JP2003014841A - Radar apparatus and coherent integration method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that a transmission pulse width could not be determined freely according to required time resolution and distance resolution conventionally. SOLUTION: The radar apparatus comprises a transmitter/receiver 1 for transmitting pulses and receiving reflection signals from a measurement target, a filter 2 for removing a signal at a frequency band that cannot be measured out of the received signals, an A/D conversion means 3 for A/D-converting at a predetermined sampling period, a gate means 4 for gating the reception signal on a time axis and extracting the reception signal in a gate, a data division means 5 for dividing the reception signal into four data, Fourier transform means 6a-6d for performing the Fourier transform of each data, complex conjugate means 7a-7c for obtaining the complex conjugate of the output signal at the Fourier transform means, complex multiplication means 8a-8c for performing the complex operation of the output of the Fourier transform means and that of the complex conjugate means, and a complex addition means 9 for adding the output signal of the complex multiplication means, thus inhibiting a system noise and improving SNR in the reception signal.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a light wave, an electromagnetic wave,
Or send and receive pulses consisting of waves such as sound waves,
The present invention relates to a radar device and a coherent integration method for detecting a property of a measurement target, such as a shape of the measurement target or a moving speed, from a frequency spectrum of a received signal.

[0002]

2. Description of the Related Art A conventional radar device of this type transmits pulse-modulated light waves, electromagnetic waves, sound waves, etc., and receives a reflection signal from a measurement object with a delay time corresponding to the distance to be measured. To do. The received signal having a time equal to the time width of the transmitted pulse is Fourier-transformed and the signal is integrated for each frequency to measure the property of the measurement target, for example, the moving speed.

In such a device, the integration time for improving the signal-to-noise power ratio (hereinafter referred to as SNR) is limited by the pulse width. In order to improve the SNR, it is conceivable to increase the pulse width or to transmit the pulse multiple times, but the reflected signal from the measurement target is the coherence time (reference [1], edited by Ryuichi Hioki,
Hikari Glossary, Ohmsha, published November 30, 1981,
(Page 84) causes phase fluctuation, and a sufficient SNR improvement effect by coherent integration cannot be obtained.

One of the radar devices that solves this problem is
It is shown in Japanese Patent Application No. 11-312876 (submitted on November 2, 1999).

A conventional radar device will be described with reference to the drawings. FIG. 3 shows, for example, Japanese Patent Application No. 11-31287.
It is a figure which shows the structure of the conventional radar apparatus shown by No. 6.

In FIG. 3, reference numeral 1 is a transceiver, and 3 is an A / D.
Converting means, 4 means for gate, 5 means for dividing data, 6a
And 6b are Fourier transform means, 7 is a complex conjugate means, 8 is a complex multiplication means, and 9 is a complex addition means.

Next, the operation of the conventional radar device will be described with reference to the drawings.

The pulse-modulated transmission signal from the transceiver 1 is reflected by the object to be measured and received by the transceiver 1. The received signal is A / D converted by the A / D conversion means 3, and then the gate means 4 extracts a time gate including a reflection signal from the measurement target from the received signal.

The received signal in the gate is data dividing means 5
Is divided into two sets of data of even and odd or the latter half. Each of the divided data is Fourier transformed by the Fourier transforming means 6a and 6b.

For one of the two Fourier transformed results, the complex conjugate is obtained by the complex conjugating means 7.
Complex multiplication with another Fourier transform result is obtained by the complex number multiplication means 8. The transmission signal is transmitted a plurality of times, the same measurement is repeated, and the result of the complex multiplication is integrated by the complex adding means 9. As a result, it was possible to compensate for the phase fluctuation of the data obtained each time the transmission signal was transmitted and to sufficiently improve the SNR by the coherent integration.

[0011]

However, in coherent integration of a desired signal using the above-mentioned conventional radar device, the transmission pulse width and the gate time width are determined by the signal coherence time. There was a problem, and the transmission pulse width could not be freely determined according to the required time resolution or distance resolution.

The present invention has been made to solve the above-mentioned problems, and even when the time width of a transmission pulse is larger than the coherence time of a desired signal, only the desired signal is in-phase in the gate. SN in the received signal by coherent integration at
An object is to obtain a radar device and a coherent integration method that can improve R.

[0013]

A radar device according to a first aspect of the present invention transmits a pulse composed of a wave and a transceiver for receiving a reflected signal from a measurement target, and an output of the transceiver in advance. A / D conversion means for A / D conversion at a predetermined sampling period, gate means for applying a gate to the output of the A / D conversion means on the time axis, and extracting a reception signal in the gate, and the gate means. A data dividing means for dividing the received signal extracted by the above into a plurality of data, a Fourier transforming means for Fourier transforming the plurality of divided data in the gate, and a complex conjugate for obtaining a complex conjugate of the output signal of the Fourier transforming means. Means and two temporally adjacent data of the plurality of divided data, the output of the Fourier transform means concerning the first half data and the latter half data. A complex multiplying means for performing a complex multiplication of the output of the complex conjugate unit related data, in which a complex adder means for adding an output signal of the complex multiplying means.

A radar device according to a second aspect of the present invention is
It further comprises a filter that removes a signal in a frequency band that does not need to be measured from the signal received by the transceiver, and the A / D conversion means may predetermine the output of the filter instead of the output of the transceiver. A / D conversion is performed according to the sampling period.

A radar apparatus according to claim 3 of the present invention is
The lower limit of the required frequency in measurement is fs, and the upper limit is fe.
The sampling period in the A / D conversion means is 2fe or more, the lower limit value of the pass frequency range of the filter is fbs, the upper limit value is fbe, and the coherence time of the reflection signal determined by the property of the measurement target is τc. In this case, 1 / (fbe-fbs) <τc / 2, fbe <f
The time width τa of the data divided by the data dividing means satisfies the relation of s, fe <fbs, and is 1 / (fbe
-Fbs) <τa, τa <τc / 2.

A coherent integration method according to a fourth aspect of the present invention comprises a step of removing a signal in a frequency band not required to be measured from a received signal by a filter, and a gate of the output of the filter on a time axis, A step of extracting a received signal in the gate, a step of dividing the received signal in the extracted gate into a plurality of data and Fourier transforming each, and a step of obtaining a complex conjugate of the Fourier transformed data, Performing a complex multiplication between the output of the Fourier transform unit for the first half of the data and the output of the complex conjugate unit for the second half of the two data that are temporally adjacent to each other among the plurality of divided data; , And adding the results of the complex multiplications.

In the coherent integration method according to claim 5 of the present invention, the lower limit value of the required frequency in measurement is fs and the upper limit value is fe, the lower limit value of the pass frequency range of the filter is fbs, and the upper limit value is fbe, When the coherence time of the reflected signal determined by the property of the measurement target is τc, 1 / (fbe-fbs) <τc / 2, fbe <f
The time width τa of the divided data which satisfies the relation of s, fe <fbs is 1 / (fbe-fbs) <τa, τ
This satisfies the relationship of a <τc / 2.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1. A radar device according to Embodiment 1 of the present invention will be described with reference to the drawings. 1 is a diagram showing a configuration of a radar device according to Embodiment 1 of the present invention. In each figure, the same reference numerals indicate the same or corresponding parts.

In FIG. 1, 1 is a transmitter / receiver for transmitting and receiving pulses composed of waves such as light waves, electromagnetic waves, or sound waves, 2 is a filter for removing a signal in a frequency band that does not need to be measured from a received signal, and 3 is a filter. A / D conversion means for A / D conversion, 4 gates the received signal on the time axis, and gate means for extracting the received signal in the gate, 5 a plurality of received signals extracted by the gate means 4. A data dividing means for dividing the data.

In the figure, 6 (6a, 6b, 6
c, 6d) is a Fourier transform means for Fourier transforming the divided data in the gate, 7 (7a, 7b, 7c) is a complex conjugate means for obtaining the complex conjugate of the output signal of the Fourier transform means 6, and 8 (8a, 8b). , 8c) is a complex multiplication means for performing a complex multiplication between the output of the Fourier transform means 6 for the first half data and the output of the complex conjugation means 8 for the second half data of two data adjacent in time among the divided data. , 9 are complex adding means for adding the output signals of the complex multiplying means 8. The complex multiplication means 8 performs complex multiplication for each same frequency.

Further, FIG. 1 shows the case where the number of data divided by the data dividing means 5 is 4 and the number of complex multiplying means 8 is 3, but these numbers are transmitted. It is determined by the relationship between the pulse time width and the number of divisions by the data division means 5, and is not particularly limited to these numbers.

Next, the operation of the radar device according to the first embodiment will be described with reference to the drawings.

FIG. 2 is a timing chart showing a transmitting / receiving operation of the radar device according to the first embodiment of the present invention.

In FIG. 2, (a) shows the transmission timing and (b) shows the reception timing. Also, τ
Is the time width of the transmission pulse, and τd is the delay time corresponding to the distance to the measurement target. Further, τa is the time width of the data divided by the data dividing means 5. Note that τc
Is the coherence time of the Doppler signal.

The radar device according to the first embodiment transmits and receives waves such as a light wave, an electromagnetic wave, or a sound wave, and detects the property of the measurement object, such as the shape or moving speed of the measurement object, from the frequency spectrum of the received signal. It is applicable to the device. Here, as a concrete example,
A case will be described in which a light wave is transmitted to the atmosphere, a reflection signal reflected from an aerosol in the atmosphere is heterodyne-detected to obtain a Doppler spectrum, and the wind velocity of the atmosphere is measured from this Doppler spectrum.

The received signal from the transceiver 1 contains the Doppler signal which is a desired signal and the system noise inside the radar device which is an unnecessary signal.

The Doppler signal is a signal having a coherence time determined by the properties of the atmosphere. The coherence time in this specification is a time within a range in which phase fluctuation of a signal does not occur, and is given by the reciprocal of the frequency spectrum width of the signal. The definition is given in the above-mentioned document [1]. Reference [1] describes the case of light waves, but it is clear that the same description can be made when the wave used is an electromagnetic wave or a sound wave.

The system noise inside the radar device is white random noise and is a signal having no coherence time.

In FIG. 1, the lower limit value of the required frequency in measurement is fs, the upper limit value is fe, and the sampling period in the A / D conversion of the A / D conversion means 3 is 2fe or more. Further, the pass frequency range of the filter 2 has a lower limit value of fbs and an upper limit value of fbe. Further, the coherence time of the Doppler signal is τc, and is set so as to satisfy the following equations (1) and (2).

[0030]   1 / (fbe-fbs) <τc / 2 (1)

[0031]   fbe <fs, fe <fbs (2)

The operation of the first embodiment will be described below with reference to FIGS. 1 and 2.

First, as shown in FIG. 2A, the transmitter / receiver 1 transmits a transmission pulse having a time width τ. This transmitted pulse is reflected by the aerosol in the atmosphere, and this reflected signal is received by the transceiver 1. In the transceiver 1, the reflected signal is heterodyne-detected (hereinafter, the heterodyne-detected reflected signal is referred to as a Doppler signal) and becomes a signal having a Doppler spectrum determined by the wind speed and the propagation speed of the light wave. The Doppler signal is a signal that has only components within the pass frequency band of the filter 2.

The Doppler signal from the transceiver 1 and the system noise pass through the filter 2 shown in FIG. As a result, the components of the frequency band that do not require measurement are removed. Since the desired Doppler signal has only the component within the pass band of the filter 2, there is no change in the signal level and the coherence time. On the other hand, the system noise is white random noise, so that components other than the pass band are removed by passing through the filter 2 to reduce the noise level. As described above, the provision of the filter 2 brings about an effect that the SNR of the received signal is improved.

When the system noise passes through the filter 2, it changes from white random noise to colored noise and has a coherence time corresponding to the reciprocal of the pass frequency bandwidth of the filter 2. Therefore,
The system noise coherence time is 1 / (fbe-
fbs).

The Doppler signal and system noise that have passed through the filter 2 are converted into digital signals by the A / D conversion means 3.

Next, the delay time τ according to the distance to be measured
A signal having the same time width τ as the time width of the transmission pulse after d is extracted by the gate means 4. Thereby, the gate including the Doppler signal from the measurement target is extracted.

The time width τ of this gate is larger than the coherence time 1 / (fbe-fbs) of the system noise and the coherence time τc of the Doppler signal. Therefore, the Doppler signal and system noise in the gate are signals with phase fluctuations.

Next, the data dividing means 5 divides the signal of the time width τ outputted from the gate means 4 into a plurality of data on the time axis. At this time, the time width τa of the divided data is set so as to satisfy the following expression (3).

[0040]   1 / (fbe-fbs) <τa, τa <τc / 2 (3)

Since the time width τa of the divided data is larger than the coherence time 1 / (fbe-fbs) of the system noise and smaller than the coherence time τc of the Doppler signal, the phase fluctuation of the Doppler signal in the divided data is However, system noise is a signal with phase fluctuation.

These divided data are as follows, where N is the number of divisions of the gate divided by the data dividing means 5, n is the data number, M is the number of samples in the divided data, and k is the sample number. expressed.

S (n, k) k = 0, 1, 2, ..., M-1 n = 0, 1, 2, ..., N-1

At this time, since the pass frequency range of the filter 2 is set by the equation (1), the time difference τa between the data adjacent in time is larger than the coherent time 1 / (fbe-fbs) of the system noise. , Smaller than the coherence time τc of the Doppler signal.

The plurality of divided data are Fourier-transformed by the Fourier transforming means 6a, 6b, 6c and 6d as shown in the following equation (4) with the sample number on the Doppler frequency axis as l.

[0046]

[Equation 1]

This operation is equivalent to an operation of decomposing a plurality of divided data for each Doppler frequency and performing coherent integration on the same Doppler frequency component.

The Fourier transform in this specification means the Fourier transform of a digital signal in the time domain, and is a DFT (Discrete Fourier).
Transform), FFT (Fast Fourier)
er Transform). DFT
Is used, there is an effect that the gate time can be finely set according to the coherent time τc of the Doppler signal, and the use of FFT has an effect that the calculation time is shortened.

These Fourier transform means 6a, 6b, 6
The Doppler spectrum obtained by c and 6d is obtained by decomposing the data divided in the gate for each Doppler frequency and coherently integrating the same Doppler frequency component. At this time, there is no phase fluctuation of the Doppler signal in the divided data, and the system noise is a signal with phase fluctuation. Therefore, the Fourier transform means 6 (6a, 6b, 6c, 6
According to d), only the Doppler signal can be coherently integrated in phase to cancel the system noise, so that the SNR is improved.

Next, the complex conjugate means 7 (7a, 7b, 7)
c) is Fourier transform means 6 (6a, 6b, 6c, 6)
Find the complex conjugate of the output of d). At this time, it is not necessary to obtain the complex conjugate for the first data on the time axis among the data divided by the data dividing means 5.

Next, the complex multiplication means 8 (8a, 8b, 8)
In c), with respect to two data which are temporally adjacent to each other among the divided data, the output of the Fourier transform means 6a, 6b, 6c for the first half data and the output of the complex conjugate means 7a, 7b, 7c for the second half data. Performs complex multiplication of. This complex multiplication is performed for each identical Doppler frequency component as in the following Expression (5).

[0052]

[Equation 2]

Here, i is a combination number of two data which are temporally adjacent to each other, A (i, l) is an amplitude term of a complex multiplication result, and φ (i, l) is a phase of a complex multiplication result. Is the phase difference of the signal between the first half data and the second half data of two data that are temporally adjacent to each other.

When the time width τ of the transmission pulse is larger than the coherence time τc of the Doppler signal, the phase of the Doppler signal between the divided data in the gate fluctuates. However, since the time difference between two data adjacent in time in the gate is smaller than the coherence time τc of the Doppler signal, the phase difference between two data adjacent in time with respect to the Doppler signal is constant in the gate. On the other hand, since the time difference between two data adjacent in time is larger than the coherence time 1 / (fbe-fbs) of system noise, the phase difference between two data adjacent in time related to system noise is within the gate. Takes a random value.

Next, the complex addition means 9 performs the complex addition process on the complex conjugate of the Doppler spectrum and the result obtained by the complex multiplication as in the following equation (6).

[0056]

[Equation 3]

As a result, even when the time width τ of the transmission pulse is larger than the coherence time τc of the Doppler signal and the phase of the Doppler signal fluctuates at the gate, only the Doppler signal can be coherently integrated in phase, and the system Since noise is integrated in a random phase, the SNR of the received signal can be improved.

In the radar device according to the first embodiment,
An apparatus for dividing a received signal into a plurality of data, obtaining respective Doppler spectra, and performing a complex multiplication of a complex conjugate result of one Doppler spectrum of two data adjacent in time and the other Doppler spectrum. Therefore, even if the time width of the transmission pulse is longer than the coherence time of the Doppler signal, in-phase coherent integration can be performed in the gate, and a sufficient SN can be obtained.
There is an effect that R can be improved. If this radar device is used, the time width τ of the transmission pulse can be set to the required time resolution,
Alternatively, the effect that it can be freely determined according to the distance resolution further occurs.

Further, since the radar apparatus according to the first embodiment is provided with the filter 2 before the A / D conversion means 3, unnecessary frequency components are removed and the system noise level is reduced as compared with the conventional apparatus. can do.

Further, the coherence time determined by the property of the measurement object is τc, the lower limit value of the required frequency in measurement is fs, the upper limit value is fe, and the lower limit value of the pass frequency range of the filter 2 is fbs and the upper limit value is fbe. if you did this,
Set so as to satisfy the equations (1) and (2), and
Since the time width τa of the divided data is set within the range of the equation (3), the time interval between two data adjacent in time after the division is larger than the coherence time of the system noise and the desired signal Can be smaller than the coherence time of. As a result, even if the system noise has the coherence time by providing the filter 2, only the desired signal is coherently integrated in phase to suppress the system noise and suppress the SNR in the received signal.
The effect of improving

In the first embodiment, the number of transmissions of the transmission pulse is one, but the number of transmissions may be increased. At this time, the phase difference between the Doppler signals of two data temporally adjacent to each other in the gate in each transmission is constant. Therefore, if complex addition is further performed on the complex addition result obtained for each number of transmissions, the Doppler signal can be coherently integrated by this complex addition, and the SNR is further improved.

[0062]

As described above, the radar device according to claim 1 of the present invention transmits a pulse composed of a wave and a transceiver for receiving a reflected signal from an object to be measured, and an output of the transceiver. A / D conversion means for performing A / D conversion at a predetermined sampling period, and the A
A gate unit that gates the output of the / D conversion unit on the time axis to extract the received signal in the gate, a data dividing unit that divides the received signal extracted by the gate unit into a plurality of data, and a gate Fourier transforming means for Fourier transforming a plurality of divided data of, and complex conjugating means for obtaining a complex conjugate of the output signal of the Fourier transforming means,
Complex multiplication means for performing a complex multiplication between the output of the Fourier transform means for the first half data and the output of the complex conjugation means for the second half data of two data that are temporally adjacent to each other among the plurality of divided data. And a complex adding means for adding the output signals of the complex multiplying means, so that even if the time width of the transmission pulse is larger than the coherence time of the desired signal, only the desired signal is in-phase in the gate. The effect of being able to improve the SNR of the received signal by suppressing the system noise by coherent integration is achieved.

A radar device according to claim 2 of the present invention is
As described above, the filter further includes a filter that removes a signal in a frequency band that does not need to be measured from the signal received by the transceiver, and the A / D conversion unit replaces the output of the transceiver with the filter. Since the output is A / D converted at a predetermined sampling cycle, even when the time width of the transmission pulse is larger than the coherence time of the desired signal, only the desired signal is coherently integrated in phase in the gate, It is possible to suppress system noise and improve the SNR of the received signal.

A radar device according to claim 3 of the present invention is
As described above, the lower limit value of the required frequency in measurement is fs, the upper limit value is fe, the sampling period in the A / D conversion means is 2fe or more, the lower limit value of the pass frequency range of the filter is fbs, and the upper limit value is If fbe and the coherence time of the reflected signal determined by the property of the measurement target is τc, then 1 / (fbe-fbs)
<Τc / 2, fbe <fs, fe <fbs, and the time width τa of the data divided by the data dividing means is 1 / (fbe-fbs) <τa, τa <τc
Since the relationship of / 2 is satisfied, even when the time width of the transmission pulse is longer than the coherence time of the desired signal, only the desired signal is coherently integrated in phase in the gate to suppress system noise and receive. The SNR of the signal can be improved.

As described above, the coherent integration method according to the fourth aspect of the present invention includes a step of removing a signal in a frequency band not required to be measured from a received signal by a filter, and an output of the filter on a time axis. , A step of extracting a received signal in the gate, a step of dividing the extracted received signal in the gate into a plurality of data and Fourier transforming each of them, and a complex conjugate of the Fourier transformed data. And a complex of the output of the Fourier transform means for the first half data and the output of the complex conjugation means for the second half data of the two temporally adjacent data of the divided plurality of data. Since the step of performing the multiplication and the step of adding the result of the complex multiplication are included, the time width of the transmission pulse is desired. In the case larger than the coherence time of the signals even, only the desired signal and coherent integration in phase in the gate, an effect that it is possible to improve the SNR in the received signal by suppressing the system noise.

As described above, in the coherent integration method according to the fifth aspect of the present invention, the lower limit value of the required frequency in measurement is fs and the upper limit value is fe, and the lower limit value of the pass frequency range of the filter is fbs and the upper limit value. When the value is fbe and the coherence time of the reflected signal determined by the property of the measurement target is τc, 1 / (fbe-fbs) <τ
c / 2, fbe <fs, fe <fbs are satisfied,
The time width τa of the divided data is 1 / (fbe−
fbs) <τa and τa <τc / 2 are satisfied, so that even if the time width of the transmission pulse is larger than the coherence time of the desired signal, only the desired signal is coherently integrated in the gate. The system noise can be suppressed and the SNR of the received signal can be improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a radar device according to a first embodiment of the present invention.

FIG. 2 is a timing chart showing a transmitting / receiving operation of the radar device according to the first embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of a conventional radar device.

[Explanation of symbols]

1 transceiver, 2 filter, 3 A / D conversion means, 4
Gate means, 5 data division means, 6a, 6b, 6
c, 6d Fourier transforming means, 7a, 7b, 7c complex conjugating means, 8a, 8b, 8c complex multiplying means, 9 complex adding means.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yoshihito Hirano             2-3 2-3 Marunouchi, Chiyoda-ku, Tokyo             Inside Ryo Electric Co., Ltd. (72) Inventor Shuzo Waka             2-3 2-3 Marunouchi, Chiyoda-ku, Tokyo             Inside Ryo Electric Co., Ltd. (72) Inventor Takahiko Fujisaka             2-3 2-3 Marunouchi, Chiyoda-ku, Tokyo             Inside Ryo Electric Co., Ltd. F term (reference) 5J070 AB01 AC06 AD01 AH02 AH31                       AH35 AH40

Claims (5)

[Claims] 1. A transmitter / receiver that transmits a pulse composed of a wave and receives a reflected signal from an object to be measured, and an A / D conversion means that A / D-converts the output of the transmitter / receiver at a predetermined sampling cycle. And gate the output of the A / D conversion means on the time axis,
Gate means for extracting the received signal in the gate; data dividing means for dividing the received signal extracted by the gate means into a plurality of data; and Fourier transform means for Fourier transforming the plurality of divided data in the gate , A complex conjugating means for obtaining a complex conjugate of the output signal of the Fourier transforming means, and an output of the Fourier transforming means for the first half data of two data adjacent in time among the plurality of divided data, and a second half A radar device comprising: a complex multiplication means for performing a complex multiplication with the output of the complex conjugation means for the data of 1. and a complex addition means for adding the output signals of the complex multiplication means. 2. A filter for removing a signal in a frequency band that does not need to be measured from a signal received by the transceiver, wherein the A / D conversion means is provided in the filter instead of the output of the transceiver. The radar device according to claim 1, wherein the output is A / D converted at a predetermined sampling period. 3. The lower limit value of the required frequency in measurement is f
s, the upper limit value is fe, the sampling period in the A / D conversion means is 2fe or more, the lower limit value of the pass frequency range of the filter is fbs, and the upper limit value is fbe. When the coherence time is τc, the relations of 1 / (fbe-fbs) <τc / 2, fbe <fs, fe <fbs are satisfied, and the time width τ of the data divided by the data dividing means is
The radar device according to claim 2, wherein a satisfies the following relationships: 1 / (fbe-fbs) <τa, τa <τc / 2. 4. A step of removing a signal in a frequency band that does not need to be measured from a received signal by a filter, a step of applying a gate to an output of the filter on a time axis, and extracting a received signal in the gate, Dividing the received signal in the extracted gate into a plurality of data and Fourier-transforming each of them; determining a complex conjugate of the Fourier-transformed data; and a time within the divided plurality of data. Adjacent to each other
A step of performing a complex multiplication of the output of the Fourier transform means for the first half data and an output of the complex conjugate means for the second half data of one data; and a step of adding the results of the complex multiplication. Coherent integration method. 5. The lower limit value of the required frequency in measurement is f
s, the upper limit value is fe, the lower limit value of the pass frequency range of the filter is fbs, the upper limit value is fbe, and the coherence time of the reflected signal determined by the property of the measurement target is τc, 1 / (fbe-fbs) <Τc / 2, fbe <fs, fe <fbs, and the time width τa of the divided data satisfies 1 / (fbe-fbs) <τa, τa <τc / 2. 5. The coherent integration method according to claim 4.