**Abstract**

For the last century, tide gauges have been used to measure sea level change along the world's coastline. However, tide gauges are heterogeneously distributed and sparse in coverage. The measured sea level changes are also affected by solid-Earth geophysics. Since 1992, satellite radar altimetry technique made possible to measure heights at sea independent of land changes. Recently various efforts started to improve the sea level record reprocessing past altimetry missions to create an almost 30 year-long combined record for sea level research studies. Moreover, coastal altimetry, i.e. the extension of altimetry into the oceanic coastal zone and its exploitation for looking at climate-scale variations of sea level, has had a steady progress in recent years and has become a recognized mission target for present and future satellite altimeters. Global sea level rise is today well acknowledged. On the opposite, the regional and local patterns are much more complicated to observe and explain. Sea level falls in some places and rises in others, as a consequence of natural cycles and anthropogenic causes. As relative sea level height continues to increase, many coastal cities can have the local elevation closer to the flooding line. It is evident that at landsea interface a single technique is not enough to de-couple land and sea level changes. Satellite radar altimetry and tide gauges would coincide at coast if land had no vertical motion. By noting this fact, the difference of the two independent measurements is a proxy of land motion. In this chapter, we review recent advances in open ocean and coastal altimetry to measure sea level changes close to the coasts over the satellite radar altimetry era. The various methods to measure sea level trends are discussed, with focus on a more robust inverse method that has been tested in the Northern Adriatic Sea, where Global Positioning System (GPS) data are available to conduct a realistic assessment of uncertainties. The results show that the classical approach of estimating Vertical Land Motion (VLM) provides values that are almost half of those provided by the new Linear Inverse Problem With Constraints (LIPWC) method, in a new formulation which makes use of a change of variable (LIPWCCOV). Moreover, the accuracy of the new VLM estimates is lower when compared to the VLM estimated from GPS measurements. The experimental Sea Level Climate Change Initiative (SLCCI) data set (high resolution along track) coastal sea level product (developed within Climate Change Initiative (CCI project) that has been also assessed in the Gulf of Trieste show that the trends calculated with the gridded and along track datasets exhibit some differences, probably due to the different methodologies used in the generation of the products.

shifts; sediment loading, glacial isostatic adjustment, etc.) but also as a consequence of man-made factors (e.g., ground water extraction; oil and gas pumping, etc.). There are various techniques for measuring VLM, e.g., geotechnical investigations using spirit levels and borehole extensometers [12]; geodetic surveying with Global Positioning System (GPS) satellite technology [13]; satellite remote sensing observations that use a technique called interferometric synthetic aperture radar (InSAR) [14]. The advantage of satellites is that they ensure almost global coverage in a repeatable manner and consistency of the measuring system over long time periods that is an important requirement for the detection of slow changes over time.

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

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

The quantification of VLM before the modern satellite era is difficult due to the poor coverage of geotechnical investigations. The advent of GPS receivers and their co-location with tide gauges made possible to continuously measure land elevation changes with simultaneous sea level reading at the same location [15]. GPS sensors return vertical and horizontal positions. The vertical position is a measure of the elevation of the land surface relative to the center of Earth, also referred as absolute. It is generally two to three times less precise than the horizontal components. The present picture is that, while there are many tide gauges around the world, not all

The InSAR tool uses repeating multiple satellite radar imagery to create a time profile of land elevation change. The advantage respect to the GPS technology is the much higher density of VLM measuring points in the imaged area. The technique to provide statistically significant results requires a sufficient number of images and reduced scattering over time for the area of interest. The availability of images from the first satellites (e.g., ERS and Envisat) can be very irregular both in time and space [17]. Only the recent Sentinel-1 constellation provides global coverage and more frequent revisiting. Other satellite missions (e.g., COSMO-SkyMed,

Sea level can be also measured with satellites using radar altimeters. These instruments send microwave pulses down along the satellite's ground track and measure their echoes, revisiting the same place every 10 days or more depending on the mission. The time their echoes take to bounce back allows the system to measure the satellite's altitude above the sea surface (the so-called range). It can be then corrected for instrumental and environmental effects. Knowing the satellite orbit with respect to Earth's center of mass, the absolute, not relative, sea level can be

Routine sea level observations began in 1992 with the TOPEX/Poseidon spacecraft on a 10-day repeat cycle, and this has subsequently been followed up by the Jason 1/2/3 series and the recent Sentinel-6 mission, providing a near-global fully consistent along track data set of sea level to understand how sea levels have changed over the past nearly three decades. Over the years, various satellite missions with different orbital configurations and other scientific objectives were launched, e.g., ERS-1/2, Envisat, Sentinel-3, SARAL, CryoSat-2 and HY-2A/B. But single satellites have limitations. The sea level is tracked along paths whose distance is relatively large. A satellite alone could not fly in the region of interest, as it is for example the case of Venice for the T/P-Jason-Sentinel-6 family. Moreover, it has been difficult to retrieve data near coast where both the presence of land and more complex ocean surface scattering make the standard processing problematic [18].

**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

have permanent GPS receivers co-located or near them [16].

RADARSAT, etc.) only provide imagery on demand.

thus calculated, and its change tracked over time.

**97**

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