Multistatic synthetic aperture radar (SAR) is a special mode of SAR system. The radar transmitter and receiver are located on different satellites, which brings many advantages, such as flexible baseline configuration, diverse receiving modes, and more detailed ground object classification information. The multistatic SAR has been widely used in interferometry, moving target detection, three-dimensional imaging, and other fields. The frequency offset between different oscillators will cause a modulation phase error in the signal. Therefore, phase synchronization is one of the most critical problems to be addressed in distributed SAR systems. This article reviews phase synchronization techniques, which are mainly divided into two methods: synchronization by direct microwave link and synchronization by a data-based estimation algorithm. Furthermore, the future development of synchronization technology is anticipated.

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.

Geosynchronous orbit (GEO) satellites are widely used in communications and navigation fields due to their special properties, such as comprehensive ground coverage and fixed subsatellite trajectories. The safety of GEO satellites can be ensured by using space-borne inverse synthetic aperture radar (SBISAR) to achieve high-precision observations or monitoring of uncatalogued targets near the GEO satellites. However, the amount of echo data is significant due to the slow relative motion between the GEO targets and SBISAR during long coherent processing intervals (CPIs). In this work, a variable sampling start echo receiving method is proposed to reduce the amount of redundant echo data, and the timing which combining the radar tracking with imaging is designed. First, the Earth center inertial (ECI) coordinate position vectors of the GEO target can be obtained through the monopulse method and coordinate transformation. Second, the initial orbit parameters of the GEO target are established by the Gibbs method. Third, the slant range of the GEO target is obtained by the interacting multiple model (IMM). Finally, the slant range is used to design the sampling start of the echo window and the timing of tracking imaging integration. Taking GEO satellite imaging as an example, experiments based on simulation data verify the effectiveness of the proposed method.

In recent years, synthetic aperture radars (SARs) have been widely applied in various fields due to their all-weather, day-and-night global imaging capabilities. As one of the most common types of antennas, the reflector antenna offers some advantages for spaceborne radars, including low cost, lightweight, high gain, high radiation efficiency, and low sidelobes. Consequently, spaceborne SAR systems based on reflector antennas exhibit significant potential. This paper reviews the main types and characteristics of reflector antennas, with particular attention to the structural configurations and feed arrangements of deployable reflector antennas in spaceborne SAR applications. Additionally, some emerging techniques, such as digital beamforming, staggered SAR, and SweepSAR based on reflector antennas, are examined. Finally, future development directions in this field are discussed, including high-resolution wide-swath imaging and advanced antenna deployment schemes.

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.

Spaceborne bistatic synthetic aperture radar (BiSAR) plays an important role in the field of remote sensing. However, frequency deviation among different oscillators will cause a modulated phase error on the echo signal. Therefore, phase synchronization is a necessary condition for the normal operation of BiSAR. Currently, the main synchronization schemes are pulsed alternate synchronization of radio frequency(RF). However, these schemes have problems such as complex synchronization hardware links, interruption of SAR imaging, and constraints on swath width coverage. This paper proposes a monochromatic continuous wave duplex synchronization with azimuth phase coding (APC). The synchronization signal is a monochromatic continuous wave at intermediate frequency (IF), sharing the same IF bandwidth with the echo signal. Both the synchronization signal and the echo signal are received simultaneously by the satellite and separated using APC techniques. By employing IF continuous wave for synchronization signal, the hardware design of the synchronization link is simplified. Operating at IF allows for a wider antenna beam, which in turn simplifies the configuration of the synchronization antennas and reduces the number of required antennas. The processing workflow of the synchronization scheme is described in detail. The simulation experiments are performed. The accuracy of phase synchronization can reach 1 degree, which verifies the effectiveness of the proposed synchronization scheme.