JPH0317314B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is a synthetic aperture radar that is mounted on a flying object (mobile platform) such as an artificial satellite or an aircraft and obtains high-resolution images of the ground surface, sea surface, etc. This is related to the improvement of.

[Conventional technology]

Figures 7a and b are for example FTUlaby, RK
Moore, AKFung paper “Microwave
"Remote Sensing (Active and Passive) - Microwave Remote Sensing (Active and Passive)" PP630-745,
Addition-Wesley Publishing Company,
(1982), each showing a configuration of a conventional synthetic aperture radar. In each figure, 1 is an antenna, 2 is a transmitter, 3 is a receiver,
4 is a pulse compression device, 5 is a phase compensation device, 6 is a
FFT (Fast Fourier Transformer), 7 is a display, 8 is an inertial navigation device, 9
is an antenna drive device, 10 is an IFFT (Inverse Fast
Fourier Transformer (inverse fast Fourier transformer), 11 is a reference signal generator, 12 is a multiplier,
13 is a transmitting/receiving switch.

Figures 8 and 9 are Figures 7a and b, respectively.
FIG. 2 is a diagram for explaining the operation of the synthetic aperture radar. In each figure, A is a transmitted signal, B is a received signal, C is an observation target, D is a certain point in the observation target C,
E is a flying object equipped with an antenna beam and F is a synthetic aperture radar.

Next, the operation of the conventional synthetic aperture radar shown in FIGS. 7a and 7b will be described. Depending on the position and speed of the flying object F measured by the inertial navigation device 8, the antenna driving device 9 directs the antenna 1 toward the observation target C. Thereafter, the transmission pulse signal generated by the transmitter 2 is radiated through the transmission/reception switch 13 and the antenna 1 as a transmission signal A shown in FIG. 8 toward an observation target C such as the ground surface or the sea surface. The radiated transmission signal A is reflected by the observation object C and is received as the reception signal B by the antenna 1 shown in FIG. After that, the received signal B is transferred to the transmitter/receiver switch 13
The signal is input to the receiver 3 via the . The receiver 3 amplifies and detects the high frequency received signal B, and converts it into a baseband video signal. This video signal is input to a pulse compression device 4 and subjected to pulse compression in order to improve distance resolution. This pulse compression improves the resolution of the beam in the radial direction, or in the terminology associated with radar, in the range direction. The range resolution ΔR is expressed as ΔR=Cτ _P /2 (1) (where C: speed of light, τ _P : pulse width after pulse compression).

Now, in FIG. 7a, the pulse compression device 4
The output signal is input to the phase compensator 5. By the way, as is clear from FIGS. 8 and 9, the distance R between a certain point D in the observation target C and the flying object F changes moment by moment from R _O to R _t within the synthetic aperture period T. . Therefore, the phase of the received signal B, in other words, the instantaneous Doppler frequency also changes accordingly. By compensating for this phase fluctuation and aligning the frequency to f ₁ based on the coordinates determined by the inertial navigation device 8, the received signal B from the observation target C can be expressed by the frequency f ₁ within the synthetic aperture period T. can. When the synthetic aperture period T is increased, the number of samples of the received signal B from the observation target C can be increased, the observation accuracy of the frequency f ₁ is improved, and the angular resolution is improved.
In this way, the frequencies of adjacent points of observation target C are
f ₂ and each point in the field of view is sequentially represented by a frequency f _o , it is possible to image each point in the field of view in correspondence with its coordinates with an accuracy according to the synthetic aperture period T. . In accordance with the instructions from the inertial navigation device 8, the phase compensator 5 uses the distance signal separated by the pulse compressor 4 to detect each observation target C at an equal distance in the azimuth direction according to the operating principle of the conventional synthetic aperture radar described above. The phase of the signal from a certain point D in the field is compensated, and a signal representing each point in the field of view at each frequency f ₁ to _{f o is} output. This output signal is separated into frequencies f ₁ to f _o by FFT 6. The amplitude values of the signals of the separated frequencies f ₁ to f _o correspond to the scattering coefficient of a certain point D in each observation target C. By displaying the output signals of each channel of the FFT 6 in correspondence with the coordinates of the display surface of the display 7, a high resolution image corresponding to the pulse compression ratio of the pulse compression device 4 and the synthetic aperture period T is displayed. The synthetic aperture period T can theoretically be made infinite if the antenna drive device 9 is controlled to continue emitting radio waves in the direction of the observation target C according to the instructions of the inertial navigation device 8.
In reality, it is limited by the range of radio waves, the time allowed for image reproduction, etc.

In addition, in FIG. 7b, the pulse compression device 4
The output signal is first separated by the frequency input to the FFT 6. At this time, as mentioned above, the received signal B from a certain point D in the observation target C instantaneously responds to changes in the Doppler frequency within the synthetic aperture period T.
Outputs occur on all channels of FFT6. however,
In this case, unlike the case shown in FIG. 7a, each channel also includes received signals from other points, but the received signals from each point are stored as phase information. Here, based on the coordinate information of the inertial navigation device 8, the output signal of the FFT 6 is multiplied by the reference signal related to the phase generated by the reference signal generator 11 by the multiplier 12. The phase information from each point is
Since the phase changes from the first channel to the n-th channel of the FFT 6 in accordance with the sample delay within the synthetic aperture period T, the phase is corrected to a constant value using the reference signal. After this, when converted into a time signal by IFFT10, the signal at a certain time τ ₁ becomes the observation target C
It is separated as coming from a certain point D inside. The signals from each time τ ₁ to _{τ o} corresponding to each phase correspond to the signals at each azimuth point, and these are displayed on the display 7.
If you display it above, you will get a high-resolution image. The angular resolution of the displayed image is determined by the pulse compression device 4.
The pulse compression ratio and the synthetic aperture period T are the same as in the case of FIG. 7a above. In addition,
In Figure 7b, without changing the fundamental essence,
There is also a method of directly processing the portion from FFT6 to IFFT10 in the time domain. An example of operation of these conventional synthetic aperture radars is as shown in FIG.

[Problem that the invention seeks to solve]

Since the conventional synthetic aperture radar described above is configured as described above, it is possible to obtain images with extremely high resolution, and it can be effectively applied to resource exploration, topographic map creation, etc. This is evidenced by this type of radar installed. However, as an example of a more flexible operation, you may want to obtain images with the highest resolution possible, even if it takes more time, you want to monitor the ground surface in a scanning manner, or you want to zoom in on a part of the ground surface and monitor it. It is not possible to respond to requests such as the desire to reproduce images as quickly as possible, even if the resolution is low, and the desire to organically combine the above functions and select them according to the situation. The point was hot. In other words, a synthetic aperture radar set to a certain mode was operated solely in that mode in most cases.

The present invention was made to solve these problems, and aims to provide a synthetic aperture radar that can flexibly handle various modes according to operational requirements.

[Means for solving problems]

The synthetic aperture radar according to the present invention includes a spectrum analyzer that gradually improves the accuracy of spectrum analysis as the synthetic aperture length increases, a mode controller that operates the system in various modes according to operational requirements, and an antenna. A beam controller, etc. that controls the direction of the emitted beam is added to diversify the operation of the system.

[Effect]

In the synthetic aperture radar of the present invention, the mode controller is prepared with control signals for operating the system in modes corresponding to anticipated operational requirements. The system is controlled so that modes for each operation such as imaging can be freely switched and imaging can be performed. In addition, the control signal of the mode controller is supplied to the beam controller, pulse compression device, step conversion FFT, etc. in order to control each of the above modes, and is radiated from the antenna during the synthetic aperture period required for each mode. The beam direction is controlled and the range and azimuth resolutions are controlled to desired values, thereby increasing system flexibility and enabling imaging to meet advanced operational requirements.

〔Example〕

FIG. 1 is a block diagram showing the configuration of a synthetic aperture radar which is an embodiment of the present invention. The synthetic aperture radar shown in Fig. 1 is of a type that forms azimuth pixels in the frequency domain, and the same or equivalent parts as those of the conventional example shown in Fig. 7 are indicated using the same symbols. Detailed explanation will be omitted. In the figure, 14 is a beam controller that controls the direction of the beam emitted from the antenna 1, and 15 is a step conversion FFT that is a spectrum analysis device that improves the accuracy of spectrum analysis step by step as the synthetic aperture length increases. Reference numeral 16 denotes a mode controller that operates the system in various modes according to operational requirements.

Next, the operation of the synthetic aperture radar, which is an embodiment of the present invention shown in FIG. 1, will be described. The beam controller 14 controls the antenna drive device 9 using signals from the inertial navigation device 8 and the mode controller 16. The signal from the inertial navigation device 8 is a signal indicating the position, speed, oscillation, etc. of the flying object F, and the signal from the mode controller 16 is an angle signal that changes with time corresponding to a desired mode. In response to these signals, the beam controller 14 outputs a control signal for setting the beam radiated from the antenna 1 in a desired direction via the antenna driving device 9. The configuration of the step transform FFT 15 is shown in FIG. 3a, and the basic principle of its operation is shown in FIG. 3b.

In FIG. 3a, the signal x(t) output from the phase compensator 5 shown in FIG. 1 is input. This signal x(t) is a signal obtained by irradiating a target with the transmission beam from the transmitter 2 shown in Fig. 1 and receiving the reflected wave at the antenna 1. This is a time-series signal received one after another. Assume that the input signal x(t) is a pulse train with an interval of Δt. The signal x(t) is divided into M, and the first signal is divided into N
FFT which is a fast Fourier transformer with a number of points _1,7
is input. The second signal is input to other N-point fast Fourier transform FFTs _{1 and 2} via a switch S ₁ and a delay element with a delay time τ (=(N+1)Δt). The Mth signal is input to another N-point fast Fourier transformer FFT _1,(M+1) via switches S ₁ to S _L and a delay element with a delay time Mτ. From each FFT _1,1 to FFT _1,(M+1) , every time N pulses of x(t), which is a time series signal of a pulse train, are input, a signal is divided into frequency regions of N points. is output. Now, since the Doppler information (that is, frequency information) contained in the signal x(t) indicates the scene within the radar field of view as described above, the output of each FFT shows each scene after being decomposed. . However, the resolution at this time is 1/NΔT corresponding to the N-point FFT. When switches S ₁ to S _L are OFF, the output of FFT _1,1 is output (1). Output (1) has a resolution of 1/
It becomes a scene of NΔT. Next, if switch S ₁ is in the ON state, the past scene will change for a time τ.
Obtained as the output of FFT _1,2 . Furthermore, as the switches S ₂ to S _L are turned on, the past scene of time 2τ...Mτ is obtained, and when all the switches S ₁ to S _L are turned on, a total resolution of (M+1) images is obtained. is 1/NΔT, and the same scene can be obtained with the time shifted by τ.

Let's consider again the case where only switch _S1 is closed. From FFT _1,11 and FFT _1,2 respectively
A total of 2N outputs are obtained, with N outputs f ₁ to f _N.
These 2N outputs are again subjected to Fourier transformation using FFT _{2,1 .} . . FFT _2,N as shown in FIG. 3a. However _{, at this} time, _the selection _of input signals to FFT _{2,1 ...} FFT _{f 2,2}
Then, it is decided to input f _N to FFT _2,N, respectively. FFT _2,X is an FFT of 2 points each,
The output will be two. Therefore, the number of outputs from FFT _2,1 to FFT _2,N is 2N. Furthermore, although the input to FFT _2,1 is two types of f ₁ , it should be noted that these two f ₁ are time series with a time interval of τ. In other words, applying a 2-point Fourier transform to two independent time series data at the same point means FFT _1,1 and
Another one is between f ₁ and f ₂ decomposed by each of FFT _{1 and} 2.
This means that the number of points has been increased by supplementing points.
Similarly, FFT _2,2 to FFT _2,N increase the number of points between each of f ₁ to f _N by 1 point, forming a total of 2N points of FFT. This will be called output (2), and its resolution will be 1/2NΔT.

Next, consider again the case where switches S ₁ to S _L are closed. In this case, similar input signals are formed for FFT _(M+1),1 to _{FFT (M+1),N} using the same rules as when output 2 was obtained. That is, time τ
The signals f ₁ , f _1, 〓, f _1,2 〓…f _1,M 〓 which are shifted by
Input that of f _N into FFT _(M+1),1 and FFT _(M+1),N , respectively. As a result, an output (M+1) is obtained.
The result of these series of operations is shown in Figure 3b. In Figure 3b, as mentioned above, FFT _1,1
~FFT _(M+1),1 ~ for the signal obtained by dividing the frequency domain shifted by time τ of ~FFT 1 _,(M+1) into 1/N
By performing the transformation of FFT _(M+1),N , the number of FFT output points is increased by interpolating (M+1) points between f ₁ and f _N . Note that Δt in FIG. 3b indicates the calculation time of Fourier transform. As described above, by appropriately switching the switches S ₁ to S _L , the resolution of the scene corresponding to the range of f ₁ to f _N can be switched.
In other words, switching the resolution within the same field of view means that a zooming effect can be obtained by using a known display method that gives the same viewing angle to each pixel when displaying. Become. An example of operation of a synthetic aperture radar using the step conversion FFT 15 is shown in FIG. Fourth
As shown in the figure, the synthetic aperture period T is divided into several unit synthetic aperture periods τ. For each τ, the first step FFT (FFT _1,1
〜FFT _1,(M+1) ) is prepared. The signals of the same channel among these FFT _1,1 to FFT _1,(M+1) are combined to create a time series signal with time delay from 0 to Mτ, and input to the second step FFT. . The second step FFT has a number of points M corresponding to the number of the first step FFT. In the second step FFT, the channel interval analyzed in the first step FFT is subdivided into M points to improve the accuracy of the analysis. In this way, the first step
By combining the FFT and the second step FFT, NM spectral analysis results are obtained. in this way,
Dividing the step transform FFT 15 into multiple steps is convenient for zooming. As shown in Fig. 4, after performing a quick turn with the minimum unit synthetic aperture period τ, as the flying object F moves, zooming is performed in stages such as zoom 1 at 2τ, zoom 2 at 3τ, etc. can be advanced to obtain the highest resolution image with the final maximum synthetic aperture period T. During this time, the data to each of the first and second step FFTs can be gradually accumulated as τ, 2τ, 3τ, . . . . If one NM point step transformation FFT 15 is used, it is necessary to resampling the data from the beginning for each synthetic aperture period τ, making practical zooming impossible. Each switch S ₁ ~ as shown in Figure 3a
S _L is provided for switching the zooming step. The mode controller 16 is for switching modes according to operational requirements, and an example of the control sequence is shown in FIG. When the mode is set according to the operational requirements, the direction of the antenna 1, pulse repetition period, pulse compression ratio, number of steps of the step conversion FFT 15, etc. corresponding to the mode are calculated, and after adjusting the timing, each controller Output to a certain beam controller 14.

Now, by combining the parts that perform individual operations as described above as shown in FIG. 1, it is possible to realize a system in which various modes are flexibly combined according to operational requirements. Typical operational requirements are: (1) Kuitkurtsk mode, which allows near real-time image playback even though the angular resolution may be low; (2) Scanning images of the ground surface. There is a normal mode, (3) a precision mode where you want to take a particularly long time to obtain high-resolution images, and (4) a zooming mode where you can see the progress of the above. In the system shown in FIG. 1, these can be continuously varied and realized. Of course, even when each mode is independent, it can be easily realized only by operating the mode controller 16. FIG. 6 shows the observation status of each mode; FIG. 6a shows the Kuytskurtsk mode, FIG. 6b shows the normal mode, and FIG. 6c shows the precision mode. In Figure 6a, the flying object (mobile platform) mounted on the radar moves from the left to the right at the top of the figure to the observation target C,
That is, the scene is irradiated with a beam width θ _B for a time τ. At this time, in the system shown in FIG. 1, the switches S ₁ to S _L of the step conversion FFT 15 are turned off by the mode controller 16, but the compression ratio of the pulse compression device 4 is obtained by the step conversion FFT 15. The modulation width Δf of the known chirp conversion frequency is set so that the distance is equivalent to the resolution in the azimuth direction.
is controlled. Furthermore, the antenna driving device 9 is controlled by the mode controller 16 so that the antenna setting angle θ _A tracks the scene C for a time τ. At this time, the antenna setting angle θ _A may have any absolute angular direction, and it is sufficient to view the same scene C for a time τ. Therefore, the antenna setting angle θ _A obtained by considering the relationship with the moving speed of the mobile platform set in advance is θ _A = θ _A0・F (V, t) 0 < tτ …(2) However, θ _A0 : initial value of θ _A ; F(V, t): function of speed of moving platform and time.

Therefore, the scene C can be tracked by changing the antenna setting angle θ _A according to the function shown in equation (2) above. As is clear from FIGS. 3a and 3b and FIG. 4, the time τ is the mode that is the quickest and easiest to realize real-time observation, and is usually called the quick-curt mode.

Next, in Fig. 6b, the antenna setting angle θ _A is moved to the range θ _A = ±π−θ _B /2 …(3), and T _S =φ _C /V …(4). Then, the mode controller 16 shown in FIG.
controls the switches S ₁ to S _L. However, in equation (4) above, φ _C is the diameter of scene C, and _{T S =} kτ
It goes without saying that it is better to choose φ _C so that . If the system of FIG. 1 is controlled in this way, after imaging a scene C along the movement of the mobile platform as shown in FIG. 6b,
When moving to the next scene C, it is possible to capture an image without creating a gap.

Next, in Figure 6c, |θ|>π−θ _B /2 …(5) T _P >φ _C /V(T _{s nax} =(M+1)τ) …(6) , the antenna driver 9 and the step conversion by the mode controller 16 of the system of FIG.
Controls switches S ₁ to S _L of FFT 15. By controlling in this manner, it is possible to select up to the maximum (M+1)τ in FIG. 3b as T _P , thereby obtaining the highest resolution. However, it goes without saying that the observation time will be the longest in this case. As is clear from Fig. 6a, it is possible to overlap adjacent fields of view in the quick-kurtsk mode, but it is also possible to overlap a part of the normal mode shown in Fig. 6b, while zooming up. It is also possible to shift to normal mode.

FIG. 2 is a block diagram showing the configuration of a synthetic aperture radar according to another embodiment of the present invention. The synthetic aperture radar shown in Fig. 2 processes only the normal mode in the time domain, and the other modes form the azimuth pixels shown in Fig. 1 above. 17
has been established. Here, only the differences from what is shown in FIG. 1 will be explained.
A signal for controlling the changeover switch 17 is added to the system, and the system is switched between the above system and the system shown in FIG. 1 in accordance with operational requirements. Most of the conventional synthetic aperture radars are time-domain processing methods, so when adding a mode to a conventional product, the one shown in FIG. 2 can be realized at a lower cost.

〔Effect of the invention〕

As explained above, the present invention provides a spectrum analysis device that gradually improves the accuracy of spectrum analysis as the synthetic aperture length increases in a synthetic aperture radar, and a mode control that operates the system in various modes according to operational requirements. By adding a beam controller, etc. that controls the direction of the beam emitted from the antenna, we have diversified the operation of the system, allowing us to flexibly deal with various modes according to operational requirements, and with advanced This has excellent effects such as making it possible to take images that correspond to the operation of the system.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a synthetic aperture radar which is an embodiment of the present invention, FIG. 2 is a block diagram showing the configuration of a synthetic aperture radar which is another embodiment of the invention, and FIG. Figure 1 is a diagram for explaining the configuration of the step transform FFT in the synthetic aperture radar and the basic principle of its operation, and Figures 4 to 6 are diagrams for explaining the operation of the synthetic aperture radar in Figure 1, respectively. 7a and 7b are block diagrams showing the configuration of a conventional synthetic aperture radar, respectively, and FIGS. 8 and 9 are block diagrams for explaining the operation of the synthetic aperture radar shown in FIGS. 7a and 7b, respectively. It is a diagram. In the figure, 1... antenna, 2... transmitter,
3... Receiver, 4... Pulse compression device, 5... Phase compensation device, 6... FFT, 7... Display, 8...
...Inertial navigation device, 9...Antenna drive device, 10
...IFFT, 11...Reference signal generator, 12...
...multiplier, 13...transmission/reception switch, 14...beam controller, 15...step conversion FFT, 16...
. . . mode controller, 17 . . . changeover switch. In each figure, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Scope of Claims] 1. Consists of a radar transceiver, a pulse compression device, a phase compensation device, a chronic navigation device, a spectrum analysis device, and a display mounted on a mobile platform such as an artificial satellite or an aircraft, and In synthetic aperture radar, which effectively synthesizes antenna apertures through signal processing and obtains high-resolution images, step conversion improves the accuracy of spectrum analysis step by step as the synthetic aperture length increases.
FFT, an antenna drive device that can arbitrarily set the synthetic aperture length, a beam controller that controls this antenna drive device, and according to operational requirements, perform observation without any gap in the scanning mode in the quick scan mode. A synthetic aperture radar characterized by comprising a mode control device for switching between a normal mode and a precision mode for observing a specific scene with as high a resolution as possible. 2. To perform colevolution of input signals.
The FFT, the multiplier, and the inverse FFT are provided as separate channels, and this channel and modes such as the quick mode, normal mode, precision mode, etc. can be switched via a changeover switch. A synthetic aperture radar according to claim 1. 3. The synthetic aperture radar according to claim 1, wherein the first step FFT of the step transform FFT in the spectrum analyzer is connected via a switch.