## 1 Introduction
Regarding the first topic, once a rough set of boundaries for the target has been defined, there are two common approaches to select the measurements corresponding to the target. The most common method is to define a polygon delimiting the area of interest, which is usually called a virtual station (VS) or water mask. VSs are typically generated based on historic water extent data obtained from satellite imagery, combined with locations where the satellite's nadir is over the targeted water surface [17; 21; 23; 25]. Nadir measurements where water is present nearby in the across-track direction are not utilized in WL\({}_{a}\) estimation approaches using nadir altimeters. In contrast to interferometric or swath altimeters, like the Surface Water and Ocean Topography (SWOT) satellite, classic nadir altimeters are not able to perform measurements across-track, i.e., to associate a cross-track angle of arrival to the measured range. Therefore, a larger distance from nadir implies a larger error in the elevation estimation, which is not trivially corrected. In this article, nadir measurements above water are referred to as \"nadir\", and measurements above land and close to water in the across-track direction are referred to as \"across-track\". A common disadvantage of VSs based on historic water extent is the neglect of potential variability in the target's extension, especially during extreme phenomena such as drought or flooding. As a way to tackle the problem of variable extension, [PERSON] et al. [22] suggested employing a waveform classification algorithm over a predefined area to discern between measurements over land, water, and the transition between them. However, validation of this classification scheme on a global scale is still pending. In regard to the retracker, specialized methods have been proposed to tackle inland waveforms, which should outperform the OCOG and ocean retracker [17; 18; 21]. Unfortunately, none has proven consistently superior to the others across the globe and over targets of different dimensions and surrounding topography. In fact, [PERSON] et al. [17] introduced the idea of analyzing the waveform suitability for processing with the ocean retracker before application. Taking into account that certain retrackers are fitted to process certain waveform shapes, this idea could be expanded to implement a multi-retracker approach for inland waters. However, this has not been globally tested yet. Outliers are usually produced by the presence of multiple bright scatterers aside from the target of interest. Even though outlier filtering is crucial for disregarding invalid measurements, it is a delicate step because it could neglect extreme events, such as flooding or drought, which are of special hydrological interest. Therefore, frequent and numerous measurements over a target are essential. Filtering can be applied vertically, horizontally, or in a combination of both. Vertical filtering means analyzing the measurements of a single satellite pass over the target, and horizontal filtering means analyzing the time series. As an example of an algorithm that combines both approaches, [PERSON] et al. [21] proposed selecting WL\({}_{i}\)s through a state-space model, which takes into account a mixed Gaussian-Cauchy distribution of WL\({}_{i}\) measurements for each date and a random walk between dates. However, such statistical methods rely on the retrieval of measurements from frequent and uninterrupted passes over the target of interest, the assumption of the distribution of measurements, and the assumption that WL\({}_{a}\) dynamics are not heavily fluctuating between passes. In studies concerning small to medium targets, the topic remains challenging. [PERSON] et al. [7] conducted research over eight reservoirs in the Ebre River Basin (Spain) using four different retrackers and UFSAR data from S3A and S3B. The researchers were unable to identify a univoque best-performing retracker, as the error of the measurements was largely target-dependent, with a best-case value of 18 cm.