JPH065276B2 - Radar reflection cross section measuring device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for measuring a radar reflection cross section of an object to be measured that reflects radio waves.

(Summary of the Invention) In the measurement of a radar reflection cross-section, the present invention rotates an object to be measured, transmits a radio wave to the object to be measured, receives a reflected wave reflected from the object, and measures the object to be measured from the received signal. By detecting the Doppler frequency shift caused by the rotation and subjecting this to a high-speed Fourier transform, it is possible to obtain the amount of reflection from each part of the object to be measured.

(Prior Art) Conventionally, various measurement methods such as a pulse method and a phase interferometry method have been established in the measurement of a radar reflection cross section (hereinafter, referred to as RCS).

(Problems to be Solved by the Invention) By the way, various conventional RCS measurement methods described above are based on the amount of reflection (strength of reflected radio wave) from the entire object to be measured.
The RCS is measured, and there is a drawback that it is not possible to measure how much reflection amount is present from a certain portion of the measured object. Therefore, for example, in the case of conducting research such as efficiently reducing the RCS of an object, there is a problem that it is not possible to evaluate in the research stage which portion should be improved by the surface shape effect or the electromagnetic wave absorber mounting effect.

In view of the above points, the present invention, when measuring the RCS, by rotating the DUT, by performing a fast Fourier transform of the Doppler frequency shift, from the various parts of the DUT with a relatively simple measurement system. It is an object of the present invention to provide a radar reflection cross section measuring device capable of measuring a reflection amount.

(Means for Solving the Problems) In order to achieve the above object, in a radar reflection cross-sectional area measuring apparatus of the present invention, a rotating unit that installs and rotates the object to be measured, and a transmission that transmits radio waves to the object. Section, a receiving section for receiving a reflected wave from the DUT, a detecting section for detecting a Doppler frequency shift caused by rotation of the DUT from a signal received by the receiving section, and the DUT The rotation time is divided into small sections, and the conversion section that acquires the data of the detection section and performs fast Fourier transform in the small sections is provided, and the rotation section is driven to rotate at a constant speed by the rotation table. And an inverted L-shape having a portion extending in the radial direction with respect to the center of rotation of the rotary table and having a radio wave absorber mounted on the entire surface or a non-reflective DUT mounting base. A corner reflector as the DUT And to detect the reference level of the reflected wave by there.

(Operation) First, as a theoretical explanation of the present invention, in the radar reflection cross section measuring apparatus of the present invention, the object to be measured is rotated, and therefore the reflection of the radio wave transmitted toward the object to be measured is reflected. The waves undergo a Doppler frequency shift. At that time, the reflected wave from the portion close to the rotation center has a small Doppler frequency shift, and the Doppler frequency shift increases as the distance from the rotation center increases. Therefore, the Doppler frequency shift included in the reflected wave from the object to be measured is detected and referred to as a fast Fourier transform (hereinafter referred to as FFT (Fast Fourier Transform)). }
By processing, the amount of reflection from each part of the measured object can be accurately measured.

That is, as shown in FIG. 4, when a radio wave arrives at a rod-shaped object that is rotating at an origin on the X axis with a length L and an angular velocity Ω from an antenna that is separated by a distance D, the amount of reflection from this object (Strength of reflected radio wave) V (t) is as follows.

V (t) = U (t) exp (jΩt) (1) where Where t is time, x is the distance from the center of rotation of the rod-shaped object,
W (x) is a weighting function that indicates the amount of reflection when each part of the object is different, k is the wave number in free space, and C is an arbitrary proportional coefficient. And when this U (t) is observed on the time axis,
The angle of rotation θ =
If it is 0 °, it becomes as schematically shown in FIG. Therefore, by dividing this into small sections TDIV, performing Fourier transform, and observing on the frequency axis, the reflection amount of each part can be known.

In the Fourier transform, the observed frequency f is | f | ≦ LΩ / λ (2) (where λ is the wavelength of the radio wave to be measured) in relation to the rotational angular velocity Ω, and the maximum frequency is f _max. Then, the sampling interval Δt for covering this band must be Δt ≦ 1 / 2f _max = λ / (2LΩ) (3). Therefore, when the time is divided into small sections, the relationship between the time T _DIV and the sampling number N is as follows.

N ≧ T _DIV / Δt = 2T _DIV LΩ / λ (4) Further, the distance resolution Δx of the object to be measured at this time is F in the frequency range of FFT processing, and the sampling number N
Can be expressed by Δx = LΔF / (2f _max ) ... (5) (where ΔF = 2F / N).

(Embodiment) Next, regarding the configuration of the radar reflection cross-section area measuring apparatus according to the present invention, a schematic block diagram of its basic configuration is shown in FIG. 1, a block diagram of the embodiment is shown in FIG. The structure of the mounting base is shown in FIG. 3, and the measuring method will be described step by step below.

In FIG. 1, reference numeral 1 denotes a transmitter, which transmits a radio wave (transmitted wave) to an object to be measured. Reference numeral 2 denotes a receiver that receives a reflected wave (received wave) from the DUT, and 3 is a detector that detects a Doppler frequency shift caused by rotation of the DUT from the reflected wave signal received by the receiver. Reference numeral 4 denotes a conversion unit that divides the rotation time of the object to be measured into small sections and FFT-processes the data of the detection unit acquired in the small sections. Reference numeral 5 denotes a rotating unit that installs and rotates the object to be measured.

FIG. 2 shows the schematic block diagram of FIG. 1 in more detail. In FIG. 2, 11 is an oscillator and 12 is an oscillator.
Is a first mixer, 13 is an amplifier, 14 is a transmitting antenna, and 15 is a local oscillator, and these constitute the above-mentioned transmitting unit. Reference numeral 16 is a receiving antenna, and 17 is an amplifier, which constitute the above-mentioned receiving section. 1
Reference numeral 8 is a second mixer, 19 is a filter, and 20 is a third mixer, and the oscillator 11 and the local oscillator 15 used as local oscillators constitute the above-mentioned detection unit. Reference numeral 21 is an FFT converter.
Further, 30 is a rotary base, 31 is an object mounting base that is rotationally driven by the rotary base at a constant speed, and these constitute the rotary unit described above.

As shown in FIG. 3, an inverted L-shaped bar-shaped mounting base 31 (having a wooden cross-sectional area of 1.2 m) having a horizontal length (the length of a portion extending in the radial direction with respect to the center of rotation of the rotating base) is 1.2 m. 4 cm x 4
cm) is fixed on the rotary table 30, and the object to be measured 40 is mounted on the mounting table. Further, a radio wave absorber 32 is mounted on the entire surface of the mounting base 31 in order to minimize reflection from the mounting base itself. In addition, the mounting base 31
It is effective to prevent the aliasing signal at the time of the FFT processing by configuring the bar shape as an inverted L shape.

In the above configuration, 1 MHz C generated by the oscillator 11
The W wave is up-converted to a frequency f ₁ in the X band or Ku band by the first mixer 12 using the local oscillator 15, amplified by the amplifier 13 to about 0.5 W, and directed from the antenna 14 to the DUT 40. Sent. Here, the DUT 40 has a period of 25.0 se, as shown in FIG.
Since it is mounted on the mounting base 31 that rotates integrally with the rotating base 30 that rotates at c, the reflected wave from the DUT 40 shifts to the transmission frequency f ₁ by the Doppler frequency deviation Δf.
Will be included. The reflected wave is received by the receiving antenna 1.
6, the received signal is amplified by the amplifier 17.

Here, in the present measurement device, since it is necessary to measure this Δf with high accuracy, it is necessary to perform the second conversion when down converting.
The local oscillator 15 used for the up-conversion is used again as the local oscillator of the mixer 18 of 1., and the frequency of 1 MHz + Δf is accurately extracted.
By thus sharing the local oscillator 15, there is an advantage that the fluctuation of the local frequency does not affect Δf.

Next, with respect to this 1 MHz + Δf, unnecessary frequency components such as harmonic components generated by frequency conversion are removed by the filter 19, and then only Δf is extracted by the third mixer 20. Note that the oscillator 1 is also used here as in the case described above.
1 is used as a local oscillator to accurately extract Δf.

Finally, this Δf is subjected to FFT processing by the FFT conversion unit 21 to be converted into the frequency domain as described above, and the reflection amount from each part of the DUT 40 is obtained. here,
The reflected wave from the portion of the DUT 40 near the rotation center has a small Doppler frequency deviation, and the Doppler frequency deviation increases as the distance from the rotation center increases. It can be distinguished and recognized in the frequency domain.

Further, in this measurement, two types of corner reflectors having different sizes (hereinafter, the smaller one is referred to as the reflector A and the larger one is referred to as the reflector B) are rotated on the 1.2 m rod-shaped mounting base 31 described above. It was mounted by changing the position from the center, and it was examined whether the position and the relative amount of the reflection level were accurately measured. Here, 15 GHz is selected as the operating frequency, and the dimensions of the two types of corner reflectors A and B used here at the above frequencies and theoretical calculation values of RCS are shown in Table 1 below.

The calculated value of RCS here was obtained by actually mounting the corner reflectors so that the incident radio waves were incident parallel to the axis of symmetry of the corner reflectors, that is, perpendicularly to the aperture. It is the calculation result in the state.

The FFT converter 21 used here has a frequency range of 500 Hz and a sampling period of 1 ms.
00 points are sampled and subjected to FFT processing. Therefore, it takes 0.8 seconds for the FFT process, and considering that the rotation cycle of the DUT is 25.0 seconds, FF
It is necessary to convert the rotation angle to 11.8 ° for the T processing. For this reason, in this measurement, the amount of reflection of each part when the DUT is directly beside the transmitting / receiving antenna is measured. This means that the average reflection amount of 0.99 is measured. However, since the corner reflector is used as the object to be measured in this measurement, the change in the RCS can be almost ignored with respect to the change in the incident angle of about ± 5.9 °.

Next, we will show the actual measurement results using this measurement device, but before that,
First, the distance resolution Δx and the frequency resolution ΔF of this measurement method were theoretically calculated using the above equations (2) to (5). That is, in this measurement, N = 800, F = 500 Hz, L / 2 =
Since it is 1.2 m, Δx = 4.98 cm. Also,
Maximum value of Doppler frequency shift in this case fmax = 3
It becomes 0.144 Hz, and from this, ΔF becomes 1.25 Hz. Furthermore, the relationship between the Doppler frequency deviation Δf after FFT processing and the distance R from the center of rotation is R (cm) = 3.981 · Δf (Hz) ... (6), and the distance from the center is calculated from this. it can.

In consideration of such basic matters, in this measurement,
The large reflector B is fixed at a distance R = 40 cm, and the distance R (hereinafter referred to as R ₁ ) of the small reflector A is 70 cm to 110 c.
The amount of reflection and the distance R ₁ in each case were measured by changing the distance up to m at intervals of 10 cm. Further, in order to know the measurement limit of RCS in this measurement method, the reflection amount from only the mounting base was also measured without mounting the reflectors A and B. These results are shown in FIGS. 6 (a) to (c). The same figure (a) shows the reflection level of the mounting table 31 when there is no DUT,
(b) shows the case where R ₁ = 80 cm, and (c) shows the case where R ₁ = 110 cm. The peak of the reflection level of the large reflector B and the peak of the small reflector A appear at different frequencies. ing.

From these figures, the frequency at which the reflector A appears and the measurement distance R ₁ (distance from the center of rotation of the reflector A) converted from this frequency using the equation (6), and the reflection level with the reflector B at that time The measurement results of the differences are summarized in Table 2 below.

As a result, FIG. 6 (a) shows the amount of reflection from the mount 31 only. This level is the background noise level of the main measurement, and this level is the minimum RCS that can be measured by the main measurement system. To decide. Therefore, by observing this pattern, it is considered that the RCS of about 10 dBcm ² can be measured in consideration of the reflection levels of the reflectors A and B in this measurement.

However, it is considered that this background noise level can be further reduced by making the mounting base 31 of the object to be measured by a non-reflective radio wave such as styrofoam. It seems possible to measure small RCS.

When this pattern is observed, the reflection level from 0 to about 30 Hz is almost flat, and the reflection level sharply decreases at about 30 Hz.
It can be seen that the shape of 1 is accurately observed.

Next, FIGS. 6 (b) and 6 (c) show the measurement results when the reflectors A and B are attached to the attachment base 31.
As a result, when observing the measured pattern, a flat portion showing the mounting base 31 from the center of rotation, a portion where the reflection level is high where the reflectors A and B are located, and further, the mounting base length of 1.2 m The portion where the reflection level sharply drops in the previous space portion is accurately measured in each figure. In addition, when these figures are observed in detail, Table 2
As shown in, the maximum distance error of the measurement is about 2 cm, and the measurement error of the reflection level is about 4 dBcm when compared with the theoretical value (10.6 dBcm ² ) obtained from Table 1 for the relative level difference between the ^two reflectors. It was confirmed to be ² .

The measurement error of the measurement distance and the reflection level increases as the distance R ₁ increases. The reason is that the beam width of the antenna used for the measurement is relatively narrow and the reflector becomes laterally away from the rotation center. This is probably because the plane waves do not reach uniformly.

When measuring a sample other than the corner reflector, which is the object to be measured, a corner reflector whose theoretical value is known in advance is used to detect the reference level of the reflection amount (strength of the reflected wave), and then the sample to be the sample. By measuring the measurement object under the same measurement conditions, the reflection level in the case of the sample can be known.

(Effects of the Invention) As described above, according to the radar cross-section measuring apparatus of the present invention, the DUT is rotated, and the Doppler frequency shift is subjected to the FFT processing, so that a relatively simple measurement system can be obtained. It is possible to measure the amount of reflection from each part of the measured object.

As a result, in the measurement method using this device, for example, it is possible to accurately measure the distance of the object to be measured within an error of about 2 cm,
Also it confirmed that that can be measured with the measurement error of about 4DBcm ² to RCS of about 10DBcm ² also reflection level, measuring devices partial RCS of the object to be measured can be realized.

In the present invention, the distance resolution is restricted due to the relationship of the sampling cycle of the FFT processing, but it is considered that more accurate distance resolution measurement can be performed by using an even faster FFT conversion unit in the future.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration of a radar reflection cross section measuring apparatus according to the present invention, FIG. 2 is a block diagram in one embodiment of the present invention, and FIG. 3 is an object to be measured in one embodiment of the present invention. FIG. 4 is a schematic view of a reflected wave and FIG. 6 is a measurement of an embodiment of the present invention. It is explanatory drawing which shows an example of a result. 1 ... Transmission unit, 2 ... Reception unit, 3 ... Detection unit, 4 ... Conversion unit, 5
... rotating part, 11 ... oscillator, 12 ... first mixer, 13
... Amplifier, 14 ... Transmission antenna, 15 ... Local oscillator, 16 ... Reception antenna, 17 ... Amplifier, 18 ... Second
, 19 ... Filter, 20 ... Third mixer, 21 ... FFT conversion section, 30 ... Rotating table, 31 ... DUT mounting base, 40 ... DUT.

Claims (1)

[Claims] 1. A rotating section for setting and rotating an object to be measured, a transmitting section for transmitting an electric wave to the object to be measured, a receiving section for receiving a reflected wave from the object to be measured, and a receiving section for receiving the wave. And a detection unit for detecting the Doppler frequency shift caused by the rotation of the measured object from the signal, and dividing the rotation time of the measured object into small sections, and acquiring the data of the detection section in the small sections to obtain a fast Fourier transform. An inverted L-shape having a converting part for converting, the rotating part having a rotary base and a part which is rotationally driven at a constant speed by the rotary base and extends in a radial direction with respect to a rotation center of the rotary base. It is characterized in that a radio wave absorber is attached to the entire surface or is configured with a non-reflective DUT mounting base, and a reference level of the reflected wave is detected by using a corner reflector as the DUT. Radar cross section Constant apparatus.