Current phase error estimation approaches, such as the signal subspace comparison (SSC) method and the orthogonal subspace (OS) method, neglect the effect of noise. Consequently, their estimation accuracy deteriorates under low signal-to-noise ratio (SNR) conditions. In contrast, the adaptive weighted least squares (AWLS) method accounts for noise. Its accuracy, however, deteriorates under highly nonuniform sampling conditions. Although the maximum normalized image sharpness (MNIS) algorithm achieves high estimation accuracy, it suffers from high computational complexity. To address these limitations, novel phase error estimation methods based on weighted subspace fitting are proposed. The proposed methods explicitly account for noise by constructing two weighted fitting models between the subspace and the steering vector, from which phase errors are estimated via optimal solutions. Depending on the employed subspace, the proposed methods are classified as weighted signal subspace fitting (WSSF) and weighted noise subspace fitting (WNSF). Simulations and airborne experiments verify the accuracy and robustness of the proposed methods.

In azimuth multichannel synthetic aperture radar (AMC-SAR) systems, channel phase errors can induce ghost targets. The estimation of signal parameters by the rotational invariance technique (ESPRIT) assumes that the principal signal subspace corresponds to the steering vector at zero Doppler frequency. However, this assumption is frequently violated in practice, resulting in performance degradation. To address this issue, a robust ESPRIT (RESPRIT) algorithm is proposed, which integrates initial ESPRIT-based phase error estimation with residual phase error correction through image contrast maximization. Both simulation and real airborne data experiments demonstrate the effectiveness and robustness of the proposed method.

Sea-surface velocity (SSV) and sea-surface height (SSH) are among the most crucial parameters in an oceanic dynamic environment. Using spaceborne interferometric synthetic aperture radar (InSAR) to obtain high-resolution, large-scale survey area, high observation frequency, and high-precision ocean dynamic parameters is advantageous. However, the hybrid baseline InSAR phase data include components from both SSV and SSH, making it challenging to distinguish them and affecting inversion accuracy without additional information. The PIE-1 constellation, the world’s first four-satellite distributed InSAR system, was successfully launched into orbit on March 30, 2023. This constellation can form multiple interferometric pairs by combining two satellites, enabling the potential to extract SSV and SSH from hybrid phase signals simultaneously. In this study, two pioneering and fundamental works were conducted: 1) an integrated current-height inversion model was developed based on the multichannel likelihood (ML) function, with error analysis performed according to PIE-1 parameters and 2) the detailed data processing scheme for the first simultaneous inversion of SSV and SSH based on spaceborne InSAR data was presented. The inversion results were compared to Doppler centroid analysis (DCA)-derived Doppler velocity and reference data from the ESA’s Copernicus Marine Service (CMEMS). Both qualitative and quantitative comparisons validated the effectiveness and accuracy of the inversion results. This approach represents an effective technique for simultaneous inversion of SSV and SSH in future multibaseline spaceborne/airborne InSAR systems.

The hybrid along-track (AT)/cross-track (XT) synthetic aperture radar interferometry (InSAR) system enables the measurement of radial sea surface velocity, where solving it through phase unwrapping (PU) is intrinsically ill-posed due to the coupling of velocity-induced and height-induced phase components. Conventional hardware-based methods impose stringent and costly system requirements, while auxiliary-data-dependent approaches are error susceptible. Under this condition, we innovatively propose an along-track two-stage programming approach (AT-TSPA) multibaseline (MB) PU method within the three-pass differential InSAR (DInSAR) framework. AT-TSPA addresses the ill-posedness by making use of the normal baseline diversity, thus enabling accurate and robust retrieval of sea surface velocity without relying on hardware upgrades or external auxiliary data. Both theoretical analysis and experimental results demonstrate that AT-TSPA is effective and practical for accurately measuring sea surface velocity under complex oceanic conditions. AT-TSPA is also applicable to other fields involving coupled velocity and height phase components, such as sea ice velocity estimation. Furthermore, AT-TSPA extends the theory of TSPA-based MB InSAR from terrestrial to oceanic domains, thereby advancing the applicability and practicability of well-posed InSAR techniques.

This article proposes a configuration for a multiple input-multiple output (MIMO) along-track interferometry (ATI) synthetic aperture radar (SAR) satellite system designed for measuring ocean surface velocity vectors. The satellite system consists of a main satellite and at least two secondary satellites. The main satellite is equipped with dual antennas for transmitting and receiving echoes, while the secondary satellites are equipped with single antennas for receiving echoes. Based on the configuration, we explore key issues related to MIMO ATI SAR for measuring ocean surface velocity vectors to form a comprehensive solution. The issues include the current measurement principle, methods for assessing velocity vector accuracy, optimal baseline determination, carrier frequency selection, echo separation methods, and sources of phase error. The corresponding methods are derived in this article to address the challenges posed by the geometry of bistatic observations in beam steering and digital beamforming (DBF) processing. In addition, a system parameter design method based on optimizing ocean surface velocity measurement accuracy is proposed, eliminating the need for repeated trade-offs on individual parameters. Finally, the article presents an example of a high-precision wide-swath ocean surface velocity vector measurement system based on the configuration. This system achieves an ocean surface velocity accuracy of 3 cm/s, a swath coverage of 200 km, and a product resolution of$1\times 1$km2 in wind speeds above 3 m/s.

Compared with the broadside imaging mode in multichannel SAR system, high-squint imaging mode in azimuth would be much more flexible to achieve high-resolution and wide-swath (HRWS) imaging and greatly improve the efficiency of application for Earth dynamic monitoring. However, the high-squint angle would result in more complex problem of spectrum folding after down-sampled, which would pose new challenge to the unambiguous signal reconstruction for multichannel SAR system. In this paper, the processing procedure of the unambiguous signal reconstruction for high-squint multichannel SAR is proposed. Then, an onboard experiment based on Chinese GF-3 dual receiving channel (DRC) spaceborne SAR system were successfully carried out to validate the practicality of high-squint imaging mode for multichannel SAR system in azimuth in January 2022. Experimental data validates the effectiveness of the proposed processing procedure.

High-squint multichannel synthetic aperture radar (SAR) in azimuth can overcome the constraint of minimum antenna area in the traditional single-channel classical SAR system, and thereby it would be much more flexible to achieve high-resolution and wide-swath (HRWS) imaging than broadside SAR. However, for the high-squint multichannel SAR imaging mode, the squint angle causes Doppler centroid varying with the range frequency, which can further result in much more complicated problem of the spectral folding after down-sampled with low PRF. Meanwhile, the Doppler centroid frequency is indispensable for the estimation of phase bias and unambiguous signal reconstruction among received channels. In this paper, the impact of high-squint angle on the aliased Doppler spectrum and Doppler centroid is analyzed in detail. Then, a novel approach to phase bias estimation as well as the underlying signal processing in the case of high-squint multichannel SAR imaging mode in azimuth is proposed. Simulation results validate the effectiveness of the proposed processing flowchart for high-squint multichannel SAR.

In the azimuth multichannel (AMC) synthetic aperture radar (SAR) system, the large receiver is divided into several small receivers with uniform distribution, and high-resolution wide-swath (HRWS) imaging is achieved by using a low pulse repetition frequency (PRF). However, a low PRF will result in Doppler ambiguity for each channel and ambiguous radial velocity estimation. In this paper, an effective unambiguous radial velocity estimation method for a ground moving target is proposed. First, the baseband radial velocity is estimated quickly by calculating the energy ratio of the reconstructed Doppler spectrum. Then, the Doppler ambiguity number is estimated by using the Radon transform (RT). Finally, unambiguous radial velocity estimation can be achieved, and the proposed method has lower time complexity than the method based on traversal search. Numerical simulation results prove the effectiveness and accuracy of the proposed method.

Random stepped-frequency chirp waveform (RSFCW) is capable of achieving high-resolution synthetic aperture radar (SAR) imagery as well as certain electronic countermeasure capabilities. The Doppler-dependent phase incoherence due to the platform motion leads to degradation of the frequency synthesis of RSFCW. In this letter, a coherence processing algorithm for RSFCWSAR is developed with full cell Doppler correction (FCDC). The waveform structure of RSFCW is exploited to decompose the signal into different Doppler cells, which paves a way to the FCDC in phase alignment for frequency synthesis. Extensive simulation has confirmed the superiority of our method’s performance in comparison to those of traditional techniques.

For high resolution sliding spotlight SAR (HRSS-SAR) on agile platform, its beam steering can be achieved by satellite attitude maneuver. In designing the satellite attitude, the variation of Doppler centroid and azimuth resolution should be considered, along with the illuminated scene width and shape. We proposed a feasible satellite attitude maneuver strategy for HRSS-SAR. The strategy developed the conventional sliding spotlight SAR geometry with moderate azimuth resolution. This paper conceive that the rotation center HRSS-SAR moves along azimuth time, and that the variation of Doppler center along range direction should be minimized. In the strategy, beam steering is realized by three-axes attitude maneuver, escaping from the scene bending phenomenon by one or two axes maneuver in large squint angle geometry. The effectiveness and feasibility of our strategy is validated by simulation experiments both in Satellite Tool Kit (STK) and MATLAB.