Gaofen-3 is China's first meter-level multipolarization synthetic aperture radar (SAR) satellite with 12 imaging modes for the scientific and commercial applications. In order to evaluate the imaging performance of these modes, the multiple mode SAR raw data simulation is highly demanded. In the paper, the multiple mode SAR simulation framework will be briefly introduced to expose how the raw data simulation guarantees the development of Gaofen-3 and its ground processing system. As an engineering simulation, the complex working modes and practical evaluation requirements of Gaofen-3 mission put forward to the higher demand for simulation simplification and data input/output (I/O) efficiency. To meet the requirements, two improvements have been proposed. First, the stripmap mode based multiple mode decomposition method is introduced to make a solid and simplified system simulation structure. Second, the cloud computing and graphics processing unit (GPU) are integrated to simulate the practical huge volume raw data, resulting in improved calculation and data I/O efficiency. The experimental results of sliding spotlight imaging prove the effectiveness of the Gaofen-3 mission simulation framework and the decomposition idea. The results for efficiency assessment show that the GPU cloud method greatly improves the computing power of a 16-core CPU parallel method about $40\times$ speedup and the data throughput with the Hadoop distributed file system. These results prove that the simulation system has the merits of coping with multiple modes and huge volume raw data simulation and can be extended to the future space-borne SAR simulation.

Multiple Mode SAR Raw Data Simulation and Parallel Acceleration for Gaofen-3 Mission

Yaw steering is a method of Doppler centroid compensation performed by attitude steering of the satellite platform. However, the conventional yaw steering method does not work well as the satellite orbit height becomes higher, such as with the geosynchronous Earth orbit synthetic aperture radar (GEO SAR) which has 36 000-km orbit height. This letter primarily analyzes the Doppler properties of the GEO SAR in the satellite local coordinate systems, and gives the calculation formulas of the Doppler centroid frequency and instantaneous Doppler bandwidth. Based on the vector analysis, a new calculation method is proposed for the 2-D attitude steering angles in GEO SAR. The new method, considering Earth's rotation and elliptical orbit effects, can accurately compensate the Doppler centroid to zero. Then, according to the 2-D attitude steering angles and the characteristics of GEO SAR, the 2-D phase scan method is put forward to carry out the highly accurate compensation of Doppler centroid, which can avoid the difficulty of rotating and stabilizing the GEO SAR system. The simulations validate the conclusion in this letter.

A New Method of Zero-Doppler Centroid Control in GEO SAR

In this paper we give a brief review of compressive sensing (CS) applied to radar. Though CS theory has been introduced only a few years ago (in 2006, see e. g. [1]), it today manifests as a kind of revolution in signal processing and sensor systems. We will discuss some properties of CS radar and present a few examples. It is also a concern of the author to point to some limitations and shortcomings if CS is `blindly' applied with great enthusiasm to any radar problem. The time has come to deliberate when CS will be an advantage and when `oldfashioned' methods should better be applied.

A brief review of compressive sensing applied to radar

Low-inclined elliptical orbits are recommended for geosynchronous synthetic aperture radar (GEO SAR) to map the regions of interest at middle latitudes. However, the radius variation of such orbit results in a time-variant nadir interference and slant range of illuminated regions. These factors shorten the available imaging time for a specific pulse repetition interval (PRI), which may be even shorter than the required integration time. Besides, the squint mode widely used in GEO SAR brings in the nonorthogonal and nonuniform ground resolution, which cannot be completely described by the traditional range and azimuth resolutions. To solve these problems, first, the ground resolution area is proved to be an ellipse, and the axis lengths of the resolution ellipse are used to present the worst and the best resolutions along various directions. Conditions for orthogonal and uniform ground resolutions are investigated, and analytical expressions of the resolution-related parameters are derived. Second, effects of the orbit radius variation on data acquisitions are analyzed, and a continuous PRI variation method is proposed to lengthen the available imaging time. By varying the PRI with a constant time interval, the acquisition window and transmit interference are changed with the time-variant illuminated region, which loosens the constraint of the transmit interference. Finally, the proposed ground resolution expressions and continuous PRI variation method are verified by computer simulations.

Radar Parameter Design for Geosynchronous SAR in Squint Mode and Elliptical Orbit

This letter presents an accurate range model, named as Doppler parameter range model (DRM), to perform signal processing for geosynchronous synthetic aperture radar (Geo-SAR). Besides that a credible approach, defined as accurate coordinate numeric model (ACNM), is proposed to simulate the raw echo signals. To improve the accuracy of DRM and ACNM, the rotating Earth is modeled as an ellipsoid and the eccentricity of the satellite's orbit is analyzed. The superiority of DRM that it is more precise than hyperbolic range equation (HRE) and advanced hyperbolic range equation (AHRE), presented before, is illuminated. The point target 2-D frequency spectrum of DRM is derived. Finally, the simulation results validate that DRM and ACNM has a good effect on an L-Band Geo-SAR system with an azimuth resolution around 5 m.

An Accurate Range Model Based on the Fourth-Order Doppler Parameters for Geosynchronous SAR

Generally the Doppler centroid of the spaceborne Synthetic Aperture Radar (SAR) system is not zero because of the earth rotation, which results in a decrease in imaging precision, interferometry accuracy, ground moving target indication performance and other factors. This paper proposes a new satellite attitude steering approach to perform yaw and pitch steering for SAR satellites to enable the Doppler centroid to be total zero. Based on the basic methodology of the SAR satellite attitude steering, the desired yaw angle and the pitch angle are derived. And comparisons with the attitude steering technique currently applied are presented. Simulation and analysis show that this method can obtain better results. (4 pages)

A new satellite attitude steering approach for zero Doppler centroid

The traditional radar system needs large bandwidth, and the increasing number of channels brings huge amount of data. These data can easily overflow the memory of the sensor or the bandwidth of the signal which transferred to the ground station. In order to solve this problem, a new method of acquiring Synthetic Aperture Radar (SAR) raw data and compressing pulse which based on the theory of Compressive Sensing (CS) theory are presented. In this method, CS SAR imaging is affected by noise, sampling rate and the sparsity of signal. Furthermore, Donoho-Tanner phase transition diagram is applied to show the performance of CS pulse compression. Engineers can intuitively find the scene and the sampling rate which is suitable for using compressed sensing synthetic aperture radar pulse compression.

Effects of noise, sampling rate and signal sparsity for compressed sensing Synthetic Aperture Radar pulse compression

Spaceborne synthetic aperture radar (SAR) has been proved highly significant for remote sensing of the Earth. The image quality is constrained by the system design and the image processing technology, azimuth ambiguity is one of the most important factors. We know the pulse repetition frequency (PRF) determine the sampling rate of the azimuth, but the spectrum of the azimuth is non-bandlimited, it is hard to find a suitable solution with the conventional approach. A new method is proposed based on PRF micro-variation to suppress the azimuth ambiguity for spaceborne SAR, which changes the sampling mode and makes the azimuth signal no longer uniform in frequency domain. At last it can improve the image quality and make the ambiguity energy disperses randomly. In this paper, the basic theory of azimuth signal sampling is reviewed, and the reason of azimuth ambiguity is also introduced in detail. Through analysis we discover the PRF micro-variation has a scope, finally, some simulation results demonstrate this method is effective and feasible.

Azimuth ambiguity suppression for spaceborne SAR based on PRF micro-variation

On compressive sensing applied to SAR imaging

A new cooperation detection method is presented to improve the detection performance of the spacebased Synthetic Aperture Radar (SAR) system, which adopts the parallel distributed detection algorithm to process the information acquired by several SARs in one constellation observing the same target area from different angles at the same time. Based on the research of the spacebased SAR system and the distributed detection theory, the Spacebased SAR Cooperation Detection (SSARCD) model is established, which determines the relationship between the detection performance and the parameters of the orbit, the platform and the SAR system. Simulation results prove that the detection performance of the SSARCD system is better than that of the individual spacebased SAR system. (4 pages)

A novel spacebased SAR cooperation detection method

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