Radar Altimetry Tutorial
301 pages
English

Radar Altimetry Tutorial

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Radar Altimetry Tutorial
December 2 006
V. Rosmorduc, J. Benveniste, O . Lauret, M. Milagro, N. Picot
J. Benveniste, N. Picot, Editors Radar Altimetry Tutorial
Radar Altimetry Tutorial
Predicting climate, monitoring mean sea level, river and lake levels, global warming, El Niño and La
Niña events, marine currents and ocean circulation, tides, geoid estimates, wind, wave and marine
meteorology models, ice sheet topography and sea ice extent, etc. Radar altimetry can provide such a
wealth of information -- and more -- from its measurements.
This Radar Altimetry Tutorial describes applications, examples (data use cases) and techniques, including
standard data processing, as well as the various satellite missions that have carried, are carrying or will carry
a radar altimeter onboard, plus a range of altimetry products (data, software and documentation).
A Basic Radar Altimetry Toolbox is also available. This is a collection of tools and documents designed to
facilitate the use of radar altimetry data. It can read most distributed radar altimetry data, from ERS-1 & 2,
Topex/Poseidon, Geosat Follow-on, Jason-1, Envisat to the future Cryosat missions, and can perform
processing and data editing, extraction of statistics, and visualisation of results.


Acknowledgments & contributors
This tutorial was produced by CLS under contract to ESA and CNES.
Citation
If using this tutorial, please cite:
Rosmorduc, V., J. benveniste, O. Lauret, M. Milagro, N. Picot, Radar ...

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Nombre de lectures 185
Langue English
Poids de l'ouvrage 6 Mo

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Radar Altimetry Tutorial December 2 006 V. Rosmorduc, J. Benveniste, O . Lauret, M. Milagro, N. Picot J. Benveniste, N. Picot, Editors Radar Altimetry Tutorial Radar Altimetry Tutorial Predicting climate, monitoring mean sea level, river and lake levels, global warming, El Niño and La Niña events, marine currents and ocean circulation, tides, geoid estimates, wind, wave and marine meteorology models, ice sheet topography and sea ice extent, etc. Radar altimetry can provide such a wealth of information -- and more -- from its measurements. This Radar Altimetry Tutorial describes applications, examples (data use cases) and techniques, including standard data processing, as well as the various satellite missions that have carried, are carrying or will carry a radar altimeter onboard, plus a range of altimetry products (data, software and documentation). A Basic Radar Altimetry Toolbox is also available. This is a collection of tools and documents designed to facilitate the use of radar altimetry data. It can read most distributed radar altimetry data, from ERS-1 & 2, Topex/Poseidon, Geosat Follow-on, Jason-1, Envisat to the future Cryosat missions, and can perform processing and data editing, extraction of statistics, and visualisation of results. Acknowledgments & contributors This tutorial was produced by CLS under contract to ESA and CNES. Citation If using this tutorial, please cite: Rosmorduc, V., J. benveniste, O. Lauret, M. Milagro, N. Picot, Radar Altimetry Tutorial, J. Benveniste and N. Picot Ed., http://www.altimetry.info, 2006. Authors V. Rosmorduc (CLS), J. Benveniste (ESA), O. Lauret (Silogic), M. Milagro (SERCO), N. Picot (CNES) Editors J. Benveniste (ESA), N. Picot (CNES) Scientific committee G. Goni (NOAA, USA), S. Laxon (UCL, UK), J.M. Lefèvre (Météo France, France), C. Maes (IRD, New Caledonia), F. Rémy (Legos/CNRS, France), J. Tournadre (Ifremer, France) Acknowledgments Thanks to M. Ablain, L. Amarouche, J. Dorandeu, J.P. Dumont, P. Escudier, S. Guinehut, F. Lefèvre, F. Mercier, P. Schaeffer, P. Thibaut, O.Z. Zanifé (CLS) for inputs and advices n n n n n l n n n m n m n l n l n n n m n m n n n l n m n m m n m m n m m l n m n n m Radar Altimetry Tutorial Contents Introduction Overview 1. Altimetry applications 1.1. Geodesy & geophysics 1.1.1. Bathymetry 1.1.2. Geodesy 1.1.3. Other geophysical applications 1.1.4. Tsunami 1.2. Ocean 1.2.1. Large-scale circulation 1.2.2. Ocean currents and eddies 1.2.3. Operational oceanography 1.2.4. Tides 1.2.5. Mean Sea Level rise and the Greenhouse effect 1.3. Ice 1.3.1. Ice sheets 1.3.2. Sea ice 1.4. Climate 1.4.1. El Niño - Southern Oscillation (ENSO) 1.4.2. North Atlantic Oscillation (NAO) 1.4.3. Decadal oscillations 1.4.4. Seasons 1.5. Atmosphere, wind & waves 1.5.1. Wind & waves 1.5.2. Cyclones, hurricanes and typhoons 1.5.3. Rain 1.6. Hydrology & land 1.6.1. Lake level 1.6.2. Land 1.6.3. River level 1.7. Coastal 2. Data use cases 2.1. Altimetry data processing for mesoscale studies 2.2. Western boundary currents 2.3. Temporal variations of the Amazon basin 2.4. El Niño and ocean planetary waves 2.5. Seasonal distribution of Significant Wave Height 3. Altimetry 3.1. How it works 3. 1.1. Basic Principle 3.1.2. From Radar pulse to the altimetric measurements 3.1.3. Frequencies used, and their impacts 3.1.4. Multi-mission combinations n n n m m n n n m n l n l m m n m n n l m n n n n m n m n m n n n l n n 3.2. Data flow 3.2.1. Data acquisition 3.2.2. Data processing 3.2.3. Data qualification 3.3. Future technology improvements 3.3.1. Ka-band 3.3.2. Constellations 3.3.3. Interferometers 3.3.4. GNSS 4. Altimetry missions 4.1. Past missions 4.1.1. Skylab 4.1.2. GEOS 3 4.1.3. Seasat 4.1.4. Geosat 4.1.5. ERS-1 4.1.6. Topex/Poseidon 4.2. Current missions 4.2.1. ERS-2 4.2.2. GFO 4.2.3. Jason-1 4.2.4. Envisat 4.3. Future missions 4.3.1. Jason-2 4.3.2. Cryosat 4.3.3. (AltiKa) 4.3.4. NPOESS 4.3.5. Sentinel 3 5. Product information 5.1. Product information 5.2. Toolbox 6. FAQs 6.1. Applications 6.2. Altimetry 6.3. Toolbox Glossary Radar Altimetry Tutorial Tutorial overview This Radar altimetry tutorial is organised into six main chapters: 1. Applications This chapter describes the main applications of radar altimetry, both current and in development, and is organised according to field of study. 2. Data use cases This chapter gives some practical examples of the applications of altimetry data. The type of data to use, the methodology and the main computations are detailed, as well as how to work with data using the Basic Radar Altimetry Toolbox. 3. Altimetry This chapter contains background information about altimetry techniques, how data are measured, processed and qualified, as well as the future technology developments being studied. 4. Altimetry missions This chapter describes the past, present and future altimetry satellites, with details about the altimetry- related instruments onboard, their orbits and ground segments. This chapter describes the past, current and future altimetry satellites, with details on the altimetry-related instruments onboard, the orbit and ground segment. 5. Product information This chapter covers altimetry products (software and documentation as well as data). It also gives access to the Basic Radar Altimetry Toolbox. 6. FAQs This chapter contains questions asked about the tutorial and the toolbox, applications, altimetry and products. In general, the deeper you go into the sub-sections, the more technical the information becomes. This document is mainly aimed at newcomers to altimetry. For this particular audience, we suggest beginning with a specific field of interest (Geodesy & geophysics, Ocean, Ice, Climate, Atmosphere, wind & waves, Hydrology & land, Coastal), then having a look at the corresponding use cases, if any. The 'Altimetry' section provides more technical information, but for an initial approach you can focus on the section headers, where an overview is given (e.g. How altimetry works : basics). A contents gives a general view of the Tutorial. This can be used as a doorway for advanced and expert users, enabling them to go directly to the technical information. l l l l l l l Radar Altimetry Tutorial 1. Altimetry applications A wealth of applications are possible using radar altimetry measurements, involving most geoscience fields and practised by more than a thousand teams of users around the world. From the 'historical' applications (geodesy, general ocean circulation) to the developing ones (solid Earth and coastal applications, etc) and the ones that have become classic (ocean variability, ice topography, hydrology), altimetry has shown over and over that it is a very productive technique. Geodesy & geophysics Ocean Ice Climate Atmosphere, wind & waves Hydrology & land Coastal l l l l Radar Altimetry Tutorial 1.1. Geodesy & geophysics Geophysics is the study of the substances that make up the Earth and the physical processes occurring on, in and above it. Information derived from altimetry data can be used to study the Earth's shape and size, gravity anomalies (geodesy), seafloor relief (bathymetry), tectonic plate motion and rifts (geophysics), etc. Although often linked to plate tectonics, tsunamis are very different, transient phenomena. However, their impact on the sea surface can be seen by altimeters in some cases, thus helping the study of their propagation. Bathymetry Geodesy Other geophysical applications Tsunami Example of geophysical information extracted from altimetry (around Italy) (Credits University of Calgary) Further information: McAdoo, D., Marine Geoid, Gravity, and Bathymetry: An increasingly clear view with satellite altimetry, 15 years of progress in radar altimetry Symposium, Venice, Italy, 2006 Radar Altimetry Tutorial 1.1.1. Bathymetry estimate from altimetry Dense satellite altimeter measurements can be used in combination with sparse measurements of seafloor depth to construct a uniform resolution map of the seafloor topography. These maps do not have sufficient accuracy and resolution to be used for assessing navigational hazards, but they are useful for such diverse applications as locating obstructions/constrictions to the major ocean currents and shallow seamounts where fish and lobster are abundant. Detailed bathymetry also reveals plate boundaries and oceanic plateaus. A detailed knowledge of topography is fundamental to the understanding of most Earth processes. In the oceans, detailed bathymetry is essential for understanding physical oceanography, biology and marine geology. Currents and tides are controlled by the overall shapes of the ocean basins, as well as by the smaller, sharp ocean ridges and seamounts. Sea life is abundant where rapid changes in ocean depth deflect nutrient-rich water toward the surface. Because erosion and sedimentation rates are low in the deep oceans, detailed bathymetry also reveals mantle convection patterns, plate boundaries, the cooling/subsidence of the oceanic lithosphere, oceanic plateaus and the distribution of off-ridge volcanoes. Since it is impossible to map the topography of the ocean basins directly from Space, most seafloor mapping is a tedious process that is carried out by research vessels equipped with echo sounders. However, completely mapping the ocean Bathymetry can be computed using altimetry basins at a horizontal resolution of 100 m would take about 125 together with other data ship-years of survey time using the latest technology, with (Credits CNES) highly non-uniform data. Thus, until recently, our knowledge of the seafloor topography was poor. Radar altimeters aboard the ERS-1 and Geosat spacecraft have surveyed the marine gravity field over nearly all of the world's oceans with high accuracy and moderate spatial resolution. In March 1995, ERS-1 completed its dense (~8 km track spacing at the equator) mapping of sea surface topography between the latitudes of 81.5° North and South. These data have been combined and processed to form a global marine geoid or gravity grid [Cazenave et al., 1996; Sandwell and Smith, 1997]. In the wavelength band 15 to 200 km, gravity anomaly variations are highly correlated with seafloor topography and thus, in principle, can be used to recover topography. l l l Gravity anomalies (left), computed from altimetry, and predicted topography (right) deduced from these gravity anomalies plus in situ measurements. (Credits NOAA/Scripps Institution of Oceanography) The basic theory for predicting seafloor topography from satellite altimeter measurements is summarised in a paper by Dixon et al. [1983]. The conceptual approach uses the sparse depth soundings to constrain the long- wavelength depth while the shorter-wavelength topography is predicted from the downward-continued satellite gravity measurements [Smith and Sandwell, 1994]. There are a number of complications that require careful treatment, e.g.: computing bathymetry from gravity anomalies is only possible over a limited wavelength band, longer wavelengths in this band are highly dependent on the elastic thickness of the lithosphere and/or crustal thickness, sediments favour filling bathymetric lows and can eventually completely bury the pre-existing basement topography. Bathymetry computed near Madagascar. Near Comoros Island, between Africa and Madagascar, and east of this island (the Mascarene ridge) the undersea mounts are very specific. (Credits Legos/CNRS) References: - Cazenave, A., P. Schaeffer, M. Bergé, and C. Brossier, High-resolution mean sea-surface computed with altimeter daat of ERS-1 (Geodetic mission) and Topex/Poseidon, Geophys. J. Int., 125, 696-704, 1996. - Dixon, T. H., M. Naraghi, M.K. McNutt, and S.M. Smith, Bathymetric prediction from Seasat altimeter data. J. Geophys. Res. 88, 1563-1571, 1983. - Smith, W.H.F., and D.T. Sandwell, Bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry. J. Geophys. Res. 99, 21,803-21,824, 1994. - Smith, W.H.F., and D.T. Sandwell, Global sea floor topography from satellite altimetry and ship depth soundings. Science 277, 1956-1961, 1997. Further information: - Sandwell D.T. and W.H.F. Smith, Bathymetric estimation Satellite altimetry and Earth sciences, L.L. Fu and A. Cazenave Ed., Academic Press, 2001 - Measured and estimated seafloor topography (UCSD, USA)
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