Retrieving Sea Level and Freeboard in the Arctic: A Review of Current Radar Altimetry Methodologies and Future Perspectives

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.

Sensitivity of Radar Altimeter Waveform to Changes in Sea Ice Type at Resolution of Synthetic Aperture Radar

“…Sea ice freeboard estimates are used to assess the potential to retrieve sea ice thickness from altimeter data over small-scale sea ice features. The procedure to convert the measured altimeter waveform to surface elevations is called retracking and an overview of commonly applied methods can be found in [27]. Common to all these methods is to identify the point on the leading edge of the waveform that corresponds to the nadir return.…”

Estimating The Sea Ice Floe Size Distribution Using Satellite Altimetry: Theory, Climatology, and Model Comparison

“…Floe chord lengths are not measurements of floe size, and do not resolve regions of small floes, as the minimum chord length retrieval D min is limited to the CryoSat-2 footprint (∼300 meters along-track) (see the discussion in Appendix B). However, surface discrimination via altimetry is highly accurate in months without melt ponds (Peacock and Laxon, 2004;Guerreiro et al, 2017;Quartly et al, 2019), giving confidence that two consecutive floe echos, the minimum length scale represented here, represent a coherent length of ice. Indeed, this raw floe chord data has been used successfully to reduce biases in altimeter-observed satellite sea ice thickness estimates from satellite altimeters with different footprint sizes (Tilling et al, 2018a(Tilling et al, , 2019).…”

Editorial for Special Issue “Advances in Satellite Altimetry and Its Application”

“…The contributions of this special issue are sorted along their application area: the first paper [1] handles method development applicable to various regions of the Earth. Afterward, oceans [2,3], polar regions [4,5], coastal areas [6,7] and inland waters [8] are addressed. The last paper [9] deals with the Moon's topography.…”

“…The last paper [9] deals with the Moon's topography. All relevant satellite radar altimetry measurement techniques are covered: from Low Resolution Mode (LRM) [4][5][6] to Synthetic Aperture Radar (SAR) [1,2,4,5,7,8] and SAR in mode [7] to SWOT [3] as well as laser altimetry [1]. In the following, each paper is briefly summarized.…”

“…The paper by Quartly et al [5] reviewed various approaches that had been used to improve satellite altimetry results in the Arctic, especially the initial classification and subsequent retracking over diverse polar surfaces, showing examples from both LRM and SAR altimeters. Moreover, issues concerning geophysical corrections were discussed, and perspectives on future developments are provided.…”