**3. Coastal sea level from satellite radar altimetry – a review**

The satellite radar altimetry system has been initially conceived for usage in open ocean. The processing of radar echoes and the development of corrections is

datasets exhibit some differences, probably due to the different methodologies used

Sea level rise is primarily an issue at the boundary line with land. It represents a potential threat to infrastructures and population living in low-elevation coastal areas [1]. The land disappears not only because the rising sea changes the coastline, but also because at a place there could be the land moving up or down, therefore contrasting or accelerating sea level rise [2]. Sea level can change significantly from one coastal location to another, as a result of a number of ocean, atmospheric and

In a global change scenario, as speculated in Li et al. [4], a slow rise of the sea level of few cm associated to climate change would make a difference to the coastline. It would not retreat from land, making it permanent. The flooding line of transient events (e.g., storm surges, tsunamis, etc.) would also uplift, increasing the risk of more frequent land inundation and more inland propagation [5]. An example is the City of Venice that has long been vulnerable to short duration flooding during winter [6]. The problem was so important that a system of 78 storm gates, known as MOSE [7] has been constructed to protect the city when high water is expected in Venice [8]. Long-term rising sea levels will represent additional challenges in the future [8].

Understanding the climate-related contribution to the sea level change and how much it will likely affect coastal regions is a major challenge, as it also requires localscale measurements of the land effects. In this chapter, we review the sea level trend measuring system involving the integration of recent satellite-based observations from radar altimeters and Global Positioning System (GPS) receivers with historical data from tide gauge stations. The latest advances in open ocean and coastal altimetry to measure sea level changes close to the coasts over the satellite radar altimetry era are also summarized. A more robust inverse method to estimate sea level trends is also presented. It has been tested in the Northern Adriatic Sea, where GPS data

Since Roman period, sea level has been measured nearby land just sticking a graduated pole within protected piscinae [9]. Since the 19th Century, tide gauges have been used in some coastal places around the world to measure the local change in sea level relative to the adjacent land [10, 11]. The baseline for measuring sea level over time is typically a mean sea level computed by averaging all the measurements over a period of years at each location. This relative sea level that will rise if ocean levels rise and/or land levels fall is the net change in the sea level and is the quantity of interest to the local coastal community in the real-time monitoring. However, understanding the future coastal sea level changes and their relative significance requires to remove the effect of waves, tides, and other short-term fluctuations. But tide gauges alone cannot determine whether the sea level is rising, the land is sinking, or both. Sea level can rise or retreat in the long-term in response to the natural processes that alter the volume of water, including the climate-related contribution. The land level changing over time (the so-called vertical land motion, VLM or subsidence/uplift) can rise or fall due to natural processes (e.g., tectonic

**Keywords:** Satellite radar altimetry, Tide gauges, GPS, Sea level, Adriatic Sea

*Geodetic Sciences - Theory, Applications and Recent Developments*

land processes that occur at various spatial and temporal scales [3].

are available to conduct a realistic assessment of uncertainties.

**2. Techniques for measuring sea levels**

**96**

in the generation of the products.

**1. Introduction**

now at mature stage in this domain, with the various datasets routinely used for global sea level studies. However, data were normally flagged as bad and therefore rejected in the coastal zone. But the situation rapidly changed in the last ten years for two reasons: (1) the prospect of recovering a valuable long-term sea level data around the global coastline; (2) the improved suitability of the new and future altimeters (like those on CryoSat-2, AltiKa, Sentinel-3, Sentinel-6, Crystal). Therefore, a new domain "coastal altimetry", i.e. the extension of altimetry into the oceanic coastal zone has been emerging, with a community around it developing a set of coastal altimetry techniques in order to get more and better sea level data closer to the coast.

The analyses of radar echoes revealed that pulse-limited missions, if reprocessed with dedicated models, could provide reliable range measurements to few km from the coastline. An example is the Adaptive Leading Edge Subwaveform (ALES) retracking algorithm, that has been validated and applied successfully to sea level research, demonstrating the ability to increase the quality and the quantity of sea level retrievals in coastal areas [19].

In addition, it was noted that geophysical corrections that must be applied to altimeter range data have a significant impact in coastal altimetry and therefore their constant improvement is crucial. There have been noticeable developments to improve the tropospheric delay [20], the tidal sea level where global models have still large errors [21] and the mean sea surface models, suitable for the observation of the coastal sea level [22]. There have been also improvements in procedures to avoid aliasing of major tidal signals and short-period ocean response to meteorological forcing aliases onto low frequency signals [23].

A single altimeter always leaves gaps along the coast between neighboring tracks: tenths to hundred km are not covered, so that the vast majority of the worldwide coast is not sampled. The coverage can be augmented with additional existing

*Main characteristics of satellite altimetry missions operating until now and planned for the future.*

*Coastal Sea Level Trends from a Joint Use of Satellite Radar Altimetry, GPS and Tide…*

*DOI: http://dx.doi.org/10.5772/intechopen.98243*

Data from the various altimeter missions were used to create several datasets. Examples include RADS [30], X-TRACK [28], etc. that also provide sea level estimates. Since 1992, at least two altimeter satellites have been operating simultaneously, and during some periods, even more than two. Such data can be combined in a single product to provide a consistent long-term sea level data set, globally with sufficient spatial coverage over almost three decades. However, altimeter missions need to be accurately homogenized and cross-calibrated to reduce biases and

A satellite-based sea level data set to analyze long-term trends that uses the available historic observations from the various radar altimeters is key requirement for the climate community [32]. A recent reprocessing within the European Space Agency (ESA) Sea Level Climate Change Initiative (SLCCI) has produced a gridded altimetry product with a spatial resolution of 0.25° (which is around 25 km resolution) from 1993 to 2015 [33, 34], thus permitting a more detailed view of sea-level

The sea level Environment Climate Variable (ECV) (at global and regional

In the case-study illustrated in the chapter, the SLCCI and C3S datasets are used to assess their maturity as state-of-the-art altimetry datasets in climatological studies. The multi-mission gridded products have not still tuned for last 10 km from the coast, where the amount of valid data might decrease. The ESA CCI + Sea Level

scales) is now operationally produced by the Copernicus Climate Change Service (C3S) [35] by applying the altimetry processing standards developed in the SLCCI initiative. The C3S product ensures a stable number of two altimeters since the beginning and the reference field used to compute sea level anomalies (SLA) is a homogeneous mean sea surface for all missions. The C3S record is a regional product, gridded at 0.125° in the Mediterranean Sea, starting in 1993 and offering ongoing coverage [36]. Both the SLCCI and the C3S datasets are state-of-the-art products designed to be a reference for climate-related sea

altimeters.

**Figure 1.**

uncertainties [31].

level studies.

**99**

change around the world coastlines.

The wet tropospheric correction is the major source of uncertainty in altimetry budget error, due to its large spatial and temporal variability: this is reason why a multi-channel passive microwave radiometer is on the same platform as the altimeter. Unfortunately, this estimate gets quickly corrupted as soon as land enters the radiometer footprint, i.e. 20–50 Km from the coast. Alternative corrections have been devised and appear to be successful at least in some particular conditions [24]. A very promising approach was the one attempting to estimate the wet tropospheric path delay from GPS measurements known as GPD (GNSS-derived Path Delay), and its latest version called GPD+ (Plus) [25].

The classical data editing used in open ocean was also considered excessively restrictive and revisited with novel editing/re-interpolation approaches (e.g., [26]). The new data from the various reprocessing efforts are now bringing altimetry around the global coastline, with a higher spatial resolution and precision that was previously not available in coastal and shelf sea areas, while constant improvement [18] and validation [27] are still ongoing. The new coastal altimetry datasets open a new opportunity to study sea level change from open ocean to coast and differences in trend and variability at various distances from the coast, also nearby tide gauges [28].
