With the development of high-resolution radar satellite for global comprehensive environmental monitoring, day-and-night and all-weather surveillance has become an active and growing research field. However, in all cases, these applications require radar to have a high-efficiency radar module (e.g., T/R module), and high system transmitting power. These requirements may put an important limitation on the performance of a radar satellite with a high-power configuration. In this paper, we report a novel waveform optimization framework. Through this framework, an advanced nonlinear frequency modulation (NLFM) waveform with lower sidelobes and a smaller main lobe, which can significantly relieve the restriction of very limited satellite power, is constructed. In addition, we apply it in a real synthetic aperture radar (SAR) system with a bandwidth of 100 MHz at 9.6-GHz carrier frequency and the whole process of the NLFM waveform for radar imaging is discussed in detail, including the system architecture and configuration, a system error compensation method, and a modified chirp scaling algorithm (CSA). The imaging results demonstrate the excellent performance of the advanced NLFM waveform. Moreover, we observe that the SAR system with the advanced waveform has a higher signal-to-noise ratio (SNR) of 1.29 dB compared with the conventional linear frequency modulation (LFM) waveform. The improvement of 1.29-dB SNR means that the real radar system can reduce transmitting power with a ratio of 25%. This effect is likely to be a potential feature of NLFM waveform, which can reduce the transmitting power requirement, especially for radar satellite.

Generally, synthetic aperture radar (SAR) system transmits linear frequency modulation (LFM) signal to obtain the high-resolution image and weighted windowing is usually employed to suppress sidelobes. However, it will cause a 1-2-dB signal-to-noise ratio (SNR) loss. Nonlinear frequency modulation (NLFM) signal, which can construct the signal's power spectral density (PSD) to reduce sidelobes without loss of SNR, is a promising candidate. However, the real-time generation of precise NLFM signal is still a technical challenge. In this letter, a high-precision NLFM signal generator with the ability of predistortion compensation is developed, and this signal generator will be employed in LuTan-1 (LT-1, i.e., TwinSAR-L) mission which is an innovative spaceborne bistatic SAR mission and planned to launch in 2020. In addition, a two-step error compensation method is developed to compensate the system error. Finally, the ground experiment is performed to validate the designed signal generator.

This article deals with the synthesis and analysis of quasi-orthogonal nonlinear frequency modulation (NLFM) waveforms to mitigate the impairments of ambiguous returns in quadrature-polarimetric (quad-pol) synthetic aperture radars (SARs). To this end, focusing on signals with a continuous piecewise linear instantaneous frequency, the design of a waveform pair exhibiting both a low cross correlation energy (CCE) and low peak to sidelobe ratios (PSLRs), is considered. To handle the resulting nondeterministic polynomial (NP) hard problem, a coordinate descent (CD) method is employed, where, at each step, the marginal minimization is tackled via a MATLAB optimization toolbox. Hence, transmission/reception schemes jointly capitalizing quasi-orthogonal NLFM waveforms and azimuth phase coding (APC) techniques are proposed to suppress ambiguity interference. Moreover, a systematic framework for the evaluation of the resulting azimuth ambiguity-to-signal ratio (AASR) and range ambiguity-to-signal ratio (RASR) is provided. Finally, detailed simulation experiments based on the LuTan (LT-1) parameters are carried out to verify the practicability and effectiveness of the newly proposed transceiver schemes.

The challenging problem to realize high-resolution and wide-swath (HRWS) synthetic aperture radar (SAR) imaging is the ambiguity suppression in the azimuth and range directions. According to the spatial angle difference of each ambiguity component, the current technical approach is to design the spatial filter for achieving the ambiguity suppression based on the 2-D multichannel system. Along with the increasing of HRWS imaging requirements, the number of system channels also gradually increase and further result in the complex structure design of the phased array antenna system. Meanwhile, the traditional phased array antenna cannot effectively control the direction of the transmit beampattern in range. Unlike the traditional phased array, frequency diverse array (FDA) employs a small-frequency increment across the whole array elements and forms the range-angle-dependent S-shaped transmit beampattern, which can be utilized to separate the different range ambiguous region. Considering the above-mentioned characteristics and the range periodicity problem of transmit beampattern, this letter devises a scheme for spaceborne SAR HRWS imaging mode in the view of transmit beampattern utilizing 2-D planar array, i.e., the FDA in azimuth for removing the range nonperiodicity ambiguity and the conventional phased array in elevation for removing the range periodicity ambiguity. Simulation results have been presented to validate the effectiveness of the proposed scheme.

The echo separation issue associated with different transmit antennas is the most technical challenge in realizing the multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) system. In this article, a novel MIMO-SAR imaging scheme based on an advanced radar waveform, namely, segmented-phase-code (SPC) waveform, is proposed. Compared with the state-of-the-art short-term shift-orthogonal (STSO) waveform beamforming schemes, this scheme relieves the short-term shift-orthogonality condition of transmitted waveforms without losing the imaging performance, which extends the optional waveform space. In this scheme, the separation of the echoes from close arrival angles is ensured by a simple time-shift weighting processing. Furthermore, a range bandpass filter bank and the digital beamforming (DBF) technique are employed to ensure that the echoes from far arrival angles are separable. Finally, detailed simulation experiments are performed to verify the feasibility of the proposed scheme, and in-depth discussions of different waveforms and MIMO-SAR imaging schemes are presented.

In the Earth observation mission of the synthetic aperture radar (SAR), wide swath can be used to complete global monitoring in a short time and high resolution can provide rich detailed information about the feature space and prominent structure and texture. However, the traditional single-channel classical SAR system cannot meet high-resolution and wide-swath (HRWS) imaging demand due to the constraint of minimum antenna area. Fortunately, this fundamental limitation can be overcome by using multiple receive subapertures in combination with advanced digital beamforming (DBF) technique. DBF in elevation can provide high gain and better system performance and has recently gained much attention in the field of SAR imaging. This article presents a 16-channel in elevation airborne X-band DBF-SAR system with 500-MHz bandwidth, characterized by high speed data acquisition and storage, as a test bed to provide the technical reserves and support for a future spaceborne DBF-SAR system in China. The hardware configuration of this system is designed according to a realistic flight mission. To verify the feasibility and operability of this advanced 16-channel DBF-SAR system, an outfield airborne flight experiment was successfully conducted in eastern Guangdong Province in November 2019. Meanwhile, considering the inevitable channel mismatch from airborne system, a precise strategy as well as the underlying signal processing is proposed to process the experiment data. In addition to the channel mismatch due to the topographic height, the Scan-On-Receive (SCORE) pattern loss (SPL) is also an inherent factor, which will deteriorate the output SNR in final SAR images. Therefore, this article also implements a quantitative assessment of SPL combined with the practical flight parameters and the real airborne data. Finally, the corresponding processing results are presented and analyzed in detail. The practical SNR improvement of 11.23 dB emphasize that DBF technology can significantly improve the quality of SAR images and will make an essential contribution to next generation of HRWS technology for environment monitoring.