2.3 Review of the current coastal altimetry products

For about a decade, many efforts have been paid by the altimetry community to overcome the above difficulties and exploit altimetry information near the coast. A number of coastal altimetry products were distributed to the community. For the Jason-2 altimeter, the most popular products are (1) X-TRACK developed by LEGOS (Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, France), (2) PISTACH (Prototype Innovant de Système de Traitement pour l'Altimétrie Côtière et l'Hydrologie, Mercier et al. [15]) developed by CLS (Collecte Localisation Satellites, France), and (3) ALES (Adaptive Leading Edge Sub-waveform, Passaro et al. [14]) developed by NOC (National Oceanography Centre, UK). PEACHI (the Prototype for Expertise on Altimetry for Coastal, Hydrology and Ice, PEACHI) is a sister product of PISTACH while it currently focuses on the Saral/Altika. Corresponding coastal altimetry products are based on dedicated analysis of the nonstandard waveforms and/or sophisticated coastal geophysical corrections [13–15].

X-TRACK is a relatively concise product but does not apply waveform retracking, while ALES and PISTACH are extended products to SGDR (Sensor Geophysical Data Record which includes the waveform distributed by AVISO officially), applying waveform retracking, conserving the official SGDR terms, and annexing new parameters (retracking results and geophysical corrections). ALES does not provide improved geophysical correction, while PISTACH provides 2–3 candidate solutions for almost every geophysical correction term.

X-TRACK is a level 3 (L3) product: using the GDR data and state-of-the-art altimetry corrections, along-track sea level time series projected onto reference tracks (points at same locations for every cycle) are computed at 1-Hz (6 km along-track resolution). It is simple to use and is based on improved geophysical corrections near the coast (see [6] for details), but its current version only includes the Jason-2 official retracker dedicated to open ocean conditions.

For the geophysical corrections, the advantages of X-TRACK are (1) a more robust median-based editing criterion for the ionosphere correction. (2) A Loess filter (locally weighted scatter plot smoother using a quadratic polynomial model) for the sea state bias correction. (3) A new set of tide solution based on empirical harmonic analysis of the altimetry data.

PISTACH outperforms its counterparts in waveform classification and wet troposphere correction. The decontaminated wet troposphere correction approach is based on the corrected brightness temperature of the on-board radiometer. Another improvement of PISTACH may be the dedicated sea state bias correction algorithm at the coast, albeit not very reliable due to limited dataset.

ALES simply focuses on the design of an adaptive retracker that can be applied to a variety of waveforms and reduces inconsistence derived from the bias among different retrackers.

The schemes of the three main state-of-the-art coastal altimetry products for Jason-2 satellite altimeter are tabulated in Table 1.

### 2.4 Altimeter waveform processing

The essential part of the altimetry processing is the so-called "waveform retracking". "Waveform", as shown in Figure 1, records the amplitude of the earth surface echo as a function of time delay. Due to the difference of contexts or traditions, the x-axis of a waveform can be time, frequency, or range, but the three items above can be transformed to each other by simple scaling factors. "Retracking" is the process of extracting useful parameters (range, amplitude, and sometimes significant wave height) from the waveform. The coastal waveforms show very diverse pattern, and it is impractical to find a unique retracker that performs best for every waveform, so it is necessary to classify the waveforms before retracking.
