Synthetic Aperture Radar (Synthetic Aperture Radar) is a high-resolution microwave imaging using the principle of synthetic aperture. It has all kinds of features such as all-day, all-weather, high-resolution, and wide-width. It is mainly airborne and spaceborne platform. With the development of technology, synthetic aperture radars equipped with various types of platforms such as missiles, ground-based SAR, unmanned aerial vehicle SAR, adjacent space platform SAR, and handheld devices have appeared, which are widely used in military and civilian fields.
The SAR uses a small antenna as a single radiating element, and the unit is continuously moved along a straight line to receive echo signals of the same ground at different positions and perform related demodulation and compression processing. A small antenna synthesizes an equivalent "large antenna" by "moving", so that a higher azimuth resolution can be obtained, and the azimuth resolution is independent of the distance, so that the SAR can be installed on the satellite platform and can be obtained. Higher resolution SAR images.
One of the hot spots of SAR research: the new system argument
The goal of SAR system design is: high image quality (high spatial and radiation resolution), wide imaging width, multi-mode (scanning, variable incident angle strip, strabismus, bunching), multi-band, full polarization, three-dimensional Imaging, moving target detection and imaging capabilities, adaptability to platform motion and posture changes. To this end, the SAR platform must be equipped with sophisticated navigation and attitude measurement systems (GPS/INS/IMU). Precise time synchronization devices (such as atomic clocks, GPS timing, etc.) must be used between multiple platforms. SAR systems must use fully polarized phase. Array antenna (flexible beam scanning capability, high power synthesis capability and good robustness), oscillator source with very high frequency stability, increased transmit signal bandwidth (sometimes subband synthesis must be used), multi-channel simultaneous reception Processing, and flexible, stable, real-time imaging and image processing algorithms that are compatible with system design. The three major synchronizations (time, space and phase), wave position design, performance index analysis and impact analysis of various error sources in the new system design are hot topics.
Since the invention of SAR, from strip mode, to spotlight and scan mode, the improvement of resolution has brought about the continuous improvement and development of many system hardware and imaging algorithms. Single polarization to full polarization also affects the continuous updating of SAR hardware. In addition, the user's requirements for the stability and quantitative characteristics of the SAR system are getting higher and higher, and the SAR is continuously enhanced and strengthened.
The second hot spot of SAR research: imaging under new systems and special application conditionsIn some new imaging systems such as SAR, small platform (such as drone) SAR, and large squint applications (such as missile load), the following situations are often encountered:
(1) Due to the increase in resolution or/and the increase in imaging width, the coupling of azimuth and distance can no longer be ignored, and the real-time performance of the algorithm is also tested;
(2) The motion compensation of the small platform is difficult, and it is difficult to continuously obtain images satisfying the quality requirements;
(3) Working at a large oblique angle, the distance needs to be accurately compensated for walking, and the image quality is difficult to guarantee;
(4) In order to reduce the computational complexity of the algorithm, reduce the dependence of the imaging algorithm on the attitude stability of the platform, or filter out the speckle, the time domain/frequency domain subaperture imaging algorithm is needed.
(5) In the absence of available radar platform motion sensor data, a self-focusing algorithm based on measured data parameter estimation must be used;
(6) Subband synthesis algorithm suitable for ultra high resolution imaging;
(7) Baseline estimation and phase unwrapping of InSAR;
(8) Dual/multi-base SAR imaging under irregular flight paths;
(9) Engineering algorithm and real-time imaging processor design.
A series of high-demand and high-level SAR features such as real-time imaging, real-time display, and real-time data transmission have prompted SAR to continue to study at a higher level. Not only will the drones bring their own SAR systems, but also achieve automatic target recognition based on GMTI technology, automatically hitting targets in real time. Perhaps this is the terrible thing of the future war. Without the arrival of the army, a group of drones wearing missiles came. . . .
The third hot spot of SAR research: image processingSpeckle reduction, image enhancement, geometric correction, digital elevation map (DEM) generation, pixel location (with ground control points and no reference points), radiation/polarization calibration, target detection, feature extraction (popular demand) ) and identification (wide military demand), multi-sensor image fusion (fusion of multi-band, multi-polarized SAR images, fusion of SAR images and optical images), and so on. The quality assessment of SAR images is also a major difficulty.
If the first two belong to the system level, hardware level, and imaging level, then this stage belongs to the subsequent image processing, the stage before the user uses. Mainly in the civil society exhibition, such as the optimization of remote sensing images, the extraction of disaster targets and so on.
The fourth hot spot of SAR research: moving target detection and imagingMilitary applications of SAR urgently need to address the detection and imaging problems of slow moving targets on the ground. Difficulties: Target non-cooperative movement, strong clutter background. Two main methods: single-channel SAR system uses detection and imaging based on motion target parameter estimation (can also use adjacent unit clutter cancellation, but the effect is not good), multi-channel SAR system uses motion detection based on clutter cancellation And imaging (DPCA, STAP). The precise positioning of moving targets is difficult.
The fifth hot spot of SAR research: ISAR motion compensation and imagingFour typical types of targets: aircraft, ships, ballistic missiles, and space vehicles. The goal of non-cooperative movements creates difficulties for motion compensation. Not all motion pose data can be imaged and data selection is required. Multi-target imaging is a major difficulty. Not long ago, I saw the moon photo, only to know that the original SAR image of the moon is through the radar on the earth (appears to be the largest pit in the world, a large semi-circular pit, is a giant radar), the principle is ISAR.
Six hot spots in SAR research: SAR interference and anti-jammingThe interference patterns mainly include noise interference, coherent interference (generated by the radar signal parameters of the reconnaissance and the scene distribution of the scattering intensity configuration of the area to be protected) and composite interference (combination of noise interference and coherent interference). The anti-jamming technology of SAR mainly includes low intercept probability waveform design, repetitive frequency and carrier frequency agility, and airspace adaptive interference cancellation. The anti-jamming effect evaluation of SAR system is also a major difficulty. It mainly considers the influence of interference pattern on image quality. The difficulty of interference device to capture and implement interference of SAR transmit signal parameters is not considered.
In these years, working with SAR images, and knowing things are gradually increasing. If SAR is a technology, it can be; if SAR is a science, it can be; it can be simply understood as a two-dimensional matrix; It is also a multi-dimensional two-dimensional matrix; it can also be understood as the result of the action of the ground object and the microwave; it can be understood as an image of a group of numbers that are tossed out in this way; of course, it is a radar product, and it is between the sensor and the target. There are specific imaging means that are formed by relative motion. The idea of ​​SAR is very beautiful. With it, we have discovered the appearance of synthetic aperture sonar, and perhaps there will be synthetic aperture *** in the future. . . In short, the development of SAR is now very hot and the future is more developed.
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