JPS5845582A - Synthetic aperture radar device - Google Patents

Abstract

PURPOSE:To extend the reception time and extend the receiving Doppler spectrum band width, by controlling the beam radiated from an antenna so that this beam is directed toward a predetermined target direction. CONSTITUTION:A synthetic aperture radar mounted on an airplane radiates radio waves to a target from a point A through a point C to receive and record reflected waves and records momentarily the ground speed, the altitude, the target azimuth, etc. At this time, the synthetic aperture radar antenna mounted on the airplane is so controlled that the target is the beam center. Thus, the beam direction is so controlled that radar radio waves are irradiated around the target to extend the reception time and extend the receiving Doppler spectrum band width, thereby obtaining a high-resolution synthetic aperture radar even if an antenna of a comparatively narrow beam is used.

Description

[Detailed description of the invention] The present invention relates to a synthetic aperture radar device.

Synthetic aperture radar (hereinafter referred to as SAR) is a branch of the SEIDLE and KING radars, and is an all-weather, high-resolution radar that is placed on a flying object such as an aircraft or an artificial aircraft and captures images of the ground using microwaves. 8A While observing the image target blood diagonally, radio waves are emitted and received over and over again on the image target surface.As a result, a coherent phase history of the received and transmitted pulses is obtained, and signal analysis technology is used to effectively High azimuth resolution equivalent to that obtained using a large diameter antenna can be obtained. This kind of SA
Regarding R, it is shown in the Radar Handbook published by McGraw-Hill, Chapter 23, Syntheti and Quaperture radar.

Generally, the azimuth resolution of BAR increases as the aperture length of the antenna becomes smaller. In other words, as the actual beam width increases, the observation time (integration time) from the moving radar becomes longer, and the received radar reflected wave throat brass vector band expands, so the resolution improves. PRF > FDm-FDI O is required so that the pulse repetition frequency (PRF) without any pulse repetition frequency (PRF) also satisfies the sampling theorem. Here, FDl is the highest frequency of the radar received waves subjected to Doppler shift, and To9Fm is the lowest frequency.

However, if the PRF t- is increased, the maximum observation width W in the distance direction becomes w, and is limited to 1 force. Here C is the speed of light. It was. The degree of freedom in terms of scan width and resolution of the system that can be realized is limited because the radio waves reflected from the intended target must be received between transmission and the next transmission time. In addition, the conditions for the altitude, speed, attitude, etc. of the platform on which the SAR is mounted, such as an aircraft, are severe and may be difficult to implement.

The present invention uses an antenna that has a narrow beam width and can change the beam direction as an antenna mounted on a satellite or aircraft, and uses the antenna to continue irradiating radar radio waves to the target target and its surroundings for as long as possible. By controlling the beam direction of the laser beam, it is possible to provide a focused 5ARt with low PRF (pulse repetition frequency) and high lateral resolution.

According to the present invention, it is installed on a mobile platform such as an aircraft or a satellite, and the beam direction is controlled so as to continue irradiating radar radio waves around the target for as long as possible, thereby extending the reception time and receiving. By widening the Doppler spectral bandwidth, it is possible to obtain a high-resolution synthetic aperture radar even when using a relatively narrow beam antenna.

The following is a first example of high-resolution SAR according to the present invention.
To explain with reference to the figure, the SA operator A installed on the aircraft
Radio waves are radiated to the target from point to zero, and the reflected waves are received and recorded, and at the same time ground speed If, altitude, target azimuth, etc. are recorded on both sides. At this time, the 8AR antenna mounted on the aircraft is constantly controlled so that the target is on the 7th beam.

Radar signals reflected from the target and the vicinity of the target are recorded as discrete data sampled at a pulse repetition frequency (hereinafter referred to as PRF). This received signal is brushed depending on the speed of the aircraft, and its frequency varies over a wide range as shown in FIG. 2 as the relative speed of the aircraft and target changes. In the conventional 8 person 8 system, the PRF needs to be greater than the frequency change width BW in Figure 2, but in the case of this device, the PRF is determined by the antenna aperture diameter, PRF'≧
4 Vain (' )/J, where V is the ground speed of the aircraft, θ is the antenna beam stability, and λ is the wavelength of the transmitted wave.

This relational expression is derived as follows. As shown in Figure 3(a), the radio waves emitted from the nine BARs mounted on the satellite or aircraft SKM moving at a speed of KV are reflected at point A and received again by the SAR. Is receiving.

Here, λ: wavelength of the transmitted wave In order to establish a SAR system, the Doppler shift range of the reflected wave from the effective area irradiated by the antenna (region where the antenna gain decrease is usually within 3 dB) must be P
RF must be high.

Now, when the beam width of the antenna is closed to 2#, Fig. 3 (b)
Let's find the Doppler shift when the beam center is tilted by ψ.

Here, fd at -200 is the maximum value and 9 4Viin# fdmax= Therefore, it is sufficient for PRF to be equal to or greater than fdmax.

In Figure 2, the minimum required PRF is fd, but in reality it is 0P.
This means that by setting PRF higher, the number of processing units, that is, the amount of processing calculations, is reduced.In this embodiment, PRF is set to fp so that all processing data can be divided into four parts. .

Figure 4 Received data is a list of received signals a certain time after transmission, that is, reflected signals from targets located at a certain distance. th 'r', e T'@'# Tl are points A and B, respectively. CA This is the data I obtained when I passed the meeting. Processing to obtain an image near the target from this data can be done using either hardware or software, but here we will show an example of processing using software.

In FIG. 2, received data a is arranged on the time axis, and is divided into four parts 111-D* to D4, and the data is reconfigured as processing units 1 to 4.

In 5TEP, frequency shift is performed for each processing unit data so that the center frequency bme t to ω becomes 0 frequency, and then in 5TEPt, a fast 7-lier transform (FFT) is used. Convert to frequency domain. FFT
The data at each 0 frequency of the output are 611 to #, respectively.
Therefore, in 5Tli:Pa, each data at 4 processing rates is arranged as t-1 frequency domain data. At this time, the portions where the low-level data in the valley processing are equal to 1 are added, and after 8TEP4 becomes one frequency domain data, as shown in Figure 5, the correlation with the reference function is calculated as in normal SAR processing. is performed in the frequency domain and converted into an image. In FIG. 5, 1 is a matched filter, 2 is an FFT circuit, 3 is '1ffl data (@wavenumber domain), 4 is a multiplier,
5triIFFT circuit, 6 indicates output image data.

As described above, the present invention can achieve high resolution with a low PRF' by configuring the focal point with a relatively narrow beam antenna system that is controlled to continue irradiating the target and its surroundings for as long as possible.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing frequency changes of received waves and processing units. Fig. 3 is a diagram for explaining the principle of the present invention, Fig. 4 is a diagram showing the first half of the processing method, and Fig. 5 is a diagram showing the processing method of the next step of Fig. 3. . l...Matched filter, 2...FFT, 3...
Received data, 4... Multiplier %5... IFFT, 6.
...Output image data. 47 Issaki A circle on the foot

Claims (1)

[Claims] Aircraft 1 A synthetic aperture radar device configured to vary the direction of a beam emitted from an antenna mounted on a mobile platform such as an artificial satellite, and to control the beam direction in a predetermined target direction. .