Abstract
Spaceborne radar altimeters record echo waveforms over all Earth surfaces, but their interpretation and quantitative exploitation over the Arctic Ocean is particularly challenging. Radar returns may be from all ocean, all sea ice, or a mixture of the two, so the first task is the determination of which surface and then an interpretation of the signal to give range. Subsequently, corrections have to be applied for various surface and atmospheric effects before making a comparison with a reference level. This paper discusses the drivers for improved altimetry in the Arctic and then reviews the various approaches that have been used to achieve the initial classification and subsequent retracking over these diverse surfaces, showing examples from both LRM (low resolution mode) and SAR (synthetic aperture radar) altimeters. The review then discusses the issues concerning corrections, including the choices between using other remote-sensing measurements and using those from models or climatology. The paper finishes with some perspectives on future developments, incorporating secondary frequency, interferometric SAR and opportunities for fusion with measurements from laser altimetry or from the SMOS salinity sensor, and provides a full list of relevant abbreviations.
Highlights
Within the Arctic Ocean, radar altimetry is used to monitor both the sea level and the freeboard of sea ice, i.e., how far the ice surface is above the surrounding water level
This paper has provided a review of the technical aspects of radar altimetry over the Arctic for both sea level and sea-ice studies and complements the scientific review provided by Johannessen and Andersen [1]
It has shown how the surface type affects the shape and strength of the return waveforms, both for low-resolution measurement (LRM) and Synthetic Aperture Radar (SAR) altimeters. It has covered the challenges of robustly classifying the waveforms and developing retracking approaches for deriving the height of the reflecting surface, especially with spurious extra signals due to off-nadir bright targets
Summary
Within the Arctic Ocean, radar altimetry is used to monitor both the sea level and the freeboard of sea ice, i.e., how far the ice surface is above the surrounding water level. The sea level is a measure of the amount of water within a basin, but assuming the geostrophic approximation to be valid, its spatial derivative is an indicator of surface currents. Arctic Ocean altimetry helps provide a number of measures for monitoring the environment, both for short-term responses to force and to understand the effects of climate change. This paper provides a review of the methodological techniques required to obtain useful environmental information from altimeters, giving an overview of the choice of approaches available to interested users. This paper does not explore the scientific results achieved because there has been a good recent summary of those by Johannessen and Andersen [1]
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