The effectiveness of swath and Doppler spectrum reconstruction algorithms in cross-track and along-track multistatic synthetic aperture radar (SAR) formations heavily relies on the precise relative positioning of each spacecraft. Ideally, a static array configuration is desired, where the satellites are evenly distributed in an Earth-fixed frame. However, maintaining this ideal configuration is not feasible without constant active flight control to counter the satellites’ relative dynamics. Alternatively, one can design natural trajectories that result in the desired formation geometry within an acceptable margin for a significant fraction of the orbital period. In this paper, we present a relative motion description in an Earth-fixed geometry for distributed SAR concepts. We derive a general expression for the transformation matrix from the Hill-Clohessy-Wiltshire (HCW) frame to the SAR-appropriate zero-Doppler (ZD) frame, and propose a general optimization method to fit the best natural non-drifting formation solution for any desired formation configuration. By applying the developed technique to optimize along-track and cross-track relative positioning requirements, we obtain conforming baselines over extended orbit stretches. We present SAR imaging examples for along-track distributed formations and formations with small cross-track baselines, using distributed platforms spanning a 72° latitude range. The demonstrated configurations achieve orbit duty cycles exceeding 20%, with baseline errors kept below 5% relative to the ideal array element positions. These results indicate the feasibility of these concepts using natural orbit solutions.

The motion of the synthetic aperture radar (SAR) sensor during the transmit and receive events introduces a space-invariant polychromatic phase, known as intrapulse (IP) effect, which may degrade the impulse response function (IRF) if left uncorrected. A straightforward solution is already available in the literature to compensate for this effect for conventional SAR systems using a single waveform during the acquisition. However, there is currently no solution in the case the SAR system transmits an interleaved up/down sequence. If left uncorrected, besides the impulse response degradation in terms of azimuth and range resolution loss, additional azimuth ambiguities will appear. This letter proposes an azimuth-reconstruction-based approach to mitigate the IP effect for both single-receiver and multireceiver SAR systems with up- and down-chirp (UDC) modulation. The performance of the approach is demonstrated with point-target simulations.

Geosynchronous synthetic aperture radar (SAR) missions offer the advantage of conducting multiple sub-daily interferometric acquisitions using a single platform. However, this comes at the expense of increased fuel and power budgets, due to the significant distance to Earth’s surface. Consequently, such missions are suited for delivering SAR products with relatively lower spatial resolutions, typically in the range of hundreds of meters, over large localized areas spanning more than one million square kilometers. Alternatively, a constellation of small SAR satellites operating in low Earth orbits (LEO) can also deliver similar interferometric products. In LEO, the reduced free-space propagation losses and simpler orbit insertion can be exploited to realize and operate a larger constellation of LEO-SAR satellites. This paper provides an equivalence framework between the two alternatives in terms of interferometric lags and coverage. It further outlines an example mission concept, equivalent to Hydroterra, one of ESA’s Earth Explorer 10 candidate missions.

We present in this paper a sensitivity analysis of swath reconstruction algorithms for multistatic SAR constellations, mainly focused on geometric aspects of the formation, such as the irregularity of the sampling and the tilt.

This paper presents our new SAR processing framework known as PRISM (Processor for Interferometric SAR Missions). This flexible approach allows the efficient and accurate processing of SAR data independent of the sensor and the acquisition mode. The two main PRISM components (the focusing and the interferometric chains) are described in this paper together with the philosophy of the software architecture. Experimental results are presented and discussed based on the impulse response function analysis of simulated data as well as the focusing and interferometric results using real TerraSAR-X data.

In the constant strive for improving the capacity of next-generation spaceborne SARs while containing the increase in system complexity, distributed along-track constellations operated below Nyquist appear as one of the most promising solutions for delivering metric resolution imagery over swaths of hundreds of kilometers. In the operation of multistatic SAR constellations, however, sampling singularities typically result in dramatic noise scaling and the impossibility to recover the entire Doppler bandwidth unambiguously. We investigate in this article the likelihood of these singularities in the case of along-track distributed constellations and justify the use of irregular sampling schemes to reduce the percentage of invalid samples of an acquisition. This article also presents a bistatic polychromatic reconstruction algorithm, which is used for the evaluation of the performance of along-track distributed constellations. With all these elements, the performance of along-track multistatic systems is assessed by means of a Monte Carlo analysis and conclusions on the scaling numbers of the constellations are drawn. Based on the performance investigations, an exemplary distributed L-band SAR constellation is proposed as a corollary of this article.

The BIOMASS DEM Product Prototype Processor (BIO-DEMPP) is being developed in the frame of ESA's Earth Explorer BIOMASS mission. The prototype includes a complete interferometric SAR chain, from the stack co-registration until the mosaicking of the derived height products (Digital Elevation and Digital Terrain Models). This paper presents an overview of the BIODEMPP architectural design and its validation strategy, as well as first results obtained with simulated BIOMASS-like data.

This paper discusses multistatic SAR, highlighting the potential of constellations to deliver scalable performance, which allows for the simplification and reduction of the single units in terms of size and power. In particular, the paper illustrates the impact of technical challenges such as navigation control, clock synchronization, and data processing. The paper also includes the discussion of the main applications of multistatic SAR, including the measurement of single-pass interferometric and tomographic signatures as part of the calibration process. The paper is closed with examples of possible constellations offering comparable performance to SAR missions of the present and next generations.

A multistatic synthetic aperture radar (SAR) system offers the potential of exceeding the capabilities of conventional SARs in various ways, one of which is to realize high-resolution imaging over wide swaths when operated under the Nyquist frequency. Due to the decrease of the operational pulse repetition frequency, the Doppler spectrum of the received data of the single channels appears strongly aliased and needs to be resolved via azimuth reconstruction. Our aim in this article is to suggest an accurate reconstruction strategy that is applicable to multistatic SAR systems also with large along-track baselines and very high resolution on curved orbits. The algorithm, which can be regarded as a generalized reconstruction in the range-Doppler domain, operates in two steps and is capable of correcting most of the polychromatic deviations over large areas, achieving accurate reconstruction in constellations with kilometric baselines, resolutions of about 15λ over swaths of hundreds of kilometers. The validity of our approach has been tested using point targets in two- and six-receiver multistatic configurations.

Future spaceborne SAR satellites will achieve both high resolution and wide swath with their multiple receivers. In order to suppress range ambiguities, cyclic up and down chirp and azimuth phase coding have been widely applied. A multiple receiver system has lower pulse repetition frequency to increase the swath width and thus, the effect of azimuth phase coding to mitigate range ambiguities is decreased. In this paper, we reveal the possible combination of the two methods to overcome this drawback.

A multi-aperture Synthethic Aperture Radar (SAR) system offers to exceed the capabilities of the conventional SARs in various ways, one of which is to realize high-resolution wide-swath mode imaging when operated under the Nyquist frequency. Due to the low operational pulse repetition frequency, the Doppler spectrum of the received data appears strongly aliased in such systems and needs to be resolved via azimuth reconstruction. Our aim in this paper is to suggest an accurate reconstruction strategy that is applicable to multi-aperture SAR point target simulation in a two-spacecraft constellation. We suggested a two-step reconstruction method and validated it with point target simulation in two-channel multistatic constellation configurations with large along-track baselines and high resolution. The results show that our method is capable of accommodating both the polychromatic and range-variant nature of the azimuth reconstruction.

This paper presents first the performance analysis of the state-of-the-art multi-channel SAR signal reconstruction algorithms that suffer from the geometrical approximations. Then, it proposes a two-step signal reconstruction method for spaceborne large along-track baseline multistatic SAR constellations operated with a pulse repetition frequency (PRF) under the Nyquist rate. The suggested algorithm first applies a bulk polychromatic reconstruction in wavenumber domain and then compensates the residual range variance in range-Doppler domain. This simple modification helps to reduce overall phase errors in the reconstruction process of the analysed case with respect to the state-of-the-art algorithms by about one order of magnitude. The performance of the algorithm is verified by showing the results in point target simulations for a multistatic X-band constellation with a resolution of about 15 times the carrier wavelength.

Large along-track baselines introduce residual polychromatic quadratic phase components which decrease the performance of state-of-the-art multichannel/multiplatform SAR reconstruction algorithms. This letter investigates the impact of the geometrical approximations in signal reconstruction methods for spaceborne multistatic SAR constellations with large along-track baselines operated with a pulse repetition frequency (PRF) under the Nyquist rate required for a single platform. We characterize and quantify the impact of these approximations, especially severe in the case of kilometric baselines and resolutions around 15λ. Finally, we put forward a generalized range-Doppler strategy to accommodate the geometry of distributed along-track constellations in an accurate manner.

This paper presents a robust mis-registration estimator based on ESD method for TOPSAR interferometry. For the in-terferometric applications of TOPSAR mode data, a master and slave SLC pair requires a co-registration accuracy in the milli-pixel order due to the large Doppler centroid frequency variation. Although ESD method estimates the mis-registration with the required sensitivity, a prior fine co-registration step is a prerequisite. This paper presents an advanced co-registration method that relaxes the fine co-registration requirement of the ESD and improves the estimation sensitivity. Experimental results with Sentinel-1A data are shown to validate the proposed method.