Tutorial FD-8: Remote Sensing with Reflected and Occulted Global Navigation Satellite System (GNSS) Signals
Presented by: James L. Garrison, Adriano Camps and Estel Cardellach
1: Purdue University, Italy
2: Universitat Politècnica de Catalunya (UPC), Barcelona, Spain
3: Institute of Space Sciences, Spain
Although originally designed for navigation, signals from the Global navigation satellite system (GNSS), ie., GPS, GLONASS, Galileo and COMPASS, pass through the Earth's atmosphere and reflect off of the Earth and ocean surface, where the effects of refraction and rough surface scattering change their properties. Several methods have been developed for inverting these effects to retrieve geophysical data, including ocean surface roughness (winds), soil moisture, and atmospheric refractivity profiles (from which water vapor profiles can be generated).
This tutorial will summarize the current state of the art in physical modeling and signal processing for these measurements, with the majority of the morning session devoted to radio-occultation (RO) signals and the remainder of the tutorial devoted to reflected (GNSS-R) signals.
Radio-occultation (RO) has been developed over the last decade and half, starting with GPS/MET (1995-1997), followed by RO receivers launched on numerous small satellites (including CHAMP, GRACE and SAC-C), culminating in the deployment of the 6-satellite COSMIC constellation in 2006. Refraction of the GNSS signal, as it passes through the Earth's atmosphere, will result in a time-dependent bending angle. This time series can then be inverted to obtain profiles of the index of refraction, from which meteorological data, including temperature and pressure, can be obtained.
GNSS reflection (GNSS-R) measurements are still in the development stage. Airborne experiments have been conducted in both the US and Europe. An extensive set of satellite data have been collected by the UK-DMC satellite, starting in 2004. The ability to sense soil moisture, through the reflectivity of the surface, and ocean roughness through the change in the correlation properties of the scattered signal have been demonstrated on numerous airborne campaigns. Ocean roughness measurements at L-band, provided by GNSS-R signals, have been studied as a source of auxiliary data to correct microwave radiometric sensing of sea surface salinity (SSS). The use of a small, passive, receiver enables GNSS-R instruments to be deployed cheaply on small satellites, light aircraft, and UAV's.
- Fundamentals of radio occultation (RO) measurement: definition of geometry and the relationships between bending angle, impact parameter and the index of refraction. The Abel transform and inverse.
- Introduction to the GNSS signal structure: Correlation properties of PRN codes; BPSK and BOC modulation;
- Summary of radio-holographic processing of RO signals: Back propagation (BP), Full Spectrum Inversion (FSI) and the Canonical Transform (CT).
- Open-Loop (OL) tracking of RO signals.
- RO from airborne platforms.
- Models for the reflected GNSS (GNSS-R) signal: Geometric optics-based models for rough surface scattering, their limitations, and current attempts to improve upon them. Geometry of the bistatic radar problem. Second-order moments of the reflected signal waveform as a stochastic process.
- Geophysical model functions: Ocean height spectrum models (Elfouhaily), and the generation of filtered mean square slope. Models for the slope statistics (Cox and Munk) and reduction of these models to account for the L-band wavelength of GNSS-R signals. Surface reflection coefficients on land and water, and the relationship to soil moisture and ocean salinity.
- Retrieval of geophysical data through inversion of scattering models. Direct inversion of scattering models, to estimate surface roughness from delay-Doppler waveform measurements. Non-linear least squares approaches and their sensitivity. Recent results on full-PDF retrievals. Faster computational methods, including series approximations, waveform peak tracking, and matched filters.
- Power calibration of the reflected signal.
James L Garrison has been a member of the faculty at Purdue University since 2000, where he is currently an Associate Professor in the School of Aeronautics and Astronautics, with a courtesy appointment in the School of Electrical and Computer Engineering. He has also a founding member of the Division of Environmental and Ecological Engineering at Purdue. From 1988 to 2000 he was employed by the National Aeronautics and Space Administration (NASA), first at the Langley Research Center in Hampton VA, and later at the Goddard Space Flight Center in Greenbelt MD. He earned a PhD from the University of Colorado at Boulder in 1997, while on graduate study leave from NASA. He also holds a BS from the Rensselaer Polytechnic Institute and an MS from Stanford University. He is the author or co-author of 24 journal articles and 44 conference proceedings. He holds 5 US Patents. He has received a number of awards, include a NASA New Investigator grant, an Institute of Navigation Early Achievement Award, and an invitation to the Keck Futures Initiative.
Adriano Camps was born in Barcelona, Spain, in 1969. He received the degree in telecommunications engineering and Ph.D. degree in telecommunications engineering from the Universitat Polit'cnica de Catalunya (UPC), Barcelona, Spain, in 1992 and 1996, respectively. In 1991 to 1992, he was at the ENS des Télécommunications de Bretagne, France, with an Erasmus Fellowship. Since 1993, he has been with the Electromagnetics and Photonics Engineering Group, Department of Signal Theory and Communications, UPC, where he was first Assistant Professor, Associate Professor in 1997, and Full Professor since 2007. In 1999, he was on sabbatical leave at the Microwave Remote Sensing Laboratory, of the University of Massachusetts, Amherst. Since 1993, he has been deeply involved in the European Space Agency SMOS Earth Explorer Mission, from the instrument and algorithmic points of view, performing field experiments, and more recently studying the use of GNSS-R techniques to perform the sea state correction needed to retrieve salinity from radiometric observations. His research interests are focused in microwave remote sensing, with special emphasis in microwave radiometry by aperture synthesis techniques and remote sensing using signals of opportunity (GNSS-R).
Dr. Camps was Chair of u Cal 2001, Technical Program Committee Cochair of IGARSS 2007, and co-chair of GNSS-R '10. Currently, he is Associate Editor of Radio Science and the IEEE Transactions on Geoscience and Remote Sensing, and President-Founder of the IEEE Geoscience and Remote Sensing Society Chapter at Spain. In 1993, he received the Second National Award of University Studies; in 1997, the INDRA award of the Spanish Association of Telecommunication Engineers to the best Ph.D. in Remote Sensing; in 1999 the Extraordinary Ph.D. Award at the Universitat Polit'cnica de Catalunya; in 2002, the Research Distinction of the Generalitat de Catalunya for contributions to microwave passive remote sensing; and in 2004 he received a European Young Investigator Award, and in 2009 the ICREA Academia award. Moreover, as a member of the Microwave Radiometry Group, UPC, he received in 2000, 2001, and 2004: the 1st Duran Farell and the Ciutat de Barcelona awards for Technology Transfer, and the "Salvà i Campillo" Award of the Professional Association of Telecommunication Engineers of Catalonia for the most innovative research project for MIRAS/SMOS related activities, and in 2010 the 7th Duran Farell award for Technological Research for the work on GNSS-R instrumentation and applications. He has published nearly 100 papers in peer-reviewed journals, and more than 200 international conference presentations.
Estel Cardellach received a PhD degree from the Polytechnic University of Catalonia, Barcelona, Spain, in 2002. She has been working on scientific applications of GNSS for remote sensing of the Earth, such as extraction of geophysical information of the GNSS reflected signals, radio occultation, and geodetic techniques. She was a National Research Council Awardee for a postdoctoral position at NASA/Jet Propulsion Laboratory, Pasadena, CA (2002-2003); a postdoctoral researcher at Harvard Smithsonian Center for Astrophysics, Cambridge, MA (2003-2005). Since 2005 she is at the Institute of Space Sciences (ICE-CSIC/IEEC), currently under the Spanish Ramon y Cajal program. She is the P.I. of the space-based experimental polarimetric Radio-Occultation mission aboard the Spanish PAZ satellite.