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The paper describes the basic components of MirrorSAR and explains how bi- and multistatic SAR acquisitions are achieved by shifting only minimal functionality to the receive satellites. It shows that synchronization is not required or reduced in complexity by a MirrorLink. The MirrorLink forwards the ground reflected radar signal from the receiving satellites in a space transponder manner to the transmit satellite. Several options for the MirrorLink are discussed. In MirrorSAR, a number of satellites enable the acquisition of dual- or multi-baselines in a single-pass. The paper discusses several application examples where MirrorSAR eases the overall system complexity, the data acquisition and improves the SAR product quality. An interferometric SAR application example is discussed in more detail.

This paper proposes the wrapped staring spotlight (WSS) SAR imaging mode, which is a new method to extend the azimuth steering capability for phased array SAR to achieve either an improved azimuth geometric or radiometric resolution. It investigates the utility of steering directions with main lobe gains that are smaller than that of the grating lobes and exposes how these directions can be exploited. Furthermore, two methods are proposed to reduce the speckle and the image noise at once, i.e., the Look-Normalized Pattern Correction and the Ω-weighting. Based on two example TerraSAR-X WSS acquisitions, the image performance of extended and point targets is discussed.

For the TerraSAR-X mission a new staring spotlight mode has been implemented delivering very high azimuth resolution. Detailed performance analyses have been conducted in order to optimize the commanding parameters for this mode. Compared to the previously available TerraSAR-X imaging modes, staring spotlight requires operating the instrument with minimum margins and at the edge of the specifications the radar has been designed for. Therefore an additional step – a global acquisition simulation and analysis – is introduced during the operationalization to ensure the suitability of the derived commanding parameters on a global scale. The paper gives an overview about SAR performance analyses conducted for the mode optimization and implementation phases and presents a novel global performance assessment approach, which is in general applicable for the verification of operational SAR modes. Additionally, measurement results and exemplary acquisitions are shown.

This paper introduces the concept of a fractionated MirrorSAR which is based on a set of mutually separated transmitter and receiver satellites. As opposed to previously published bi- and multistatic SAR systems, the receiver satellites are considerably simplified, as their main functionality is reduced to a kind of microwave mirror (or space transponder) which routes the radar echoes towards the transmitter. The forwarded radar signals are then coherently demodulated within the transmitter by using the same oscillator that had been used for radar pulse generation. This avoids the necessity of a bidirectional phase synchronization link as currently employed in TanDEM-X. Since the needs for fully equipped radar receivers, on-board memory and downlink are also overcome, the weight and costs of the receiver satellites can be significantly reduced. This allows for a scaling of their number without cost explosion, thereby paving the way for novel applications like multi-baseline SAR interferometry and single-pass tomography. Several additional opportunities make the MirrorSAR concept even more attractive. First, the separation of the transmitter and receiver front-ends reduces not only RF losses by avoiding switches and circulators, but it may also lower the peak power in the transmitter satellite by employing a frequency-modulated continuous wave (FMCW) illumination. This simplifies the design of the high-power amplifier and increases its efficiency. Second, the opportunity for continuous radar data collection enables new modes for the imaging of ultra-wide swaths with very high resolution, thereby overcoming an inherent limitation of conventional monostatic SAR systems. Third, the joint availability of all receiver signals in a centralized node offers new opportunities for efficient data compression, as the multistatic radar signals from close satellite formations are characterized by a high degree of mutual redundancy. Fourth, the use of a sufficiently separated transmitter satellite can avoid the risk for mutual illumination, which challenges the design and operation of fully-active multistatic SAR systems. Further advantages arise from the scalability and reconfigurability, which support new redundancy concepts and pave at the same time the way to new modes like MIMO-SAR tomography.

This paper addresses several important aspects that need to be considered for the processing of spaceborne synthetic aperture radar (SAR) data with resolutions in the decimeter range. In particular, it will be shown how the motion of the satellite during the transmission/reception of the chirp signal and the effect of the troposphere deteriorate the impulse response function if not properly considered. Further aspects that have been investigated include the curved orbit, the array pattern for electronically steered antennas, and several considerations within the processing itself. For each aspect, a solution is proposed, and the complete focusing methodology is expounded and validated using simulated point targets and staring spotlight data acquired by TerraSAR-X with 16-cm azimuth resolution and 300-MHz range bandwidth.

This paper introduces the bidirectional synthetic aperture radar (BiDi SAR) imaging mode, i.e., the simultaneous imaging of two directions by one antenna into one receiving channel, and presents short-term time series of images and interferograms acquired by the TerraSAR-X and TanDEM-X satellites. A comparison to alternative approaches for the acquisition of short-term time series is provided. The BiDi acquisition geometry is defined, and a TerraSAR-X BiDi antenna pattern is analyzed. BiDi raw data are simulated, sampled with different pulse repetition frequency values, and compared with real BiDi raw data. The spectral separation of simultaneously acquired forward- and backward-looking images is explained. This paper presents the image results of BiDi acquisitions with TerraSAR-X and TanDEM-X satellites flying with 20-km along-track separation. This pursuit configuration allowed for the acquisition of up to six short-term repeated images and up to three interferograms in a single pass. An overview of potential applications for the new BiDi SAR imaging mode concludes this paper.