SARin mode, and a window delay approach, for coastal altimetry
Introduction
The scientific exploitation of satellite altimetry missions data during the last decades has contributed to a huge step forward in the understanding of the ocean dynamics on a global scale. Measurements of the Sea Surface Height (SSH), Significant Wave Height (SWH), wind speed and also –indirectly- the geostrophic currents have been retrieved all over the world’s oceans, and validated with in-situ data and models (Legeais et al., 2015, Shum et al., 1995, Fu and Cazenave, 2001, Rio et al., 2014).
A series of satellite altimetry missions, from ERS-1/2 to EnviSat and AltiKa (Europe), and the Jason series (collaboration between USA & France), has carried on-board altimeters operating in a pulse-limited mode that in recent years has been named as Low Resolution Mode (LRM). The basics of this operational mode of satellite altimetry are thoroughly addressed in (Chelton et al., 2001).
The CryoSat-2 (CS-2) altimetry mission, launched in April 2010, was intended to help in an area within the altimetry community: the cryosphere science. This Earth Explorer altimeter was built following a specific instrumental design that fulfilled the hard requirements of monitoring the elevation of the steep polar ice caps, mountain glaciers and sea-ice thickness.
The CS-2 orbit is also specially designed for its ice mission, flying 700 Km above the Earth, only two degrees from a perfect polar orbit inclination. The repeat cycle is close to one year, with sub-cycles of 30 days. Each CS-2 complete cycle gives a pattern of ground-tracks much more dense than its predecessors altimeters. The ground track separation at equator is of 8 km, while for instance, for Jason-3 is of 315 Km. The study of the sea state in restricted areas of interest has benefited from this increased spatial resolution, and this is the case of this paper investigation.
The CS-2 altimeter (SIRAL: Synthetic aperture Interferometric Radar ALtimeter) was meant to be the first of a new generation, operating in addition to LRM in two new modes: (1) Synthetic Aperture Radar (SAR) and (2) the SAR Interferometric mode (SARin). SAR mode benefits from a better accuracy, thanks to an improved Signal-to-Noise Ratio (SNR). The conventional pulse-limited systems generate circular footprints from 1.5 to around 10 km wide, a coarse resolution if compared to the SAR along-track footprint, forming a series of across-track stripes with a distance of around 300 m between each other, while preserving the footprint diameter across-track (Chelton et al., 1989, Wingham et al., 2006). SARin mode adds interferometric capabilities to the SAR mode (Wingham et al., 2004). This mode allows to distinguish facets across-track, by enabling the reception of the signal by the two antennas placed perpendicular to the flight direction. The phase of the complex signal received by the two antennas is computed, and from the difference between the two echoes phases the Angle of Arrival (AoA) is derived, allowing the user to know the position of a particular target across-track within the SAR footprint.
The SARin mode is, so far, a unique special feature of the CryoSat-2 altimeter system, not present in the more recently launched Sentinel-3A mission and its Sentinel-3 constellation partners. The Sentinel-6 mission, due to be launched in 2020, will carry an altimeter on-board that will operate in an interleaved mode which offers the two options of LRM and SAR mode simultaneously, but again, will not offer SAR interferometric mode.
For many years the common interest in altimetry data was restricted to studies on the open ocean scenario. In coastal areas the level of contamination of the radar signal usually causes a degradation of the science waveforms data up to a point that the ground processing fails at retrieving valid oceanographic results. Due to the complex technical solutions needed to overcome the problem of coastal echoes contamination, for years the coastal ocean altimetry data has remained underexploited.
Recently, the major agencies have been boosting the research on Coastal Altimetry. Dedicated international workshops are organised yearly, while the Coastal Altimetry community is growing. Projects as COASTALT, funded by ESA (Cipollini et al., 2012) and PISTACH, funded by CNES (Dufau et al., 2012, Mercier et al., 2008) are also encouraging the users to work with coastal altimetry datasets offering improved performances. A number of studies have been done on coastal zones SSH trends determination, monitoring storm surges and studying the coastal sea state (Desportes et al., 2010, Mangiarotti, 2007, Madsen et al., 2007).
The work described in this paper was developed under the framework of the CryoSat Plus for Ocean (CP4O) project (Cotton et al., 2013, Benveniste et al., 2012), within the ESA/ESRIN funded STSE program.
The paper is structured as follows. Section 2 explains the Area of Interest and the CS-2 data used for the study. Section 3 is the core part of the paper, and it illustrates two different approaches of the algorithms and their results for the coastal ocean processing, including a statistical analysis for the second solution. The paper is finally concluded in Section 4, with a summary of the most remarkable subjects.
Section snippets
Area of interest and dataset
For the development of this study, a particular SARin mode area was requested: the Cuban Archipelago, between latitudes 19 and 24 North and longitudes 73 and 86 West, as in Fig. 1(a). It was approved by ESA and implemented on the 1st of October 2012, with the activation of the CS-2 mode mask V3.4.
The area of interest, highlighted in green in Fig. 1(a) includes a wide range of coastal topography types: cliffs, reefs, islands, lowlands, and swamps. It should present a large number of echo types,
Algorithms
Several investigations have addressed the problem of science waveforms contamination near the coast developing different techniques, generally focusing the solution in the assessment of the echo shape features and the retracking algorithms. Some studies modify the ocean surface analytical model to add Gaussian peaks for modelling coastal specular targets in the retracking processing (Halimi et al., 2012), some combine different retracking solutions and let the user select the best choice as in
Conclusions
In this paper we have described two solutions that aim to improve the SSH series at coastal areas. A comparison is done with respect to the CS-2 ESA L2 products (IPF Processor Baseline B version).
The first one is specific to the SIRAL CS-2 instrument, in SARin mode, which enables across-track interferometric measurements by processing the echoes received by two antennas placed in an across-track baseline. The difference between the two scattered signal phases allows to derive the across-track
Acknowledgements
This study has been developed within the CP4O project, funded by ESA (contract n°. 4000106169/12/I-NB).
We wish to acknowledge David Cotton (SatOC) for the reviews made to our work in its different stages along the CP4O project. Also to thank Albert Garcia-Mondéjar (isardSAT) for the fruitful technical discussions about the algorithm approaches. Finally, I want to thank Róisín for her patience and help in reviewing and proofreading this paper.
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