The interactions among geometrical Doppler, beam pattern, and normalized radar cross section (NRCS) result in the unwanted coupling leakage of geometrical Doppler into the perceived geophysical Doppler. Starting from high-resolution synthetic aperture radar (SAR) data, we model this leakage for synthesized real aperture radar (RAR) observations of ocean motion. The uncertainty introduced by leakage is ~1 m${}\cdot {}$s−1. Corrections proposed in this work exploit the known interactions between beam pattern and geometric Doppler with NRCS gradients retrieved from simulated low-resolution RAR to estimate and correct for the incurred leakage, reducing the uncertainty to$\mathcal {O}$(0.1 m${}\cdot {}$s−1). Further reduction of instantaneous leakage may be achieved through temporal averaging, since the NRCS gradients that cause leakage appear mostly atmosphere-induced and decorrelate rapidly. The azimuth resolution and number of independent samples determine a system’s sensitivity to leakage. C-band systems are inherently prone to suffer from greater leakage and worse corrections than their Ku- and Ka-band counterparts. With DopSCA, the propagated effect of leakage is only secondary compared with the pulse-pair uncertainty. In similar systems where pulse-pair uncertainty is suppressed, leakage will dominate instead.

The Harmony mission features two bistatic synthetic-aperture radar (SAR) companions of Sentinel-1. As with any multistatic system, frequency deviations among the oscillators of the receivers cause a phase error in the phase of the demodulated SAR signal. Given that interferometry will be used to retrieve geophysical parameters from Harmony’s radar instruments, an erroneous phase difference between the SAR signals of the two companions will bias the retrieval. The companions will use a global-navigation system (GNSS)-based method to synchronize the phase of the signals. The residual phase that remains after the synchronization is significant enough to make the retrieval of relative sea-surface height (RSSH) impossible. In this paper, we present Multisquint with Overlaps (MuSO), a data-driven algorithm to remove the synchronization residual. The algorithm uses the multisquint processing approach, together with the overlap regions of the TOPSAR acquisition mode, to estimate the derivative of the residual. After running the algorithm, simulations suggest that the error signal reduces from a standard deviation of 4° to 0.01°, allowing the retrieval of RSSH from Harmony data.

The paper presents a novel concept of SwarmSAR for improving the azimuth resolution of fast-decorrelating targets such as ocean surfaces to generate high-resolution SAR images. The SwarmSAR concept consists of multiple simple nodes in a close formation cooperating in a MIMO-like fashion and illuminating a common footprint. Each individual node is basic but self-sufficient, guaranteeing decent target-resolving capabilities, even when operating individually. However, when operating in a MIMO-like fashion, they significantly improve the target resolution and imaging capabilities.In this paper, we promote an S-band SwarmSAR considering a simplified geometry for resolving a fast-decorrelating point target in the azimuth direction. The results demonstrated in this work show the superiority of distributed SwarmSAR architecture over traditional monostatic SAR systems in resolving fast-decorrelating targets, and provide insight into the potential of the concept for future SAR missions.

Earth Explorer 10 mission Harmony will consist of two satellites that fly in formation with Sentinel-1. It will operate as a multistatic radar in which Sentinel-1 transmits signals and all three satellites receive signals from different lines-of-sight. To prepare for Harmony and other possible future bistatic missions, transforms are derived to map the ocean-wave spectrum into bistatic synthetic aperture radar (SAR) spectra. The SAR mapping follows the standard derivation using the multi-dimensional characteristic function, but with adjustments for the modulation transfer functions compared to the monostatic case. This paper focuses on the SAR modulations caused by velocity bunching as it is the dominant distortion mechanism. We argue that a multistatic system, such as Harmony, leads to an inversion that constrains the real aperture radar (RAR) response on a scene-by-scene basis. A benefit of having additional receivers for wave-spectra estimation is that the three lines-of-sight enables to capture a larger fraction of the wave spectrum. Improvements are especially expected in high wind speed conditions such as tropical cyclones, where large energetic surface motions strongly deteriorate the (azimuth) resolution of the SAR data. Enhanced directional wave spectral characteristics will further help to improve the interpretation of the new bistatic Harmony high-resolution scatter and Doppler combined directional measurements.

Assessing the performance of interferometers and processing interferograms require accurate knowledge of the temporal lag, sensitivity, and spectral shift. While these parameters are well-defined for conventional interferometric configurations, their definition becomes opaque for complex configurations, such as bistatic systems with formation-flying satellites. According to the principle of diffraction tomography, each instrument samples a distinct region of the scattering surface’s Fourier domain. Using this principle, we introduce a wavenumber-domain method for calculating the temporal lag, spectral shift, and sensitivity to height of synthetic-aperture radar (SAR) interferometers. The method calculates interferometric parameters by aligning the ground-projected wavenumber support of the SAR images forming the interferogram. Although the wavenumber-support method agrees with the conventional geometric formulations of the temporal lag and sensitivity in geometrically simple cases, the two methods diverge in more complex geometries. We show that when the two SAR satellites fly in a close-formation or have lines of sight that are squinted with respect to the zero-Doppler direction, then the geometric formulations are inadequate and the wavenumber-support method is needed to accurately estimate the interferometric parameters.

The closure phase, which is a circular summation of the phases of the three multilooked interferograms, comprises a geophysical component and phase noise. In agricultural regions of southern Spain, encompassing both open crop fields and greenhouses, the closure phases constructed from Sentinel-1 acquisitions consistently exhibit positive signatures. The evolution of these observations appears to be related to the phenological stages of plants, as evidenced by crop calendars. Moreover, the signatures of closure phases stand out as a potential indicator of vegetation development under dense vegetation conditions when compared with coherence and normalized radar cross section (NRCS). Two existing models, one based on dielectric variation in the subsurface and another on volume scattering combined with perpendicular baselines, do not explain observed time series. Therefore, the presence of these positive closure phases implies the existence of supplementary factors contributing to closure phases associated with plant development. In this context, we explore two potential factors: variations in dielectric properties within crop canopies and the line-of-sight (LoS) motion of crops. These factors are considered to establish connections between temporal changes in vegetation parameters and observed closure phase signatures. Regarding the first factor, we characterize the crop canopies using the dielectric constant of an equivalent medium, thereby capturing changes in wave propagation within the canopies due to leaves and vertical stalks’ development throughout the crop growth stages. We then model their contributions to closure phases in a manner analogous to an existing soil moisture model. Using realistic vegetation parameters derived from in situ measurements, this forward model generates synthetic data comparable in magnitude to the observations. As for the second factor, we propose an additional contributing mechanism to closure phases—skewed motion in the radar LoS direction induced by plant growth. This motion model is mathematically verified under a small-motion approximation. Both the models offer valuable insights into the origins of geophysical closure phases.

We present a method that uses the spectral wavenumber domain to estimate the temporal lag of synthetic-aperture radar (SAR) interferometers. The method uses the surface-projected region of support of a given resolution cell of each of the two SAR images that forms the interferogram. Despite the spectral method agreeing with the conventional geometric method for the monostatic interferometer case with an along-track separation, the two methods diverge in bistatic cases of formation flying interferometers. We show that the geometric approximation overestimates the temporal lag when the two SARs fly in a close-formation or have a squinted line of sight with respect to the zero-Doppler direction.

Simultaneous multi-angle 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 multi-dimensional deformations from the mixing differential tropospheric delay (DTD), which generally distorts deformation signals in single interferograms. This paper focuses on the multi-dimensional deformation estimation by simultaneous multi-angle spaceborne D-InSAR. A multi-channel Wiener filter (MWF)-based multi-dimensional deformation and DTD joint estimation method is proposed in this paper. 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 multi-angle spaceborne D-InSAR in multi-dimensional deformation measurement. The proposed method is effective in achieving good estimation accuracy and spatial resolution preservation.

Estimating sea surface height using cross-track interferometry requires high sensitivity because the ocean surface signal is in the order of 10 cm. Additionally, the interferometer requires a temporal delay of a few milliseconds to ensure coherency of the moving ocean surface. We show that a squinted line of sight, in combination with a Helix satellite formation allows optimizing the effective perpendicular and along-track baselines to satisfy these conditions. This paper presents a model to estimate the performance of a formation-flying cross-track interferometer with a squinted line of sight. The tenth Earth Explorer, Harmony, which features two bistatic SAR companions, and a theoretical system with one monostatic and one bistatic SAR are used as case studies. The standard deviation of the height estimate is 1 cm to 10 cm between 29° to 41° and increases to 30 cm at the far range (46°) at a wind speed of 5ms-1. The power spectral density of the elevation shows that spatial scales of 47km can be resolved. The performance improves at higher wind speeds due to higher backscattering. At a wind speed of 15ms-1, wavelengths of 27km to 11km can be resolved, depending on the elevation spectrum. The performance over a 250km swath enables the instantaneous estimation of the surface elevation at the submesoscales for the first time.

This paper presents an approach to implement a forward model for Sentinel-1 copol and crosspol backscatter and coherence using crop bio-geophysical parameters namely leaf area index, biomass, canopy height, soil moisture and root zone moisture as inputs for the maize. These required input parameters are generated using Decision Support System for Agrotechnology Transfer (DSSAT), one of the state-of-the-art crop growth models. The predicted SAR signal is generated using Support Vector Regression (SVR) over all the maize fields in an agricultural region, Flevoland, Netherlands. The correlation between simulated signal and observed signal is evaluated.

A wide-swath synthetic aperture radar (SAR) image is known to be achievable by merging multistatic aliased data that is collected with an appropriate along-track displacement. However, in terms of both flexibility requirements like potentiality of both along-track and cross-track (XT) interferometry and design requirements like orbital safety, etc., a XT baseline is not only demanded but also unavoidable. In this work, a method is proposed in order to merge SAR data to recover the Doppler spectrum unambiguously for the nonzero XT baseline case. The algorithm is compared and validated by simulating an azimuth invariant multistatic SAR satellite constellation. The results are promising in terms of dealing with XT baselines.

Several researchers have shown that the Capon algorithm can be applied to reprocess SAR images, resulting in super-resolution reconstructed scenes with lower sidelobe levels. Thus by employing the Capon-based reprocessed images in Persistent Scatterer Interferometry (PSI), the persistent scatterer (PS) density can be increased. In this letter, we exploit the Capon-based PS densification method for Sentinel-1 (S-1)Terrain Observation by Progressive Scans(TOPS) data. We propose a revised robust approach of the Capon algorithm, which applies the automatic diagonal loading (DL) method when the condition number of the covariance matrix is big enough. The proposed approach is robust and can avoid spurious persistent scatterer candidate (PSC) points introduced by DL approaches. We also consider and analyze the spectral property caused by the scanning mode of TOPS in the reprocessing. We applied the revised-robust-Capon-based reprocessing algorithm to a stack of real-life S-1 data and selected PSCs from them. The final result shows that the number of PSs increases by approximately 30% with respect to the original stack.

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 Tromsø, Norway.

Bistatic scattering from rough surfaces is typically approached through the analysis of the scattered field in the conventional H and V polarization basis, which coincides with the zenith and azimuth unit vectors in a spherical reference frame. This study delves into the impacts of different choices of the transmit and receive linear basis on the performance and design of a synthetic aperture radar (SAR) mission receive-only companion. This article formalizes the rotation of the scattered wave orientation at the antenna axes of the companion with respect to the transmitted one and introduces a novel set of linear polarizations, named principal polarizations, in transmit and receive, deemed more suited to represent the scattering mechanisms of rough surfaces. Such a set is defined by the polarization bases that maximize the radar cross section. It is shown that the theoretical estimates from the proposed geometrical framework provide a good agreement with analytical and numerical simulations, performed considering state-of-the-art numerical solutions. In addition, this article promotes the hypothesis that a bistatic radar configuration, defined through the conventional H and V linear basis, presents a strong similarity, from a target information retrieval standpoint, to a monostatic compact$\varphi $-pol mode, i.e., with the transmission of a linear polarization rotated by an angle$\varphi $. The rotation$\varphi $varies over the swath and as a function of satellite separation. For baselines of 250–300 km, such as those envisioned by the European Space Agency (ESA) Harmony Earth Explorer candidate, and for steep incidence angles, an equivalent$\pi /8$-pol can be achieved for rough surfaces.

Multiple CubeSat altimeters can work independently or corporately to form altimeter constellations. Different configurations of the constellations can acquire distinguished advantages: improved spatial/temporal sampling and high cross-track resolution, which will be helpful for observations of oceanic small-scale structures and weather forecasting. Compared to single conventional altimeters, CubeSat altimeter constellations may achieve better performances with lower costs. To fully understand these systems, this article focuses on the performance analysis and methodology for CubeSat altimeter constellations. Besides the typical analyses of the resolution, revisit, and absolute sea surface height (SSH) accuracy, the performance analysis was conducted by considering the characteristics of multiple measurements provided by CubeSat altimeter constellations. Local and global spatial sampling performances are investigated for various constellations and compared by sampling density and swath size. Moreover, relative SSH accuracy is introduced and evaluated based on the spatial structure functions of errors to effectively evaluate the measurement performance. Related system requirements on power, delta-v, etc., to achieve the performance are also discussed, which ensures that the analysis fits the boundary conditions of implementation. Finally, different concepts of the CubeSat altimeter constellations are compared, where their limitations and possible solutions are also discussed.

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 andin situmeasurements. 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.

Distributed SAR systems provide imaging capabilities that cannot be achieved by traditional monolithic satellites, thanks to the multiple angles and times of observation. In this paper the opportunities offered by a swarm of small satellite nodes operating in S-Band are discussed. The nodes fly in a close formation and operate in MIMO mode. All the$N$satellites transmit and receive the$N$pulses at the same time, following a Frequency Division Multiplexing (FDM) scheme. The work focuses in particular on the impact of the cross-track baselines on the resolution and on the quality of the signal reconstructed from the$N^{2}$channels. A first iteration of a distributed target processor based on adaptive frequency and channel treatment is hence proposed with the aim of effectively accounting for the slope-induced spectral shifts.

The paper investigates the polarimetry of bistatic Synthetic Aperture Radar (SAR) acquisitions over rough surfaces, with focus on the rotation of the scattered wave orientation at the companion antenna axes and on the optimal linear polarization in transmission. This latter is defined as the polarization achieving the maximum radar cross section and will be herewith recalled as principal polarization. The paper outlines a geometrical framework for the interpretation and the estimation of the principal polarizations. It is shown that the theoretical formulation provides a good agreement with the second-order analytical approach in [1]. The paper finally postulates that a bistatic illumination in the traditional$\mathrm{H}$and V linear modes can be considered equivalent to a compact$\psi$-pol, i.e. with the transmittion in a linear polarization rotated by$\psi$. For long baselines, such those as those envisioned by the ESA Harmony EE10 candidate, and for steep incidence angles, an equivalent$\pi/4$-pol might be possible for rough surfaces.

Using a combination of multi-directional SAR and TIR measurements, the Harmony Earth Explorer 10 mission candidate will provide high resolution simultaneous measurements of surface stress, surface currents SST and wave spectra over oceans, 3-D deformation vectors over solid Earth, and time-series of surface elevation changes over volcanic areas and land ice masses. This will serve a series of science objectives aimed at better understating multi-scale processed and feedbacks in the Earth System.

Ocean topography using SAR interferometry requires coherent observations of the sea surface. To observe the surface coherently, the along-track baseline between observations of the same scene must be kept to a minimum. Minimising the along-track baseline while maintaining a cross-track baseline that allows good sensitivity to relative surface height is difficult to achieve in satellite missions. This paper shows how a squinted line of sight allows single-pass cross-track interferometry with a wide swath over oceans. The Harmony candidate mission will have a formation that uses such an acquisition geometry to coherently observe the oceans.

A recent publication claims that closure phases in SAR interferometry bear no relationship to physical changes of the scatterer, but only to the statistical properties of the averaged pixels. We disprove this claim with a simple counterexample and remind the reader of cases in which closure phases indicate a clear physical content, including the exploitation of closure phases in other fields.

Existing microwave remote sensing instruments used for Earth observation face a clear tradeoff between spatial resolution and revisit times at global scales. The typical imaging capabilities of current systems range from daily observations at kilometer-scale resolutions provided by scatterometers to meter-scale resolutions at lower temporal rates (more than ten days) typical of synthetic aperture radars (SARs). A natural way to fill the gap between these two extremes is to use medium-Earth-orbit SAR (MEO-SAR) systems. MEO satellites are deployed at altitudes above the region of low Earth orbits (LEOs), ending at around 2000 km and below the geosynchronous orbits (GEOs) near 35 786 km. MEO SAR shows a clear potential to provide advantages in terms of spatial coverage, downlink visibility, and global temporal revisit times, e.g., providing moderate resolution images (some tens of meters) at daily rates. This article discusses the design tradeoffs of MEO SAR, including sensitivity and orbit selection. The use of these higher orbits opens the door to global coverage in one- to two-day revisit or continental/oceanic coverage with multidaily observations, making MEO SAR very attractive for future scientific missions with specific interferometric and polarimetric capabilities.

The work investigates staggered and random PRF (Pulse Repetition Frequency) strategies for a close formation of small Synthetic Aperture Radar (SAR) satellites operating in a multistatic configuration. The satellites are positioned within a fraction of the along-track critical baseline, hence allowing for the application of Displaced Phase Center image formation approaches. The performance of regular and random pulse sampling schemes is in particular assessed for a single-input multiple-output (SIMO) S-Band constellation, whose feasibility is further analyzed in relation to the number of satellites and their antenna size.

This paper debates the performance of the HARMONY (STEREOID) ESA EE-10 candidate mission in measuring the two-dimensional (2D) terrain deformation. Thanks to its Stereo configuration, where the two passive spacecrafts span a large along-track baseline centered on the Sentinel-1 satellite, a large observation angle diversity in azimuth can be achieved. This theoretically leads to promising deformation performance in the north-south direction component, which will play an extremely important role for the surface displacement analysis in the future.

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 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.

Persistent scatterers (PSs) are coherent measurement points obtained from time series of satellite radar images, which are used to detect and estimate millimeter-scale displacements of the terrain or man-made structures. However, associating these measurement points with specific physical objects is not straightforward, which hampers the exploitation of the full potential of the data. We have investigated the potential for predicting the occurrence and location of PSs using generic 3-D city models and ray-tracing methods, and proposed a methodology to match PSs to the pointlike scatterers predicted using RaySAR, a ray-tracing synthetic aperture radar simulator. We also investigate the impact of the level of detail (LOD) of the city models. For our test area in Rotterdam, we find that 10% and 37% of the PSs detected in a stack of TerraSAR-X data can be matched with point scatterers identified by ray tracing using LOD1 and LOD2 models, respectively. In the LOD1 case, most matched scatterers are at street level while LOD2 allows the identification of many scatterers on the buildings. Over half of the identified scatterers easily correspond to identify double or triple-bounce scatterers. However, a significant fraction corresponds to higher bounce levels, with approximately 25% being fivefold-bounce scatterers.

Capon's minimum-variance method (MVM) and amplitude and phase estimation (APES) spectral estimation algorithms can be applied to synthetic aperture radar (SAR) processing to improve the resolution and suppress sidelobe levels. In this paper, we use Capon-/APES-based SAR reprocessing algorithms to increase the persistent scatterer (PS) density in PS interferometry (PSI). We propose a PS candidate (PSC) selection algorithm applicable to the superresolution reprocessed images and the corresponding processing chain. The performance of the proposed algorithm is evaluated by a number of simulations and a stack of TerraSAR-X data. The results show that the Capon algorithm outperforms others in PSC selection. We present a full PSI time-series analysis on the PSCs extracted from the Capon-reprocessed stacks. The results show that the PS density is increased between 50% and 60%, while their interferometric quality is maintained.

This paper discusses the use of the along-track interferometric coherence as a radar observable with geophysical information rather than as a mere quality indicator. The use of dual co-polarized data to estimate the temporal decorrelation factor is explained, and preliminary results using TanDEM-X data are shown.

Future spaceborne synthetic aperture radar (SAR) systems are expected to deliver enormous quantity of data. Accordingly, the observation plan becomes crucial for a proper allocation of the mission resources. A central challenge is to reduce the number of acquisitions by making the same image available for more applications. Here, a relevant restriction is the interferometric incompatibility between fully polarimetric (QP) and single polarimetric (SP) SAR images, which typically occurs when the acquisitions are performed in burst mode, as in case of SAR products with wide coverage acquired by means of ScanSAR or terrain observation by progressive scanning (TOPS) technique. In order to overcome this limitation, a new operational mode, denoted as beam-switch wide-swath (BSWS), was conceived. The BSWS mode exploits the operational flexibility of future SAR systems to generate SP images, interferometrically compatible with the QP ones, keeping the wide coverage characteristic of the SP products. This letter presents in detail the BSWS mode, analyzing its functional principle, and imaging performance, based on a realistic future SAR system, the high-resolution wide-swath (HRWS) SAR, considered for the next generation of the Sentinel-1 mission.

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.

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.

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 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.

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.

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.

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.

This paper presents a methodology to design a spaceborne dual-beam along-track synthetic aperture radar interferometer to retrieve ocean surface velocity vectors. All related aspects and necessary tradeoffs are identified and discussed or reviewed, respectively. This includes a review of the measurement principle and the relation between baseline and sensitivity, the relation between wind and radar backscatter, a discussion of the observation geometry, including the antenna concept, polarization diversity, and all main error contributions. The design methodology consists of a sensitivity-based derivation of explicit instrument requirements from scientific requirements. In turn, this derivation is based on a statistical model for the interferometric phase error. This allows a quantitative, wellgrounded instrument design offering an additional degree of freedom to the approach, which we call “noise-equivalent-sigmazero requirement space.” Crucial tradeoffs for the system design, such as the resolution, the number of independent looks, the minimum wind speed, and the coherence and ambiguities, are pointed out and discussed. Finally, this paper concludes with a single platform system concept operating in Ku-band, which provides the measurement quality needed to achieve a surface velocity estimation accuracy of 5 cm/s, 200-km swath coverage, for 4×4 km2L2-product resolution and winds starting at 3 m/s.

Tandem-L is a highly innovative SAR satellite mission for the global observation of dynamic processes on the Earth's surface with hitherto unknown quality and resolution. Thanks to its novel imaging techniques and its unprecedented acquisition capacity, Tandem-L will deliver urgently needed information for the solution of pressing scientific questions in the areas of the biosphere, geosphere, cryosphere and hydrosphere. The feasibility of Tandem-L has been analyzed and confirmed in the scope of a phase A study, which has been conducted in close cooperation between the German Aerospace Center (DLR) and the German space industry. This paper provides an overview of the Tandem-L mission concept and summarizes the actual development status.

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.

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) 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.

SAR instruments with multiple transmit/receive channels allow for a variety of operation modes. The trade-space includes the instrument and antenna parameters designed to enable various operation modes. These are utilized to achieve a required SAR performance derived from the mission and user requirements. Finding a solution in this multi-dimensional trade space is not trivial and often involves the compromise of parameters. This paper addresses the topic of SAR instrument design tailored to a set of operation modes. It explains and compares the SAR techniques and gives the performance.

This paper presents the correlating synthetic aperture radar (CoSAR) technique, a novel radar imaging concept to observe statistical properties of fast decorrelating surfaces. A CoSAR system consists of two radars with a relative motion in the along-track (cross-range) dimension. The spatial autocorrelation function of the scattered signal can be estimated by combining quasi-simultaneously received radar echoes. By virtue of the Van Cittert-Zernike theorem, estimates of this autocorrelation function for different relative positions can be processed by generating images of several properties of the scene, including the normalized radar cross section, Doppler velocities, and surface topography. Aside from the geometric performance, a central aspect of this paper is a theoretical derivation of the radiometric performance of CoSAR. The radiometric quality is proportional to the number of independent samples available for the estimation of the spatial correlation, and to the ratio between the CoSAR azimuth resolution and the real-aperture resolution. A CoSAR mission concept is provided where two geosynchronous radar satellites fly at opposing sides of a quasi-circular trajectory. Such a mission could provide bidaily images of the ocean backscatter, mean Doppler, and surface topography at resolutions on the order of 500 m over wide areas.

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.

SAOCOM-CS denotes a satellite formation based on SAOCOM, Argentina's L-band SAR satellite, and a small passive companion satellite (CS) of the European Space Agency (ESA). The realization of this mission is currently under evaluation by ESA. A possible acquisition geometry of particular interest for its scientific and innovative value is a large baseline bistatic (LBB) configuration. This paper discusses the impact of the LBB geometry on the SAR imaging performance computation and presents the expected performance results in a reference scenario based on a stripmap, full polarization operational mode.

The paper discusses the implementation of a quasi-geostationary MIMO Synthetic Aperture Radar swarm, also defined as ARGOS in [1]. Such system combines the all-time-all-weather image capability of a SAR, with the resolution enhance, robustness and scalability of a swarm and the continuous observation provide by the geostationary orbit. Two C-band concepts are analyzed in details that differ for the image time and azimuth resolution.

In this paper we propose a new hybrid technique to suppress range ambiguities in spaceborne SAR systems with multiple elevation beams. First, conventional scan-on-receive (SCORE) is performed in real-time onboard the satellite by employing a set of dispersive beams that maximize the collected signal energy for each transmitted pulse. The range ambiguities are then removed in a second step by a joint processing of the signals collected by the multiple elevation beams. The suggested two-stage approach has the advantage that a more robust range ambiguity suppression, which may involve advanced nulling techniques to account for local topography as well as satellite attitude and instrument phase errors, can be performed on ground without tremendously increasing the onboard processing demands or the data downlink volume.

This paper analyzes the impact of a spatially inhomogeneous ionosphere on spaceborne single-pass SAR interferometry. For this, linear TEC gradients and higher-order irregularities are considered. It is shown that TEC gradients as low as 0.01 TECU/km may already noticeably affect the accuracy of an L-band cross-track interferometer, causing, e.g., horizontal and vertical offsets in the order of 1-2 m. Higher-order perturbations of the electron plasma lead to additional errors that vary nonlinearly with the length of the interferometric baseline. To predict these errors, we model the ionospheric irregularities as the product of a vertical profile and a second-order stationary stochastic process with a 3-D power-law spectrum. The interferometric errors are then derived via a set of projection integrals that express the expected phase error variance as a function of the baseline length and the angular extent of the synthetic aperture. With this model, we show that the phase errors of an L-band single-pass SAR interferometer may reach several tens of degrees under medium turbulence conditions. Since these phase errors are highly correlated among neighboring resolution cells, they cannot be reduced by multi-looking, thereby posing a possible challenge for multiple baseline interferometry and SAR tomography.

ALOS-Next/Tandem-L is a proposal for a highly innovative L-band SAR satellite mission for the global observation of dynamic processes on the Earth's surface with hitherto unparalleled quality and resolution. It is based on a collaboration between DLR and JAXA which started with a pre-phase A study in 2013 and is currently undergoing a phase A study. Thanks to the novel imaging techniques and the vast recording capacity with up to 8 Tbytes/day, it will provide vital information for solving pressing scientific questions in the biosphere, geosphere, cryosphere, and hydrosphere. By this, the new L-band SAR mission will make an essential contribution for a better understanding of the Earth system and its dynamics. ALOS-Next/Tandem-L will, moreover, open new opportunities for risk analysis, disaster management and environmental monitoring by employing especially designed acquisition modes and techniques in combination with a reconfigurable tandem satellite configuration and an L-band SAR instrument with advanced digital beamforming techniques.

This contribution presents two experiments performed with the TerraSAR-X (TSX) and TanDEM-X (TDX) satellites working in the pursuit monostatic configuration. Their objective is to estimate the along-track component of the motion in the scene in repeat-pass scenarios with an accuracy better than the one given by the stripmap azimuth resolution. Such performance is possible by exploiting the angular diversity of the bidirectional (BiDi) SAR mode and the π-shifted (or staggered) TOPS.

The potential of differential SAR interferometry (D-InSAR) techniques for the study of the 3-D deformation phenomena has been extensively demonstrated. In particular, two different performance approaches (one analytical and one numerical solutions) have been implemented and analyzed. The primary objective is to investigate the capabilities of the Sentinel-1 follow-on SAR mission (named here HRWS) and its preferred acquisition modes for this particular application domain, which might help to the mission optimization.

This paper presents a spaceborne SAR mission concept in Ku-band for the measurement of the two-dimensional ocean surface current. The measurement concept is based on dual-beam along-track interferometry, which requires two highly squinted antennas each looking simultaneously in fore and aft direction. A single side-looking geometry and a ScanSAR imaging mode is chosen to complete the observational concept in order to achieve the requirements on revisit-time and coverage. The applied hybrid polarization is intended to increase the information level about different ocean scattering mechanisms. Furthermore a methodology of instrument requirement derivation is briefly discussed, ensuring the ocean current measurement accuracy requirements. Finally the instrument concept, including digital beamforming, and the SAR performance are presented.

The signal-to-noise ratio of a radar system is described through the well-known radar equation, which can be adapted to apply to imaging Synthetic Aperture Radar (SAR). Extension to multi-channel SAR systems utilizing digital beam-forming turns out to be treacherous; the pivotal question being, if and how to include the gain of the multiple antenna apertures. This paper puts an effort into answering this question in a simple and comprehensible, yet mathematically rigorous way.

The orbit for Earth observation satellite synthetic aperture radar (SAR) missions is maintained within an Earth-fixed orbital tube to ensure ground-track coverage repeatability and, consequently, to enable repeat-pass interferometric compatibility between data takes. In this letter, it is shown that the size of the orbital tube may affect the interferometric performance in terms of azimuth spectral decorrelation and azimuth coregistration accuracy under the presence of squint. These effects require special consideration for SAR burst modes, such as ScanSAR or TOPS (i.e., Terrain Observation by Progressive Scans). This letter presents and analyzes these aspects in the frame of the Sentinel-1 mission.

With three coherent synthetic aperture radar images, it is possible to form three interferograms. In some cases, the phases of the three averaged interferograms will be significantly inconsistent and indicate a sort of phase excess or deficit (which we call lack of triangularity or inconsistency). In this paper, we illustrate theoretically which models can explain such phenomenon and provide some real-data examples. It is also shown that two or more independent scattering mechanisms are necessary to explain phase inconsistencies. The observation of lack of consistency might be useful to derive information on the target and as a warning that the scatterer presents a temporal covariance matrix, which is not intrinsically real, with consequences for the processing of interferometric stacks.

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.

The reliability of any final differential synthetic aperture radar (SAR) interferometry (DInSAR) product is directly related to the phase quality of the interferograms involved in the processing. Performing an appropriate selection of high-quality pixels is hence mandatory in order to avoid the inclusion of noisy data in the processing and thus obtain reliable deformation information within the area of study. In this framework, a new full-resolution pixel selection method based on exploiting the spectral properties of pointlike scatterers is presented. The method is developed from the concept of the so-called coherent scatterers, which are characterized by having a correlated spectrum in a single acquisition, but now involving the temporal axis in the detection. Using the spectral properties of scatterers to perform the DInSAR pixel selection step presents some advantages with respect to the well-known permanent scatterer (PS) approach. On the one hand, the radiometric calibration of data is not required since the detection is now not dependent on the amplitude behavior of scatterers. On the other hand, a reliable pixel selection can be performed even with a reduced number of images due to the nature of the estimator. In contrast, its main limitation relies on the loss of range resolution involved in the sublook generation process. The method is more suited for urban scenarios where the density of temporally stable pointlike scatterers is generally higher. In this context, the final deformation maps obtained using the new method presented are compared, in terms of density and phase quality, with the ones obtained with the well-known traditional PS approach, showing a similar performance. Finally, since both methods characterize pointlike scatterers from different points of view, the combination of both techniques is proposed leading to a significant increase in the pixels' density and giving place to a meaningful improvement in the final DInSAR results.

There is a need for scattering models that link quantitatively synthetic aperture radar (SAR) interferometric observables to soil moisture. In this paper, we propose a model based on plane waves and the Born approximation, deriving first the vertical complex wavenumbers in the soil as a function of geometrical and dielectric properties and successively the complex interferometric coherences. It is observed that soil moisture behaves on the phase in a similar way as tomography does, breaking the phase consistency in triplets of interferograms. The proposed model is validated with L-band airborne SAR data; preliminary inversion results based on interferogram triplets and coherence magnitudes are presented.

This paper presents a new evolution of SIGNAL's instrument concept that uses novel Digital Beamforming (DBF) techniques to further extend the coverage of the mission. In addition, by introducing two phase centers in azimuth, which are needed to achieve the swath extension, the system recovers the desired along-track interferometric capabilities.

Synthetic Aperture Radar (SAR) systems construct the equivalent to a very long antenna array by coherently combining radar echoes of a scene acquired along the trajectory of a space- or airborne radar. A key underlying assumption, or requirement, is that the observed scene stays coherent during the acquisition. This is true for static scenes, in particular when the acquisition time is kept short. For decorrelating targets, the azimuth resolution achievable by a SAR system is limited by the coherence time of the targets, τcoh, which limits the length of the synthetic aperture to the product of τcohand the azimuth velocity of the system. Typical cases where this can be an issue is for the observation of water surfaces, where the coherence times at microwave frequencies are typically well below 100 ms, which makes them short compared to the typically associated integration times.

The term MIMO SAR is used for Synthetic Aperture Radar (SAR) systems utilizingmultiple-receive andmultiple-transmit channels (Multiple-Input Multiple Output: MIMO). The trade-space for such SAR systems includes the instrument and antenna parameters taking into consideration the techniques, i.e. the operation modes of the radar. The purpose of exploring this trade-space is to improve and optimize the performance of the system, described through the performance parameters. The aim of this paper is to show examples of how MIMO SAR offers new and unexpected ways of trading the system and performance parameters versus each other. Adequately exploring the trade-space yields a high flexibility and allows to simultaneously improve multiple performance parameters; a feature not available in conventional single channel SAR systems. The content of this paper is considered novel in the sense that it deals with highly advance SAR techniques.

This letter reports about some observations over rainforests (in Brazil and Indonesia), where the spectra of TanDEM-X interferograms show distinct features, almost a signature, which is explained and modeled in terms of the scattering properties. Supported by comparisons with simulations, the observations exclude any homogeneous horizontally layered forest; instead, they are compatible with a model with point scatterers clustered in clouds. Such a model, with high extinction and large gaps that allow significant penetration, is able to explain to a good degree the observations.

A contrast-based phase calibration algorithm for digital beamforming remote sensing radars using three contrast metrics is presented. The algorithm corrects time-varying antenna array phase errors that defocus digital beamforming remote sensing radar imagery. Amplitude errors are treated by equalizing the received powers in all elements. As such, the algorithm does not produce an absolute (or radiometric) calibration vector for the array. The performance of the algorithm is studied using a combination of simulated and real radar data under various conditions and is compared with a clutter-based calibration algorithm. An analytical proof showing that maximizing the expected value of the 4-norm metric is equivalent to phase-calibrating the image, except for a linear phase offset, is provided. We find that the clutter calibration algorithm performs best for statistically homogeneous scenes but that the contrast-calibration algorithms perform better with scenes with larger contrast ratios.

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.

In this paper a baseline (height) self-calibration concept is introduced and applied to simulations of two different single-pass interferometric SAR systems with a single-swath and a dual-swath configuration. The results will show that the general performance is very promising and that, by simultaneously acquiring two separated swathes, accuracies in the sub-decimetric range can be reached.

This paper presents the processing of TanDEM-X acquisitions for the monitoring of the topography of the Petermann ice island. In this particular case the area under study is continuously moving and the acquisition geometry is changing, so the processing of the iceberg's DEMs is challenging and additional effects are to be considered. The SAR processing chain used is presented and the results obtained summarized, showing the effects and limitations observed during the process.

This paper reviews radar architectures that employ multiple transmit and multiple receive channels to improve the performance of synthetic aperture radar (SAR) systems. These advanced architectures have been dubbed multiple-input multiple-output SAR (MIMO-SAR) in analogy to MIMO communication systems. Considerable confusion arose, however, with regard to the selection of suitable waveforms for the simultaneous transmission via multiple antennas. In this paper, it is shown that the mere use of orthogonal waveforms is insufficient for the desired performance improvement in view of most SAR applications. As a solution to this fundamental MIMO-SAR problem we had previously suggested to exploit the special data acquisition geometry of a side-looking imaging radar equipped with multiple receiver channels in addition to appropriately designed waveforms transmitted by multiple antennas. Here, we extend this approach to a more general set of radar waveforms with special correlation properties that satisfy a short-term shift-orthogonality condition. We show that the echoes from simultaneously transmitted pulses can be separated if the short-term shift orthogonality is combined with digital beamforming on receive in elevation. This enables the implementation of a fully functional MIMO-SAR without correlation noise leakage for extended scattering scenarios.

This paper provides an overview of the BIOMASS Mission End-to-End simulator (BEES) and of the mission performance analysis performed with it. The end-to-end performance, in terms of biomass estimates error, is close to the 20% error goal set for the mission. The main sources of errors are temporal decorrelation and the limited available bandwidth, while system induced errors have a negligible impact on the final performance.

This paper discusses the tomographic potential of interferometric SAR acquisitions under crossing tracks, which results from the associated baseline variation. A simple model is proposed to describe the received signal. Experimental results confirm the expected tomographic potential.

This paper presents an initial analysis of the possibilities for velocity and acceleration measurement with the Bi-Directional SAR imaging mode (BiDi). It comprises single satellite single path acquisitions as well as a constellation of two satellites. The translational velocity components into azimuth and range directions are simulated. A BiDi approach for measuring the azimuth velocity from one satellite with one receiving channel is proposed. Image examples acquired with the TerraSAR-X (TSX) and TanDEM-X (TDX) satellites show velocity and acceleration effects on ships and the proposed BiDi velocity approach is verified. Interferometric fringes were observed on anchoring ships. A first approach into the understanding of rotational effects was achieved by simulation of rotational fringes.

This paper presents investigations on a one year time series of crossing orbit differential interferometry SAR acquisitions with a 1-, 5- and 6-days temporal baseline. The conditions for crossing orbit interferometry are briefly discussed as the requirement of a common ground spectrum has to be satisfied. The uniquely short revisit times give the opportunity to perform interferometry on a glacier area, i.e. the Ronne ice-shelf, in X-band. The coherence results over one year as well as surface velocity measurements are shown and discussed.

Synthetic Aperture Radar (SAR) operating at Ka-band frequency is increasingly being considered for spaceborne applications. The benefits are seen in the compactness of the instruments, the high bandwidth and resolution. Besides this the high carrier frequency facilitates single-pass interferometry by allowing for an accommodation of all transmitters and receivers on the same platform. This paper presents the instrument architecture and SAR performance for a system utilizing an advanced operational mode enabled through digital beamforming techniques.

This paper reports about some SAR interferometric obervations over rainforest (in Brazil and Indonesia). The spectra of TanDEM-X interferograms exhibit a bimodal character, which can be explained by the fact that vertical components of the trees are present in the interferograms. This is explained by interpreting the periodogram as a histogram of the different slopes present in the scene. Simulations based on based on clouds of point scatterers are enough for a first modeling of the observations. It is possible to show -theoretically and with the support of simulations- that a horizontally homogeneous forest model is not able to justify our observations. This analysis has been made possible by the characteristics of the TanDEM-X system: high resolution, almost no temporal decorrelation and relatively small heights of ambiguity.

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 discusses cross-track SAR interferometry under crossing orbits. The underlaying theory is briefly outlined, showing that a crossing angle between the ground tracks needs to be compensated by applying different squint angles in order to have overlap in the the ground-spectral domain. Two sets of crossing orbits InSAR experiments are described. The results of the first experiment, are discussed.

This paper discusses the implementation of an end- to-end simulator for the BIOMASS mission. An overview of the system architecture is provided along with a functional description of the modules that comprise the simulator.

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.

TanDEM-X is a high-resolution interferometric mission with the main goal of providing a global and unprecedentedly accurate digital elevation model (DEM) of the Earth surface by means of single-pass X-band SAR interferometry. De spite 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 non-nominal modes have been conducted with the main purpose of testing the performance of both space and ground segments in very demanding scenarios. In particular, this paper reports about the first bistatic acquisition and the first single-pass interferometric (mono/bistatic) acquisition with TanDEM-X. Even in the absence of essential synchronisation and calibration information, bistatic images and interferograms with similar quality to pursuit monostatic have been obtained.

Large datasets of satellite SAR missions dedicated to deformation monitoring will require intensive and dedicated interferometric processing techniques. Decorrelation effects will prevent simple processing approaches, but optimal algorithms are computationally intensive. This paper shows results of a novel efficient algorithm that aims at exploiting scatter component that are stable over time, without being isolated point targets. The concept is proved valid for simulated data and limited examples in X-band and L-band.

This letter describes the first interferometric acquisitions and results obtained by the TerraSAR-X add-on for Digital Elevation Measurements mission. Due to the large along-track separation between the two satellites during the approaching maneuver and the Earth's rotation, useful interferometric acquisitions were only possible at high latitudes. This resulted in a crossing angle between the ground tracks whose impact was corrected by acquiring the two synthetic-aperture radar images with an opposite squint. The still very large 2-km cross-track baseline resulted in a 3.8-m interferometric height of ambiguity, producing extremely detailed images of the topography of the target area. Results acquired over the October Revolution Island, Russia, are shown and discussed.

This letter shows that in a repeat-pass data set of synthetic aperture radar (SAR) images, a long-term coherent component, when present, can be recovered by coherent temporal filtering of the SAR images and can successively form interferograms with higher signal-to-noise ratio. The validity of the idea is confirmed through simulations and one example with real TerraSAR-X data. The theoretical necessity of using long-term interferograms is also discussed and linked to autoregressive processes, starting from the observation that the optimal weighting is given by the inverse of the covariance matrix.

SIGNAL is an innovative earth exploration mission proposal with the main objective to estimate accurately and repeatedly topography and topographic changes associated with mass change or other dynamic effects on glaciers, ice caps and polar ice sheets. Elevation measurements are complemented with glacier velocity measurements, providing valuable additional information for a better understanding of the hydrology of glacierized basins and of the Arctic and Antarctic water cycle. SIGNAL is capable of monitoring all critical regions with a high spatial resolution and an adequate revisit time. This paper gives an overview about the actual mission design status and provides a brief description of the topography (DEM - digital elevation map) self-calibration strategy and the estimated global interferometric performance.

The paper presents the development of a ground based bistatic receiver using TerraSAR-X as a transmitter. The receiver subsystems like antennas, low-noise amplifiers, mixers, filters, synthesizers, etc. have been developed using low-cost monolithic devices in order to allow affordable deployment and at the same time offer final year students a challenging SAR engineering project. First raw data have been acquired on the Barcelona harbor area that has been focused producing geocoded images well matched with existing maps. A preliminary interferogram have been also produced.

This paper presents across-track tomography applied to a bistatic geometry with fixed receivers. This kind of geometry can overcome some of the classical monostatic tomography limitations such as temporal decorrelation and irregular baseline distribution. The Remote Sensing Laboratory (RSLab) of the Universitat Politècnica de Catalunya (UPC) has implemented a SAR Bistatic Receiver for INterferometric Applications, SABRINA, with 4-channels. SABRINA has been used to carry out a bistatic tomographic experiment. The acquired data has been processed with different tomographic methods and their performances compared.

This paper presents an along - track interferometry (ATI) study for a bistatic or multiestatic SAR configuration with fixed ground receivers. This technique can be useful for sea current estimation or for any problem of Ground Motion Target Indicator (GMTI). The proximity of the ground receivers to the scene allows to be very sensitivite to velocities with small baselines. This paper also proposes a multibaseline approach for ATI able to diferenciate among different velocity contributions in the same resolution cell. At the end of this paper, some results over real acquired bistatic data will be presented and discussed. The data have been acquired using the C-band SAR Bistatic Receiver for INterferometric Applications (SABRINA) and ESA's ENVISAT satellite, as a transmitter of opportunity.

Tandem-L is a proposal for an innovative interferometric and polarimetric radar mission that enables the systematic monitoring of dynamic processes on the Earth surface. Important mission objectives are global forest height and biomass inventories, large scale measurements of millimetric displacements due to tectonic shifts, and systematic observations of glacier movements. The innovative mission concept and the high data acquisition capacity of Tandem-L provide a unique data source to observe, analyze and quantify the dynamics of a wide range of mutually interacting processes in the bio-, litho-, hydro- and cryosphere. By this, Tandem-L will be an essential step to advance our understanding of the Earth system and its intricate dynamics.

This paper presents an analysis of the performance of TerraSAR-X for subsidence monitoring in urban areas. The city of Murcia has been selected as a test-site due to its high deformation rate and the set of extensometers deployed along the city that provide validation data. The obtained results have been compared with those ob-tained from ERS/ENVISAT data belonging to the same period and validated with the in-situ measurements.

Radar data have already proven to be compressible with no significant losses for most of the applications in which it is used. In the framework of information theory, the compressibility of a signal implies that it can be decomposed onto a reduced set of basic elements. Since the same quantity of information is carried by the original signal and its decomposition, it can be deduced that a certain degree of redundancy exists in the explicit representation. According to the theory of compressive sensing (CS), due to this redundancy, it is possible to infer an accurate representation of an unknown compressible signal through a highly incomplete set of measurements. Based on this assumption, this paper proposes a novel method for the focusing of raw data in the framework of radar imaging. The technique presented is introduced as an alternative option to the traditional matched filtering, and it suggests that the new modes of acquisition of data are more efficient in orbital configurations. In this paper, this method is first tested on 1-D simulated signals, and results are discussed. An experiment with synthetic aperture radar (SAR) raw data is also described. Its purpose is to show the potential of CS applied to SAR systems. In particular, we show that an image can be reconstructed, without the loss of resolution, after dropping a large percentage of the received pulses, which would allow the implementation of wide-swath modes without reducing the azimuth resolution.

In this paper, bistatic interferometry using fixed-receiver configurations is addressed both theoretically and experimentally. The analytical expressions for interferometric phase and height sensitivity are derived, and a full interferometric processing chain for digital elevation model (DEM) generation is presented. The derived expressions are general, and they can be applied to two possible acquisition geometries:backscatteringandforward scattering. The theoretical developments are complemented with experimental results done with the bistatic receiver Synthetic Aperture radar Bistatic Receiver for INterferometric Applications. The obtained DEMs are compared with a DEM from the Shuttle Radar Topography Mission and a digital terrain model from the Institut Cartografic de Catalunya. The comparison allows one to validate the results and demonstrate to which particular features of the scene that the bistatic radar is sensitive.

This paper discusses the use of Capon's minimum-variance method (MVM) and Amplitude and Phase EStimation (APES) spectral-estimation algorithms to synthetic aperture radar range-azimuth focusing. The rationale of the algorithms is discussed. An implementation of a Capon or APES processing chain is explained, and processing parameters such as chip-image size, resampling factor, and diagonal loading are discussed. For multichannel cases, a joint-processing approach is presented. A set of Monte Carlo simulations are described and used to benchmark Capon- and APES-based processing against conventional matched-filter-based approaches. Both methods improve the resolution and reduce sidelobes. APES yields generally better estimates of amplitude and phase than Capon but with worse resolution. Results with RADARSAT-2 quad-polarization data over Barcelona are used to qualitatively study the real-life performance of these algorithms.

The presented work discusses the processing of repeat-pass interferometric data acquired with SABRINA (SAR Bistatic Receiver for INterferometric Applications), where the receiver is fixed and the baseline is due to the two different satellite orbits. The paper addresses the particularities of bistatic interferometry like the coregistration of the images and the decorrelation factors affecting the interferometric phase. Theoretical developments are complemented with the first repeat-pass results. The characteristics of the scene and the 35 days temporal baseline have caused the interferogram to be severely affected by the temporal decorrelation. This first result opens the discussion of what kind of targets are seen in a bistatic geometry and which of them can be coherent along time.

This paper gives a quick overview of inversion methods for bistatic tomographic processing. The methods are tested over simulated and real data. The real data has been acquired from a set of indoor experiments carried out in the Radiation Laboratory (RadLab), at the University of Michigan. A scale model with a house, some trees and a rough surface on the ground has been built to reproduce an urban scenario.

This paper discusses the use of Capon's Minimum Variance Method (MVM) and Amplitude and Phase EStimation APES spectral estimation algorithms to SAR range-azimuth focusing. The rationale of the algorithms is explained. An implementation of a Capon or APES processing chain is described. Results using RADARSAT-2 Quad Pol data over Barcelona are used to qualitatively study the real-life performance of these algorithms.

This paper characterizes the interferometric phase for a fixed-receiver bistatic SAR system. Also, the expressions for the interferometric phase and image resolution cell are summarized. It will be distinguished between two particular acquisition geometries : back-scattering and forward-scattering. A bistatic interferometric chain has been implemented whose particularities are discussed. The theoretical developments are complemented with the comparison of the Digital Elevation Models (DEM) generated from the bistatic interferometric data acquired with our fixed receiver, named SABRINA (SAR Bistatic Receiver for INterferometric Applications) with a SRTM DEM and a Digital Terrain Model (DTM) from the Institut Cartografic de Catalunya (ICC).

This paper describes the design and construction of a bistatic SAR receiver suitable for airborne applications, using orbital SAR systems (ENVISAT, ERS-2, RADARSAT, TerraSAR-X among others) as opportunity transmitters. The challenge of this design is to reduce the required data throughput of the recorded data. This is achieved storing data only in the time intervals when scattered signal from the target area appears. The task of detecting this time intervals is performed in real time using a matched filter of the signal received directly from the SAR transmitter.

This paper aims at introducing the recent theory of compressive sensing to radar imaging systems in order to retrieve the imaged scene with better resolution and a reduced amount of collected samples. As a result of the application of the alternative imaging technique proposed, the use of matched filtering is avoided and the effect of its sidelobes in the images is drastically diminished. Furthermore, the amount of data to be stacked in the sensor and then downlinked to the ground station is meaningfully lower. This permits a more efficient management of resources.

This paper discusses temporal and phase synchronization in bistatic synthetic aperture radar (SAR) systems that use orbital sensors as coherent sources of opportunity and receivers at a fixed location. The discussion is particularized to SAR Bistatic Receiver for INterferometric Applications (SABRINA), a ground-based bistatic receiver that uses ENVISAT and ERS-2 as transmitters. Transmitter–receiver synchronization is hindered by the presence of independent reference oscillators at the transmit and receive end and by the lack of a common time frame. Phase synchronization and pulse alignment are achieved using a dedicated channel that receives a clean signal directly from the satellite. How to align the acquired data with the satellite orbit and how to estimate the Doppler centroid (DC) are studied. It is shown that in the bistatic configuration considered, the receiver provides an implicit control point which limits the negative impact of a DC misestimation on the resulting images. An algorithm to achieve this temporal alignment using the apparent phase history of the received pulses is proposed. Finally, this algorithm is validated through Monte Carlo simulations and experimental data acquired by SABRINA.

Bistatic SAR systems are an emerging research field. In which context, the Universitat Politecnica de Catalunya is developing a ground based bistatic system using ESAs ENVISAT and ERS-2 as transmitters. In this paper we characterize the bistatic interferometric phase and the different sources of decorrelation. We will also present a bistatic interferometric simulator which is able to generate realistic synthetic bistatic interferograms. The simulator is validated with real data obtained with our bistatic acquisition system, named SABRINA.

The Universitat Politecnica de Catalunya is developing a ground based bistatic system using ESA's ENVISAT and ERS-2 as transmitters. The spatial resolution of this configuration is similar to that of their monostatic counterpart, although foreshortening effects have a lesser impact. First single-pass interferometric images corresponding to a local test-site are presented and compared to a synthetic interferogram.

This paper discusses temporal and phase synchronization in SABRINA, a bistatic system that uses ENVISAT and ERS-2 as transmitters of opportunity. Phase synchronization and pulse alignment is achieved using a dedicated channel that receives a clean signal directly from the satellite. It is studied how to temporally align the acquired data with the satellite orbit using the apparent range migration history.

This letter discusses the implementation of SABRINA, Synthetic Aperture radar Bistatic Receiver for Interferometric Applications. The ground resolution of a fixed-receiver bistatic system is studied, showing that it is comparable to that of a monostatic system. Due to the short distance from target to receiver, large sensitivity is obtained. The noncooperative nature of the bistatic system forces a conservative data-acquisition strategy based on continuously sampling the scattered signal during a temporal window around the predicted satellite overpass time. Also, to be able to synchronize the system in time and in frequency, sampling of a direct signal obtained through an antenna pointed at the satellite is required. Besides the signal processing required to phase-lock the received signal, the bistatic synthetic aperture radar processing needs to take into account the azimuth-dependent phase history. First focused images obtained with the SABRINA-ENVISAT combination are discussed