The European Space Agency's (ESA) BIOMASS mission is a pioneering Earth observation satellite mission launched on April 29, 2025. Utilizing a P-band synthetic aperture radar (SAR), the objective of BIOMASS is to deliver estimates of above-ground forest biomass, forest height (FH), and forest disturbance (FD), with unprecedented accuracy. The mission's primary scientific goal is to quantify the distribution and changes in forest biomass, thereby reducing uncertainties in carbon flux estimates and informing climate models. The satellite's advanced instrumentation and innovative approach allow it to penetrate dense forest canopies, capturing data even in challenging environments. The mission will operate in two distinct phases: the tomographic phase and the interferometric phase, which will support polarimetric interferometric SAR (Pol-InSAR) and tomographic SAR (TomoSAR) processing. Additionally, BIOMASS will provide valuable observational data for ice sheets, deserts, the ionosphere, below canopy topography, and other domains.

BIOMASS is new ESA Earth Explorer, scheduled for launch in 2024. It will be the first P-band spaceborne Synthetic Aperture Radar (SAR), featuring full polarimetry, repeat pass interferometry and tomography. Its main objective is global routine monitoring of forested areas, its secondary objectives include exploration of glacier dynamics, subsurface geology and sub-canopy terrain topography. In order to correctly retrieve information, interferometric calibration has to address main disturbances affecting repeat pass P-band SAR: ionosphere, baseline errors and troposphere. In this paper we present the scientific design of BIOMASS interferometric calibration processor, detailing the state-of-the-art correction steps implemented. Some preliminary results obtained by processing simulated acquisitions are illustrated to motivate the proposed techniques.

The Copernicus Programme is a joint European initiative developed by the European Commission (EC) and the European Space Agency (ESA) to provide accurate, up-to-date, and comprehensive Earth observation data for environmental monitoring, climate change analysis, disaster management, and security. The Copernicus program comprises a series of dedicated satellite missions i.e., the Sentinels spanning a wide range of the electromagnetic spectrum with different sensing techniques.Sentinel-1 [1] is the radar imaging component of Copernicus. It is a two-satellites constellation placed in the same orbit and spaced 180º apart. The Sentinels are providing systematically data to the Copernicus service component and scientific users.The Copernicus operational services are covering key operational applications tailored to support the definition and monitoring of European policies such as:–Observation of the marine environment and sea-ice monitoring–Surveillance of maritime transport zones–Land surface mapping, e.g., vegetation cover and surface deformation,–Polar environment monitoring (e.g., ice shelves and glaciers)–Climate Change–Emergency Response and ManagementThe Sentinel-1 first generation comprises four satellite units developed in two batches. The first two (A and B) have been already launched and two recurrent (C&D) ones planned for launch ensuring continuity of measurement throughout the 2020’s. Sentinel-1C and D will overlap with other Copernicus SAR mission in development like ROSE-L [2] and the Sentinel-1 Next Generation [3].In this paper we discuss the status and evolution of the Sentinel-1 first generation.

The Copernicus program, and particularly Sentinel-1, are among the largest Earth Observation SAR data providers, serving an ever-increasing number of services, users, and applications. A key aspect of the program is the constant provision of quality data, which requires long term engagement to carefully monitor, preserve, and even improve the system performances. These tasks are mainly carried out within the SAR Mission Performance Cluster (SAR-MPC), an international consortium of SAR experts in charge of the continuous monitoring of the Sentinel-1 instruments status and of the L1 and L2 products quality. This paper provides an update on the monitoring and the actions implemented by the SAR-MPC during 2022 and 2023. The analysis regards the monitoring of the instrument status, the evaluation of the instrument radiometric and geometric accuracy, and the updates of the S-1 Instrument Processing Facility.

The Extended Timing Annotation Dataset (ETAD) product provides timing corrections for Sentinel-1 (S-1) single-look complex (SLC) data for improved geometric accuracy. Given the close relationship between path delay and radar signal phase, it also has the potential to serve as the basis for phase corrections in SAR interferometry (InSAR) applications with Sentinel-1. The provided timing corrections may be converted to phase offsets, which can be used to derive interferometric phase screens between pairs of repeat-pass acquisitions. In this work we explore the capabilities and limitations of ETAD to provide accurate interferometric phase corrections and introduce new features currently under investigation in the context of S1-ETAD scientific evolution study.

The Copernicus Sentinel-1 mission ensures continuity of C-band SAR observations for Europe. The mission is characterized by large-scale and repetitive observations, systematic production and a free and open data policy. Sentinel-1 data are routinely used by Copernicus and many operational services, as well as in the scientific and commercial domain. Sentinel-1 is based on a constellation of two SAR satellites, Sentinel-1A and Sentinel-1B that have been launched from Kourou on a Soyuz rocket on 3rd April 2014 and 25th April 2016 respectively. At the time of finalising this paper (31 May 2022), the operations of Sentinel-1B have been interrupted due to an anomaly on the satellite power sub-system, that occurred on 23 December 2021. This is a major anomaly that will probably result in a permanent loss of the satellite. Detailed investigations are close to completion, and unfortunately, all attempts to bring back the faulty power unit to nominal operations have failed. The paper provides some details on the Sentinel-1B power anomaly and on the mitigation actions performed at mission level to compensate, to some extent, the lack of Sentinel-1B data.

This article reports the Phase A study results of the interferometric extension of the high-resolution wide-swath (HRWS) mission with three MirrorSAR satellites. According to the MirrorSAR concept, small, low-cost, transponder-like receive-only satellites without radar signal demodulation, digitization, memory storage, downlink, and synchronization are added to the planned German X-band HRWS mission. The MirrorSAR satellites fly a triple helix orbit in close formation around the HRWS orbit and span multiple single-pass interferometric baselines. A comprehensive system engineering and performance analysis is provided that includes orbit formation, MirrorLink, Doppler steering, antenna pattern and swath design, multi-static echo window timing, SAR performance, height performance, and coverage analysis. The overall interferometric system design analysis of Phase A is presented. The predicted performance of the global digital elevation model (DEM) is improved by one order of magnitude compared to presently available global DEM products such as the TanDEM-X DEM.

The GeoWAM project aims to achieve high-resolution and highly-accurate digital elevation models (DEM) to monitor the evolution of tidal flats over the German North Sea. To achieve this, the dual-frequency/dual-baseline (DFDB) DEM generation approach, recently developed by DLR, was further expanded to include L-band data. Based in the experience obtained in the 2019 GeoWAM campaign with X- and S-band only, the addition of L-band for the 2020 campaign helps to deal with the decorrelation found in repeat-pass data and potentially allows the creation of new merged products. For the computation of phase and coherence, a new local fringe frequency approach was included to improve the final generated DEMs.

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

Permanent Scatter Interferometry (PSI) is a established technique to measure terrain deformation. In order to monitor natural disasters and to better understand their causes and impact, it is important to precisely estimate the mean velocity of the terrain deformation, eventually making use of higher order deformation models. The estimation is usually performed using Grid-Search (GS) to find the location of the maximum of the periodogram, operation which may be very time demanding depending on the densities of the Persistent Scatter (PS) grid and of the stack. In this paper we propose the use of the Dividing Rectangles (DIRECT) optimization algorithm as an alternative to GS. DIRECT offers a good balance between local and global search, largely reducing the number of evaluations of the optimization functional. The approach was tested with different datasets and the mean velocity could be determined more precisely and faster than GS (speedup up to 20 was obtained).

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

The NASA-ISRO SAR (NISAR) mission will map the Earth's surface every 12 days using a reflector-feed system with scan-on-receive (SweepSAR) capability. An alternative for continuous swath coverage is to employ staggered SAR operation, i.e., to vary the pulse repetition intervals over time. In this way, the blocked (missing) samples are spread in range and azimuth and can be interpolated prior to focusing. However, the relatively low azimuth oversampling intended for NISAR in combination with the strong non-uniformity of the staggered data grid forbids the use of conventional interpolation kernels. This paper discusses a processing approach for the NISAR staggered data and validates the strategy with simulated data. Moreover, the impact of the staggered operation for contrast-based and interferometric applications is discussed with examples.

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.

This contribution will present the investigations carried out with stacked time-series of TOPS acquisitions to retrieve azimuth displacements. The exploitation of the burst overlapping areas allows to retrieve highly accurate ground displacement estimates in that direction, mitigating the limited performance of classical correlation techniques, due to the low resolution of the mode. The focus of this contribution is the application to time-series of Sentinel-1 data, where slow azimuthal motion is expected. Results with Sentinel-1 data on the proposed methodology are provided for the region of Balochistan, in Pakistan.

In this paper data acquired at different frequencies, namely X- and L-band are exploited to perform DInSAR analysis. The objective of this work is to present strategies to apply DInSAR by evaluating the required processing aspects to jointly estimate the deformation for multi-frequency time series. The X-band data are acquired with the TerraSAR-X sensor, while the L-band ones are ALOS/ALOS-2 data. The present work is the result of the ongoing “inter agency arrangement for strategic partnership” between the German Aerospace Center (DLR) and the Japanese Aerospace Exploration Agency (JAXA) concerning Multi-temporal InSAR analysis.

Augmenting traditional spaceborne SAR sensors with additional receive-only satellites in close formation enhances the observation space, allowing for single-pass interferomtry in along-and/or across-track with flexible baselines and the potential to build tomographic stacks with reduced temporal decorrelation properties. The feasibility of such add-ons is presently investigated by ESA and other national space agencies and for a variaty of master satellites operating from X-to L-band. This paper presents a simulation framework for complete scenes dedicated specifically to the analysis of the additional technical requirements imposed by the bistatic SAR imaging geometry with relatively large along-track separation of illuminating master/chief and the receive-only slaves/deputies.

Nonlocal filters show outstanding performance in the field of interferometric phase restoration by providing strong filtering power together with high spatial features preservation. In this work we focus on the generation of Digital Elevation Models (DEM) from a pair of interferometric SAR images. In the specific, we aim at comparing the performance of state-of-the-art InSAR filtering approaches on the basis of their noise suppression and detail preservation capabilities. We exploit a dataset of TanDEM-X SAR data relative to the volcanic area of the Kamchatka region (Russia).

In this paper, a dual-frequency and dual-baseline (DFDB) processing framework for the extraction of high-precision terrain information from airborne interferometric synthetic aperture radar (SAR) data is presented. Specifically, we propose the use of two single-pass data sets acquired simultaneously in two different frequency bands and two large-baseline repeat-pass data sets also acquired simultaneously in two frequency bands. The configuration profits from the stability of the single-pass derived elevation maps in relation to spatially correlated artifacts as well as from the increased sensitivity associated with large-baseline acquisitions. Moreover, the dual-frequency nature of the data set enables the tackling of the phase unwrapping issue, promoting the retrieval of unambiguous measurements. Several algorithms for the interferometric processing of the DFDB airborne data set are proposed, including the outline of multichannel phase calibration and unwrapping error correction strategies and approaches to remove spatially correlated artifacts and extract the common underlying topography. Elevation models generated from a DFDB data set acquired with the airborne F-SAR sensor over tidal flats in northern Germany are presented, and comparisons with an airborne laser scanner reference show errors with a standard deviation of around 14 cm and a mean absolute deviation of less than 10 cm.

The contribution focuses on the current status of the ESA study entitled “InSARAP Sentinel-1 Constellation Study”, which investigates the interferometric performance of the S1A/S1B units. General aspects like the interferometric compatibility in terms of common range and Doppler bandwidth and the burst synchronization are addressed. Besides the first interferometric results with both units, time series results over the pilot sites combining both satellites are also shown, as well as some investigations with fast moving (i.e., glaciers) scenarios.

The TanDEM-X mission aims the generation of an accurate global Digital Elevation Model (DEM). On the other hand, due to their flexibility regarding baseline configuration, frequency of acquisition, time and revisit time, airborne SAR interferometers remain viable for the generation of DEMs with very high vertical and horizontal accuracies at a local scale. In this paper, we present an experiment demonstrating the beneficial use of a TanDEM-X intermediate DEM to support the generation of an elevation model from airborne SAR with relative vertical accuracy in the order of centimeters.

A number of synthetic aperture radar (SAR) systems might work in interrupted operation for different purposes. Examples are cooperative bistatic SAR systems with a synchronization link between the transmitter and receiver or multistatic systems operating in receive-only mode, among others. As a direct consequence, the acquired raw data contain missing echoes presented in a periodical or random pattern. Since the missing raw data introduce artifacts in the processed images, recovery methods have to be applied. Usually, spectral-estimation-based interpolators can be used to recover data. Although such algorithms show good performance for pointlike targets, their efficiency is decreased for distributed scatterers. In this paper, we propose, for a coherent pair of SAR images, the use of the common information in one image to reconstruct the other and vice versa. The conditions required for the proper use of the approach are discussed, and the method is verified using simulated data. One special case of study is the TanDEM-X mission, where the cooperative nature of the bistatic operation requires the periodic exchange of information between the satellites in order to gather information for calibration and synchronization, creating a periodic missing data pattern in the raw data. For this case of study, the reconstruction methods based on spectral estimation are analyzed, and the proposed reconstruction using cross-information is validated.

The use of repeat-pass-large-baseline SAR interferometry to generate highly accurate digital elevation models (DEMs) has not been as exploit as the use of single-pass interferometry. This is mainly due to issues associated with the small height of ambiguities (HoAs) necessary to deliver the required height accuracy, and with the temporal and geometrical decorrelation typical of repeat-pass systems. Although airborne systems are very attractive for the flexible baseline choice in repeat-pass mode, when using airborne datasets, the processing of the DEMs should account for uncompensated residual motion errors and other phase artefacts. In this paper we describe a methodology for generating accurate DEMs combining single- and repeat-pass data from dual frequency (X- and S-band) F-SAR acquisitions.

This paper presents the first fully polarimetric high-resolution circular synthetic aperture radar (CSAR) images at L-band (1.3 GHz). The circular data were acquired in 2008 by the Experimental SAR (E-SAR) airborne system of the German Aerospace Center (DLR) over the airport of Kaufbeuren, Germany. The obtained images resulting from the coherent integration of the whole circular flight are investigated and discussed in terms of two of the main CSAR properties, namely, the theoretical subwavelength resolution in the horizontal plane (x, y) and the 3-D imaging capabilities. The 3-D imaging capabilities are of special interest due to the penetration of L-band in vegetated areas. These results were compared with images processed by the incoherent addition of the full synthetic aperture. The coherent approach showed a better performance since scatterers are focused at their maximum resolution. Due to the nonlinearity of the tracks and the high-computational burden, an efficient fast factorized back-projection (FFBP) has been developed. Unlike frequencydomain processors, it accommodates azimuthal variances and topography changes. Limits and considerations of the proposed algorithm are described and discussed. To further accelerate this process, the FFBP was also implemented in a graphics processing unit (GPU). Processing performance has been assessed with the direct BP (DBP) as a reference, obtaining speedup factors up to 1800. Residual motion errors have been estimated with a new frequency-based autofocus approach for CSAR configurations based on low signal-to-clutter ratio (SCR) isotropic scatterers. High-resolution images of man-made and distributed scatterers have been analyzed and compared with a stripmap SAR, both concerning anisotropic and isotropic-like scatterers. Results include a single-channel tomogram of a Luneburg lens and a fully polarimetric tomogram of a tree.

Missing echoes in the raw data introduce artifacts that might be noticeable in processed SAR images. As a consequence, recovery methods must be applied to ensure the high quality of the images. Although spectral estimation based interpolators are suitable for the recovery of point-like targets, their efficiency is poor for distributed ones. In this paper we propose a new reconstruction method for interferometric systems, which uses the common information in one image of a coherent pair to recover the other and vice versa. Since a typical example of missing data happens when acquiring with the bistatic mode of TanDEM-X, the proposed method is verified with data from a bistatic TanDEM-X survey in addition to simulated data. The results show that the proposed reconstruction can significantly improve the accuracy of the resulting interferograms.

This paper analyses the use of an algorithm to estimate the unknown interferometric phase offsets without the use of ground control points or already calibrated data. Modifications in order to enhance the precision and robustness of the algorithm are presented and the new implementation is validated with X-band airborne SAR data. Furthermore, issues related with the use of the algorithm for P-band datasets are discussed and experimental results are shown for this kind of data. The stability of the algorithm according to the choice of the area of the image used for the estimation and to the choice of the coherence threshold is also verified for both X- and P-band data sets.

The cooperative nature of the TanDEM-X mission bistatic operation allows the periodically exchange of information between transmitter and receiver over the SAR acquisition. With the information gathered during the synchronization (sync) events it is possible to perform the phase and time referencing necessary for the calibration of the bistatic SAR image. However, given that transmitter and receiver are dedicated to the link during the sync events, the reception of SAR signal is periodically interrupted leading to gaps in the SAR raw data. Consequently, the processed SAR image will be corrupted, especially when acquiring close to the Nyquist rate. The paper addresses different approaches based on parametric and non-parametric spectral estimation to reconstruct the missing data and presents results with SAR data from TerraSAR-X and TanDEM-X.

The F-SAR airborne SAR instrument represents the successor of the E-SAR system of the German Aerospace Center (DLR), which had previously been extensively used over the last three decades. Its development was triggered by the current demand for simultaneous acquisitions at different wavelengths and polarisations as well as by the demand for very high resolution in the order of decimetres. F-SAR is a modular development utilising state of the art hardware. As for E-SAR, DLRs Dornier DO228-212 aircraft is the first choice as platform for the new system. With the recently completed X-band single-pass interferometer and the P-band subsystem, F-SAR is now ready for fully replacing the E-SAR. This paper presents the two new subsystems and analyses their performance based on fully polarimetric imagery acquired during recent system test and calibration campaigns.

Launched in June 2010, TanDEM-X is an interferometric mission with the main goal of providing a high-resolution, global and unprecedentedly accurate digital elevation model (DEM) of the Earth by means of single-pass X-band 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 was operated in pursuit monostatic mode. During that time, some pioneering bistatic SAR experiments with both satellites commanded in non-nominal modes were conducted with the main purpose of testing the performance of both space and ground segments in very demanding scenarios. In particular, this article includes results of the first bistatic acquisition and the first single-pass interferometric (mono/bistatic) acquisition with TanDEM-X, addressing their innovative aspects and focussing on the analysis of the experimental results. Even in the absence of essential synchronisation and calibration information, bistatic images and interferogramswith similar quality to pursuit monostatic have been obtained. Some months later, with TanDEM-X already in its operational DEM-acquisition phase a further challenging bistatic acquisition carried out in cooperation with DLR's airborne radar F-SAR has been carried out. The objective was to acquire fully polarimetric interferometric data with a high range of available bistatic angles. A dedicated commanding of both TanDEM-X and F-SAR was required, including irregular sampling schemes, partially missing bistatic echo reception and bistatic synchronisation, which pose a number of technological challenges in SAR data processing before the calibrated bistatic SAR images are obtained. This article reports about these two set of experiments.

This paper analyzes the multi-path component effect in the airborne SAR context, proposing a model as an approach to correct the generated disturbances in the processed phase and amplitude images. The method assumes along-track interferometric or polarimetric data to estimate the unknowns of the model. Airborne data acquired by the F-SAR system of DLR are used to evaluate the performance of the proposed approach.

The study of data acquired over a circular trajectory has raised an increasing interest in the SAR community. Two main reasons summarize the interest in such geometry. First, sub-wavelength resolution can be achieved, as the targets in the spotted area are observed under a 360oaperture. Second, the use of the information from different azimuthal directions allows one to obtain information of the scene in the third dimension, making possible a 3D target reconstruction. In any case, both applications require certain target reflectivity homogeneity. This paper shows several processing results and analyzes the potentials and limitations of circular SAR to perform tomography of semi-transparent media. Special processing aspects, like the estimation of residual motion errors due to inaccuracies in the navigation data, are also addressed. Data acquired at L-band by DLR's E-SAR system are used to demonstrate the high resolution and tomographic imaging capabilities of circular SAR. The results include the tomogram of a Luneburg lens, as well as preliminary results over man-made targets and vegetation.