Surface water storage and fluxes in rivers, lakes, reservoirs and wetlands are currently poorly observed at the global scale, even though they represent major components of the water cycle and deeply impact human societies. In situ networks are heterogeneously distributed in space, and many river basins and most lakes—especially in the developing world and in sparsely populated regions—remain unmonitored. Satellite remote sensing has provided useful complementary observations, but no past or current satellite mission has yet been specifically designed to observe, at the global scale, surface water storage change and fluxes. This is the purpose of the planned Surface Water and Ocean Topography (SWOT) satellite mission. SWOT is a collaboration between the (US) National Aeronautics and Space Administration, Centre National d’Etudes Spatiales (the French Spatial Agency), the Canadian Space Agency and the United Kingdom Space Agency, with launch planned in late 2020. SWOT is both a continental hydrology and oceanography mission. However, only the hydrology capabilities of SWOT are discussed here. After a description of the SWOT mission requirements and measurement capabilities, we review the SWOT-related studies concerning land hydrology published to date. Beginning in 2007, studies demonstrated the benefits of SWOT data for river hydrology, both through discharge estimation directly from SWOT measurements and through assimilation of SWOT data into hydrodynamic and hydrology models. A smaller number of studies have also addressed methods for computation of lake and reservoir storage change or have quantified improvements expected from SWOT compared with current knowledge of lake water storage variability. We also briefly review other land hydrology capabilities of SWOT, including those related to transboundary river basins, human water withdrawals and wetland environments. Finally, we discuss additional studies needed before and after the launch of the mission, along with perspectives on a potential successor to SWOT.

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The SWOT Mission and Its Capabilities for Land Hydrology

The Surface Water and Ocean Topography (SWOT) satellite mission planned for launch in 2020 will map river elevations and inundated area globally for rivers >100 m wide In advance of this launch, we here evaluated the possibility of estimating discharge in ungauged rivers using synthetic, daily ‘‘remote sensing’’ measurements derived from hydraulic models corrupted with minimal observational errors Five discharge algorithms were evaluated, as well as the median of the five, for 19 rivers spanning a range of hydraulic and geomorphic conditions Reliance upon a priori information, and thus applicability to truly ungauged reaches, varied among algorithms: one algorithm employed only global limits on velocity and depth, while the other algorithms relied on globally available prior estimates of discharge We found at least one algorithm able to estimate instantaneous discharge to within 35% relative root-mean-squared error (RRMSE) on 14/16 nonbraided rivers despite out-of-bank flows, multichannel planforms, and backwater effects Moreover, we found RRMSE was often dominated by bias; the median standard deviation of relative
residuals across the 16 nonbraided rivers was only 125% SWOT discharge algorithm progress is therefore encouraging, yet future efforts should consider incorporating ancillary data or multialgorithm synergy to improve results

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An intercomparison of remote sensing river discharge estimation algorithms from measurements of river height, width, and slope

Flooding is one of the most costly natural disasters and thus mapping, modeling and forecasting flood events at various temporal and spatial scales is important for any flood risk mitigation plan, disaster relief services and the global (re-)insurance markets. Both computer models and observations (ground-based, airborne and Earth-orbiting) of flood processes and variables are of great value but the amount and quality of information available varies greatly with location, spatial scales and time. It is very well known that remote sensing of flooding, especially in the microwave region of the electromagnetic spectrum, can complement ground-based observations and be integrated with flood models to augment the amount of information available to end-users, decision-makers and scientists. This paper aims to provide a concise review of both the science and applications of microwave remote sensing of flood inundation, focusing mainly on synthetic aperture radar (SAR), in a variety of natural and man-made environments. Strengths and limitations are discussed and the paper will conclude with a brief account on perspectives and emerging technologies.

Microwave remote sensing of flood inundation

This paper presents an application of the recent advances in the field of spherically invariant random vector (SIRV) modeling for coherency matrix estimation in heterogeneous clutter. The complete description of the polarimetric synthetic aperture radar (POLSAR) data set is achieved by estimating the span and the normalized coherency independently. The normalized coherency describes the polarimetric diversity, while the span indicates the total received power. The main advantages of the proposed fixed-point (FP) estimator are that it does not require any a priori information about the probability density function of the texture (or span) and that it can directly be applied on adaptive neighborhoods. Interesting results are obtained when coupling this FP estimator with an adaptive spatial support based on the scalar span information. Based on the SIRV model, a new maximum-likelihood distance measure is introduced for unsupervised POLSAR classification. The proposed method is tested with both simulated POLSAR data and airborne POLSAR images provided by the Radar Ae?roporte? Multi-Spectral d'Etude des Signatures system. Results of entropy/alpha/anisotropy decomposition, followed by unsupervised classification, allow discussing the use of the normalized coherency and the span as two separate descriptors of POLSAR data sets.

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Coherency Matrix Estimation of Heterogeneous Clutter in High-Resolution Polarimetric SAR Images

This book presents a unique collection of state-of-the-art contributions by international remote sensing experts focussing on methodologies to extract information about urban areas from Synthetic Aperture Radar (SAR) data. SAR is an active remote sensing technique capable to gather data independently from sun light and weather conditions. Emphasizing technical and geometrical issues the potential and limits of SAR are addressed in focussed case studies, for example, the detection of buildings and roads, traffic monitoring, surface deformation monitoring, and urban change. These studies can be sorted into two groups: the mapping of the current urban state and the monitoring of change. The former covers, for instance, methodologies for the detection and reconstruction of individual buildings and road networks; the latter, for example, surface deformation monitoring and urban change. This includes also investigations related to the benefit of SAR Interferometry, which is useful to determine either digital elevation models and surface deformation or the radial velocity of objects (e.g. cars), and the Polarization of the signal that comprises valuable information about the type of soil and object geometry. Furthermore, the features of modern satellite and airborne sensor devices which provide high-spatial resolution of the urban scene are discussed.

Radar Remote Sensing of Urban Areas

The principal instrument of the NASA/CNES wide-swath altimetry mission Surface Water and Ocean Topography (SWOT) is the Ka-band Radar Interferometer (KaRIn), a bistatic synthetic aperture radar (SAR) system operating on near-nadir swaths on both sides of the satellite track. There are limited reports on backscattering from natural surfaces at this short wavelength and particular observation geometry. Near-field backscattering measurements on water, as well as the first interferometric airborne SAR acquisitions at Ka-band covering the 0.6 °-3.9 ° incidence range of KaRIn, were therefore conducted. The experimental results confirm expected characteristics of near-nadir Ka-band interferometric SAR imagery, such as strong water/land radiometric contrast (typically in the order of 10 dB) and very high interferometric coherence on water.

KaRIn on SWOT: Characteristics of Near-Nadir Ka-Band Interferometric SAR Imagery

The last SAR campaigns of the French ONERA RAMSES system have been dedicated to various applications, military or civilian, including very-high resolution, FOPEN (Foliage penetration) in cooperation with the FOI, MTI, bistatic SAR and vegetation characterization. These different projects gave us the opportunity to improve our SAR processing procedure (very high resolution) and to develop new algorithms (bistatic scenario). In parallel, the ONERA team has developed and applied many techniques (polarimetric analysis, PolInSAR, wavelet analysis) which proved to be efficient in these different thematic context. The current aircraft platform, a Transall C160 will be decommissioned at the end of 2008. In order to maintain our activities, we are developing a new system family compatible with smaller aircrafts. The challenge is two fold : migrating the existing radars in a airplane four times smaller and maintaining our innovation capabilities with the development of new radar concepts. These paper presents the most recent results on SAR techniques and describes the status of the development of the RAMSES successors : the first, SETHI, dedicated to military applications and earth observation, the second, BUSARD more focused on UAV applications.

The ONERA RAMSES SAR: latest significant results and future developments

UAVs from different countries and with different payloads have proven their capabilities within military applications. Future UAV employment in the civilian areas of surveillance (pollution, natural risks prevention, fire prevention), monitoring (traffic control, environmental monitoring, earth observation) and communication relays becomes unavoidable. Many payload configurations may be used in UAV operations, but compared to systems working in other spectral regions (such as optical or infrared sensors), radar has the main advantage to be able to operate in all-weather condition. A concept of low-cost imaging Ka-Band radar is presented in this paper. This radar is integrated into under-wings pods that are fixed on a STEMME S10VT motorglider. This radar concept combines real aperture in the cross-track direction, by the antennas geometrical aperture, and synthetic aperture in the along-track direction, realized with the aircraft motion. Radar front-end uses FMCW technique which allows to reduce the power emission to a few Watts. In addition, the use of the millimeter band induces antennas size reduction, and makes possible the radar integration into pods. Thus, radar particularities are a low-size, a low-weight and a low-cost basis, making this radar suitable for future integration on board small vehicles, such as UAV. The radar definition and specifications will be detailed, together with the first results obtained on Fall 2006. Two ways of operating the radar will be presented: An application as vertical sounder, using horn antennas, and an application as SAR radar, using rectangular antennas. The two cases will be illustrated by samples of acquisitions results. The DRIVE radar has been developed and built to fly on board an UAV vehicle. Indeed, this radar is designed and will be used by ONERA as a UAV radar test bench. Current airplane used for testing applications is a STEMME S10-VT motorglider as the geometry of this airplane may be representative of a MALE UAV: wingspan is 23 m large and weigh is about 900 kilograms. DRIVE Project started at ONERA on January 2005, and first flights happened in the beginning of 2006. DRIVE radar uses FMCW techniques and operates in Ka Frequency Band: Central frequency is 35 GHz and bandwidth is about 800 MHz width.

A low-cost imaging radar: DRIVE on board ONERA motorglider

A concept of low-size and low-weight imaging Ka-Band radar is presented in this paper. This radar is embedded into an under-wing pod. This radar particularities are a low-size, a low-weight and a low-cost basis, making this radar suitable for future integration on board small vehicles, such as UAV. The radar definition and specifications will be detailed, together with the results obtained from in flight campaigns executed on 2006 and 2007. This SAR radar has been developed and built to fly on board an UAV-like vehicle. Indeed, this radar is designed and used by ONERA as a UAV radar test bench. Different upgrades are under study, such as a squint-angle acquisition mode or an increase of the polarimetry capacities. In addition, a complementary X band sensor is also manufactured. Current airplane used for testing applications is a STEMME S10-VT motorglider as the geometry of this airplane may be representative of a MALE UAV: wingspan is 23 mm large and weigh is about 900 kilograms.

The ONERA Compact Ka-SAR

Since a few years ONERA has been investing in high performance radar demonstrators. It has been contracted by the French MoD to develop a X-Band high peak power and large bandwidth sensor for airborne experiments dedicated to UHR (Ultra High Resolution) imagery. This paper will describe the RAMSES-NG X-UHR sensor design and results with airborne High Resolution Imagery.

J.F. Nouvel

Papers

KaRIn on SWOT: Characteristics of Near-Nadir Ka-Band Interferometric SAR Imagery

The ONERA RAMSES SAR: latest significant results and future developments

A low-cost imaging radar: DRIVE on board ONERA motorglider

The ONERA Compact Ka-SAR

Ultra-High Resolution and Long Range X-Band Airborne SAR System