[0041] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.
[0042] For such as figure 1 The airborne downward-looking three-dimensional imaging radar system shown in the present invention is designed from the perspective of analyzing the array layout form of its orbiting direction, so as to realize high-resolution wide-range imaging of the observation scene.
[0043] Specifically, the airborne sparse array down-view three-dimensional imaging radar system of the present invention includes an airborne platform, a sparse array antenna system, and a distributed POS (Position and Orientation System).
[0044] The carrier platform is an aircraft for carrying the sparse array antenna system and distributed POS. The aircraft can be equipped with array antennas in the middle and both sides, and can fly smoothly at an altitude of 1 to 3 km.
[0045] The sparse array antenna system of the present invention is composed of a plurality of sub-arrays, and the plurality of sub-arrays are distributed in three areas of the carrier platform, namely, a main area and two auxiliary areas. The main area is distributed in the middle of the aircraft carrier platform, and the two secondary areas are respectively located on both sides of the aircraft carrier platform.
[0046] According to the present invention, an array antenna composed of multiple sub-arrays densely arranged is installed in the main area, one or two sub-arrays are installed in the sub-area, and the array antennas in the three areas form a larger sparse array antenna . The size of the sub-array in the cross-track direction can be 0.25 m, and the size in the along-track direction can be determined according to the actual required along-track resolution.
[0047] The distributed POS of the present invention is distributed at the sub-array position of the sparse array antenna, and is used to realize high-precision measurement of the relative position of the multiphase center of the array antenna.
[0048] Refer below figure 2 To describe specific embodiments of the present invention.
[0049] In an embodiment of the present invention, the carrier platform is figure 2 The aircraft shown, such as a transport aircraft. However, according to the present invention, the aircraft carrier platform can be any aircraft that can be equipped with array antennas in the middle and on both sides, and can fly smoothly at an altitude of 1 to 3 km above the ground.
[0050] As mentioned above, the carrier platform of the present invention is used to carry the sparse array antenna and distributed POS, and therefore should have a location and area for arranging the sparse array antenna and distributed POS.
[0051] In this embodiment, figure 2 The position 1 shown is the belly of the aircraft, which is the main area where the sparse array antennas are distributed. In this main area, 8 densely arranged sub-arrays are installed along the cross-track direction to form an array antenna with a length of 2 m. figure 2 The position 2 shown is the position of the wing mounted pods on both sides of the aircraft. It is the two sub-regions where the sparse array antennas are distributed. The distance between the two sub-regions is about 9m. Install 2 sub-arrays respectively to form a 0.5m array antenna. The sub-arrays distributed in the three regions form a sparse array antenna with a length of about 9.6m.
[0052] image 3 In this article, the Yun-12IV aircraft is used as a specific example of a carrier platform to show the specific positions of the belly and wing pods on the carrier platform.
[0053] Each sub-array of the sparse array antenna simultaneously sends out radio frequency sub-orthogonal signals, using the principle of multi-phase central aperture synthesis, the obtainable equivalent phase center distribution is as follows Figure 4 Shown. The equivalent phase center is the virtual phase center generated at the middle position of the transceiver sub-array when the transceiver sub-array is split. The positional relationship between the equivalent phase center and the transceiver sub-array is as Figure 5 Shown.
[0054] The sub-array adopts a two-dimensional phase-scan active array, that is, its beams can be scanned in two directions (along the track and the cross-track), especially when the cross-track beam needs to be scanned to expand the imaging during a single pass Width. The phase scan refers to the method of controlling the phase shift amount of the phase shifter on the array antenna to change the excitation phase of each sub-array, so as to realize beam scanning.
[0055] The distributed POS is distributed at the sub-array positions of the sparse array antennas in the three regions, and the distributed spatial structure realizes accurate measurement of the multi-phase center of the array antenna, thereby realizing precise motion compensation and improving imaging quality.
[0056] Refer below Image 6 To describe the method of using the airborne sparse array down-looking three-dimensional imaging radar system of the present invention to achieve high-resolution wide-range three-dimensional imaging of the observation scene.
[0057] First of all, for a sparse array antenna composed of multiple sub-arrays in the cross-orbit, the relative spatial position of the multi-phase center of the array antenna is measured by the distributed POS distributed in each sub-array position, so that high-precision motion compensation can be realized.
[0058] Then, using the DBF (Digital Beamforming) pattern of the array antenna in the main area as a weighting function, weighting is performed on the sparse array pattern composed of the equivalent phase centers obtained by multiple transmissions and multiple receptions of the sub-arrays to improve the peaks. Lobe ratio and integral sidelobe ratio.
[0059] According to the principle of pattern product, the array pattern F(θ) is composed of the sub-array pattern F e (θ) multiplied by the array factor S(θ), namely
[0060] F(θ)=F e (θ)S(θ)
[0061] One consists of N intervals as d, and the beam direction is θ 0 The array factor of the equally spaced linear array antenna composed of sub-arrays is
[0062] S ( θ ) = 1 + e jkd ( sin θ - sin θ 0 ) + e j 2 kd ( sin θ - sin θ 0 ) + · · · + e j ( N - 1 ) kd ( sin θ - sin θ 0 )
[0063] = sin [ Nkd ( sin θ - sin θ 0 ) / 2 ] sin [ kd ( sin θ - sin θ 0 ) / 2 ]
[0064] The cross-orbit dimension of the sub-array is 0.25m. If the wavelength of the transmitted signal is 0.02m, then the 0.25m sub-array can be composed of 25 omnidirectional antenna elements (ie F e (θ) = 1) composition, the pattern is as Figure 7a Shown. Since each sub-array has multiple transmissions and multiple receptions, 39 equivalent phase centers will be formed in the crossing direction, such as image 3 Shown. The normalized array factor of a sparse array composed of equivalent phase centers is as Figure 7b As shown, the pattern is obtained by multiplying the pattern of the sub-array and the array factor, such as Figure 7c Shown.
[0065] Since the array is sparsely distributed, its peak sidelobe ratio and integral sidelobe are relatively high. The DBF pattern of the array formed by the 8 sub-arrays of the distributed main area (such as Figure 7d As shown), the result of weighting the above pattern is as Figure 7e Shown. Through the pattern weighting process, the peak sidelobe ratio and integrated sidelobe ratio of the sparse array antenna can be improved.
[0066] The pattern of the array antenna corresponds to the imaging result, and the method of weighting the pattern can be introduced in the imaging algorithm to improve the imaging quality.
[0067] Then, the sparse array antenna sub-array adopts a two-dimensional phase-scan active array, and in the front-down view working mode, adopts a scanning mode combining ScanSAR mode and SweepSAR mode to expand the observation imaging width.
[0068] The ScanSAR mode and SweepSAR mode are two commonly used beam scanning methods. The ScanSAR mode expands the observation amplitude at the expense of reducing the on-track resolution, and the SweepSAR mode expands the observation amplitude at the expense of increasing the system pulse repetition frequency. width. The scanning method that uses the combination of ScanSAR mode and SweepSAR mode can take into account the pulse repetition frequency and along-track resolution of the system.
[0069] Figure 8 A specific example of scanning mode combining ScanSAR mode and SweepSAR mode is given. 3 SweepSAR mode wave standing positions are used to form a ScanSAR mode sub-strip, and the total observation width is composed of 5 such ScanSAR mode sub-strips. . Assuming that when the beam is not scanned, the pulse repetition frequency of the system is 4kHz, and the on-track resolution is 0.2m. In the scanning mode, the pulse repetition frequency of the system will increase to 12kHz, and the on-track resolution will drop to 1m. The observation width can be expanded 15 times.
[0070] Then, when the carrier platform is operating at high altitude, a sparse re-overhead flight mode is adopted to obtain an equivalent array antenna with a larger length in the cross orbit direction to improve the cross orbit resolution.
[0071] The expression of the cross-orbit resolution of the airborne sparse array down-view 3D imaging system is
[0072] ρ c = λR 2 L
[0073] It can be seen that the cross-orbit resolution is related to the transmitted signal wavelength λ, the slant distance R and the effective length L of the cross-orbit array.
[0074] For a given transmission signal, when the altitude of the carrier is increased, if the cross-orbit resolution is to be improved, the length of the cross-orbit array can only be increased, and the length of the cross-orbit array is limited by the wingspan of the carrier. Increase arbitrarily. By adopting the method of re-tracking, a larger-length array antenna can be obtained in the cross-orbit direction to achieve the purpose of improving the resolution of the cross-orbit direction.
[0075] The schematic diagram of the airborne sparse array down-looking three-dimensional imaging radar flying through the air Picture 9 Shown.
[0076] In order to reduce the number of re-passing flights, a sparse re-passing plan is considered. The Barker code is a random signal with equal sidelobe characteristics and can be used as a sampling criterion for sparse re-pass.
[0077] Picture 10 The autocorrelation function of the 13-bit Barker code sequence ([1 1 1 1 1 0 0 1 1 0 1 0 1]) is given, and it can be seen that the side lobe values are all equal. Figure 11a The pattern of the array obtained when the 13-bit Barker code is used as the sampling criterion for random sparse re-air flight is given, and the DBF pattern of the array composed of 8 sub-arrays in the distributed main area (such as Figure 7d As shown), the result of weighting the above pattern is as Figure 11b Shown.
[0078] When the flying height of the carrier is low, the shorter Barker code can be used as the sampling criterion to realize random sparse sampling of re-passing flights, and using fewer re-passing times to obtain a cross-orbit array antenna that meets the conditions.
[0079] Finally, according to the echo signal obtained in the above steps, using a three-dimensional wavenumber domain algorithm, and introducing the distributed POS to the measured value of the relative spatial position of the equivalent phase center and the method of weighting the pattern, it can be realized Three-dimensional imaging of the observation scene.
[0080] The specific embodiments described above further describe the purpose, technical solutions and beneficial effects of the present invention in further detail. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Within the spirit and principle of the present invention, any modification, equivalent replacement, improvement, etc., shall be included in the protection scope of the present invention.
