# Inland Water Level Monitoring from Satellite Observations:
## 1 Introduction
The first mission operated both in OLTC and SAR mode was the Sentinel-3A, launched in 2016 [24]. The timeline of modern radar altimeters from the nineties to the next decade is shown in Figure 1. Past missions are in red and satellites in operations are in orange, while future missions and unconfirmed mission extensions are in yellow. The latest altimetry mission named SWOT was launched in December 2022 and is the first mission specifically dedicated to the observation of continental surface water bodies. This mission was developed by NASA and CNES with contributions from the Canadian Space Agency and the United Kingdom Space Agency. The SWOT mission can acquire 2-D swaths as opposed to conventional 1-D profiles used for approximately 50 years. It might be the most important technological breakthrough in the history of altimetry missions since the T/P mission's capability to reach centimetre-level accuracy. The SWOT mission has opened a new era of scientific advances for continental hydrology, and therefore the extraction of inland water levels [25]. Inspired by the launch of so many altimetry missions, and the development of services to provide inland water level time series to supply data for Earth system understanding, hydraulic studies are becoming more important than ever. After the launch of GEOS-3 and Seasat missions in the 1970s, two reviews focused on marine geodesy and ocean circulation [26; 27]. Other reviews focused on continental surface waters [28], and specifically on surface water storage [14]. The role of satellite altimetry in monitoring the world's inland water resources is highlighted in [29; 30; 31], while another study presents the capabilities of ESA altimeters to monitor inland water heights [32]. Furthermore, the various achievements made by altimetry missions, and their key contribution to global climate change monitoring are presented in [33], while a different analysis emphasized the importance of radar altimeters in terms of water level and discharge [34]. Moreover, a comprehensive analysis summarizing the progress of 25 years of satellite altimetry was made by more than 300 co-authors [35], while the most recent review paper, summarizes the scientific achievements of altimetry using a total of 8541 publications covering the years from 1970 to 2021 [36]. Scoping studies have become a progressively more prevalent approach for systematically searching the literature to address a specific research question [37]. The current scoping review paper presents the achievements made by satellite missions, and hence by altimetry missions to the field of inland water level monitoring between 2018 and 2022. Following the methodological framework developed by [PERSON] and [PERSON] [38], the purpose of this review is to summarize research findings in inland water surface height monitoring, while for the analysis of the current research question five electronic literature databases are used. Researchers used the PRISMA statement to improve the reporting of scoping review and meta-analyses [39]. This study includes data related to the different literature databases and the number of publications per year. Moreover, both the publications associated with specific satellite missions and the different algorithms used to retrack the waveforms are analyzed to mirror the trend in the field of inland water level monitoring. This study provides a comprehensive review using spaceborne missions during the past 5 years, aids new researchers in their study of this specific field, and assists experienced researchers in keeping their knowledge up to date. The remainder of this paper is organized as follows: Section 2 describes the methodological framework of the  
Figure 1: Timeline of modern radar altimetry missions.