JPH03175386A - Measuring apparatus for reflecting cross-sectional area of radar - Google Patents

Abstract

PURPOSE:To measure the quantity of reflection from each part of an object to be measured by rotating the object to be measured and subjecting Doppler frequency deviation to fast Fourier transform processing. CONSTITUTION:The CW wave of 1MHz generated from an oscillator 11 is upwardly converted to be set to the frequency of an X-band or Kw-band by the first mixer 12 using a local oscillator 15 and amplifier by an amplifier 13 to be transmitted to an object to be measured from a transmission antenna 14. Since the object to be measured is mounted on the mounting table 31 integrally rotated along with a turntable 30, the reflected wave from the object to be measured becomes one wherein Doppler frequency deviation DELTAf is contained in transmission frequency to be received by a receiving antenna 16. The local oscillator 15 used in upward conversion is again used to accurately take out frequency of 1MHz+DELTAf. When the oscillator 15 is jointly used as mentioned above, the fluctuation of local frequency exerts no effect on DELTAf. Next, the reflected wave is passed through a filter 19 with respect to 1MHz+DELTAf and, further, only DELTAf is taken out by the third mixer 20. Finally, DELTAf is subjected to FFT processing in a fast Fourier transform (FFT) part 21.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an apparatus 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 radio waves to the object, receives the reflected waves, and uses the received signal to determine the object to be measured. By detecting the Doppler frequency shift caused by rotation and subjecting it to fast Fourier transform processing, it is possible to determine the amount of reflection from each part of the object to be measured.

(Prior Art) Conventionally, various measurement methods such as the pulse method and the phase interferometry have been established for measuring the radar reflection cross section (hereinafter referred to as RC5).

(Problems to be Solved by the Invention) By the way, the various conventional RC3 measurement methods described above are based on the amount of reflection (intensity of reflected radio waves) from the entire object to be measured.
This method measures the RC3, and has the drawback that it cannot measure the amount of reflection from a certain part of the object to be measured. For this reason, when conducting research to efficiently reduce the RC3 of an object, for example, there was a problem in which it was not possible to evaluate at the research stage which parts should be improved by the effect of surface shape or the effect of installing a radio wave absorber.

In view of the above points, the present invention, when measuring RC3, rotates the object to be measured and fast Fourier transforms the Doppler frequency deviation, thereby measuring the RC3 from each part of the object using a relatively simple measurement system. It is an object of the present invention to provide a radar reflection cross section measurement device that can measure the amount of reflection.

(Means for Solving the Problems) In order to achieve the above object, the radar reflection cross section measurement device of the present invention includes a rotating section for installing and rotating the object to be measured, and a transmitter for transmitting radio waves to the object to be measured. a receiver that receives reflected waves from the object to be measured; a detector that detects a Doppler frequency shift caused by rotation of the object from the signal received by the receiver; and an object to be measured. The rotation time of the sensor is divided into small sections, and the converting section acquires the data of the detecting section in the small sections and performs fast Fourier transform on the data.

(Function) First, a detailed explanation of the present invention will be explained. In the radar reflection cross section measurement device of the present invention, the object to be measured is rotated, and therefore the reflected wave of the radio wave transmitted toward the object to be measured is undergoes a Doppler frequency shift. At this time, the Doppler frequency deviation of the reflected wave from a portion close to the rotation center is small, and the Doppler frequency deviation increases as the distance from the rotation center increases. Therefore, the Doppler frequency shift included in the reflected wave from the object under test is detected and fast Fourier transform (
Hereinafter, F F T (Fast Fourier Tr
ansform). ), the amount of reflection from each part of the object to be measured can be accurately measured.

In other words, as shown in Figure 4, when a radio wave arrives from an antenna a distance away from a rod-shaped object that is located at the origin on the X-axis and is rotating at an angular velocity Ω, the amount of reflection from this object is (Intensity of reflected radio waves) V (t) is as follows.

V (t) = U (t)expUΩt)
-(1) Here, 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 differs, k is the wave number of free space, and C is an arbitrary proportionality coefficient. When observing this U (t) on the time axis, if the rotation angle θ = 0° when the rotation angle is θ = 0゜ when facing directly sideways to the direction of arrival of the radio waves, it becomes roughly as shown in Fig. 5. Therefore, by dividing this into small sections T o Iv, Fourier transforming it, and observing it on the frequency axis, it is possible to know the amount of reflection at each part.

In addition, during Fourier transformation, the frequency f observed in relation to the rotational angular velocity Ω is f1≦LΩ/λ (2) (where λ is the wavelength of the radio wave to be measured), and the maximum frequency is f. sex, the sampling interval Δt to cover this band is Δt≦1/
2fo, = λ/(2LΩ), 13). Therefore, the relationship between the time T D + V when the time is divided into small sections and the number of samplings N is as follows.

N≧TDIv/Δt=2T,,vLΩ/′λ ・(4
) Also, the distance resolution ΔX of the object to be measured at this time is FFT
Letting the frequency range of processing be F and using the above sampling number N, it can be expressed as Δx=LΔF/(2f,...) (5) (where ΔF=2F/N).

(Example) Next, regarding the configuration of a radar reflection cross section measuring device according to the present invention, a schematic block diagram of its basic configuration is shown in FIG. 1, a block diagram of an example is shown in FIG. The structure of the mount is shown in FIG. 3, and the measurement method will be explained below layer by layer.

In Fig. 1, 1 is a transmitter, which sends radio waves (
transmit waves). 2 is the reflected wave from the object to be measured (
3 is a detection unit that detects the Doppler frequency shift caused by the rotation of the object to be measured from the reflected wave signal received by the receiver, and 4 is a small section of rotation time of the object to be measured. A conversion section performs FFT processing on the data of the detection section acquired in this small section, and 5 is a rotation section in which the object to be measured is installed and rotated.

FIG. 2 shows the schematic block diagram of FIG. 1 in more detail, and in this FIG. 2, 11 is an oscillator, 12
13 is a first mixer, 13 is an amplifier, 14 is a transmitting antenna, and 15 is a local oscillator, which constitute the above-mentioned transmitter.

16 is a receiving antenna, and 17 is an amplifier, which constitute the above-mentioned receiving section. 18 is the second mixer, 1
9 is a filter, 20 is a third mixer, and these together with the oscillator 11 and local oscillator 15 used as local oscillators constitute the above-mentioned detection section. 21 is an FFT conversion section. Also, 30 is a turntable, 3
Reference numeral 1 denotes a measuring object mounting table which is rotated at a constant speed by a rotary table, and constitutes the above-mentioned rotating section.

As shown in FIG. 3, a rod-shaped mounting base 31 (made of wood with a cross-sectional area of approximately 4 es + x 4 cm) having a horizontal length of 1.2 m is fixed on the rotary base 30, and the 5S40 to be measured is mounted on the rotating base 30.
can be installed. Furthermore, a radio wave absorber 32 is attached to the entire surface of the mounting base 31 in order to reduce reflection from the mounting base itself as much as possible.

In the above configuration, the IMHz CW wave generated by the oscillator 11 is up-converted to a frequency f of the X band or Ku band by the first mixer 12 using the local oscillator 15, and then converted to approximately 0.5 W by the amplifier 13. It is amplified and measured from the antenna 14.

It is transmitted toward Motonori 40. Here, the object to be measured 40 is
As shown in FIG. 3, since it is mounted on a mounting table 31 that rotates together with a rotary table 30 that rotates at a period of 25.0 seconds, the reflected wave from the object to be measured 40 is
The transmission frequency f includes the Doppler frequency shift Δf. The reflected wave is received by the receiving antenna 16, and the received signal is amplified by the amplifier 17.

Here, in this measuring device, it is necessary to measure this Δf with high accuracy, so when down-converting, the second
The local oscillator 15 used for up-conversion is used again as the local oscillator of the mixer 18, and the frequency of IMHz+Δf is extracted with high accuracy.

In this way, by sharing the local oscillator 15, there is an advantage that fluctuations in the local frequency do not affect Δf.

Next, for this IMHz+Δf, a filter 19 removes unnecessary frequency components such as harmonic components generated by frequency conversion.
After removing Δr, the third mixer 20 extracts only Δr. Note that here, as in the case described above, the oscillator 11 is used as a local oscillator, and Δf is accurately adjusted.
I'm trying to take it out.

Finally, this Δf is subjected to FFT processing in the FFT conversion unit 21 to convert it into the frequency domain as described above, and the amount of reflection from each part of the object to be measured 40 is determined. here,
The reflected waves from the parts near the center of rotation of the object to be measured 40 have a small Doppler frequency shift, and the further away from the center of rotation, the larger the de Knobler frequency shift. Therefore, the distance from the center of rotation of each part and the reflection Quantities can be distinguished and recognized in the frequency domain.

In addition, in this measurement, two types of corner reflectors of different sizes (hereinafter, the smaller one will be referred to as reflector A and the larger one will be referred to as reflector B) are rotated on the rod-shaped mount 31 of 1 or 2 m as described above. We installed it at different positions from the center and examined whether the relative amount of reflection level and the position could be measured accurately. Here, the frequency used is 15 GHz.
The dimensions of the two types of corner reflectors A and H used here at the above frequency and the theoretically calculated value of RC8 are shown in Table 1 below.

Table 1 The calculated value of RC3 here is based on the fact that in actual measurements, the corner reflector was mounted so that the incident radio waves were incident parallel to the axis of symmetry of the corner reflector, that is, perpendicular to the aperture surface. Here are the calculation results in that state.

Further, the FFT conversion unit 21 used here has a frequency range of 500 Hz and a sampling period of 1+ms, and samples 800 points and performs FFT processing. Therefore, it takes 0.8 seconds for FFT processing,
Considering that the rotation period of the object to be measured is 25.0 seconds, the rotation angle is converted to 11.8° for FFT processing.
It will require. For this reason, in this measurement, the amount of reflection from each part is measured when the object to be measured is directly horizontal to the transmitting and receiving antenna, but strictly speaking, if the directly horizontal state is 0°, in this measurement, the amount of reflection is ±5 This means that the average amount of reflection of .9° is being measured. However, since the measurement here uses a corner reflector as the object to be measured, ±5
．． For a change in the angle of incidence of about 9°, the R
Changes in CS can be ignored.

Next, we will show the actual measurement results using this measuring device, but before that,
First, the distance resolution ΔX and 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=500Hz,
Since L/2 = 1.2m, Δx = 4.98cn + roar. In addition, the maximum value of the Doppler frequency shift in this case f,...=30.144Hz, and from this, ΔF is 1
．． It becomes 25Hz. Furthermore, the relationship between the Doppler frequency shift Δf after FFT processing and the distance R from the center of rotation is R (cm) −3,981・Δf (Hz) = −(
6), from which the distance from the center can be calculated.

Considering these basic matters, in this measurement,
The distance R of the large reflector B is fixed at 40 cm, and the distance R of the small reflector A (hereinafter referred to as R1) is set to 70c-~1.
The distance was changed at intervals of 10c within a range up to 10cm, and the amount of reflection and distance R1 in each case were measured. In addition, in order to know the measurement limit of RCS in this measurement method, reflector A,
The amount of reflection from only the mounting base was also measured without attaching B. These results are shown in FIGS. 6(a) to 6(c). FIG. 6(a) shows the reflection level of the mount 31 when there is no object to be measured, and FIG. 6(b) shows the reflection level of R, - In the case of the 80c mouse, the figure (c) shows the case where R+'' is 110 cm, and the peak of the reflection level due to the large reflector B and the peak due to the small reflector A appear at different frequencies.

From these figures, we can see the frequency at which reflector A appears and the measurement distance R1 (reflector A) converted from this frequency using equation (6).
(distance from the center of rotation) and the measurement results of the difference in reflection level with reflector B at that time are summarized in Table 2 below.

Table 2 As a result, first of all, Figure 6(a) shows the amount of reflection from only the mounting base 31, but this level is the background noise level of this measurement, and this level is the minimum that can be measured with this measurement system. Determine the RCS of Therefore, observing this pattern, it is thought that in this measurement, an RCS of about 10 dBcm'' can be measured, considering the reflection levels of reflectors A and B.

However, it is believed that this background noise level can be further reduced by making the mounting base 31 of the object to be measured made of a material that does not reflect radio waves, such as polystyrene foam. It seems possible to measure small RCS.

Furthermore, when observing this pattern, the reflection level from 0 to approximately 30 Hz becomes almost flat, and the reflection level rapidly decreases at approximately 30 Hz, indicating that the shape of the rod-shaped mount 31 can be observed accurately. I can see that it is.

Next, FIGS. 6(b) and 6(c) show the measurement results when reflectors A and B are attached to this mounting base 31. As a result, when observing the measured pattern,
The reflection level suddenly increases from the center of rotation to the flat part indicating the mounting base 31, the part where the reflection level is high where reflectors A and B are located, and furthermore, the part of the space beyond the length of the mounting base of 1.2 m. The area of decline is precisely measured in each figure. In addition, when observing these figures in detail, as shown in Table 2, the distance error in measurement is approximately 2 c at maximum.
m, and the measurement error of the reflection level is the theoretical value (10.6 dB) obtained from Table 1 regarding the relative level difference between the two reflectors.
cm2), it was confirmed that it was approximately 4 dBc++2.

Note that as the distance R1 increases, the measurement errors in the measurement distance and reflection level increase; the reason for this is that the beam width of the antenna used for measurement is relatively narrow and as it moves away from the center of rotation in the horizontal direction, the beam width becomes uniform across the reflector. This is thought to be because the plane wave did not reach the area.

In addition, when measuring a sample other than the corner reflector that is the object to be measured, first detect the reference level of the amount of reflection (strength of reflected waves) using a corner reflector whose theoretical value is known, and then By measuring the object to be measured under the same measurement conditions, it is possible to know the reflection level of the end metal of the sample.

(Effects of the Invention) As explained above, according to the radar reflection cross section measuring device of the present invention, a relatively simple measurement system can be achieved by rotating the object to be measured and subjecting the Doppler frequency shift to FFT processing. This makes it possible to measure the amount of reflection from each part of the object to be measured.

As a result, in the measurement method using this device, it is possible to accurately measure the distance of the object to be measured, for example, within an error of about 2 cm, and the reflection level can be measured with an RC of about 10 dBcm2.
It can be confirmed that measurements can be made with a measurement error of 4 to 3, and a device for measuring a partial radar reflection cross section of an object to be measured can be realized.

In the present invention, the distance resolution is limited due to the sampling period of the FFT process, but it is believed that if a faster FFT conversion unit is used in the future, it will be possible to measure the distance resolution with higher accuracy.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the basic configuration of a radar cross-sectional area measuring device according to the present invention, FIG. 2 is a block diagram of an embodiment of the present invention, and FIG. 3 is a block diagram of an object to be measured in an embodiment of the present invention. A front view showing the installation of a mounting base and a corner reflector as an object to be measured, FIG. 4 is an explanatory diagram showing an analytical model, FIG. 5 is a schematic diagram of reflected waves, and FIG. 6 is a measurement in an embodiment of the present invention. It is an explanatory diagram showing an example of a result. 1... Transmitting section, 2... Receiving section, 3... Detecting section, 4
... Conversion section, 5... Rotating section, 11... Oscillator, 12
...first mixer, 13...amplifier, 14...transmission antenna, 15...local oscillator, 16...
Receiving antenna, 17... Amplifier, 18... Second mixer, 19... Filter, 20... Third mixer, 21... FFT converter, 30... Rotating table, 31...
...Measurement object mounting base, 40...Measurement object.

Claims (2)

[Claims] (1) A rotating part that installs and rotates the object to be measured, a transmitting part that transmits radio waves to the object to be measured, a receiving part that receives reflected waves from the object to be measured, and a signal received by the receiving part. a detection unit that detects a Doppler frequency shift caused by the rotation of the object to be measured; and a detection unit that divides the rotation time of the object to be measured into small intervals, and acquires data from the detection unit in the small intervals and performs fast Fourier transform on the rotation time of the object to be measured. A radar reflection cross-sectional area measuring device characterized by comprising a converting section. (2) Claim 1, wherein the reference level of the reflected wave is detected by using a corner reflector as the object to be measured.
The radar reflection cross section measurement device described.