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The article proposes a methodology to perform azimuth focusing of spaceborne transmitter-stationary receiver bistatic synthetic aperture radar data across multiple along-track apertures to increase azimuth resolution. The procedure uses as input several azimuth apertures (continuous groups of range compressed pulses) from one or more satellite bursts and comprises the following stages: antenna pattern compensation, slow time resampling, reconstruction of missing azimuth samples between neighboring sets of pulses using an autoregressive (AR) model and back-projection focusing of the resulting multiaperture range image. A novel, highly efficient method is proposed to estimate the optimal order for the AR model. It differs from the traditional approach that uses the Akaike Information Criterion to directly estimate the order because the proposed method estimates the order indirectly by detecting the number of targets using principle component analysis. Spatial smoothing is used to obtain a full rank covariance matrix, whose eigenvalues are then analyzed using minimum description length. The optimal order is an integer multiple of the number of targets, which depends on signal-to-noise ratio. The approach is evaluated with real bistatic data acquired over an area of Bucharest city, Romania.

We present a new perspective on Earth’s land surface, providing a normalised microwave backscatter map from spaceborne Synthetic Aperture Radar (SAR) observations. The Sentinel-1 Global Backscatter Model (S1GBM) describes Earth for the period 2016–17 by the mean C-band radar cross section in VV- and VH-polarisation at a 10 m sampling. We processed 0.5 million Sentinel-1 scenes totalling 1.1 PB and performed semi-automatic quality curation and backscatter harmonisation related to orbit geometry effects. The overall mosaic quality excels (the few) existing datasets, with minimised imprinting from orbit discontinuities and successful angle normalisation in large parts of the world. Regions covered by only one or two Sentinel-1 orbits remain challenging, owing to insufficient angular variation and not yet perfect sub-swath thermal noise correction. Supporting the design and verification of upcoming radar sensors, the obtained S1GBM data potentially also serve land cover classification and determination of vegetation and soil states. Here, we demonstrate, as an example of its potential use, the mapping of permanent water bodies and evaluate against the Global Surface Water benchmark., Measurement(s)C-band radar backscatter • extent of permanent water bodiesTechnology Type(s)Synthetic Aperture Radar (SAR) • Synthetic Aperture Radar (SAR) normalised mosaicFactor Type(s)radar backscatterSample Characteristic - EnvironmentlandSample Characteristic - Locationglobal Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.16432983

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.

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.

The systematic high temporal coverage of Sentinel-1 Synthetic Aperture Radar (SAR) is ideal for agricultural monitoring. The availability of these data on cloud computing infrastructure eliminates the need for massive computing power to process imagery. However, their distribution as SAR imagery still limits their accessibility for non-expert users. In Agricultural SandboxNL, Copernicus Sentinel-1 imagery on the Google Earth Engine (GEE) was mined to produce a database of spatially-tagged, parcel-level backscatter for every agricultural parcel in the Netherlands from 2017 to 2019. The database includes descriptors from the publicly available Basisregistratie Gewaspercelen, allowing the user to query the database by crop type and administrative boundary for any region of interest within The Netherlands. Publication of this database reduces the burden of processing and extracting a large volume of Sentinel-1 SAR data for experts. More importantly, it provides easy access to the Sentinel-1 data for agriculture/agronomy experts with limited SAR processing experience. In addition, the GEE package Sen1byParcel developed for Agricultural SandboxNL is made publicly available so that Sentinel-1 imagery can be extracted for any user-provided shapefile.

The Biomass mission is the 7th Earth Explorer Mission within the frame of the ESA Earth Observation Programme. The primary objective of Biomass is to determine the worldwide distribution of forest above-ground biomass in order to reduce the major uncertainties in calculations of carbon stocks and fluxes associated with the terrestrial biosphere. To meet these objectives Biomass will carry, for the first time in space, a fully polarimetric P-band synthetic aperture radar (SAR) supporting also interferometric acquisitions. This paper provides an overview of the Biomass system design and development status.

Monitoring large built structures, landslides, volcanoes, or glaciers implies relative height measurements and line-of-sight (LOS) displacement measurements, which are typically performed with monostatic spaceborne synthetic aperture radar (SAR) sensors. In this case, for each satellite that illuminates a given location, we can usually exploit only the information available from one ascending/descending orbit, and if the satellite uses multiple subswaths, the information is obtained only from one subswath per orbit. A bistatic configuration with spaceborne transmitter and fixed ground-based (GB) receiver opens the possibility to exploit the information available from more than one orbit and one subswath per orbit. Additionally, such a configuration offers new perspectives for target tracking and characterization (e.g., multiple LOSs and bistatic scattering signatures). This article presents an opportunistic C -band bistatic SAR differential interferometry architecture that uses a multichannel GB stationary receiver and a separate transmitter, which can be either the Sentinel-1A/B satellite, used opportunistically, or a specially designed GB sliding transmitter. The combination of the operation modes based on spaceborne and GB transmitter allows monitoring a small critical area of interest with many acquisitions triggered by the GB transmitter and surveying of the whole surrounding area with a small number of acquisitions corresponding to the satellites passes. The hardware platforms are presented along with the bistatic interferometric processing flow, and the potential of the proposed architecture is assessed in the context of monitoring large built structures.

This paper presents the first experimental results of a study on the ingestion in the Weather Research and Forecasting (WRF) model, of Sentinel satellites and Global Navigation Satellite Systems (GNSS) derived products. The experiments concern a flash-floodevent occurred in Tuscany (Central Italy) in September 2017. The rationale is that numerical weather prediction (NWP) models are presently able to produce forecasts with a km scale spatial resolution, but the poor knowledge of the initial state of the atmosphere may imply an inaccurate simulation of the weather phenomena. Hence, to fully exploit the advances in numerical weather modelling, it is necessary to feed them with high spatiotemporal resolution information over the surface boundary and the atmospheric column. In this context, the Copernicus Sentinel satellites represent an important source of data, because they can provide a set of high-resolution observations of physical variables (e.g. soil moisture, land/sea surface temperature, wind speed) used in NWP models runs. The possible availability of a spatially dense network of GNSS stations is also exploited to assimilate water vapour content. Results show that the assimilation of Sentinel-1 derived wind field and GNSS-derivedwater vapour data produce the most positive effects on the performance of the forecast.

International audience; 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.

Synthetic Aperture Radar Tomography (TomoSAR) is one primary remote sensing technique for deriving elevation estimations and recovering the 3D spatial information of an observed scene. This article presents a novel method for improved single-pass bistatic SAR tomography estimation, in the case of a spaceborne transmitter-stationary receiver architecture, with only three imaging channels at the receiver. The approach is based on the use of the coarray space to create a virtual set of acquisitions, as received from an array with a larger number of antenna elements. The proposed configuration is evaluated through simulations.

This paper presents a time-domain synthetic aperture radar (SAR) processor designed for Sentinel-l Terrain Observation by Progressive Scans (TOPS) monostatic data. The processor focuses Interferometric Wide (IW) swath and Extra Wide (EW) swath data on a selected region-of-interest (ROI) and consists of the following main stages: decoding of Level-0 data, selection of the relevant burst and pulses for the targeted ROI, range compression, azimuth frequency unfolding and resampling, and image focusing by a fast subaperture-based version of the back-projection algorithm. The performances of the developed processor are assessed on datasets acquired in IW and EW imaging modes. Such a time-domain processor can be regarded as a first step towards a geometry/frequency-agnostic SAR processing kernel for future monostatic/multi static spaceborne SAR missions.

The Mediterranean region is frequently struck by severe rainfall events causing numerous casualties and several million euros of damages every year. Thus, improving the forecast accuracy is a fundamental goal to limit social and economic damages. Numerical Weather Prediction (NWP) models are currently able to produce forecasts at the km scale grid spacing but unreliable surface information and a poor knowledge of the initial state of the atmosphere may produce inaccurate simulations of weather phenomena. The STEAM (SaTellite Earth observation for Atmospheric Modelling) project aims to investigate whether Sentinel satellites constellation weather observation data, in combination with Global Navigation Satellite System (GNSS) observations, can be used to better understand and predict with a higher spatio-temporal resolution the atmospheric phenomena resulting in severe weather events. Two heavy rainfall events that occurred in Italy in the autumn of 2017 are studied—a localized and short-lived event and a long-lived one. By assimilating a wide range of Sentinel and GNSS observations in a state-of-the-art NWP model, it is found that the forecasts benefit the most when the model is provided with information on the wind field and/or the water vapor content.

The paper proposes a methodology to perform azimuth focusing of spaceborne transmitter-stationary receiver bistatic synthetic aperture radar (SAR) data across multiple along-track apertures to increase azimuth resolution. The procedure uses as input several azimuth apertures (continuous groups of range compressed pulses) from one or more satellite bursts and comprises the following stages: azimuth antenna pattern compensation, slow time resampling, reconstruction of missing azimuth samples between neighbouring sets of pulses using an auto-regressive model and back-projection focusing of the resulting multi-aperture range image. The approach is evaluated with real bistatic data acquired over an area of Bucharest city, Romania.

European Space Agency's SnowScat instrument is a real aperture scatterometer which was developed by Gamma Remote Sensing AG. It operates in a continuous-wave mode, covers a frequency range of 9.15–17.9 GHz in a user-defined frequency-step and has a full polarimetric capability. The measurement campaigns were started first in February 2009 at Weissfluhjoch, in Davos, Switzerland, as an initial test of the instrument over a deep alpine snowpack. Physical characterizations of the snowpack and meteorological measurements were carried out, which formed a detailed in situ dataset. SnowScat was then moved to Sodankyla in Finland in early November 2009, a site of the Finnish Meteorological Institute in Lapland. In addition to the in situ snowpack characterizations and meteorological observations, continuous passive microwave observations were also performed. During the 2012–2013 winter period, a vertical time-domain snow profiling experiment was carried out in addition for resolving the scattering contributions from the snow layers of different physical properties. This paper summarizes the results of the SnowScat observations and initial comparisons against the in situ meteorological and snowpack data. The Sodankyla campaign data evidenced the high variability of the radar backscatter behavior of snowpack from year to year, which indicates its strong dependency on changing snow microstructure. Indeed, the snow microstructure is continuously driven by snow metamorphism, which are further affected by meteorological conditions and their interannual variability. The backscattering property of snowpack in the range X - to Ku -band for all polarizations appeared to be dominated by its microstructural morphology and underlying ground conditions, and to a lesser extent by the snow depth, or its snow-water-equivalent.

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 European Space Agency (ESA) STEAM (SaTellite Earth observation for Atmospheric Modelling) project aims at investigating new areas of synergy between high-resolution numerical atmosphere models and data from spaceborne remote sensing sensors, with focus on Copernicus Sentinels 1, 2 and 3 satellites. An example of synergy is the ingestion of surface information derived from Sentinel data in numerical weather prediction models. The rationale is that Sentinels 1, 2 and 3 are able to provide high spatio-temporal resolution information on the surface boundary (as well as the atmosphere column) and that an inaccurate representation of the boundary conditions represents a major source of uncertainty for weather forecasts. For a profitable ingestion of EO data in numerical weather prediction models, a critical aspect is the choice of a suitable model. Once the numerical model is chosen, the problem of the selection of the Sentinel-derived surface variables that have to be ingested in the model has to be tackled. While some data, such as sea and land surface temperature, are directly available, other surface data, such as soil moisture, have to be retrieved. Being STEAM currently in its initial phase, this paper gives a general overview of the project and focuses on the first activities performed in its framework. In particular, it describes the rationale behind the choice of the Numerical Weather Prediction Model and the multi-temporal approach designed to retrieve soil moisture from Sentinel-1 data. Moreover, the first results of the ingestion of Sentinel derived soil moisture, land surface temperature and sea surface temperature data into the selected model are shown. These results concern an extreme weather event that occurred in Tuscany (central Italy) in September 2017.

Space-based radio echo sounding of the continental ice sheets can potentially provide full coverage with uniform sampling and data quality as well as detection of change in environmentally sensitive areas. This paper addresses the feasibility of sounding the Antarctic ice sheets with a space-based P-band radar. The assessment firstly makes use of an electromagnetic model of the ice sheets where the key model parameters are determined from data that have been acquired in Antarctica with an airborne P-band ice sounding radar. The performance of a space-based radar with a nadir-looking geometry but otherwise similar to ESA's Biomass SAR, is then simulated for a set of Antarctic scenarios that are defined based on the statistics of key ice regions. It is found that in about 2/3 of the simulation scenarios clutter and/or thermal noise will obscure the echo from the ice bed.

The STEAM (SaTellite Earth observation for Atmospheric Modelling) project, funded by the European Space Agency, aims at investigating new areas of synergy between high-resolution numerical weather prediction (NWP) models and data from spaceborne remote sensing sensors. An example of synergy is the incorporation of high-resolution remote sensing data products in NWP models. The rationale is that NWP models are presently able to produce forecasts with a spatial resolution in the order of 1 km, but unreliable surface information or poor knowledge of the initial state of the atmosphere may imply an inaccurate simulation of the weather phenomena. It is expected that forecast inaccuracies could be reduced by ingesting high resolution Earth Observation derived products into models operated at cloud resolving grid spacing. In this context, the Copernicus Sentinel satellites represent an important source of data, because they provide a set of high-resolution observations of physical variables (e.g. soil moisture, land/sea surface temperature, wind speed, columnar water vapor) used NWP models runs. This paper presents the first results of the experiments carried out in the framework of the STEAM project, regarding the ingestion/assimilation of surface information derived from Sentinel data into a NWP model. The experiments concern a flood event occurred in Tuscany (Central Italy) in September 2017. Moreover, in view of the assimilation of water vapor maps obtained by applying the SAR Interferometry technique to Sentinel-1 data, the results of the assimilation of Zenith total delay data derived from global navigation satellite system (GNSS) are also presented.

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.

We have investigated the retrieval of total aboveground, stem and branch biomass using multi-temporal SAR data acquired at C-, L-, and P-band by Radarsat-2, ERS-2, ALOS PALSAR, and SETHI for a forest site in Sweden. A semi-empirical model was used to model C- and L-band backscatter as function of total aboveground, stem and branch biomass; a linear regression model was used for P-band. Calibrated with the aid of inventory plots, the models were used to estimate total aboveground, stem, and branch biomass for each individual scene in the multi-temporal and multi-frequency stack of observations. Individual estimates were then combined using weights reflecting each scenes' sensitivity to the biomass component in question. At the plot scale, the retrieval errors were 37, 40, and 33 % for total aboveground, stem, and branch biomass in terms of the relative RMSE; the error further reduced to 33, 38, and 27 % when calibrating models per species. We find that i) the retrieval of total aboveground, stem and branch biomass benefits from the availability of multi-temporal/-frequency data as the lowest retrieval errors with the best single image (P-band) were about 6 to 8 % higher, ii) there is no benefit in estimating total aboveground biomass via independent estimates of stem and branch biomass, iii) consideration of species specific differences associated with different allocations of biomass to stems and branches allows for improving the retrieval.

The paper provides an overview of the Copernicus Sentinel-1 mission, which includes Sentinel-1A (S-1A) and Sentinel-1B (S-1B) satellites, and the characteristics of its SAR system. Sentinel-1 uses pre-programmed SAR mode operations to provide a high revisit frequency and systematic global SAR image coverage. This is mainly based upon the operational use of the novel TOPS (Terrain Observation with Progressive Scans in azimuth) SAR imaging mode. In particular, we present results of the SAR system performance analysis focusing on the instrument stability and the achieved radiometric accuracy, as well as the Noise Equivalent Sigma Zero (NESZ). In addition, we discuss the cross-S-1A/S-1B SAR Interferometry (InSAR) performance considering the effects of burst synchronization and SAR antenna pointing on the achievable common Doppler bandwidth. Results of differential cross interferograms are presented showing the coseismic surface displacement caused by the central Italy earthquake.

In this paper, we propose a bistatic synthetic aperture radar (SAR) imaging procedure using a Sentinel-1 satellite as transmitter of opportunity and a stationary ground receiver. The procedure is designed for the standard operating mode of Sentinel-1 satellites - Terrain Observation with Progressive Scans SAR (TOPSAR) and essentially comprises the synchronization between satellite transmitter and ground receiver and the bistatic focusing algorithm. The proposed imaging method takes into account the particularities imposed by the antenna beam sweep from back to forth (specific for TOPSAR mode) and exploits the position vectors and transmission timestamps annotated in the ancillary data from the monostatic raw image. The methodology is demonstrated with a bistatic acquisition performed with Sentinel-1A over an area of Bucharest city, Romania.

The planned launch of a spaceborne P-band radar mission and the availability of C- and L-band data from several spaceborne missions suggest investigating the complementarity of C-, L-, and P-band backscatter with respect to the retrieval of forest above-ground biomass. Existing studies on the retrieval of biomass with multi-frequency backscatter relied on single observations of the backscatter and were thus not able to demonstrate the potential of multi-temporal C- and L-band data that are now available from spaceborne missions. Based on spaceborne C- and L-band and airborne P-band images acquired over a forest site in southern Sweden, we investigated whether C- and L-band backscatter may complement retrievals of above-ground biomass from P-band. To this end, a retrieval framework was adopted that utilizes a semi-empirical model for C- and L-bands and an empirical parametric model for P-band. Estimates of above-ground biomass were validated with the aid of 20 m-diameter plots and a LiDAR-derived biomass map with 100 m × 100 m pixel size. The highest retrieval accuracy when not combining frequencies was obtained for P-band with a relative root mean square error (RMSE) of 30% at the hectare scale. The retrieval with multi-temporal L- and C-bands produced errors of the order of 40% and 50%, respectively. The P-band retrieval could be improved for 4% when using P-, L-, and C-bands jointly. The combination of C- and L-bands allowed for retrieval accuracies close to those achieved with P-band. A crucial requirement for achieving an error of 30% with C- and L-bands was the use of multi-temporal observations, which was highlighted by the fact that the retrieval with the best individual L-band image was associated with an error of 61%. The results of this study reconfirmed that P-band is the most suited frequency for the retrieval of above-ground biomass of boreal forests based on backscatter, but also highlight the potential of multi-temporal C- and L-band imagery for mapping above-ground biomass, for instance in areas where the planned ESA BIOMASS P-band mission will not be allowed to acquire data.

In the frame of the Global Monitoring for Environment and Security (GMES) Space Component programme, the European Space Agency (ESA) undertook the development of a European Radar Observatory (Sentinel-1), a polar orbiting two-satellite constellation for the continuation and improvement of SAR operational services and applications. Satellite and payload are being built to provide routine, day-and-night, all-weather medium (typically 10 m) resolution observation capability. Ground infrastructure is provided for planning, mission control, data processing, dissemination and archiving. Free and open data access is provided. Data quality of the Sentinel-1 data products is shown along with uncertainty estimation of retrieved information products confirming specified performance and indicating application growth potential. The unique data availability performance of the Sentinel-1 routine operations makes the mission particularly suitable for emergency response support, marine surveillance, ice monitoring and interferometric applications such as detection of subsidence and landslides.

The ESA Sentinels constitute the first series of operational satellites respondin g to the Earth Observation needs of the EU-ESA Global Monitor ing for Environment and Security (GMES) programme . The GMES space component relies on existing and planned space assets as well as on new complementary developments by ESA. This describes the Sentinel- 1 mission, as imaging synthetic aperture radar (SAR) satellite constellation at C-band . It provides an overview of mission requirements, its appl ications and the technical concepts for the system .

The choice of synthetic aperture radar (SAR) system design parameters such as radiometric calibration uncertainty and noise-equivalent sigma zero has a significant impact on applications exploiting SAR image data. However, methods for quantitatively translating the impact of system and mission parameter choices into the application context are lacking. This letter addresses this subject and derives a closed-form algebraic expression for translating radiometric biases due to calibration uncertainties-an important SAR engineering specification-into estimates of classification uncertainties for the restricted case of two homogeneous classes characterized by different backscatter intensity levels. The expression is exploited within this letter to estimate the potential impact of radiometric uncertainty on SAR-derived thematic maps. The results indicate that classification results based on the joint use of data from future SAR constellations in particular may be significantly degraded due to calibration uncertainties.

The telecommunications sector has proposed to use the 5.350–5.470 GHz frequency band for ground-based communication services. The Sentinel-1, RadarSAT-2, and future RadarSAT constellation SAR satellites are operating in the same band. Here we assess the impact of these ground-based uncorrelated radio frequency interference (RFI) sources to radar interferometry-based ground motion analysis. Apart from a theoretical assessment, a Persistent Scatterer Interferometry analysis is performed based on a combination of real ENVISAT data and simulated RFI noise at different levels. The analysis shows that the RFI sources significantly reduce the applicability of radar interferometry, with a reduction of detected Persistent Scatterers with more than 50%. If the telecommunication sector would proceed with this plan, operational ground motion services based on Sentinel-1 data would no longer be possible.

Ka-band RADAR frequency range has not yet been used for Synthetic Aperture Radar (SAR) from space so far, although this technology may lead to important applications for the next generation of SAR space sensors. Therefore, feasibility studies regarding a Ka-band SAR instrument have been started [1][2], for the next generation of SAR space sensors. In spite of this, the lack of trusted references on backscatter at Ka-band revealed to be the main limitation for the investigation of the potentialities of this technology. In the framework of the ESA project “Ka-band SAR backscatter analysis in support of future applications”, this paper is aimed at the study of wave interaction at Ka-band for a wide range of targets in order to define a set of well calibrated and reliable Ka-band backscatter coefficients for different kinds of targets. We propose several examples of backscatter data resulting from a critical survey of available datasets at Ka-band, focusing on the most interesting cases and addressing both correspondences and differences. The reliability of the results will be assessed via a preliminary comparison with ElectroMagnetic (EM) theoretical models. Furthermore, in support of future technological applications, we have designed a prototypal software acting as a “library” of earth surface radar response. In our intention, the output of the study shall contribute to answer to the need of a trustworthy Ka-Band backscatter reference. It will be of great value for future technological applications, such as support to instrument analysis, design and requirements’ definition (e.g.: Signal to Noise Ratio, Noise Equivalent Sigma Zero).

The paper provides an overview of the Copernicus Sentinel-1 system capabilities and applications. In particular, the characteristics of the Sentinel-1 SAR imaging modes and their key performance parameters are described. In addition, the Sentinel-1 SAR interferometry (InSAR) capabilities, especially for TOPS InSAR and the strategy for maintaining the orbital baseline as well as the requirements for TOPS image co-registration are discussed.

A new simple scatterometer concept combines the advantages of both the fixed, multiple beam, sidelooking radar such as AMI-Wind (ERS-1/2) and NSCAT (ADEOS), and the conically scanning pencil-beam radar such as SeaWinds. A wide, fanbeam antenna is rotated around a vertical axis with a slow rotation rate. For a satellite at an altitude of 725 km, the antenna footprint sweeps a circular donut of 1500 km diameter. Such a slow conical scan combined with the motion of the satellite at /spl ap/7 km/s ground speed results in highly overlapping successive sweeps such that an image pixel is revisited up to 10-11 times during an overpass. The pixels in the radial direction are resolved by range-gating the radar echo. Depending on the across-track position of the imaged pixel, the measurement acquisitions during an overpass consist of a set of /spl sigma//spl deg/ at different combinations of the azimuth and incident angles. A preliminary optimization of the system resulted in a C-band radar concept with a 15 km multiple-look spatial resolution and global coverage in two days. A sketch of the developed concept, preliminary system design, and predicted performance are described.

The ESA Sentinels constitute the first series of operational satellites responding to the Earth Observation needs of the EU-ESA Global Monitoring for Environment and Security (GMES) programme. The GMES space component relies on existing and planned space assets as well as on new complement ary developments by ESA. In contrast to SAR systems already existing in C-band like ASAR/ENVISAT or RADARSAT-2, high demands on the radiometric stability and accuracy are made for Sentinel-1. The mission depends on th e method of calibrating the entir e Sentinel-1 system in an efficient way. This paper describes the strategy and provides a plan of all activities required for in-flight calibration of Sentinel-1. Keywords: GMES, Sentinel-1, CSAR , Calibration, Validation 1. INTRODUCTION The Sentinel-1 synthetic aperture radar (SAR) constellation represents a completely new approach to SAR mission design by ESA in direct response to the operational needs for SAR data expressed under the EU-ESA Global Monitoring for Environment and Security (GMES) programme. The Sentinel-1 constellation is expected to provide near daily coverage over Europe and Canada, global coverage at least every 6 days – all vast improvements with respect to the existing SAR systems [1, 2]. The Sentinel-1 Synthetic Aperture Radar (SAR) instrument operated in two main operational modes: Interferometric Wide Swath Mode, Wave Mode, and two additional imaging modes: Strip Map Mode, and Extra-wide Swath Mode. Except of the Wave Mode all modes can be operated in dual polarization using two receiving channels. The front end of the instrument is based on an active phased array antenna driven by 280 transmit/receiver modules. Thus, by switching the instrument over a multitude of different beams, progressive scanning in azimuth is possible in order to reduce the scalloping effect (TOPS) [3] and by scanning in elevation a wide range of swath positions can be covered (up to 400 km ground range). The most important point with respect to the in-orbit calibration of this flexible SAR system is the tight performance with an absolute radiometric accuracy of only 1 dB (31 ) for all operational modes. Never before such a strong requirement (a few tenths of dB) has been defined for a satellite SAR system. Following the experience gained with ERS and ENVISAT it is necessary to have a set of precision radar transponders to act as point targets which can be automatically programmed and accessed du ring the Sentinel-1 commi ssioning phase and the remainder of the mission lifetime in order to achieve the absolu te radiometric accuracy requirement. This paper will describe aspects of the in-orbit calibration of the Sentinel-1 constellation. The concept of the Sentinel-1 transponder design and its development status will be presented.

Recent space-based Synthetic Aperture Radar (SAR) observations are discussed of large reflector antennas on the ground. The results are elaborated with suggestions for further work to arrive at more precise information. Such targets might eventually receive further attention in applications with SAR operating at lower frequency than for which the ground-station antenna operates. Being below cut-off, the antenna becomes a passive scattering structure with a Radar Cross Section (RCS). Such antenna may function as stable target of opportunity, additional to transponders. Its response should be stable (even in complex vectorial sense). Precise prediction requires accurate modelling. A target is automatically available (“free of charge”), when the low-frequency SAR instrument is switched on over the ground-station antenna for data downloading and when the latter antenna is located within the SAR image coverage.

The ESA Sentinels constitute the first series of operational satellites responding to the Earth Observation needs of the EU-ESA Global Monitoring for Environment and Security programme. The GMES space component relies on existing and planned space assets as well as on new complementary developments by ESA. In particular, as part of the GMES space component, ESA is currently undertaking the development of 3 Sentinels mission families. Each Sentinel is based on a constellation of 2 satellites in the same orbital plane. This configuration allows to fulfil the revisit and coverage requirements and to provide a robust and affordable operational service. The launch of the 2nd satellite is scheduled 18 months after the launch of the 1st spacecraft of the constellation. The lifetime of the individual satellite is specified as 7 years, with consumables allowing mission extension up to 12 years. The lifecycle of the space segment is planned to be in the order of 15-20 years. The strategy for Sentinel procurement and replacement over this period is being elaborated, but will likely result in a need for 4-5 satellites of each type if the desired robustness for the service that GMES will provide is to be achieved. This paper will describe the operational and observational capabilities of the Sentinel-1 mission based on the user requirements, including potential emergency requests. An example of a pre-defined mission timeline for each and every cycle will be given.

Spaceborne Interferometric Synthetic Aperture Radar (InSAR) is a well established technique useful in many land applications, such as landslide monitoring and digital elevation model extraction. One of its major limitation is the atmospheric effect, and in particular the high water vapour spatial and temporal variability which introduces an unknown delay in the signal propagation. However, the sensitivity of SAR interferometric phase to atmospheric conditions could in principle be exploited and InSAR could become in certain conditions a tool to monitor the atmosphere, as it happens with GPS receiver networks. This paper describes a novel attempt to assimilate InSAR derived information on the atmosphere, based on the Permanent Scatterer multipass technique, into a numerical weather forecast model. The methodology is summarised and the very preliminary results regarding the forecast of a precipitation event in Central Italy are analysed. The work was done in the framework of an ESA funded project devoted to the mapping of the water vapour with the aim to mitigate its effect for InSAR applications.

Spaceborne Interferometric Synthetic Aperture Radar (InSAR) is a well established technique useful in many land applications, such as tectonic movements, landslide monitoring and digital elevation model extraction. One of its major limitation is the atmospheric effect, and in particular the high water vapour spatial and temporal variability which introduces an unknown delay in the signal propagation. This paper describes the general approach and some results achieved in the framework of an ESA funded project devoted to the mapping of the water vapour with the aim to mitigate its effect in InSAR applications. Ground based (microwave radiometers, radiosoundings, GPS) and spaceborne observations (AMSR-E, MERIS, MODIS) of columnar water vapour were compared with Numerical Weather Prediction model runs in Central Italy during a 15-day experiment. A dense network of GPS receivers was deployed close to Como, in Northern Italy, to complement the operational network in order to derive Zenith Total Delay as well as Slant Delay which can support InSAR processing. A comparison with Atmospheric Phase Screens (APS) derived from a sequence of Envisat multi pass interferometric acquisitions processed using the Permanent Scatters technique on the two test sites has been also performed. The acquired experimental data and their comparison give a valuable idea of what can be done to gather information on water vapour, which, besides InSAR applications, plays a fundamental role in weather prediction and radio propagation studies. The work has been carried out in the framework of an ESA funded project, named “Mitigation of Electromagnetic Transmission errors induced by Atmospheric Water Vapour Effects” (METAWAVE). This paper presents the general approach an the various methodologies exploited in the project, together with the overall intercomparison of the results. In deep details on the comparison with the InSAR APS maps derived by the PS technique, as well as on GPS receiver processing and water vapour tomography are reported in two companion papers.

International audience; In the companion paper, classical information content (IC) analysis was used to measure the potential synergy between the microwave (MW) and infrared (IR) observations from Atmospheric Microwave Sounding Unit-A, Microwave Humidity Sounder, and Improved Atmospheric Sounding in the Infrared instruments, used to retrieve the atmospheric profiles of temperature and water vapor over ocean, under clear-sky conditions. Some limitations of IC were pointed out that questioned the reliability of this technique for synergy characterization. The goal of this second paper is to develop a methodology to measure realistic potential synergies and to construct retrieval methods able to exploit them. Three retrieval methods are considered: the k nearest neighbors, the linear regression, and the neural networks (NN). These statistical retrieval schemes are tested on an application involving IR and MW synergy. Only clear-sky, near-nadir radiances over ocean are considered. The IR/MW synergy is expected to be stronger in cloudy cases, but it will be shown that it can also be observed in clear situations. The inversion algorithms are calibrated and tested with simulated observations, without any loss of generality, using similar theoretical assumption (same radiative transfer model, observational noise, and a priori information) in order to truly compare the IC and the direct statistical retrieval approaches. Multivariate and nonlinear methods such as the NN approach show that there is a strong potential for synergy. Synergy measurement tools such as the method proposed in this study should be considered in the future for the definition of new missions: The instrument characteristics should be determined not independently, sensor by sensor, but taking into account all the instruments together as a whole observing system. Copyright 2011 by the American Geophysical Union.

The Sentinel-1 synthetic aperture radar (SAR) constellation represents a completely new approach to SAR mission design by ESA in direct response to the operational needs for SAR data expressed under the EU-ESA Global Monitoring for Environment and Security (GMES) programme. The Sentinel-1 constellation is expected to provide near daily coverage over Europe and Canada, and weekly global coverage all independent of weather with delivery of radar data within 1 hour of acquisition - all vast improvements with respect to the existing SAR systems. In addition to responding directly to current needs of the GMES program, the design of the Sentinel-1 satellite mission with its focus on stability, reliability, global coverage, consistent operations and quick data delivery is expected to enable the development of new applications and meet the evolving needs of GMES, for instance in the area of climate change and associated monitoring needs. It is expected that Sentinel-1A will be launched in 2012.

Spaceborne Interferometric Synthetic Aperture Radar (InSAR) is a well established technique useful in many land applications, such as monitoring tectonic movements and landslides or extracting digital elevation models. One of its major limitations is the atmospheric variability, and in particular the high water vapor spatial and temporal variability, which introduces an unknown delay in the signal propagation. On the other hand, these effects might be exploited, so as InSAR could become a tool for highresolution water vapor mapping. This paper describes the approach and some preliminary results achieved in the framework of an ESA funded project devoted to the mitigation of the water vapor effects in InSAR applications. Although very preliminary, the acquired experimental data and their comparison give a first idea of what can be done to gather valuable information on water vapor, which play a fundamental role in weather prediction and radio propagation studies.

The ESA Sentinels constitute the first series of operational satellites responding to the Earth observation needs of the EU-ESA Global Monitoring for Environment and Security (GMES) programme. The GMES space component relies on existing and planned space assets as well as on new complementary developments by ESA. This paper describes the Sentinel-1 mission, an imaging synthetic aperture radar (SAR) satellite constellation at C-band. It provides an overview of the mission requirements, its applications and the preliminary technical concept for the system.

The ESA Sentinels constitute the first series of operational satellites responding to the Earth Observation needs of the EU-ESA Global Monitoring for Environment and Security (GMES) program. The GMES space component relies on existing and planned space assets as well as on new complementary developments by ESA. This paper describes the Sentinel-1 mission, an imaging synthetic aperture radar (SAR) satellite constellation at C-band. It provides an overview of the mission requirements, its applications and the preliminary technical concept for the system. Keywords: GMES, C-band, Space-borne SAR 1. INTRODUCTION As part of the GMES space component, ESA is undertaking the development of a European Radar Observatory (Sentinel-1), a European polar orbiting satellite system for the continuation of SAR operational applications. Sentinel-1 is an imaging radar mission at C-band consisting of a constellation of two satellites aimed at providing continuity of all-weather day-and-night supply of imagery for user services. Special emphasis is placed on services identified in ESA’s GMES service elements program and on projects funded by the European Union Framework Programmes. Three priorities (fast-track services) for the mission have been identified by user consultation working groups of the European Union: Marine Core Services, Land Monitoring and Emergency Services. These cover applications such as: - Monitoring sea ice zones and the arctic environment - Surveillance of marine environment - Monitoring land surface motion risks - Mapping of land surfaces: forest, water and soil, agriculture - Mapping in support of humanitarian aid in crisis situations

At the World Radiocommunication Conference 2003 (WRC-03) of the International Telecommunication Union (ITU) several issues related to Earth observation will be on the agenda and decisions will be taken with respect to these so-called agenda items. In this paper the current status of some of the WRC-03 agenda items will be given where Earth observation is directly involved through its active instruments, the so-called Earth Exploration Satellite Service (active), or its passive instruments, the so-called Earth Exploration-Satellite Service (passive). The paper discusses and presents the status up-to-date for the following agenda items for WRC-03: (1) Agenda Item 1.5: introduction of Radio Local Area Networks (RLANs) in the bands 5150-5350 MHz and the band 5470-5725 MHz. The 5250-5350 MHz band is an extensively used band for remote sensing by SARs, scatterometers and altimeters. This agenda item also asks for an extension of the EESS (active) band from 5460-5570 MHz to facilitate wideband (up to 320 MHz) active sensors. (2) Agenda Item 1.38: the possible allocation of 6 MHz for EESS (active) in the frequency range 420-470 MHz (P-band). A new frequency allocation in P-band would enable a future satellite project to carry a P-band radar for e.g. penetrating forest canopies for biomass estimation and ice-sheet sounding on Antarctica for retrieval of the bedrock topography. (3) Agenda Item 1.8.2: the protection of EESS (passive) sensors from unwanted emissions in adjacent and nearby bands. The issue of unwanted emissions is difficult to tackle both from a technical and regulatory point of view especially when linked to the protection of exclusive passive band At the time of the IGARSS conference itself, the outcome of these WRC-03 agenda items including the upcoming agenda items for the next World Radiocommunication Conference (planned in 2007) will be known and will be presented. Consequentially, the Earth observation community will informed at a very early stage which issues are selected for the next World Radiocommunication Conference where EESS (both active and passive) is directly involved as well as the implications of the outcome of the issues leading to WRC-03.