The EarthExplorer 10 mission Harmony by the European Space Agency ESA, scheduled for launch around 2029–2030, consists of two passive C-band synthetic-aperture-radar companion satellites flying in a flexible constellation with one Sentinel-1 radar satellite as an illuminator. Sentinel-1 will serve as transmitter and receiver of radar waves, and the two Harmonys will serve as bistatic receivers without the ability to transmit. During the first and last year of the 5-year mission, the two Harmony satellites will fly in a cross-track interferometric constellation, such as that known from TanDEM-X, about 350 km ahead or behind the assigned Sentinel-1. This constellation will provide 12-day repeat DEMs, among other regions, over most land-ice and permafrost areas. These repeat DEMs will be complemented by synchronous lateral terrain displacements from the well-established offset tracking method. In between the cross-track interferometry phases, one of the Harmony satellites will be moved to the opposite side of the Sentinel-1 to form a symmetric bistatic “stereo” constellation with ±~350 km along-track baseline. In this phase, the mission will provide opportunity for radar interferometry along three lines of sight, or up to six when combining ascending and descending acquisitions, enabling the measurement of three-dimensional surface motion, for instance sub- and emergence components of ice flow, or three-dimensional deformation of permafrost surfaces or slow landslides. Such measurements would, for the first time, be available for large areas and are anticipated to provide a number of novel insights into the dynamics and mass balance of a range of mass movement processes.

Simultaneous multiangle spaceborne synthetic aperture radar (SAR) can provide spatially diverse SAR images of the same scene without time lags. Through differential SAR interferometry (D-InSAR), the system can extract accurate multidimensional deformations from the mixing differential tropospheric delay (DTD), which generally distorts deformation signals in single interferograms. This article focuses on the multidimensional deformation estimation by simultaneous multiangle spaceborne D-InSAR. A multichannel Wiener filter (MWF)-based multidimensional deformation and DTD joint estimation method is proposed in this article. The method can achieve optimal estimation accuracy and reduce the loss of scene details. It was first validated by the simulations based on the system parameters of the future European Space Agency (ESA) Harmony mission. Additionally, the method was confirmed through the utilization of the real TanDEM-X bidirectional (BiDi) SAR data acquired over two scenes in California, USA. We analyzed the performance of the method in the presence of multiple error sources and investigated the impact of different observation geometries on estimation performance. Finally, the results demonstrate the potential of simultaneous multiangle spaceborne D-InSAR in multidimensional deformation measurement. The proposed method is effective in achieving good estimation accuracy and spatial resolution preservation.

Observations of wind and ocean surface velocity vectors by along-track interferometry (ATI) with the synthetic aperture radar (SAR) are not only important for direct applications but also to increase understandings of ocean upper layer mixing, air-ocean interactions, and mapping submesoscale (1-10 km) structures. An experimental bidirectional (BiDi) ATI acquisition mode of TanDEM-X observes with two squinted beams separated by an angle of approximately 13.2° in azimuth on the ground. The baseline is very short, and the along-track interferometric phase (ATI phase) of the ocean surface in the line-of-sight direction of the beams can be interpreted as a total Doppler velocity. The 2-D Doppler velocity field will thus include wind-wave detected motions. In this article, Doppler velocity fields acquired from this experimental acquisition mode are presented. The sequential retrieval of wind vector and total surface current vectors (TSCV) is demonstrated on the BiDi TanDEM-X data. The retrieval algorithm builds on existing geophysical model functions (GMFs) of normalized radar cross section (NRCS) and Doppler velocity. XMOD2 and a GMF based on the Elfouhaily ocean wave spectrum coupled with a Kirchhoff approximation (EOWS&KA) are used. The retrieved wind fields are generally consistent with the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-5. While the ATI phase errors are small, the retrieved TSCV field looks promising. Acquisitions were located at sea over the tip of the Novaya Zemlya in Russia and over an area near Tromso, Norway.

Tropospheric delays are one of the main contributors to the interferometric phase in synthetic aperture radar (SAR) interferometry. When the phase contributions from surface deformation, topography, and ionospheric delays are negligible or known, the interferogram can be used to estimate the differential tropospheric delay (DTD), which can help to improve tropospheric delay predictions from weather models and in situ measurements. In conventional repeat-pass interferometric SAR (InSAR), however, the estimation of the DTD can still be significantly hindered by baseline errors. In addition, a single interferogram provides only relative DTDs, as the delays can be retrieved up to an unknown offset. To address such issues, this article presents a method for the estimation of DTDs on large scales by using repeat-pass simultaneous multi-angle SAR systems. Complementary simultaneous observations of the correlated troposphere from multiple angles are used to retrieve estimates of the absolute DTD and, at the same time, to mitigate the effect of baseline knowledge errors. Finally, a performance evaluation is presented for the Harmony Earth Explorer 10 candidate mission. A centimeter-level absolute accuracy and a submillimeter-level relative accuracy of the DTD estimation are achieved under the multistatic Harmony case when at least one companion satellite has an inter-satellite distance longer than 300 km to provide enough sensitivity.

This paper addresses the aspects of image formation and interferometric processing in the frame of the STEREOID Earth Explorer 10 candidate mission. In particular, the large along-track baseline configuration (approx. 250 km), which results in a high Doppler centroid, will be addressed in terms of bistatic SAR image formation and interferometric processing. The proposed focusing algorithm is validated with point-target simulations, while the interferometric investigations are performed using raw data generated with a bistatic end-to-end simulator.

Ambiguities in short-baseline ATI interferometry need to be treated not as noise that lowers the coherence, but as a source of bias. A mathematical formulation of the interferometric ambiguity model is given, and an approach to correct ambiguities is proposed and illustrated with simulation results.

This paper provides a compact overview of Harmony, an Earth Explorer 10 mission candidate dedicated to the observation of dynamic deformations of ice, solid earth and ocean surfaces. Harmony consists of two receive-only small Synthetic Aperture Radar (SAR) satellites using Sentinel-1D as illuminator, which will alternate close formation phases, dedicated to single-pass cross-track interferometry, with StereoSAR phases dedicated to the study of ocean surface motion and 3-D land surface deformations.

This paper addresses the performance in the retrieval of 3-D mean deformation maps by exploiting simultaneous or quasi-simultaneous squinted synthetic aperture radar (SAR) interferometric acquisitions in a repeat-pass scenario. In multisatellite or multibeam low earth observation missions, the availability of two (or more) lines of sight (LOSs) allows the simultaneous acquisition of SAR images with different squint angles, hence improving the sensitivity to the north-south component of the deformation. Due to the simultaneity of the acquisitions, the troposphere will be highly correlated and, therefore, will tend to cancel out when performing the differential measurement between the interferograms obtained with the different LOSs, hence resulting in a practically troposphere-free estimation of the along-track deformation measurement. In practice, however, the atmospheric noise in the differential measurement will increase for increasing angular separations. This paper expounds the mathematical framework to derive the performance by properly considering the correlation of the atmospheric delays between the simultaneous acquisitions. To that aim, the hybrid Cramér-Rao bound is exploited making use of the autocorrelation function of the troposphere. Some performance examples are presented in the frame of future spaceborne SAR missions at C and L band.

Companion missions provide a path to add single-pass interferometric capabilities to existing monostatic SAR missions. Large baselines, only achievable through formation flying, are necessary both to achieve the required performance in many interferometric applications and to enable SAR tomography. Some of the main challenges include meeting tight formation flying requirements, often calling for multiple-companion solutions, and oscillator synchronization.

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 provides a compact overview of SESAME, a mission concept in which two receive-only small Synthetic Aperture Radar (SAR) satellites flying in close formation would allow single-pass interferometric observations using Sentinel-1 as transmitter.

SESAME (SEntinel-1 SAR companion Multistatic Explorer) is a passive SAR satellite mission proposed for the ESA Earth Explorer Program. SESAME comprises two receive-only C-band SAR satellites flying in close formation to build a single-pass SAR interferometer (SP-InSAR) using the active signal of the European Sentinel-1 satellite. The SESAME mission addresses applications in geoscience and climate research that require repeat measurements of high precision elevation data over land surfaces including ice covered areas and forests, exploiting the SP-InSAR and multistatic observation geometry of the satellite formation. The objectives, the measurement approach and geo-biophysical products of the mission are described.

Companion SAR missions offer a cost-effective solution to enhance the added value of existing SAR satellites. In particular, they offer new possibilities to extend the observation space and provide one or several single-pass interferometric channels for the acquisitions. The paper aims to provide a description of an end-to-end performance simulation for companion SAR missions, including the evaluation of two exemplary configurations under realistic conditions and with the incorporation of the major error sources affecting these systems. A critical discussion of the results will be included of the final version of the paper drawing from the analysis relevant conclusions for the design of payload and concepts for future companion SAR missions.

The contribution focuses on the technical aspects related to the focusing and interferometric processing of bistatic data acquired by companion satellite (CS) SAR missions. In particular, the processing aspects related to the large along-track baseline configuration will be addressed, for the processing needs to properly consider a potential high squint angle. The technical challenges encompass synchronization, image formation and coregistration. On the other hand, the availability of a large squint angle has some advantages in terms of the sensitivity to the north-south component of the deformation for interferometric applications. The theoretical derivations are complemented with point-target simulations and Monte-Carlo simulations.

This paper puts forward a reverse backprojection algorithm for the moderately efficient generation of raw data of natural scenes acquired with general SAR geometries; in particular, we are interested in monostatic and bistatic geosynchronous (GSO) geometries. The backprojection algorithm has the advantage of being arbitrarily precise for all acquisition modes and systems, and allows an exact accommodation of space-variant effects introduced by topography and the propagation through the atmosphere. Its exactness allows both to simulate any real scenarios and understand further optimization potentials. The reverse backprojection algorithm is presented and analyzed in the following pages. The algorithm is also validated using both simulated clutter and data from the Sentinel-1 mission.

Correlating SAR (CoSAR) has been recently proposed as a geosynchronous remote sensing mission capable of delivering continental coverage of ocean surfaces with a high repeat cycle. The main specific measurements provided by CoSAR are estimates of the normalised radar cross section (NRCS), Doppler shifts, and surface topography with higher resolution than microwave radiometers and larger coverage than state-of-the-art LEO SAR constellations. This paper analyses the specific geometrical characteristics of CoSAR surveys and discusses the challenges of efficient CoSAR image formation approaches. The space-variance of CoSAR surveys is expected to be small, hence CoSAR image formation can benefit from the available knowledge in efficient bistatic and multistatic SAR image formation approaches.

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.

Companion SAR missions offer a cost-effective solution to boost the performance and capabilities of existing SAR missions. With the use of companion satellites, a whole suite of coherent single-pass observations become feasible, allowing for the interferometric and tomographic exploitation of the received data. Interferometric and tomographic measurements are however very sensitive to even the slightest degradations in the phase of the received data, and impose stringent requirements on the calibration concept of the mission. Weakly-synchronised companion SAR missions may have significant residual clock errors, which challenge the attainment of this requirements. We put forward an innovative calibration concept based on autonomous (data-based) calibration at different product levels and present an estimation of the expected performance for ESA's SAOCOM/CS mission.

Correlating SAR (CoSAR) is an innovative remote sensing concept which delivers large-scale coverage of ocean surfaces with a high repeat cycle. The main specific measurements provided by CoSAR are estimates of the normalised radar cross section (NRCS), Doppler shifts, and surface topography with higher resolution than microwave radiometers and larger coverage than state-of-The-Art LEO SAR constellations. This paper analyses the specific geometrical characteristics of CoSAR surveys, keeping an eye in the future development of efficient CoSAR image formation approaches. Far from being foreign to SAR, the paper will show that CoSAR image formation can benefit from the available knowledge in efficient bistatic and multistatic SAR image formation approaches.