**2. Techniques for measuring sea levels**

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

### *Coastal Sea Level Trends from a Joint Use of Satellite Radar Altimetry, GPS and Tide… DOI: http://dx.doi.org/10.5772/intechopen.98243*

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.

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

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, RADARSAT, etc.) only provide imagery on demand.

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 thus calculated, and its change tracked over time.

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].
