Distributed SAR imaging from space exploits the distribution of the key system resources, normally concentrated in a single, large and complex satellite, among many small-sized and simpler sensors, thanks to the proper combination of the signals from each single node of the swarm. The simultaneous operation of the satellites represents a Multiple-Input-Multiple-Output SAR system (MIMO-SAR), for which several concepts have been presented in the scientific literature, [1]–[6].Indeed, the theoretical maturity of this technique is significant, however the practical feasibility has not yet been demonstrated. In 2020, the demonstrative Mission SATURN [7] was proposed and is currently being studied with the support of the Italian Space Agency (ASI) being part of ALCOR programme promoting the development of the next generation Italian CubeSats. The main target of the SATURN mission is to demonstrate the key technology "Cooperative MIMO Swarms of SAR MicroSats" for innovative, low cost and versatile Earth Observation applications.In this work, we present the architecture and the implementation of a miniaturized SAR payload, called MiniSAR, suitable for operation in a MIMO SAR system. The paper reports the design solutions, the architecture and the qualification of the SAR Electronics by means of functional, performance and environmental testing in thermal-vacuum chambers and vibration facilities.

Along-track constellations of SAR satellites are a promising technology capable of imaging large areas with fine resolution. The enhanced performance is achieved by reducing the sampling frequency of each spacecraft, and the caused aliasing is later solved in processing by combining data from several sensors. This work presents a flexible and robust method to combine the channels based on back-projection. A secondary step further suppresses residual azimuth ambiguities to obtain the proper high-resolution image. The method allows for the incorporation of additional calibration steps before the final signal reconstruction. Testing and validating were accomplished with simulated data using point targets and a realistic scenario.

This work investigates a new concept to finely resolve the vertical structure of natural media, like snow, ice, vegetation, by using a formation of spaceborne Synthetic Aperture Radars (SAR) mounted onboard different satellites. The formation is assumed to operate in Multiple Input Multiple Output (MIMO) mode by implementing a Frequency Division Multiplexing (FDM) access scheme, where all satellites transmit simultaneously on different frequency bands and receive the echoes scattered by the Earth’s surface in all transmitted bands. In so-doing, a formation onNsatellites is used to produceN2SAR images. By the principle of Diffraction Tomography, each of these images represents a distinct set of wavenumbers, i.e. a distinct region of the spatial spectrum of the observed scene. The vertical separation between any two sets of wavenumbers defines the interferometric differential wavenumber, which determines the sensitivity of that particular pair to the vertial structure of the observed scene. Fine vertical resolution is achieved by developing a novel approach to set the satellite positions in such a way that the resulting interferometric differential wavenumbers form an almost uniformly-spaced array of maximum length under the constraint of a given height of ambiguity and interferometric coherence magnitude. As a result, we show two examples where formations of 4 or 5 satellites are deployed to provide the equivalent of 17 and 26 monostatic acquisitions, respectively. Such figures are comparable to the best airborne and ground-based systems available as of today, and indicate the concrete possibility to image the vertical structure of natural targets from space at fine resolution. The concept here developed to deploy the formation is referred to as Minimum Redundancy Wavenumber Illumination (MRWI), as it is shown to be a generalization to distributed targets of the principle of Minimum Redundancy Virtual Array (MRVA) used in array theory. The analysis is supported by results from synthetic data generated by numerical simulations.

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 distribution of the space key resources, usually concentrated in a single, large, and complex satellite, can be shared among small-sized and simpler systems, thanks to the proper combination of the signals from each single node of the swarm. A proper sensor positioning, and on-ground data recombination allow the azimuth ambiguities to be cancelled out, guaranteeing gain in terms of signal to noise ratio. However, sensors synchronization, position control and trajectory knowledge errors are significant impairments which may make ineffective the recombination algorithms since the error contributions are hard to estimate.In this paper, a Deep Learning based approach is proposed for performing the recombination of data coming from a swarm of satellites and where no a priori knowledge error is exploited. The proposed system provides promising results in terms of ambiguity rejection.

This paper investigates the potential for accurate retrieval of Snow Water Equivalent (SWE) using an across-track formation of Synthetic Aperture Radar (SAR) satellites.

We discuss a coherent synthetic aperture radar (SAR) formation where$N$identical sensors transmit at the same time, code, and frequency. This is a particular multiple-input–multiple-output (MIMO) configuration, where the transmitted waveforms interfere together, resulting in an illumination pattern that randomly changes in space and time. Similar to the single-input–multiple-output (SIMO) formations, the diversity provided by the$N$receiver phase centers can be used to mitigate this interference and reduce the pulse repetition frequency (PRF) for achieving large swath coverage. The good point, in the MIMO case, is that the signal-to-noise ratio (SNR) gain of the system increases, theoretically, with the square of the number of elements. However, residual spurious sidelobes may appear as ghosts of the multiple illuminators. In practice, the power gain is to be optimized, together with ambiguity rejection, sidelobes, and azimuth resolution. The actual performances achievable by these formations in terms of impulse response function (IRF), SNR, and sensitivity to the precise positioning of the sensors are discussed theoretically and based on simulations.

The paper provides the main results of the SAOCOM-1B independent commissioning phase carried out by Aresys in the framework of a collaboration with Argentinian space agency CONAE. The reported results include: azimuth and elevation pointing, IRF assessment, geolocation accuracy and radiometric and polarimetric calibration.

Radio Frequency Interference (RFI) is a well-known issue since the early days of telecommunications. Traditionally considered an occasional event in the Earth Observation domain, RFI is becoming a severe problem due to the surge of wireless LANs and the increasing number of operational EO missions. In the near future, RFI will eventually be treated as an additional source of noise, to be addressed not only for mitigation, but also during mission design. Sentinel-1 (S-1) is a constellation of two spaceborne C-band synthetic aperture radar (SAR) sensors developed and operated by the European Space Agency under the Copernicus programme. In this paper, S-1 thermal noise measurements are used to create an automatic monitoring system generating world RFI maps with a periodicity of 12 days. For tuning the system, the RFI power is calibrated against a radiometer operating at the same range of frequencies. Analysing the strength and geographical distribution of the RFI, the most affected regions are identified and probability maps are extracted.

A coherent formation of$N$identical SAR satellites is considered in this paper. These sensors transmit at the same time, code, and frequency with no synchronization whichever, but for the timing of the arrivals, creating a Multiple Input Multiple Output (MIMO) configuration. The contemporaneous transmission of N signals is expected to result in a reciprocal interference that leads to an illumination pattern that changes in space and time. In spite of this, the increased number of phase centres (w.r.t. single satellite and SIMO cases) can be used to mitigate the interference and to raise the SNR of the equivalent system. The principles of a recombination algorithm are summarized. In order to evaluate the effectiveness of the MIMO reconstruction and to fully characterize the IRF of a MIMO system from a quantitative point of view, new performance figures of merit are defined.

Recent availability of low-cost small satellites and innovation of constellations have resulted in an increasing number of commercial companies having established information services fed by their own satellite systems. ESA aims to foster cooperation and collaboration with not only other national agencies, but also these New Space players, inviting some missions as potential candidates for ESA's long standing Earthnet Programme. The Earthnet Data Assessment Pilot (EDAP), headed by Telespazio UK, provides a consortium of expertise to assess data quality and suitability of individual missions for this purpose. The service framework has developed a common quality assessment methodology adapted for instrument-specific domains covering Optical, Synthetic Aperture Radar (SAR) and Atmospheric missions, with emphasis on multi-mission studies. This paper describes the EDAP framework and provides a summary of specific tasks performed by the service to assist the ESA Earthnet programme in supporting various commercial mission providers in preparation for future EO missions.

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.

Scheduled for launch in 2023, ESA's seventh Earth Explorer Mission, BIOMASS, will carry the first P-band synthetic aperture radar (SAR) to be flown in space, to gather fully polarimetric acquisitions over forested areas worldwide in interferometric and tomographic modes. This paper presents the algorithms developed to estimate biophysical parameters from BIOMASS measurements and their implementation in the BIOMASS level 2 (L2) prototype processor. The L2 processor will generate global maps of forest Above Ground Biomass (AGB), Forest Height (FH), Forest disturbance (FD). Accurate generation of these products requires the L2 processor to be closely inter-linked with the BIOMASS interferometric processor, in order to produce phase-calibrated interferometric stacks, retrieve sub-canopy terrain topography, and generate a 3D representation of forest structure by use of SAR tomography. AGB estimation results are here shown using BIOMASS-like acquisitions derived from campaign data acquired over six tropical forests in South America and Equatorial Africa.

This paper describes the rationale and development of the estimation techniques for the level-2 data products of the European Space Agency's 7th Earth Explorer BIOMASS mission. BIOMASS is planned for launch in 2023 and will carry the first-ever P-band synthetic aperture radar (SAR) onboard a satellite. It has been designed to produce consistent global maps of the Earth's forests during a nominal five-year lifetime. Fully polarimetric SAR data will be collected and the satellite orbit will be selected for repeat-pass interferometry and tomography in separated mission phases. Mission requirements call for three level-2 data products: above-ground biomass, forest height and forest disturbance. The paper also discusses the expected limitations of the estimation techniques and remaining problems to be addressed.

The ESA BIOMASS mission will be the 7th Earth Explorer measuring the above-ground biomass (AGB) in the world's forests. The current ESA Level-2 (L2) implementation study focuses on defining and implementing the main algorithms for forest parameter retrieval from BIOMASS data. After the first year of L2 study innovative results were achieved: the development of ground cancellation, in particular, has proved to be huge value, since it removes from the data the effects of environmental variability and contributions unrelated to the forest carried in the ground scattering. In this paper the current processor implementation and validation activities of the L2 team will be described.

In this work we investigate the role of volume scattering obtained from tomography and ground/volume decomposition in retrieving AGB (Above Ground Biomass). Results here presented originate from the BIOMASS L2 study, aimed at defining and implementing the tomographic and interferometric processors of the BIOMASS mission. In particular we aim at discussing whether, and to what extent, ground/volume decomposition can provide a valid alternative to tomography. To do this, both are tested based on the P-Band data collected at the forest site of Paracou, French Guiana, during the TropiSAR campaign, and validated against in-situ AGB measurements in terms of correlation and sensitivity of the retrievals. Quite surprisingly, results indicate that volumebackscattered power as obtained by ground/volume decomposition is almost unsensitive to AGB, notwithstanding different solutions for volume scattering are tested, and lead to conclusion that forest structure actually plays a non-negligible role in AGB retrieval in tropical areas.

The ESA BIOMASS mission will be the first spaceborne mission specifically dedicated to the study of forests. In this framework, ESA is running the “Level-2 implementation study”, focused on defining and implementing the main algorithms for forest parameters retrieval from BIOMASS data. During the first year, the science group involved in the study has carried out R&D activities in order to identify the most viable solutions. In this paper, an overview of the concepts and techniques behind the development of the BIOMASS L2-prototype processor are described, with a focus on each of the main mission products: biomass, forest height and forest disturbance map.

The paper describes an end-to-end simulator for Geosynchronous SAR data. The tool is composed of two modules: a raw data time domain simulator and a processor for the generation of the L1 products. The simulated raw data include the effects of atmospheric and clutter decorrelation. The end-to-end simulator is a powerful and flexible tool to be used during the system design phase for the verification of the expected performance.

This paper presents a Synthetic Aperture Radar (SAR) simulator designed to assess the performance of bi-static SAR missions. It is a modular software able to simulate realistic SAR data (including injection of the system non-idealities), to estimate the SAR performance at radiometric and interferometric level. In order to ensure accurate measures of the bi-static performance, it simulates raw data exploiting a time-domain approach, without assumptions on the acquisition geometry. This feature makes the simulator a useful tool for the design of new SAR mission concepts. We also report sample results considering the SAOCOM-CS mission case as an example of application of the simulator.

The BIOMASS mission gives completely new challenges in external calibration arising from the orbital pattern needed for the tomographic and Pol-InSAR phases of the mission, the strong effects of the ionosphere at P-band, and the lack of pre-existing P-band data except over very limited parts of the globe. Together these create problems that can only be solved by combining infrequent visits to instrumented calibration sites with systematic exploitation of the properties of distributed targets and targets of opportunity. Proposed approaches to performing radiometric and polarimetric calibration are described, together with meeting geolocation accuracy requirements.

The Sentinel-1 constellation provides high resolution radiometrically calibrated SAR images with 6 days revisit and global coverage. The radiometric calibration was established during the Commissioning Phase exploiting transponders and natural targets. The paper describes the elevation antenna pattern verification based on the S-1 Antenna Model and the necessary refinements performed to achieve the requested radiometric accuracy.

Delay/Doppler processing for radar altimeters allows to improve the along-track resolution, but it can be affected by Doppler ambiguities depending on the design of the instrument. If not accounted for, Doppler ambiguities can cause a distortion of the multilooked waveforms and a loss of accuracy in retrieved geophysical parameters. This paper presents a waveform model based approach to define a Doppler ambiguity mask to filter out the samples affected by the ambiguous returns before multilooking, assuming a uniform rough scattering surface. The effectiveness of the proposed method for Doppler ambiguities masking has been verified on Sentinel-6 simulated data and on in-orbit data from CryoSat SARin acquisitions over ocean.

In this paper, the full focused processing in the frequency domain for high Pulse Repetition Frequency (PRF) radar altimeter science data is proposed. Considering a radar altimeter as a Synthetic Aperture Radar (SAR) instrument operating in near nadir-looking geometry, a SAR frequency-domain focusing algorithm, namely Range-Doppler algorithm, is analyzed. Then a properly modified processor for radar altimeters is described and implemented. Simulation set-up using parameters from two missions (CryoSat and Sentinel-6) and a preliminary result from in-orbit CryoSat data confirm the effectiveness of the full focused processing in the frequency domain. The proposed approach, compared to the one formerly proposed in literature [1], is less complex and more computationally efficient.

In this paper, the synthetic aperture radar (SAR) calibration for low-frequency missions by means of stable point targets is presented. Calibration at low frequency involves the absolute radiometric calibration, the antenna pattern and pointing characterization and validation, and the distortion system parameters' estimation. The use of traditional instrumentation, such as a polarimetric active radar calibrator, a corner reflector, or an active transponder, may be costly and can reduce the time the instrument is used for operational acquisitions. The purpose of this paper is to evaluate the potentiality in calibration of point targets for which the radar cross section and the time stability have been characterized. Given a calibration site, once that a set of the stable point targets have been detected by the analysis of an interferometric stack of SAR acquisitions, they may be used as passive calibrators for the validation of radiometry, elevation antenna pattern, and pointing estimation. We show that, although less targets are expected to be found in P- or L-band than in C- or X-band, a sufficient amount (about 250 targets per acquisition) can provide an accuracy in antenna pattern estimation of about 0.04 dB, if the target accuracy is 0.1 dB at 1σ.

Synthetic aperture radar (SAR) altimeters reduce the along-track footprint size exploiting the coherence of the transmitted pulses and achieve at the same time a noise reduction. Consequently, a large effort has been aimed at the formulation of theoretical models that apply to SAR altimeters, in order to fully exploit the improvement in spatial resolution obtained from the along-track aperture synthesis. This letter presents a novel semianalytical waveform model for SAR interferometric altimeters that preserves high accuracy even in the presence of mispointing. Starting from the waveform model proposed by Wingham et al. that provides a unified formulation for pulse-limited and SAR interferometric altimeters which can only be computed numerically, here, we describe a semianalytical approximation for small variations of the mispointing angles around an arbitrary combination of pitch and roll angles (μ̈, θ̈). The proposed semianalytical waveform model allows to reduce the high dimensionality of the model proposed by Wingham et al. and it has been proven to be accurate for variations of mispointing angles up to 0.4 deg around the (μ̈, θ̈). The performance of the proposed formulation has been evaluated on simulated data from Sentinel-6 configuration and on real data from CryoSat-2 SARin acquisitions over ocean.

The availability of accurate trajectory information is paramount for the processing and exploitation of synthetic aperture radar (SAR) data. Considering the particular case of spaceborne SARs designed for repeat-pass interferometric applications, errors in the trajectory translate into phase artifacts that affect the interferometric performance. In this paper, we propose a model-based procedure to calibrate the trajectories of spaceborne SAR systems by the multisquint (MS) phase. The technique allows to estimate the along and the derivative of across track geometric errors. The geometric model of the InSAR phase is derived as a function of positioning errors and the MS phase model as derivative of the InSAR phase geometric model, with respect to the squint angle. We perform a sensitivity analysis of the model in order to define which geometric errors can be estimated by the MS phase, justifying the assumption that the MS phase is very poorly affected by the atmospheric phase screen. We particularly concentrate on the TOPSAR acquisition mode, where the phase is very sensitive to geometric errors. We start from the classical two-image case and then consider the extension to the multibaseline case. Experimental results obtained by processing of interferometric pairs acquired by the Sentinel-1A sensor are reported.

The paper introduces a couple of techniques for the enhanced processing of TOPSAR data. Both techniques are based on repeated passes of the sensor over the same area. The first technique combines two “de-synchronized” acquisitions, useless for interferometric purposes, to obtain an “enhanced resolution” image of the coherent scatterers in the scene. The second technique exploits the Permanent Scatterers imaged in the overlap between sub-swaths to perform a relative cross-calibration of the TOPSAR beams. Results on real TOPSAR Sentinel-1A data are shown.

This paper concerns the on-board data volume reduction for a single-platform multichannel Synthetic Aperture Radar (SAR) system. In such a system, the unambiguous Doppler spectrum reconstructed on-ground is enlarged at the cost of an increased amount of acquired data. Since the signals received by a multichannel SAR instrument are intrinsically correlated, such correlation can be exploited to represent the signals in a smaller set of components, introducing a controlled distortion. This way, a reduced amount of data can be quantized and transmitted on-ground. Data simulations have been performed to evaluate the performance of the proposed approach. By analysis of the simulation results, efficient reduction of data volume is achieved, even keeping low enough the additional distortion introduced in the reconstructed data.

The paper provides the description of a flexible frequency domain background clutter simulator for GMTI applications. The clutter module is able to simulate different acquisition geometries, including very high squint cases, and accounts for scene temporal decorrelation. The clutter simulator operates in frequency domain allowing good computational performance. Details on the simulation algorithm and an overview of the simulator results are provided.

The paper proposes two different architectural solutions for a quasi-geostationary monostatic SAR, hosted on-board of a COMSAT satellite, aiming at monitoring water-vapor, for Numerical Weather Forecast on a sub-continental scale, and hazards like hydrogeological landslides and deformations on a local scale. The two architectures address a system that combines an L-band, wide coverage beam, for coarse resolution imaging, and an X-band, spot beam, for fine resolution imaging. The challenge is to allow the X-band spot to be positioned everywhere within the L-band coverage, that should be hold fixed, but exploiting the same wide reflector.

This paper concerns the performance of the Flexible Dynamic Block Adaptive Quantizer (FDBAQ), the on-board data compression scheme used by Sentinel-1A (S-1), the C-band Synthetic Aperture Radar (SAR) system for Earth Observation launched on 3rd April 2014. FDBAQ has been designed to deal with the high instrument data rate due to the high spatial resolution and large swath width. S-1 SAR is able to acquire long data take, corresponding to large volumes of data stored on-board. Since FD-BAQ has been implemented for the first time on S-1, the capabilities of such a compression scheme have not been tested on real data before. The purpose of this paper is to present the results of the analysis of real acquisitions acquired after the end of the Commissioning Phase, so in its full efficiency. FDBAQ is compared with the traditional Block Adaptive Quantizer (BAQ) compression scheme in terms of quantization noise level, average bit rate and signal to noise ratio.

In this paper, we propose a model-based procedure to estimate the accuracy of Sentinel-1 orbit products by the Multi-Squint (MS) phase. The technique exploits the results of single baseline MS analyses collected for each possible master and slave combination in a stack to estimate the absolute orbit error. Accordingly, as first step we state the geometric model of the InSAR phase and the MS phase model as derivative of the In-SAR phase geometric model with respect to the squint angle, then we describe the algorithm to estimate two components of baseline error using the theoretical model. In this paper we focus on the TOPSAR acquisition modes of Sentinel-1 assuming at the most a linear error in the known slave trajectory. In particular, we describe a dedicated methodology to measure baselines accuracy using bursts and swaths overlaps in data acquired by IW and EW acquisition modes. Finally, we suggest a technique to estimate, by a weigthed least-squase inversion, the absolute orbit error of each image in a stack. Experimental results of single and multi-baseline MS analysis obtained on Sentinel-1 data will be displayed.

Ka-band may lead to important applications for the next generation of SAR spaceborne sensors (e.g.: DEM, disaster management, GMTI, subsidence, ocean currents, vegetation height). Nevertheless, the lack of trusted references on backscatter at Ka-band revealed to be the main limitation for the investigation of future applications. The paper, supported by the ESA project “Ka-band SAR backscatter analysis in support of future applications”, is aimed at studying the wave interaction at Ka-band for a widely varying range of targets in order to define a set of well calibrated and reliable KA-band backscatter coefficients. Here, we propose several examples of backscatter data resulting from a critical survey of available datasets at Ka-band. The reliability of the results will be assessed via a preliminary comparison with Electromagnetic Models (EM).

This paper addresses the results of the instrument and product performance verification, radiometric and geometric calibration achieved during the since commissioning and routine phase.

The paper provides a theoretical model for the Geosynchronous SAR focused data, including clutter and atmosphere decorrelation effects. The focused data model is exploited to generate realistic GEOSAR demonstrative products, on the basis of the preliminary design of a regional GEOSAR system. The provided demo products are useful for a preliminary evaluation of the expected system performances.

BIOMASS mission [1] was selected as the 7th Earth Explorer Mission within the frame of the ESA Earth Observation Envelope Programme. Being the BIOMASS instrument a P-band Synthetic Aperture Radar (SAR) and being that frequency bandwidth already in use by different services, the BIOMASS acquisitions are expected to be occasionally affected by Radio Frequency Interference (RFI). In the framework of a comprehensive strategy for the BIOMASS mission to cope with the interference signals, in addition to design choices at both mission and instrument level, in this paper it is presented a study aimed at verifying the feasibility of an on-ground processing approach to mitigate the quality degradation on BIOMASS Level1 data due to RFI.

This paper concerns the performance of the flexible dynamic block adaptive quantizer (FDBAQ), which is the onboard data compression scheme used by Sentinel-1 (S-1), i.e., the C-band synthetic aperture radar (SAR) constellation whose first satellite (Sentinel-1A) has been launched on April 3, 2014. The data rate that results from the fine resolution and the wide swath of S-1 would exceed, without data compression, the S-1 hardware limitations. The FDBAQ has been proposed as an efficient method to reduce the instrument data rate and limit the onboard storage requirement. The capabilities of such a compression scheme have not been tested on real data before. Preflight simulation and analysis were based on a simplified processing scheme and radar backscattering information taken from past missions. The purpose of this paper is to present the first results of the analysis of real acquisitions, comprising thousands of products, in every S-1 acquisition mode, acquired during and after the commissioning phase. Advantages and limits are presented in detail, comparing FDBAQ and the traditional block adaptive quantizer (BAQ) compression scheme, in terms of quantization noise level, signal-to-noise ratio, and average bit and data rates.

In this paper, a numerical method optimizer based on covariance matching is proposed for synthetic aperture radar (SAR) polarimetric calibration. The method makes use of the information provided by a distributed target and a corner reflector in order to jointly estimate the system polarimetric distortion parameters and the Faraday rotation. A preliminary analysis is conducted to show the expected accuracy values and to identify the intrinsic ambiguities of the problem. Results from simulations are shown to assess the accuracy and convergence of the method. Finally, tests have been conducted on stack of repeated full polarimetric ALOS PALSAR images to check the stability of the retrieved distortion parameters in a realistic case.

This paper focuses on the generation of very high resolution digital elevation models (DEMs) mainly from single-baseline spaceborne synthetic aperture radar (SAR) systems. The stringent requirements on vertical accuracy demand for frequent and accurate measures of the baselines, subject to mechanical vibrations of the boom that may be severe when long booms are exploited. This paper proposes a solution that exploits an existent coarse-resolution DEM, for example, the Shuttle Radar Topography Mission, as a set of ground control points (GCPs). A computational model and an inversion scheme are introduced to jointly take into account the information coming from the grid of GCP and the local shifts measured during image coregistration. The model properly takes account for the quality of the GCP, its location with respect to the target of interest, and the interferogram noise in correspondence to the GCP. A novel “split-swath” approach is proposed to enhance the quality of the baseline estimates. The mode exploits two narrow strips with significant diversity in incidence angle, achieved, for example, by ScanSAR or TOPSAR mode. An example of design is provided by assuming a Ka-band topographic spaceborne SAR mission.

In this work, a numerical method optimizer for SAR polarimetric calibration is proposed. The method makes use of the information provided by a Distributed Target and a Corner Reflector in order to jointly estimate the system polarimetric distortion parameters and the Faraday Rotation. The different order of magnitude of cross-talks and Faraday Rotation is exploited to overcome the intrinsic ambiguity related to the problem. Tests have been conducted both on simulated data, confirming the ability of the proposed method to provide realistic estimation of the distortion parameters, even in case of low quality Corner Reflector.

In this paper, we revise the relative radiometric calibration of synthetic-aperture-radar stacks which exploits natural persistent scatterers (PSs). We introduce a new model to estimate a slight error in the sensor pointing in elevation and a new coherent method that makes use of phases evaluated by averaging the complex data on the local window. We show that the proposed approach outperforms the conventional one, as it is mostly insensitive to the “differential” biasing that affects the noncoherent permanent scatterer calibration (PScal). Results from processing COnstellation of small Satellites for the Mediterranean basin Observation (COSMO)-SkyMed and European Remote Sensing (ERS) satellites stacks are presented.

The paper discusses methods for radiometric calibration of repeat-pass interferometric SAR stacks based on Persistent Scatterers. Coherent and non-coherent approaches for the estimation of the radiometric gain are compared. Furthermore, an estimator of the beam pointing in elevation in proposed. Evaluation of performances is carried out basing on both simulated and COSMO-SKYMED data.

An assessment of the impacts of the SAR system performance parameters on the Level-2 products quality is here presented for CoReH2O, that is one of the candidate Earth Explorer Core Missions and whose main objective is to estimate the water stored in the snow pack. A sensitivity analysis has been conducted exploiting the end-to-end mission performance simulator that has been developed in the framework of the feasibility studies to provide a tool for the evaluation of the mission performance up to Level-2 products. Simulations have been run for a subset of the SAR system instrumental errors, such as additive noise, radiometric bias and speckle noise. The obtained results allow to verify how the accuracy of the retrieved snow parameters, e.g. the SWE, depends on the quality of the Level-1 data and, as a consequence, on the SAR system performance parameters.

A refinement technique of the antenna model considered to compute Synthetic Aperture Radar (SAR) data pattern is proposed in this work. The accurate knowledge of the antenna patterns of a SAR sensor is of main importance for precise SAR image processing. This requirement, coupled with tight needs for a short duration of the calibration process, have made necessary the investigation of innovative methods to calibrate complex SAR systems. When considering active arrays, the antenna patterns computed from homogeneous SAR data is often in disagreement with those obtained from a mathematical model (the antenna model). In this paper a correction factor is introduced within the antenna model, in order to overcome this discrepancy. This parameter is expressed as a corrected version of the matrix indicating the working status of the antenna. It is estimated through inversion by exploiting the patten information of SAR data, and it can be used for effective antenna model calibration. The effectiveness of the proposed method is assessed by considering ENVISAT ASAR rain forest-images.

In this paper a method for calculating the antenna contribution to the Doppler Centroid is presented. The Antenna Doppler Contribution (ADC) is originated by the non idealities affecting the TR modules of the active antenna arrays. It is an undesired effect for SAR applications requiring stringent performances on the Doppler Centroid control capability. The first part of this paper introduces the problem and shows how it is possible to predict the ADC by exploiting a suitable Antenna Model. The second part of the paper presents a comparison between the ADC calculated with an Antenna Model and the ADC retrieved from real ASAR data.