Coastal Altimetry

This chapter describes the radar altimetry principle and relates the radar altimetry missions since the first demonstration mission, Skylab in 1975, to the most recently planned mission, Sentinel-3, extending to a proposed revolutionary two-dimensional topography mission, SWOT. A historical recall and a description of each mission are provided, recalling their achievements with a focus on precursor applications of radar altimetry to coastal zones.

The Global Reservoir and Lake Monitor (GRLM) records variations in surface water height for approximately 70 lakes and reservoirs worldwide using a combination of satellite radar altimetry data sets. The project was initiated by the U.S. Department of Agriculture’s (USDA) Foreign Agricultural Service (FAS) in cooperation with the National Aeronautic and Space Administration’s (NASA) Goddard Space Flight Center (GSFC) and the University of Maryland (UMD). On-line since the end of 2003, the program focuses on the delivery of near-real-time products within an operational framework and exists within the USDA’s decision support system (DSS) through the larger cooperative USDA/NASA Global Agricultural Monitoring (GLAM) program. Currently, near-real-time products are derived from the NASA/Centre National d’Etudes Spatiales (CNES) Jason-1 mission (post-2002) with archival products derived from the NASA/CNES TOPEX/Poseidon mission (1992–2002) and the US Naval Research Lab’s (NRL) Geosat follow-on (GFO) mission (2000–2008). Validation exercises show that the products vary in accuracy from a few centimeters RMS (root mean square) to several tens of centimeters RMS depending on the target size and surface wave conditions. On a weekly basis, new satellite data are retrieved and products updated. Output is in the form of graphs and text files with web links to other imaging and information resources. The next phase of the program sees an expansion to over 500 lakes and reservoirs via the incorporation of products derived from the European Space Agency (ESA) remote sensing satellites (ERS-1 and ERS-2, 1992–2008) and the ESA environmental satellite ENVISAT (post-2002). Near-real-time products will also be continued via data from the follow-on Jason-2 mission (post-2009). The USDA/FAS utilize the products for irrigation potential considerations and as general indicators of drought and high-water conditions. The monitoring system thus has relevance to water resources management and agriculture efficiency at both the national and international level.

By providing a large number of sea surface height observations globally and continuously, satellite altimetry provides a key contribution to the study of the global ocean. In coastal oceans, however, the use of altimetry is not as mature as in the deep oceans, owing to a number of problems that are discussed in other chapters of this book. Given that a number of applications in regional and coastal areas would benefit from improved altimetry datasets, there is a clear scope for the development of coast-dedicated altimetry products where processing of the radar signals in the vicinity of the shoreline is optimized with respect to specific problems. The requirements for such products were not predetermined; instead, they have been decided after a consultation with the potential users of the coastal altimetry community carried out within the PISTACH and COASTALT projects in collaboration with CNES and ESA. This chapter describes the collection of users’ views on the basis of two questionnaires developed and distributed by the project teams, and presents the resulting recommendations on the definition of the new coastal altimetry products.

There has been considerable interest in the past few years in addressing some of the long-term technical difficulties associated with retrieving valid measurements from satellite altimeters in coastal areas, where high levels of human activities are putting increasing demand for information about sea level, wind and wave conditions. Developments of altimeter waveform retracking techniques, together with the now-established practice of giving users access to altimeter waveform data, has led to rapid progress in our understanding of the challenges posed by waveform shapes in the vicinity of land. In this chapter, we present observational evidence of the huge diversity and complexity of waveforms seen by contemporary altimeters in coastal areas. We proceed with a review of waveform retracking methods, examining first empirical methods, then so-called physically-based methods, including discussion of some of their implementation intricacies. We proceed with providing examples of the application of waveform retracking methods to coastal altimeter waveforms in coastal regions around the world. Finally, we explore some of the new ideas on how it may be possible to exploit prior knowledge, for example about the statistics or the along-track evolution of ocean properties in the coastal domain, to improve the estimation of geophysical parameters. Innovative schemes, such as iterative retracking or simultaneous batch retracking, are discussed as new ways to yield unbiased parameter estimation for land-contaminated waveforms much closer to the land/water interface than is currently possible.

Satellite altimetry works conceptually by the satellite transmitting a short pulse of microwave radiation with known power towards the sea surface, where it interacts with the sea surface and part of the signal is returned to the altimeter where the travel time is measured accurately. The determination of sea surface height from the altimeter range measurement involves a number of corrections: those expressing the behavior of the radar pulse through the atmosphere, and those correcting for sea state and other geophysical signals. A number of these corrections need special attention close to the coast and in shallow water regions.

The altimeter range should be corrected for tropospheric path delays due to atmospheric pressure at sea level and atmospheric humidity. Over open ocean, these corrections are performed with enough accuracy using the microwave radiometer for the wet path delay and meteorological model analyses for the dry path delay. In coastal areas, specific studies are needed to assess the quality of the standard products and to propose specific processing if necessary. For the wet tropospheric correction, new promising approaches are presented based on optimal combination of radiometer, meteorological model, GNSS and land information. For the dry tropospheric correction, an assessment of the accuracy of the model-based estimation is provided.

Surge Models as Providers of Improved “Inverse Barometer Corrections” for Coastal Altimetry UsersP.L. Woodworth and K.J. Horsburgh

The challenges of supplying accurate tide corrections for satellite altimetry in shelf and near-coastal environments are reviewed. Relative to the deep ocean, tides in shallow water are generally larger and shorter wavelength. Nonlinearity further complicates tide prediction because of the multitude of additional compound tides and overtides that must be accounted for and because standard deep-ocean methods of inferring minor tides are inapplicable. Development of accurate shelf and coastal tide models with high spatial resolution requires assimilation of high-quality data; for most of the globe, the Topex/Poseidon and Jason time series constitutes the primary source. Data assimilation with nested high-resolution models tied to lower resolution global models appears feasible, but it places severe demands on the accuracy and resolution of bathymetry data. Some envisioned next-generation altimeter missions, capable of mapping the ocean topography at very high resolution, could relax these demands by providing direct tidal measurements at the requisite scales.

Altimetry missions in the last 16 years (TOPEX/Poseidon, ERS-1/2, GFO, Jason-1 and ENVISAT) and the recently-launched Jason-2 mission have resulted in great advances in deep ocean research and operational oceanography. However, oceanographic applications using satellite altimeter data have become very challenging over regions extending from near-shore to the continental shelf and slope (Cipollini et al. 2008). In these regions, intrinsic difficulties in the corrections (e.g., the high frequency ocean response to tidal and atmospheric loading, the mean sea level, etc.) and issues of land contamination in the radar altimeter and radiometer footprints result in systematic flagging and rejection of these data. Forthcoming altimeter missions (SARAL/AltiKa, SWOT, Sentinel-3, etc.) are designed to be better-suited for use in the coastal ocean. However, a number of studies have dealt with the problem of re-analysing, improving and exploiting the existing archive to monitor coastal dynamics. The early encouraging results (Vignudelli et al. 2005; Bouffard et al. 2008, Birol et al. submitted J Mar Syst 2009) support the need for continued research in coastal altimetry, with the opportunity of providing input and recommendations for future missions.

This chapter reviews the current status of the X-TRACK processing application (Roblou et al. 2007), whose objectives are to improve both the quantity and quality of altimeter sea surface height (SSH) estimates in coastal regions by reprocessing a posteriori (the standard Geophysical Data Records) (GDR) as delivered by operational centres, i.e. by improving the post-processing stage. Latest improvements on along-track spatial resolution (high rate data streams and removal of large-scale errors) that promise improved monitoring of coastal dynamics are also detailed. In addition, with a view to integrating coastal-oriented altimeter datasets into models for coastal ocean state analysis, methodologies for matching models with observations are discussed.

The production, formatting, and dissemination of altimeter data products have long been focused on users interested in the open ocean domain. The volume of that data set and the historical constraints on computer resources and storage media have been the driving forces behind the philosophy of the data formatting. As a result, we have dense products with a focused goal: ocean applications. Development of higher-level products has taken advantage of the consensus of approach possible over ocean domains to provide simplified, even further condensed products. As computing resources become increasingly less expensive and more regional users have entered the realm of altimetry, new data requirements have arisen: the need for standardization of products across the altimeter missions and applications, the full internal description of the data, and the flexibility to extend the products with local model information. One approach to answer these requirements is to move from a simple data product concept, to a more extensive data service approach. Projects like COASTALT, PISTACH, and RADS are already providing standardization in formats and more extensive metadata, required to underpin this concept. RADS and the JPL Access07 project take a further step in allowing generation of either standard or user defined products at the time of data delivery. A fully developed service, as envisioned here, would be capable of offering a range of specialized, tailored products, on demand, for a range of coastal users, who could select and combine altimeter data and models from various services simultaneously and then assimilate those data directly into their own application. These services alleviate the historical requirement of needing vast knowledge and time to understand the intricacies of the data products of each individual altimeter mission and would benefit all altimetry users.

The determination of global and regional mean sea level variations with accuracies better than 1 mm/year is an important yet challenging problem, the resolution of which is central to the current debate on climate change and its impact on the environment. To address this, highly accurate time series from both satellite altimetry and tide gauges are needed. In both cases, the desired accuracy represents a significant challenge for the geodetic community. From the perspective of space borne altimetry, systematic errors from the orbit, reference frame and altimeter systems are all important limiting factors and must be minimized in order to derive data products of greatest geophysical value. Indeed, the objective for the overall accuracy of future altimeter systems is 1-cm (RMS) along with a stability of 1 mm/year. From the terrestrial perspective, estimating the vertical velocity of tide gauge sites to sufficient accuracy is also one of the most important and challenging problems in modern geodesy. Essential to reaching these goals in the measurement of mean sea level variation are ultra-precise validation and calibration techniques, including in situ absolute calibration experiments. Most of the present calibration experiments are on or near the coast, reinforcing the need for developing such techniques to unify the altimetric error budget for both open-ocean and local (coastal) conditions.

Improved coastal altimetry strategies are validated over the northwestern Mediterranean Sea with tide gauge (TG) records. Cross-comparisons made with a standard altimetric product highlight significant qualitative and quantitative improvements. The data processed by improved methods are able to detect smaller dynamical processes compared to the standard altimetric products. Lastly, the improved datasets allow us to recover additional coastal data, principally closer to the coast. The improved altimetric data have been used to monitor the Liguro-Provençal-Catalan current coastal dynamics in the Gulf of Lion. For the first time, we have both altimetric and sea surface temperature (SST) observations of intrusions impinging on the continental shelf following a strong southeasterly wind event, which are consistent with past studies based on numerical simulations.

The Caspian Sea is the largest continental water body on the Earth. Owing to its size and geographical location, the sea is truly a large-scale climatic and ecological indicator. One of the most important parameters of the state of the Caspian Sea is significant interannual sea level changes that puzzle the scientists of many countries. Significant water level fluctuations have serious consequences for the region, leading to displacement of thousands of shoreline inhabitants and damaging industrial constructions and infrastructure. Continuous weather-independent observations from radar altimetry are a natural choice to complement the existing in situ sea level observations and to provide new information for open sea regions that have never been covered by direct sea level observations. The Caspian Sea can be considered as an intermediate target between the Open Ocean and closed lakes; it is therefore an interesting object to evaluate the potential of satellite altimetry for enclosed water bodies.

Physical processes in the coastal zone of the Black Sea which can contribute to a correlation between altimeter-derived SLA (sea level anomaly) data and corresponding in situ measurements are briefly reviewed. Examples of using the SLA data in studying long-term variability of the Black Sea level and mesoscale water dynamics are presented. Results of comparison between observed (in situ) and altimeter-derived SLA and wind speed values made for several coastal locations are discussed. A new approach is proposed for better consistency between altimeter-derived and observed wind speed values, which is on the basis of the decomposition of the wind directions in four quadrants in relation to the coastline orientation.

The Barents and White Seas are the seas of the Arctic Ocean. Today complicated hydrodynamic, tidal, ice, and meteorological regimes of these seas may be investigated on the basis of remote sensing data, specifically of satellite altimetry data. Results of calibration and validation of satellite altimetry measurements (sea surface height and sea surface wind speed) and comparison with regional tidal model show that this type of data may be successfully used in scientific research and in monitoring of the environment. Complex analysis of the tidal regime of the Barents and White Seas and comparison between global and regional tidal models show advantages of regional tidal model for use in tidal correction of satellite altimetry data. Examples of using the sea level data in studying long-term variability of the Barents and White Seas are presented. Interannual variability of sea ice edge position is estimated on the basis of altimetry data.

In many ways applications of satellite altimetry along the coasts of North America are representative of similar applications elsewhere in the world. On the east coast, North America has a fairly broad continental shelf which impacts coastal altimetry applications more than the relatively narrow shelf of the west coast. The Gulf of Mexico is a unique coastal region where altimetry mapping of ocean currents is significantly more successful than using traditional satellite measurements of sea surface temperature and ocean color. Meteorological conditions differ strongly between the east and west coasts with corresponding impact on the use of altimetry. Also there have been many more studies of altimeter data on the west coast than on the east coast of North America. We summarize here past studies of altimetric applications for all three coastal regions (West and East Coasts and the Gulf of Mexico) and briefly discuss the implications of tidal effects, impacts of water vapor corrections, and the need for the retracking of near coastal altimetric waveforms.

A major limitation of conventional altimetry is that most data in coastal seas are flagged as useless partly because of land contamination of altimeter return waveforms. The land effect on waveforms is analyzed at different coast terrains, shoreline shapes, and when the altimeter is approaching and leaving the land. Retracking algorithms (Ocean, Ice-2, OCOG (offset center of gravity), Threshold, and Beta5) are implemented and tested using Jason1 waveforms for 1 year (March 2006 to February 2007; cycle 155 to cycle 188) around China and adjacent coastal seas (14–45°N, 105–130°E). We use three methods to compare the retracking algorithms. First, the bias, root mean square, and standard deviation of sea level anomaly differences between ascending and descending passes are calculated at crossover points; Second, in situ sea surface height measurements from tide gauge stations are used to compare with the retracked altimetry sea surface height. Finally, in situ and altimetry significant wave height measurements are compared. We found that in coastal seas the OCOG algorithm provides more accurate results than others.

This chapter provides an overview of recent research applications utilizing satellite altimetry around Australia. Topics covered include improving the quality of altimeter sea surface height (SSH) data in coastal regions, observing and understanding the structure and variability of the major boundary current systems, estimating regional sea-level changes, and determining and verifying the marine gravity field using altimetry. The approaches highlighted in this chapter use altimetry synergistically with all available oceanic data including other remote sensing techniques, drifting buoys, and in situ data such as coastal tide-gauges. The results presented are an integration of altimetric and in situ data with a high-resolution computer model in order to simulate the sea-level changes in Australian coastal and offshore regions. Through such synthesizing research approaches, satellite altimetry continues to make an important contribution to a number of key strategic research areas in the Australasian region.

Accurate and continuous monitoring of lakes and inland seas has been possible since 1993 thanks to the success of satellite altimetry missions: TOPEX/POSEIDON (T/P), GFO, JASON-1, and ENVISAT. Global processing of the data of these satellites can provide time series of lake surface heights over the entire Earth at different temporal and spatial scales with a subdecimeter precision. Large lakes affect climate on a regional scale through albedo and evaporation. In some regions, highly ephemeral lakes provide information on extreme events such as severe droughts or floods. On the other hand, endorheic basin lakes are sensitive to changes in regional water balance. In a given region covered by a group of lakes, if the records of their level variations are long enough, they could reveal the recurrence of trends in a very reliable and accurate manner. Lakes are thought to have enough inertia to be considered as an excellent proxy for climate change. Moreover, during the last century, thousands of dams have been constructed along the big rivers worldwide, leading to the appearance of large reservoirs. This has several impacts on the basins affected by those constructions, as well as effects on global sea level rise. The response of water levels to regional hydrology is particularly marked for lakes and inland seas of semiarid regions. Altimetry data can provide a valuable source of information in hydrology sciences, but in-situ data (river runoff, water level, temperature, or precipitation) are still strongly needed to study the evolution of the water mass balance of each lake.

Conventional ocean-viewing radar altimeters (RA) have inherent limitations that become especially apparent when considering applications in the coastal environment. This chapter looks at three new technologies, two of which are to be on forthcoming missions: the delay-Doppler (or SAR mode) altimeter, to be embarked on CryoSat-2 and Sentinel-3, and Alti-Ka, on India’s SARAL mission (previously Oceansat-3). These instruments are nadir-viewing, each featuring a smaller footprint, improved tracking, and finer measurement precision. Wide swath techniques also are reviewed, represented by KaRIn. That concept offers altimetric coverage over an area that extends nominally 80 km to both sides of its nadir track. Relative to coastal applications, all three approaches have their respective advantages and limitations, highlighted in the discussion.