IEEE Geoscience and Remote Sensing Magazine - September 2017 - 44

SPACE AGENCIES
BY LIONEL GOURDEAU, BUGHSIN DJATH,
ALEXANDRE GANACHAUD, FERNANDO NIÑO,
FLORENCE BIROL, JACQUES VERRON, AND NICOLAS FULLER

Altimetry in a Regional Tropical Sea

he satellite for Argos and AltiKa (SARAL/AltiKa) is
the first ocean altimeter mission to operate in the
Ka-band frequency. The objective of this article is to
investigate the extent to which SARAL/AltiKa sea-level
measurements provide valuable information in a complex bathymetric region, i.e., the semienclosed Solomon Sea. The data-editing procedure is revisited, and
we propose two new data-editing criteria. The first is
based on the detection of erroneous sea-level values
after computation, and the second directly analyzes
the radar measurements and geophysical corrections.
We show that both methods are significantly more efficient than the standard procedure used in operational
processing chains.
Sea-level variations derived from altimeter data are
very sensitive to the choice of the mean sea-surface
height (MSSH) used in the processing. The MSS_CNES_
CLS11 solution provides the best result in our area. The
performance of the SARAL/AltiKa mission is finally evaluated in comparison with the classical Ku-band Jason-2
altimeter. SARAL/AltiKa provides significantly more
observations in the near-shore region. The lower level
of instrumental noise in SARAL/AltiKa measurements
modifies the shape of along-track sea-level wavenumber
spectra, which appear closer to the information provided by regional numerical model studies.

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Digital Object Identifier 10.1109/MGRS.2017.2679484
Date of publication: 14 September 2017

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AN AREA OF EL NIÑO INTEREST
Satellite altimeters, by providing accurate SSH anomaly
(SSHA) measurements and associated geostrophic velocity anomaly estimations at synoptic scales, have led
to important progress in our understanding of physical
oceanography [1]. However, in coastal regions or in areas
including complex topography with numerous islands,
the use of satellite altimetry remains a challenge today.
In this context, the Solomon Sea in the southwest
tropical Pacific (Figure 1) is clearly a challenging area for
altimetry. The Solomon Sea is of particular interest in a
climatic context and is a focal point of the Climate and
Ocean Variability, Predictability, and Change/Southeast

Pacific Ocean Circulation and Climate Experiment
program (http://www.clivar.org/clivar-panels/pacific/
spice). It is a transit area for the low-latitudes boundary currents that are key elements in El Niño-Southern
Oscillation (ENSO) dynamics and their low-frequency
modulation [2].
To monitor the transport entering the Solomon Sea,
glider experiments are routinely operated in the area [3],
but such in situ experiments are costly and always difficult to achieve in this remote region. In this context,
altimetry must be considered as a serious alternative to
access the Solomon Sea circulation. A first attempt was
made by the authors of [4], using Topography Experiment/POSEIDON data, and they were able to document
the seasonal variation of the low-latitudes boundary currents and estimate interannual transport anomalies entering the Solomon Sea.
However, one major limitation in monitoring the
ocean circulation in this area is that the performance of
altimetry products decreases significantly near land [5];
some geophysical corrections become unreliable [6], and
the radar signal is perturbed by the surrounding land. As

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FIGURE 1. The SARAL (thick lines) and Jason-2 (thin lines) tracks

over the Solomon Sea used for this study. The segments in color
(SARAL: blue; Jason-2: red) illustrate the spatial extension to perform the spectral wavenumber analysis (see the section "Benefits
of Ka-Band Altimetry"). The bathymetry is plotted with land in
gray, the first 20 m of depth in light gray, and the 100-m depth in
black dashed lines.
IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE

SEPTEMBER 2017

http://www.clivar.org/clivar-panels/pacific/

Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - September 2017