Synthetic aperture radar (SAR) is a crucial technology in marine surveillance. Moving ships are always defocused in SAR images, which affects the following identification process. Inverse synthetic aperture radar (ISAR) technology has been employed to focus moving targets in SAR images, typically involving two steps: translation motion compensation and rotation motion compensation. Range envelope alignment is generally considered a well-established method for translation motion compensation. However, in rotation motion compensation, current approaches rely on signal parameter estimation and optimization algorithms, which lack robust methods. This paper presents a rotational motion compensation framework, named "azimuth envelope alignment", which extends the range envelope alignment to the azimuth direction. First, a novel concept of "azimuth envelope" is introduced, and the process of azimuth envelope alignment is discussed. Then, a specific method is presented: the use of the iterative adaptive approach (IAA) and resampling for implementation "azimuth envelope alignment". This method first estimates the alignment parameter using IAA, followed by achieving alignment through resampling. This method exploits the high-precision estimation capability of the IAA while avoiding its sparse estimation results, enabling the robust, nonsparse refocusing of moving targets. Finally, simulations and experimental data validate the effectiveness of the proposed algorithm. The azimuth envelope alignment framework introduced in this paper can serve as a general approach for moving target imaging, similar to range envelope alignment.

Phase synchronization is a necessary condition for the normal operation of spaceborne multistatic SAR. The research on bistatic SAR phase synchronization is relatively mature, and some phase synchronization schemes have been verified in practical systems. Because of the limitations that existing synchronization schemes extended to the case of multistatic SAR, this paper proposes a phase synchronization scheme for spaceborne multistatic SAR. As the scheme suggests, we used OFDM-Chirp signal to be synchronization signal, exploited the orthogonality of subcarriers to separate the mixed synchronization signal. The decomposed signals are independently compressed to extract phase error. This scheme can simultaneously achieve phase synchronization for multiple radar platforms on the basis of satisfying the high accuracy of phase synchronization, greatly improves the efficiency of phase synchronization. Furthermore, the performance prediction and phase synchronization simulation for this synchronization scheme is presented, which verifies the feasibility of the proposed scheme.

The acquisition of spaceborne tomographic data usually requires a longer period of time due to the satellite’s long revisit period. To address this issue, it is possible to leverage the similarity of neighboring pixels in order to perform tomographic reconstruction of building targets in urban areas. As a result, the insufficient number of samples can be approximately substituted by pixels with similar scattering characteristics. However, the current utilization of building structures is often limited to horizontal characteristics such as contour lines (CL); in addition, extraction methods either rely on external data as prior information or are constrained by the need to fit operations, which limits the shape of the contour lines. This paper proposes a spaceborne tomographic reconstruction method based on building characteristics, starting from data and fully utilizing the horizontal and vertical characteristics of buildings for reconstruction. First, interferometric information is used to assist in tomographic processing and a strategy combining multi-point growth with multi-level fusion is employed to extract contour lines. Additionally, the vertical characteristics of buildings are established to provide constraints on the solution space for tomographic processing. The three-dimensional reconstruction of isolated and vertical buildings is then achieved by combining signal elimination techniques. By more fully exploiting the structural characteristics of buildings, the proposed method is capable of recovering building structures even with a limited number of samples. The effectiveness of the proposed method is validated through simulated data and TerraSAR-X data.

Synthetic aperture radar tomography (TomoSAR) enables three-dimensional SAR imaging. However, most spaceborne TomoSAR data is currently acquired through repeated passes of a single satellite, during which the atmospheric and deformation phases complicate processing and degrade height accuracy. Satellite formations provide a potential solution by deploying multiple satellites flying in close proximity to simultaneously capture SAR images from different perspectives, forming multiple baselines. This paper proposes a multistatic SAR satellite formation design method for single-pass TomoSAR data acquisition. This method employs the equivalent phase center (EPC) approximation to explore the relationship between the number of satellites and EPCs, determining the optimal satellite number and transceiver configurations. Specifically, only two satellites are designated as T&R satellites equipped with both transmitters and receivers, while the rest are R satellites carrying receivers only. Both odd- and even-numbered satellite formations are considered, with the two T&R satellites placed at the outermost positions to enhance tomographic resolution. To maximize the unambiguous tomographic range, a parameter design method is developed to ensure uniform EPC spacing, resulting in uniformly distributed baselines. In addition, two formation schemes, T2R3-Helix and T2R4-Helix, are provided as examples. The method provides a viable approach for future TomoSAR data acquisition and broadens the application prospects of low-mass, low-cost small satellites.