The exploitation of synthetic aperture properties in nadir-looking radars is opening new scenarios in the framework of satellite radar altimetry. Both recent and upcoming missions including Cryosat-2, Sentinel-3, Sentinel-6 and SWOT take benefit from the coherent processing of radar data, aimed at improving range measurements in particular contexts, such as ice, open ocean, coastal zone, and even inland waters. This work investigates the possibilities offered by current and future satellite radar altimetry missions for the study of inland water bodies, probing into the peculiarities of the expected radar returns and their potential usage. In this regard, signals collected by the RA-2 instrument (Radar Altimeter 2) onboard the Envisat mission offer an unprecedented possibility, even with a relatively low pulse repetition frequency, to analyze the peculiarities of actual signals for detecting and ranging small water surfaces. In particular, the RA-2 instrument offers a global archive of Individual Echoes (IEs), collected at the native sampling rate of 1795 Hz, in addition to the 18 Hz data obtained by incoherent averaging, which are typically delivered to the users as standard products. RA-2 shares with future radar platforms such as Sentinel-6 a continuous and interleaved working modality, as was recommended by the scientific community in designing next missions’ requirements. This is a further reason to consider the usage of RA-2 IEs as particularly attractive. Whilst only available for a small percentage of the earth’s surface, sufficient IE data exist to study the height retrieval capability of these echoes, in particular for what concerns small water bodies, where we show that enough coherence is exhibited for focusing relatively narrow surfaces and range them correctly. A peculiar aspect of this work lies in the assumption that most of the returned echoes (in RA-2 IEs) are specular. A theoretical framework is developed according to this assumption, which is validated by investigating real RA-2 data and observing their related specular features. In particular, we discuss how specular echoes are expected to be very common in inland altimetry, and are most often associated with small to medium size lakes and rivers. This paper illustrates the expected electromagnetic behavior of specular water targets by exploiting the classical radar cross-section (RCS) theory for specular surfaces. Results from the model are compared with real IE data in three selected case studies, regarding two rivers of variable width and one flood plain, in order to check different hydrological regimes. The model very closely matches the data in all cases, making the results of this validation activity very promising. In particular, we demonstrate the feasibility of using satellite radar altimetry in rivers much smaller than what was considered possible until now.

The velocity of sea surface gravity waves depends on the wave wavenumber, k, and the surface current, U. The surface current can thus be inferred from the displacement of waves in as sequence of ocean surface images. This has been demonstrated using both marine radars and aerial video to image the ocean waves. In this paper we describe the implementation with IKONOS satellite images. We discuss special considerations for obtaining and processing satellite images: the need for precise co-registration of the images; the optimum sun angle for imaging ocean waves; the optimum oceanographic conditions; and the use of multi-spectral bands to edit non wave surface features (e.g., whitecaps). In the test case a current velocity map was produced at 200 m grid spacing. The rms error in the velocity estimates is 10 cm/s.

The Golfe d’Arguin (NW Mauritania) is a 15,000 km2 shelf, 150 km in N-S, and 150 km offshore.  The Golfe is subdivided into inner and outer shelves.  The shallow (<10m) inner shelf, also known as Banc d’Arguin, is part of a UNESCO World Heritage Site and renowned for a richly diverse marine ecosystem.   A steep escarpment separates the deeper (20-200 m) outer shelf.    The bottom morphology of both zones is a complex system of shoals, canyons, and channels.               The water is too turbid for traditional optical remote sensing bathymetry.  The alternative is bathymetry derived from ocean wave celerity as waves shoal over the shelf.              The celerity method has been practiced for several decades, but only with small sensor footprints (typically 10-100 km2).    The Sentinel 2 satellites (now two, with plans to expand to four) have a 290 km push-broom image swath, capturing 2100 km2 per second.  The entire Earth is imaged every 4-5 days.  The imagery is free, very accessible, and easy to process.   The Sentinel 2 image archive is five years and growing daily.  Mapping large regions such as the Golfe d’Arguin is made possible.              This presentation describes the Sentinel 2 bathymetry workflow.   The Golfe d’Arguin is an excellent case study.  Long swell waves from mid-Atlantic storms are frequent.  Long waves are best for sensing the bottom depth.  (Conversely, the method does not work in locations such as the Persian Gulf shielded from oceanic wave systems.)              For the test case, the horizontal resolution is 200m, covers depths 0 to 35 m, and accuracy is 5% of depth.   Comparisons are made with the General Bathymetric Chart of the Oceans (GEBCO), EMODnet DTM, and various literature sources.    GEBCO compares very poorly.  EMODnet is better but with much lower spatial resolution and does not capture the morphological detail seen in Sentinel 2 derived bathymetry.              There is very limited ground truth bathymetry, e.g., Multibeam, in this area for rigorous validation.   Other validation methods are presented.  One is Sentinel 2 bathymetry of the Perth Australia area,  with wave conditions similar to Mauritania, and a LiDAR survey for ground truth.   Another is repeatability with images from different dates.              Below links preview Golfe d’Arguin bathymetry with Sentinel 2 and EMODnet for comparison.  GoogleEarth can be used to view.                                       http://jOmegak.com/vEGU/28QBH.Sentinel2.kmz                                       http://jOmegak.com/vEGU/28QBH.EMOD.kmz

ESA's Envisat mission carried a RA-2 radar altimeter since its launch in 2002 to sense sea state and especially measure sea surface height (SSH). The onboard processing combined multiple echoes incoherently to reduce Speckle noise and benefit from data compression. In fact, according to past literature the amplitudes were generally expected to be independent. Nevertheless, samples of complex data time series of individual echoes (IE) were down-linked and archived since 2004 for research studies. In this note we demonstrate that there is sufficient inter-pulse coherence for Doppler processing and we suggest that the archived data can be re-processed into improved SSH. This is of particular interest in challenging domains (e.g., coastal zone) where coherent processing can mitigate errors from ocean surface backscatter inhomogeneity and nearby land backscatter. A new method called zero-Doppler to process IEs is thus proposed and discussed.

Currents transport the sediment discharge of the Amazon River as far as the Orinoco Delta (Venezuela). The combined actions of waves (predominately from the NE) and the Guiana Current create mud banks of 30 km in width. A continuous process of mud erosion and accretion propagates the mud banks westward The talk demonstrates tracking the mud banks with satellite-derived bathymetry (SDB). The SDB method used here is not the familiar Lyzenga bottom radiance to depth inversion which works only in clear waters. Here there is no bottom visibility. Instead, the SDB uses the interaction of ocean waves with the bottom. Ocean waves exhibit refraction, slower celerity, and reduced wavelength as they ‘feel’ the bottom. These phenomena are observable regardless of water turbidity.WKB has been successfully implemented with X-band radars on coastal towers and ships (by German and UK researcher groups); and with the WorldView and Pleiades satellites (by this author and others). However, all these sensor modalities have small ground footprints (~10 km2 to 100 km2).The European Sentinel-2 satellites have dramatically increased WKB coverage to a regional scale. This talk presents a Sentinel-2 view of the 1500 km muddy coastline, extending up to 50 km offshore (a total area of 75,000 km2). The leap in WKB possibilities was made possible by a 220 km image swath, repeat visits every five days, and the free distribution of the images from the Copernicus portal.