This work is focused on the forthcoming ROSE-L mission, developed by the European Space Agency (ESA) as part of the Copernicus Expansion Programme. Among the different advanced engineering solutions that will be exploited by the system, we concentrate on the ScanSAR acquisition mode and show how to enable ROSE-L for a high gain two-look configuration. More specifically, we propose a solution aimed at achieving different goals.The first goal is focused on better tailoring the ROSE-L system, originally designed for a one-look ScanSAR mode configuration, to a more attractive two-look one. The second goal is to retain the azimuth resolution and the range swath of the original system design. The third goal is to be compliant with the main system specifications, with respect to the one- look mode configuration.To achieve these goals, we propose to properly shape the radiated azimuth beam, without upsetting the original design of the ROSE-L radar antenna and taking advantage of the degrees of freedom offered by its current layout.

This work shows that additional ScanSAR capabilities, with respect to those achievable through the current synthetic aperture radar (SAR) system of the forthcoming Radar Observation System for Europe at L-band (ROSE-L) mission, may be easily enabled through a proper shaping of the azimuth pattern of the TX radar antenna. In particular, we show that by properly acting only on the distribution of the input excitations of the elements of the currently designed TX antenna array, we can move from a single azimuth-look ScanSAR configuration to a more attractive two-look one. This result is achieved without upsetting the current system architecture and without changing the current mission parameters [such as azimuth resolution, range swath, and pulse repetition frequency (PRF)], all tailored to a one-look ScanSAR configuration. On the other hand, the proposed two-look mode requires to double the azimuth beamwidth with respect to the one-look mode, thus leading to an unavoidable antenna gain decrease, whose amount is approximately 3 dB. The presented analysis also shows that, by still acting on the distribution of the input excitations of the TX antenna array, the scalloping effect can be significantly mitigated with respect to that of the current system design, in both scenarios relevant to the original one-look ScanSAR configuration and the proposed two-look one.

The paper provides a summary on the context and on the updated status of the ROSE-L mission, which carries an L-Band SAR instrument and represents one of the six ESA Copernicus Sentinel Expansion missions. The overview will include the description of the SAR imaging modes and performance, as expected at the current mission phase (start of phase C), and of the synergic illumination design with Sentinel-1.

This paper provides an overview of the Copernicus L-band SAR mission referred to as ROSE-L (Radar Observing System for Europe at L-band).In particular, the paper provides an overview of the ROSE-L spacecraft and focuses on the ROSE-L SAR instrument architecture, performance and critical technology developments.The current status of the instrument activities (project is in Phase C) will be presented, highlighting the main achievements and challenges.

The Sentinel-1 mission represented an unprecedented source of SAR imagery, on which Copernicus and other providers conveniently leveraged to build a continuously expanding catalogue of operational services. In the next two decades, the family of SAR instruments under the Copernicus umbrella will feature additional wavelength diversity, with the ROSE-L mission providing operational illumination in L-band, and will introduce with the next generation Sentinel-1 (Sentinel-1 NG) a significant step ahead in terms of imaging quality and coverage performance, while carefully guaranteeing data and service continuity with the current generation. The massive volumes of data in space, time, polarizations and frequencies are expected on one hand to significantly enhance the applications and services already in place and on the other to open the way to new mapping and monitoring solutions in land, water and cryosphere domains. This paper will address such new perspectives.

In support of ESA's Mission Advisory Group for ROSE-L (Radar Observing System for Europe at L-band), a project team consisting of experts from operational ice services, universities and research institutes has been investigating the benefits of using data from L-band SAR in addition to C-band SAR imagery for monitoring of sea ice and detection of icebergs. The project work is still ongoing. Here, we introduce the technical background and present first results.

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.

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.

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.

Reliable and timely mapping of crop growth conditions and of their water resources is considered a prioritary application in light of the abrupt climate changes. With this view, the paper presents a novel approach that makes use of dense C-Band time-series for the timely estimation of crop growth stages and for the detection of changes in crop water conditions, especially related to precipitation and irrigation events. Aided by vegetation indexes extracted from Landsat and Sentinel-2 imagery, the proposed Sentinel-l centered method exploits both temporal patterns of crop growth and spatial patterns of water anomalies to enhance its classification robustness.

SAR as an active remote sensing technique is capable of providing insights into the physical features of agricultural vegetation. However, the noisy nature of SAR signals makes the direct conversion to effective productivity metrics challenging. This study sheds light on the effect of gaps present in a sugarcane field on L-band and C-band SAR signals and demonstrates the variability of this effect with changing spatial averaging windows, changing precipitation conditions and changing vegetation height.

In this study, we performed ground validation to support the interpretation of Sentinel-1 imagery during a full growing season of five key crop types in the Netherlands. Crop height and growth stage were monitored weekly in a total of 25 parcels of maize, potato, sugar beet maize and English rye grass in the province of Flevoland. Hydrometeorological data were collected throughout the season. Here, these results are used to interpret time series of Sentinel-1 data processed for the province of Flevoland. Results demonstrate that Sentinel-1 data follow the phenological stages and can be used to identify key moments in crop development. Combined with the guaranteed availability of observations regardless of cloud cover, this makes Sentinel-l data a valuable resource for agencies and commercial entities providing advice to farmers and agro-industrial co-operatives.

Multi-temporal and multi-sensor solutions are essential to increase timeliness and reliability of land monitoring systems. This paper advocates the exploitation of the temporal contextual information provided by temporally dense SAR and optical data series series through the use of a Hidden Markov model (HMM)-based approach. An efficient strategy to incorporate the C-Band SAR data into the HMM framework, relying so far on Landsat, will be debated and assessed over a dynamic agricultural scenario, i.e. characterized by high temporal and spatial diversity in cropping practices. The site is located in the state of São Paulo (Brazil), where recent ground surveying activities has been conducted.

The paper debates a novel approach for land cover (LC) mapping based on the Hidden Markov Model. The proposed methodology is aimed to address both the urgent demand of off-line (or historic) LC information retrieval and of near-real time LC monitoring. The discrete-time model employs short steps of 16 days, that conveniently fits the Landsat revisit time while providing a continuous and temporally dense representation of the land cover dynamics. Two temporal pattern typologies were identified and modeled within the proposed Markov chain architecture: a seasonal and synchrounous behavior which can be associated to the observables of LC classes such as forest and grasses, and a highly asynchronous behaviour, which characterizes the crop observables. The first typology is addressed by introducing time-dependency in state output probabilities, whereas the latter is rendered through a sequence of (sub-class) states interlinked by means of a `left-right' based model. Such model inherently incorporates crop growth tracking functionalities as an added value. In this paper the methodology has been tailored to Sao Paulo state (Brazil) scenario, showing overall accuracy above 80% on the test sample. A particular emphasis is attributed to the identification of sugarcane plantations, that are indeed responsible for major land use changes.

During the 2014–2015 sugarcane growth season in São Paulo, Brazil, a considerable dataset was acquired consisting of space-based remote sensing images from radar and optical sensors, together with intensive ground measurements. In this work, images from the Sentinel-1, Radarsat-2 and Landsat-8 satellites are used to test the effectiveness of satellite-based indicators in sugarcane growth monitoring. A two-fold hypothesis testing is applied, in order to find statistically significant emerging hot spots and cold spots, both in space and time. Especially the comparison of results from the radar and optical sensors gives an insight into the difference in capability of these sensors to detect spatial and temporal patterns and trends.

The paper debates a novel approach for sugarcane identification and characterization based on multi-spectral and multi-temporal profile matching. A parametric model aimed at identifying sugarcane among pasture/grasses/shrubs, annual crops and forest is proposed. Differently from other supervised and unsupervised classification techniques, the discussed profile-based parametric model accounts for variability in growth date, that becomes valuable information to be extracted, rather than simply a nuisance parameter, and delivers an effective extrapolation of the cane vigor. The approach is then applied to Landsat 5 TM and ERS/ENVISAT SAR time-series over the Orindiuva area attaining preliminary promising although perfectible results.

The objective of this paper is to provide a better understanding of tomographic capabilities in characterization of dense forested areas at P-and L-band. The analysis is carried out on airborne data acquired by ONERA over the site of Paracou, French Guyana, during the ESA campaign TropiSAR. The results shown support the idea that ground- and -volume interactions play a negligible role at L-band, whereas they are significant at P-band. For a dense forest of 30 m and more, there is very weak ground contribution at L-band. The L-band tomographic profile is quite disturbed as compared to the P-band profile in dense tropical forest areas. In this condition, the use of tomographic imaging at L-band in tropical forests appears limited. However, when the forest top height is roughly below 20 m (e.g., in forest regrowth), the tomographic results are expected to be the same as in boreal forests. Whereas P-band tomography allow us to retrieve the whole forest vertical structure, better characterizing of the ground and/or volume scatterings and providing an unique solution in high biomasss ranging from 150-600 t/ha.

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.

The paper debates a novel solution for the longterm monitoring of the system polarimetric quality based on radiometrically and polarimetrically stable targets, herewith named Polarimetric Permanent Scatterers (PPS). The technique is completely scene-based and thus cost effective. It allows, aided by the integration with the available Distributed Target (DT) information, for the relative calibration of the channel imbalance and cross-talk parameters. Their complex time-series information is indeed extracted with respect to the absolute unknown values of an arbitrary image of the stack. The performance achieved on a Radarsat-2 stack show that the accuracy is consistent with that returned by DT techniques. The attention shall be however focused on the fact that the more information can actually be extracted thanks to the proposed monitoring method.

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 the above-named article [ibid., vol. 8, no. 3, pp. 537-541, May 2011], an error in equation (4) appears. In it, the value 0.2589 is corrected to 77.6. The error concerns only the transcription of (4), and not the results presented in the letter, where the proper formulation was indeed used. The authors would like to thank the reviewer who helped correct this.

The paper debates an external calibration approach based on the stable natural targets, namely Permanent Scatterers (PS), that can be spotted in the illuminated frame. The method, hereby called PolPSCal, allows for relative calibration of the full 4 by 3 polarimetric distortion matrices (PDMs) affecting the stack images. The algorithm is neither constrained to a particular PDM model (thus its implementation is practically feasible for any SAR sensor) nor to any external information. These latter are eventually demanded afterwards in order to normalize the returned PDM stack to an absolute reference. The PolPSCal mathematical framework is reported, and a performance analysis with concern to the PS detection and the PDM estimates accuracy is carried out on both synthetic data and a 29 images RADARSAT-2 dataset.

The aim of this paper is the definition of a series of quantitative phase preserving tests that would be a necessary tool to evaluate the performance of different focusing algorithms and to define some necessary conditions to validate their efficacy and precision. The tests have been thought and defined to form a general tool that can cope with a wide number of systems and acquisition geometries.

The paper proposes a calibration technique based on Permanent Scatterers (PS) for the full-pol L-band and future P-band spaceborne SAR systems. It will be shown how both a radiometric and polarimetric monitoring of the system parameters can be carried out by exploiting the stable targets in the scene. The procedure aims to estimate the overrall system gain, the channel imbalances, and the Faraday rotation angles which must be accounted at such frequencies. The performance achievable by the technique is investigated with concern to the number of images and PSs which shall be supplied to the algorithm in order to meet the desired accuracy requirements.

In this letter, we face one of the main issues in ground-based radar applications, i.e., the evaluation and removal of the atmospheric phase screen (APS). The time-varying delay statistics are assessed by means of both radars and meteo simulated data sets and are critically interpreted with particular reference to the entailed compensation issues. A compensation approach based on the available on-site meteo parameters (pressure, temperature, and humidity) is then investigated. The technique proposes an initial calibration step on humidity which leads to significant improvements in the APS removal. The results of the technique are discussed in the case of a real campaign data set (Bolzano, Italy) that covers a temporal baseline of about one week.