This contribution addresses the efficient evaluation of Fourier-based kernels for synthetic aperture radar (SAR) image formation. The goal is to evaluate the quality of the focused impulse response function and the residual phase errors of the kernel without having to implement the processor itself nor perform a costly point-target simulation followed by the processing. The proposed methodology is convenient for situations where the assumption of a hyperbolic range history does not hold anymore, and hence a compact analytic expression of the point target spectrum is not available. Examples where the hyperbolic range history does not apply include very high-resolution spaceborne SAR imaging or bistatic SAR imaging. The approach first computes numerically the two-dimensional (2D) spectrum of a point target and then uses the transfer function of the focusing kernel to match it, and hence obtain the impulse response function (IRF). The methodology is validated by comparing the matched IRFs with the ones obtained using point-target simulations.

Bistatic and multistatic SAR constellations offer increased performance at the expense of increased operational complexity. Due to geometric or cost constraints, multistatic SAR constellations might be forced to operate in a partially cooperative manner, i.e., without a direct synchronisation link. In demanding scenarios, like high-resolution bistatic SAR imaging or cross-platform SAR interferometry or tomography, the data need undergo a calibration step to compensate the lack of synchronisation between transmitter and receiver master clocks. Autonomous synchronisation, based on the inversion of the phase and positioning errors of the bistatic SAR images caused by the lack of synchronisation, is used to calibrate the time and phase references of the system with the sole help of the received radar data, which drastically reduces the requirements on the hardware of the system.

TanDEM-X (TerraSAR-X Add-on for Digital Elevation Measurements) is a high-resolution interferometric mission with the main goal of providing a global and unprecedentedly accurate digital elevation model of the Earth surface by means of single-pass X-band synthetic aperture radar (SAR) interferometry. Despite its usual quasi-monostatic configuration, TanDEM-X is the first genuinely bistatic SAR system in space. During its monostatic commissioning phase, the system has been mainly operated in pursuit monostatic mode. However, some pioneering bistatic SAR experiments with both satellites commanded in nonnominal modes have been conducted with the main purpose of validating the performance of both space and ground segments in very demanding scenarios. In particular, this letter reports about the first bistatic acquisition and the first single-pass interferometric (mono-/bistatic) acquisition with TanDEM-X, addressing their innovative aspects and focusing on the analysis of the experimental results. Even in the absence of essential synchronization and calibration information, bistatic images and interferograms with similar quality to pursuit monostatic have been obtained.

This paper presents the strategy for the development of TOPMEX-9, an innovative concept for Earth observation based on synthetic aperture radar (SAR) and nanosatellite clusters. The concept is intended as a, as a start up project for future collaboration between the Microwaves and Radar Institute (HR) of the German Aerospace Center (DLR), the Mexican Space Agency (AEM) and the Mexican Talent Network (RDTM). The idea is based on a nanosatellite formation flying around a microsatellite using a distributed constellation in multistatic SAR mode. This is an analogy with the sun providing illumination to passive optical receivers or cameras. The microsatellite acts as a speaker (Tx) while the nanosatellites around behave as listeners (Rx). Multistatic SAR mode allows the separation of radar payloads, thus decreasing volume, weight, power and consequently the mission costs. It allows permits retrieval of multi-angle measurements, thus obtaining more information about the illuminated scene than the monostatic SAR systems. The design of each TOPMEX-9 nanosatellite is based on the CubeSat standard and includes a single receiver reflector antenna in H or V polarisation in the Ka-band (32.6-37.0 GHz). The SAR system distributed architecture (i.e. radar, TM/TC, tracking and intercommunication) has the advantage of maximizing the energy of the radar antenna, thus having a better signal-to-noise ratio. TOPMEX-9 is predicted to a great impact in future low-cost Earth observation missions. This mission is focused on applications in oceanography such as ocean wave spectra and sea surface roughness measurements, coastal area monitoring, wind speed estimation and atmosphere studies. Other applications are ice roughness in cryosphere research and ship detection. The limited lifetime of a nanosatellite is compensated by the fact that new radar cluster configurations can be launched based on lessons learned, contributions in the acceleration of technology development and proving innovative data acquisition schemes.

This paper reports on several experiments performed with the TerraSAR-X (TSX) and the TanDEM-X (TDX) satellites. The experiments consist in the generation of improved image products by coherently combining the images acquired individually by both satellites, hence the name distributed imaging. In the literature it has already been suggested the use of two or more satellites in close formation not only to have interferometric capabilities, but also to improve the azimuthal or range resolutions by performing the acquisitions simultaneously (no temporal decorrelation). This idea can be carried out by acquiring different portions of the spectrum and adding them coherently afterwards. Another possibility is to synthesize quad-pol acquisitions using dual-pol ones, an idea already carried out with airborne systems. Such experiments have been performed with TSX and TDX and are described in this paper.