the frequency for each transmitted pulse and uncorrelating the reflected components received by the radar.The present invention will be described in detail with reference to the drawings below.

FIG. 6 is a configuration diagram of a tracking radar device showing an embodiment of the present invention, in which the transmitting frequency changes for each transmitted pulse or every finite time instead of the stabilizing local oscillator 1 of FIG. variable oscillator 14
is used, and an averaging circuit 15 is newly added. As an example, it is assumed that the oscillator 14 is composed of M oscillators 16 that each oscillate continuous waves at frequencies f ₁ ′, f ₂ ′, ...f _M ′, and the outputs of the M oscillators 16 are switched by a switch. and from the oscillator 14, f ₁ ' at t=t ₁ , t=t ₂
Assume that f ₂ ′...t=t _M and the frequency of f _M ′ is output. It is assumed that the constant frequency of the coherent oscillator 2 is added to the frequency of the oscillator 14 by the mixer 9, and radio waves having frequencies f ₁ ', f ₂ ', . . . f _M ' are radiated from the antenna 12. The process up to estimating the elevation angle of the target for each transmitted pulse or for each finite time is the same as that described in FIG. 1.

However, as a method for estimating the elevation angle of the target, for example, the following maximum likelihood estimation method can be considered. (I.
Kupiec, “Experimental Verification of the
Performance of the Aperture Sampling
Technique”, Tech Note 1975−45, Lincoln
Lab., MIT, 15.Sept.1975) The output of each antenna is _Vo = _K 〓 〓 ^k=1 f(α _k )・A _k・exp(jφ _k )・exp[j2π/λ・(
n-1) d・sinα _k ]+F _o (A-1). However, n-1, 2, 3,...,N.

Here, K is the number of reflection sources from the target and the ground, α _k is the arrival angle (elevation angle) of the radio wave reflected by the k-th reflection source, and f (α _k ) is α _k of each antenna.
, A _k and _k are the amplitude and phase of the reflected wave from the above k-th reflection source arriving at the antenna, respectively, λ is the transmission wavelength of this tracking radar device, d is the distance between each antenna, and F _o is the reception It's machine noise. If formula (A-1) is expressed as a matrix, the following formula is obtained.

V = A・B + E (A-2) here A _ok = exp [j2π/λ(n-1)dsinα _k ] (A-5) B _k =f(α _k )A _k exp(jφ _k ) (A-7) It is. In this case, the likelihood function is is given by Here δ is receiver noise F ₁ , F ₂ ...,
The standard deviation of F _N , * represents the conjugate transpose of the matrix.

Furthermore It is. Each incident wave is estimated by finding V' that maximizes the likelihood function Z expressed by equation (A-9), but the information needed now is the arrival angle of each incident wave, and the amplitude and phase are especially necessary. isn't it. Under such conditions, the arrival angle of each incident wave can be easily determined.

That is, The result is to find the angle of arrival of each incident wave that can maximize the angle of arrival of each incident wave. Since V is given,
After all, α ₁ , α ₂ when q becomes maximum by variously changing the value of α _k of A expressed by formulas (A-4) and (A-5)
...α _k is the angle at which each incident wave is found.

The largest of these angles becomes the estimated angle θ ₁ sought by the target. As shown in Figure 3, in cases where diffuse reflection occurs because the surface of the sea or the ground is uneven compared to the transmission wavelength, if a radio wave with frequency f ₁ is transmitted from antenna 12 at t = t ₁ , diffuse reflection will occur. The apparent signal-to-noise ratio decreases due to the influence of , and the estimated elevation angle θ ₁ of the target is obtained as a value that deviates from the true elevation angle θ _T of the target. Next, a radio wave of frequency f ₂ is transmitted at t = t ₂ , and the diffuse reflection component included in the received radar echo is f ₁ and _f It is known that if the difference between ₂ is large enough, the correlation will be close to non-correlation. The estimated value θ ₂ of the target elevation angle obtained by setting f ₂ to such a value has a deviation from the true elevation angle θ _T that is different from θ ₁ .

Below, at t=t ₃ , radio waves of frequency f ₃ , t
= f ₄ at t ₄ ...When f _M radio waves are transmitted at t _M and the estimated elevation angles of the target θ ₁ , θ ₂ , ..., θ _M are obtained, these true elevation angles θ _T
The deviation from this is almost random. FIG. 5 shows how the estimated elevation angle of the target changes over time at this time. If the averaging circuit 15 calculates θ ₁ +θ ₂ +...+θ _M /M (1) from these M elevation angle estimation values, the variance of the values given by equation (1) is θ ₁ , θ ₂
... is close to 1/M of the variance of the random variable θ with θ _M as the sample value, and the average value of the values given by equation (1) becomes the true target elevation angle. Therefore, the accuracy of the estimated elevation angle of the target is improved compared to the conventional method. In order to confirm the above-described improvement in accuracy, the angular error (=|estimated elevation angle of the target - true elevation angle of the target|) with respect to the true elevation angle of the target was determined by computer simulation, as shown in FIG. However, Figure 8 is an example of the results obtained by computer simulation under the same conditions as those obtained in Figure 5, except for changing the transmission frequency, and M in equation (1) was calculated as 100. . As is clear from comparing FIG. 8 with FIG. 5, even when diffuse reflection exists, the present invention can reduce the angular error, improve the accuracy of estimating the elevation angle of the target, and perform accurate tracking. I can do it. However, the vertical and horizontal axes in FIG. 8 are normalized by the _3dB width of the antenna beam that would be formed when N antennas 12 are excited in the same phase.

As described above, in the low elevation angle tracking radar device according to the present invention, the transmission frequency is changed for each transmission pulse to decorrelate the diffuse reflection component received by the device, and the elevation angle of the target obtained for each transmission pulse is By averaging the estimated values, the variance of the estimated values is reduced, and the accuracy of estimating the elevation angle of the target is improved, thereby allowing accurate tracking.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of a conventional low elevation angle tracking device, Figure 2
The figure shows the relationship between the direct wave from the target, the specular reflection wave from the image, and the radar device when the sea surface or ground is almost flat. Figure 3 shows the unevenness of the sea surface or ground compared to the radar transmission wavelength. Fig. 4 is a diagram showing an example of the angle error due to the conventional array aperture sampling method in the case shown in Fig. 2. 5
The figure shows an example of angular error due to the conventional array aperture sampling method in the case shown in FIG. 3, and FIG. 6 is a configuration diagram of an embodiment according to the present invention.
Figure 7 is a diagram showing the estimated elevation angle value of the target in chronological order by changing the transmission frequency for each transmission pulse and correspondingly when there is diffuse reflection as shown in Figure 3. 3 is a diagram showing an example of the angle error according to the present invention in the case of FIG. 3, in which 12 is an antenna, 13 is a calculator, 14 and 16 are variable frequency oscillators, and 15 is an averaging circuit. Note that the same or corresponding parts in the figures are indicated by the same reference numerals.

Claims (1)

[Claims] 1. A tracking radar device that tracks a target located at a low elevation angle includes a transmitter whose transmission frequency can change for each transmission pulse or every finite time, and a plurality of transmitters arranged in the elevation direction to change the transmission frequency from the transmitter. an antenna that emits radio waves toward a target and receives reflected waves from the target; an estimating means that estimates the elevation angle of the target using complex signals obtained by each of the plurality of antennas; and a transmission obtained by the estimating means. 1. A tracking radar device comprising means for averaging elevation angle information of a target corresponding to frequency.