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The Institute of Electrical and Electronics Engineers (IEEE) Geoscience and Remote Sensing Society (GRSS) created the GRSS “Standards for Earth Observation Technical Committee” to advance the usability of remote sensing products by experts from academia, industry, and government through the creation and promotion of standards and best practices. In February 2019, a Project Authorization Request was approved by the IEEE Standards Association (IEEE-SA) with the title “Standard for Spaceborne Global Navigation Satellite Systems Reflectometry (GNSS-R) Data and Metadata Content.” At present, 4 GNSS constellations cover the Earth with their navigation signals: The United States of America (USA) Global Positioning System GPS with 31 Medium Earth Orbit (MEO) operational satellites, the Russian GLObal’naya NAvigatsionnaya Sputnikovaya Sistema GLONASS with 24 MEO operational satellites, the European Galileo with 24 MEO operational satellites, and the Chinese BeiDou-3 with 3 Inclined GeoSynchronous Orbit (IGSO), 24 MEO, and 2 Geosynchronous Equatorial Orbit (GEO) operational satellites. Additionally, several regional navigation constellations increase the number of available signals for remote sensing purposes: the Japanese Quasi-Zenith Satellite System QZSS with 1 GSO and 3 Tundra-type orbit operational satellites, and the Indian Regional Navigation Satellite System IRNSS with 3 GEO and 4 IGSO operational satellites. On the other hand, there are different GNSS-R processing techniques, instruments and spaceborne missions, and a wide variety of retrieval algorithms have been used. The heterogeneous nature of these signals of opportunity as well as the numerous working methodologies justify the need of a standard to further advance in the development of GNSS-R towards an operational Earth Observation technique. In particular, the scope of this working group is to develop a standard for data and metadata content arising from past, present, and future spaceborne missions such as t, This work was supported in part by the National Aeronautics and Space Administration (NASA) Science Mission Directorate with the University of Michigan under Contract NNL13AQ00C., Peer Reviewed, Article signat per 25 autors/es: Hugo Carreno-Luengo, University of Michigan (UM), Ann Arbor, MI, USA / Adriano Camps, CommSensLab-UPC, Universitat Politecnica de Catalunya (UPC), Barcelona, Spain / Chris Ruf, University of Michigan (UM), Ann Arbor, MI, USA / Nicolas Floury, European Space Research and Technology Center (ESTEC), European Space Agency (ESA), Noordwijk, The Netherlands / Manuel Martin-Neira, European Space Research and Technology Center (ESTEC), European Space Agency (ESA), Noordwijk, The Netherlands / Tianlin Wang, University of Michigan (UM), Ann Arbor, MI, USA / Siri Jodha Khalsa, National Snow and Ice Data Center (NSIDC), University of Colorado, Boulder, CO, USA / Maria Paola Clarizia, Deimos Space UK, Didcot, U.K. / Jennifer Reynolds, Deimos Space UK, Didcot, U.K. / Joel Johnson, Electrical and Computer Engineering, The Ohio State University, Columbus, OH, USA / Andrew O’Brien, Electrical and Computer Engineering, The Ohio State University, Columbus, OH, USA / Carmela Galdi, Universita degli Studi del Sannio, Benevento, Italy / Maurizio Di Bisceglie, Universita degli Studi del Sannio, Benevento, Italy / Andreas Dielacher, RUAG Space GmbH, Vienna, Austria / Philip Jales, Spire Global, Boulder, CO, USA / Martin Unwin, Surrey Satellite Technology Ltd. (SSTL), Guildford, U.K. / Lucinda King, Surrey Space Centre, University of Surrey, Guildford, U.K. / Giuseppe Foti, National Oceanography Center (NOC), Southampton, U.K. / Rashmi Shah, Jet Propulsion Laboratory (JPL), California Institute of Technology, Pasadena, CA, USA / Daniel Pascual, Deimos Space UK, Didcot, U.K. / Bill Schreiner, University Corporation for Atmospheric Research (UCAR), Boulder, CO, USA / Milad Asgarimehr, German Research Centre for Geosciences (GFZ), Potsdam, Germany / Jens Wickert, German Research Centre for Geosciences (GFZ), Potsdam, Germany, Institute of Geodesy and Geoinformation Science, Technische Universität Berlin, Berlin, Germany / Serni Ribo, Instiut d’Estudis Espacials, Postprint (published version)

Measuring ocean mesoscale variability is one of the main objectives of next generation satellite altimeters. Current radar altimeters make observations only at the nadir sub-satellite ground track, which is not sufficient to sample the ocean surface with the required spatial and temporal sampling. The GNSS-R concept has been proposed as an alternative observation system in order to overcome this limitation, since it allows performing altimetry along several points simultaneously over a very wide swath. Latest proposed GNSS-R altimeter configurations allow measuring sea height with an accuracy of few decimeters over spatial scales of 50-100 km, by means of a single-pass. This paper proposes an innovative processing and retracking concept for GNSS-R altimeters based on the acquisition of the full delay-Doppler map (DDM), which allows to acquire multiple waveforms at different Doppler frequencies, whose footprints are located outside the typical pulse-limited region. The proposed processing adapts the Synthetic Aperture Radar (SAR) delay-Doppler concept of spaceborne radar altimeters for use in a GNSS-R system. This processing yields additional multi-look with respect to conventional GNSS-R concepts and translates into an improvement of the altimetry performance estimated to be at least 25%-30%, and even higher, depending on the wanted along-track spatial resolution. The proposed processing can also provide measurements with high spatial resolution at best possible performance, and more generally, offers various possibilities for optimal trade-off between spatial-resolution and height estimation accuracy., Peer Reviewed, Postprint (published version)

We present the implementation of a facet-based simulator to investigate the forward scattering of L-band signals from realistic sea surfaces and its application to spaceborne ocean Global Navigation Satellite System (GNSS) Reflectometry. This approach provides a new flexible tool to assess the influence of the ocean surface roughness on scattered GNSS signals. The motivation stems from the study by Clarizia et al., which revealed significant differences between delay–Doppler maps (DDMs) obtained from UK-DMC satellite data and DDMs simulated with the Zavorotny–Voronovich (Z-V) model. Here, the scattered power and polarization ratio (PR) are computed for explicit 3-D ocean wave fields, using a novel implementation of the Kirchhoff approximation (KA), which we call the Facet Approach (FA). We find that the FA is consistent with the full KA and the Geometrical Optics (GO) used in the Z-V model, while being less computationally expensive than the KA and able to represent polarization effects not captured by the GO. Instantaneous maps of the bistatic normalized radar cross section computed with the FA show clear patterns associated with the underlying waves. The wave field is particularly visible in the PR, indicating that the scattering is generally dominated by the HH component, particularly from ocean wave troughs. Polarization effects show, for the first time, a strong correlation to the explicit sea surface from which the scattering originated. DDMs of the scattered power computed with the FA reveal patchy patterns and power distributions that differ from those obtained with Z-V and show closer similarities with observed DDMs from UK-DMC.