JPH0410996B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to synthetic aperture radar.

[Conventional technology]

Side Looking Radar (Side Looking Radar) is installed on mobile platforms such as aircraft and satellites.
Radar, side monitoring radar) is installed, and it is relatively small by emitting radio waves to the ground on the side of the direction of travel, receiving reflected waves and synthesizing them while traveling along a preset course. A synthetic aperture radar device that can equivalently synthesize a large-diameter antenna using an aperture plane antenna is well known.

FIG. 2 is an operating principle diagram showing the operating principle of the synthetic aperture radar device.

A mobile platform P at an altitude H moves at a low speed from the plane of the paper toward the back while irradiating an observation point Q on the ground G with a transmission pulse beam at an inclination α and a transmission pulse beam angle θ. do.
In this case, the reflected wave from the observation target point Q irradiated by the transmission pulse with the transmission pulse beam angle of θ is obtained as observation data that spreads in two dimensions in the distance and azimuth directions. In order to reproduce it as a target observation object, range and azimuth compression are required to compress it in the distance and azimuth directions.

In this case, transmission and reception are performed while maintaining a coherent state, and therefore, the reflected signals from many scattered points on the ground are combined with a specific phase relationship, and this gives a fluctuation in strength to the received signal and is reproduced as an image. It is unavoidable that a significant amount of so-called speckle noise, which appears as a flickering of shading, occurs, and for this reason, observation data in so-called Lutsk units, which are spread in two dimensions and obtained for each observation target, are usually compared to the direction of travel, or azimuth. Azimuth compression is performed after range compression on each of these divided looks, and the space is calculated by averaging the data by adding the absolute values of the data after azimuth compression in units of divided looks that are incoherent with each other. A direction multi-track process is being implemented to reduce block noise.

On the other hand, there is a distance multi-look method that compresses image data in look units in the distance direction and reduces speckle noise based on the same concept as the azimuth multi-look process.

This usually involves dividing the frequency changes in the transmitted pulse signal due to LFM (Linear FM) modulation into several independent sections, or divided looks. For each of these divided looks, range compression is first performed, then azimuth compression is performed, and these are summed. This is a method of overlapping averaging.

Transmission pulse 1 shown in Fig. 2 is a general transmission pulse used in synthetic aperture radar equipment, and is an LFM modulated wave whose frequency changes linearly from ₁ to ₂ on the time axis t with c ₁ as the center. .

The reason why such an LFM modulated wave is used as a transmission pulse is because LFM is used to widen the band, and during reception processing, the frequency vs. delay characteristic is opposite to that of the fractional delay line that is normally used when converting to LFM during transmission. It is based on the fact that the energy is reproduced as a sharp pulse concentrated at one point through a characteristic distributed delay line, and although it has a relatively small output, it can be used as a transmission pulse equivalent to a large output.

Now, the transmission pulse 1 used in the LFM format is sent out for a time T for each pulse, and L
The reflections are input to the side-lucking radar of the moving platform while forming a rangewise illumination width spanning a distance of . Apart from the level, the frequency structure of this reflected wave is also the same as the transmission pulse 1 except for the Doppler change, and is input over time T. Now, suppose that such a received signal is divided into a plurality of equal parts. For example, in the case of FIG. 2, it is assumed that the data is divided into two and divided into data for every T/2 hours. Assuming that the difference between ₁ and ₂ is Δ, the frequency difference every T/2 time is Δ/2, and the two-divided clocks with the frequency band of Δ/2 are superimposed, in fact, they are added. This makes distance multi-look processing possible, but in reality, most of the multi-look processing is related to the moving direction of the mobile platform, that is, the azimuth direction.Distance multi-look processing by dividing the transmission signal is based on the bandwidth of data processing. Currently, it is hardly used for reasons such as the increase in

[Problem that the invention seeks to solve]

As mentioned above, distance multi-look processing is hardly used at present, and this is due to the following reasons.

That is, the distance resolution is inversely proportional to the bandwidth of the processed data, and therefore, as shown in FIG.
When superimposing the divided looks, the original frequency band Δ becomes Δ/2, so the distance resolution becomes 1/2.
decreases to Generally, if n-divided look processing is used, the distance resolution will be reduced to 1/n. This reduction in resolution is basically the same in azimuth multi-look processing and is inversely proportional to the number of look divisions, but distance multi-look processing has the following problem.

Transmission pulse in synthetic aperture radar equipment
The frequency change due to LFM is Δ in the case of Figure 2.
However, it is desirable to make this as high as possible for purposes such as improving distance resolution and sharpening received pulses, and taking into account the upper limit of stable operation of the sampling frequency during A/D conversion in received data processing. This uniquely determines the upper limit of the processing speed.

On the other hand, if you want to suppress the distance resolution that decreases to 1/n, you can operate the frequency bandwidth as n・Δ, but if you expand such a frequency bandwidth, you will finally reach the upper limit of the stable operating frequency of the A/D converter. Distance multi-look processing has the disadvantage that it is hardly used due to these constraints. It is also an object of the present invention to provide a synthetic aperture radar position that eliminates such drawbacks.

[Means for solving problems]

The device of the present invention is a synthetic aperture radar device that reproduces ground conditions as an image using radio waves using a side looking radar mounted on a mobile platform such as an aircraft or a satellite. It emits a transmission pulse with a frequency with a wide bandwidth of Δ, receives the reflected wave from the observation target, and then divides this into multiple frequency bands, each with a bandwidth of Δ and different from each other, to coherently detect the received data. Received data division detection means independently performs range and azimuth compression on the plurality of received data acquired by the reception data division detection means, reproduces a plurality of image data, and then reproduces each of these plurality of image data. and processing direction multi-look processing means for executing multi-look processing in the distance direction by performing superposition by addition as a look.

〔Example〕

Next, the present invention will be explained in detail with reference to the drawings.
FIG. 1 is a block diagram showing one embodiment of the configuration of a synthetic aperture radar device according to the present invention.

The configuration of the embodiment shown in FIG. It is comprised of frequency oscillators 10 and 11, filters 12 and 13, range/azimuth compression units 14 and 15, multi-lux processing unit 16, and the like.

A reference frequency oscillator 1 generates a reference signal of a stable frequency that serves as a reference for transmitting and receiving frequencies used in the coherent radar system constructed according to this embodiment, and supplies this to a dispersion delay device 2.

The dispersion delay device 2 includes a frequency multiplier circuit, a pulse forming circuit, and a SAW (acoustic surface wave).
It is equipped with a delay line, etc., and after multiplying the input reference frequency to the center frequency of the SAW delay line, the pulse forming circuit converts it into a pulse and outputs it every time it receives a trigger pulse for reception timing output from the trigger generator 3. do. Each time the SAW delay line receives a pulse input from the pulse forming circuit, it generates a signal with a different delay time depending on the frequency. The frequency is set so that the frequency changes linearly over a wide range of n·Δ from the minimum frequency to the maximum frequency at the same center frequency as the center frequency.
In addition, Δ is the value in normal synthetic aperture radar equipment.
This is the frequency change of the LFM transmission pulse, and is generally about several tens of MHz.

The output of the dispersion delay device 2 is sent to the broadband transmitter 4, where the center frequency is converted into a transmission carrier wave, and after level amplification, it is transmitted as a transmission pulse to an observation target on the ground via an osolator 5 and an antenna 6, and the reflected wave is sent to the antenna 6. , are supplied to the broadband receiver 7 via the isolator 7.

Transmission pulse 2 shown in FIG. 2 shows the transmission pulse in this embodiment, and transmission pulse 1, which is a normal transmission pulse, changes in frequency during time T.
While it is an LFM wave that changes linearly by Δf from ₁ to ₂ with c ₁ as the center, the transmission pulse 2 has a wide band that is expanded to n·Δ, or in this example, 2Δ where n = 2. , frequency ₂ to ₃ c ₂
is spreading as its center frequency.

The wideband receiving section 7 amplifies the received signal of the reflected wave from the wideband transmission pulse 2 to a predetermined level and sends it to the synchronous detectors 8 and 9 at a frequency c ₁
Supplied as a signal containing ±Δ/2 and c ₁ ±Δ/2.

The local frequency oscillators 10 and 11 divide the received signal Δ by the bandwidth of the transmitted pulse 2 and use the local oscillation frequency necessary for synchronous reception, which in this embodiment is based on c ₁ and c ₂ shown in FIG. 2. These signals c ₁ and c ₂ are generated in synchronization with the reference frequency received from the frequency oscillator 1 and are supplied to the synchronous detectors 8 and 9, respectively.

The synchronous detectors 8 and 9 perform synchronous detection of the input signal using the local oscillation frequencies of c ₁ and c ₂ , respectively, and extract the modulated wave number component contained in the input signal. In this case, the synchronous detector 8 has a modulation frequency component of Δ/2 bandwidth from ₁ to c ₁ folded into a modulation frequency component from c ₁ to ₁ .
The synchronous detector 9 extracts the modulation frequency components from c ₁ to ₃ , and the synchronous detector 9 extracts the modulation frequency components from c ₁ to ₂ with respect to the modulation frequency components with a bandwidth of Δ/2 from _c 2 to _3. Extracted in a folded and superimposed state. The thus extracted outputs are then supplied to filters 12 and 13, which respectively filter and output only the modulation frequency components within the divided bands. As a result, the filter 12 outputs the modulated wave component of the bandwidth Δ in the divided band where the frequency of the received signal is ₁ to ₂ , and the filter 13 outputs the modulated wave component of the bandwidth Δ in the divided band where the frequency of the received signal is ₂ to ₃ . The modulated wave components are output as observation data and supplied to range/azimuth compression units 14 and 15, respectively.

The range/azimuth compression section compresses the two-dimensionally spread observation data obtained for each subband in the distance and azimuth directions using known compression means and reproduces it as an observation target as a point target. For example, the processing contents of the range/azimuth compression section 14 are as follows.

The range/azimuth compression unit 14 converts the input time domain signal into frequency domain data by FFT (Fast Fourier Transform), and then converts the input time domain signal into frequency domain data for each subband using the built-in dispersion delay line of the dispersion delay unit 2. Range compression is achieved through multiplication with a range reference function formed by a SAW distributed delay line with opposite frequency vs. delay characteristics.
After that, the data is converted into time domain data again using IFFT (Inverse FFT, Inverse Fast Fourier Transform), and this data is rearranged in the azimuth direction, and then azimuth compression processing is performed.

Prior to this azimuth compression, related processing such as Doppler frequency included in the input data, frequency shift corresponding to changes in distance between the moving platform and the observation target, and correction is performed.
The data is converted into frequency domain data using FFT, and azimuth compression is performed by multiplying it with an azimuth reference function.

Of course, azimuth compression can be performed on the range-compressed data of the same observation target obtained for each subband, that is, in units of looks, but usually this look is divided equally into multiple pieces in the azimuth direction. In this embodiment, the azimuth multi-look processing is performed to average the data by performing a superimposition process in which the azimuth compression is performed independently for each divided look in so-called multi-look units. Azimuth multi-look processing is performed using a two-part look.

The range/azimuth compression section 15 also performs range/azimuth compression for other subbands in exactly the same way, and the range/azimuth compression for the above two subbands is
The output of the azimuth compression section is output to the multi-look processing section 16 as image data.

The multi-look processing unit 16 thus receives image data of two decomposed looks output from each range/azimuth compression unit for each divided band, and
After determining the absolute values of these mutually incoherent image data, they are superimposed by non-coherent addition, the images are averaged, and distance multi-color processing is performed to reduce speckle noise.
As mentioned above, this distance multi-lux processing uses an occupied frequency band of 2Δ, and therefore, the distance resolution is transmitted in a frequency band of Δ through range/azimuth compression and multi-lux processing, which are performed by dividing the frequency band into each occupied frequency band of Δ. Accuracy similar to that for pulse 1 is maintained.

The present invention uses a plurality of signal detection circuits to extract information signals from a received signal in a plurality of divided bands, suppresses an increase in processing bandwidth in range and azimuth compression, and distance multi-track processing, and prevents a decrease in distance resolution. The basic feature is that it performs distance multi-look processing.
The figure shows only one embodiment of the present invention, and various modifications are possible.

For example, in the embodiment shown in FIG. 1, the case where n=2 for the frequency band n·Δ of the transmission pulse, and therefore the case where there are two plurality of information signal detection circuits and two divided bands is taken as an example. It is clear that the same implementation is possible even if n is set to an arbitrary number.

Further, in this embodiment, the azimuth multi-look process is also executed with the number of divisions being 2, but the azimuth multi-look process and the number of divisions can be executed and set arbitrarily.

Furthermore, the built-in distributed delay line of the distributed delay device 2 may be replaced with other digital means having equivalent functions, and all of the above can be easily implemented without detracting from the spirit of the present invention. be.

〔Effect of the invention〕

As explained above, according to the present invention, by providing means for extracting baseband information from a received signal for each subband using a plurality of detectors and performing distance multi-track processing, multi-track processing in the distance direction is performed. The present invention has the advantage that it is possible to realize a synthetic aperture radar that can significantly reduce speckle noise without causing any deterioration in distance resolution.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the synthetic aperture radar device of the present invention, and FIG. 2 is a diagram showing the principle of operation of the synthetic aperture radar device. 1... Reference frequency oscillator, 2... Distributed delay device,
3...Trigger generator, 4...Wideband transmitter, 5...
... Isolator, 6 ... Antenna, 7 ... Wideband receiver, 8, 9 ... Synchronous detector, 10, 11 ...
Local frequency oscillator, 12, 13...filter, 1
4, 15... Range azimuth compression section, 16...
Multi-look processing section.

Claims (1)

[Claims] 1 Siderukking radar installed on a mobile platform such as an aircraft or an artificial satellite
Looking Radar) is a synthetic aperture radar device that reproduces ground conditions as an image using radio waves.
It emits a transmission pulse with a preset wide bandwidth frequency of n/Δ, receives the reflected wave from the observation target, and then divides this into multiple frequency bands, each with a bandwidth of Δ and different from each other, to make it coherent. Received data division detection means for detecting received data, and a plurality of received data acquired by the received data division detection means, each independently subjected to range and azimuth compression to reproduce a plurality of image data, and then reproduced from the plurality of image data. A synthetic aperture radar device comprising distance direction multilook processing means for performing distance direction multilook processing by superimposing image data by addition as looks.