**3.2 Gravity models**

One of the basic geodetic tasks is determining the Earth's shape and size. The satellite altimetry gave an insight into the topography of the oceans, which later enabled the reconstruction of the Earth's gravity field over the oceans through gravity recovery. Gravity recovery stands for the geodetic operations and procedures of fitting the (altimeter) data to a gravity field that allows for the determination of the gravity information at any location [6]. Three standard procedures can be used to compute the gravity field from the altimetry: (1) employing the least-squares collocation on the altimeter measurements with the computed slopes of the sea surfaces along the satellite tracks or (2) along with the computed deflections of the vertical (e.g. [50, 51]), and (3) using the Vening Meinesz formula for the computations of the gravity field from the deflections of the vertical derived from satellite altimetry [52] (**Figure 6**).

Today, the global gravity field models are usually derived from gravity satellite mission(s) only or from combined observations (both ground and satellite data). When using combined data, satellite altimetry is most often included in

**87**

**Table 2.**

*data.*

*Radar Satellite Altimetry in Geodesy - Theory, Applications and Recent Developments*

modeling. Such combined models are, e.g., XGM2019e\_2159 [54], GAO2012 [55], EIGEN-6C4 [56], and EGM2008 [57]. Models derived from altimetry only are

Due to the expenses of the traditional bathymetric measuring methods (e.g., weighted lines/poles), the information about the water depths and topography of the seafloor remained mainly unexplored over the open ocean until the utilization of satellite altimetry. Today, with the global and uniform coverage, satellite altimetry is crucial in computations of the global bathymetric models fulfilling the

Predicting the bathymetry from the altimetry relies on the method developed in 1983 by [59], who have shown the potential of such modeling using the Seasat altimetry data. Over the years, the methods were further developed (e.g., [58]). Today most of the bathymetric models integrate the same altimetry-derived bathymetry. **Table 2** presents some of the most common global bathymetric models starting from the most recently updated: (1) GEBCO\_2019 (The General Bathymetric Chart of the Oceans) [60], (2) SRTM15+ (Shuttle Radar Topography Mission: Global Bathymetry and Topography at 15 arcseconds) [61], (3) EMODnet (European Marine Observation and Data Network) [62], (4) SRTM30\_PLUS [63], (5) S&S V19.1 (Smith & Sandwell) [59], (6) DTU10BAT (Technical University of Denmark) [57], and (7) ETOPO1 (National Oceanic and Atmospheric

Bathymetric models derived from satellite altimetry are not reliable enough for underwater navigation, construction works, or similar, as the errors of the bathymetric estimates sometimes exceeds 100 m but do offer general insight onto the seafloor topography and make the best available bathymetric data for many areas (see e.g., [60, 66]). **Figure 7** presents an example of the global bathymetric model.

As mentioned above, the satellite altimeter data for geodetic purposes can be integrated with tide gauges when estimating the sea-level change, with shipborne bathymetry obtained by echo sounders when modeling the bathymetry, and with discrete gravity measurements or satellite gravity when computing Earth's gravitational field. Furthermore, the satellite altimetry can be used to access the vertical land motion over the coastal area by comparing the sea level change trends from

**Name Year of issue/update Resolution** GEBCO\_2019 2019 15" SRTM15 + V2.1 2019 15" EMODnet 2018 1/16" SRTM30\_PLUS 2014 30" S&S V19.1 2014 1' DTU10BAT 2010 1′-2′ (Equator) ETOPO1 2008 1'

*Basic details on the most common global bathymetric models derived from satellite altimetry and shipborne* 

**3.4 Altimeter data with the other technologies and potential studies**

*DOI: http://dx.doi.org/10.5772/intechopen.97349*

given in, e.g., [53, 58].

**3.3 Bathymetry**

in-situ data gaps.

Administration's dataset) [64, 65].

**Figure 6.** *Altimetry-derived global ocean gravity map (data downloaded from [53]).*

*Radar Satellite Altimetry in Geodesy - Theory, Applications and Recent Developments DOI: http://dx.doi.org/10.5772/intechopen.97349*

modeling. Such combined models are, e.g., XGM2019e\_2159 [54], GAO2012 [55], EIGEN-6C4 [56], and EGM2008 [57]. Models derived from altimetry only are given in, e.g., [53, 58].

## **3.3 Bathymetry**

*Geodetic Sciences - Theory, Applications and Recent Developments*

progress in coastal altimetry and altimetry, in general, is crucial.

**3.2 Gravity models**

[52] (**Figure 6**).

The mean sea surface and its change are one of the bases for vertical height system modeling and implementation. A wide initiative on unifying the vertical height reference systems (for details see [46–48]) most usually encompasses absolute sealevel modeling from satellite altimetry extended for the tide gauge measurements at the coast (see e.g., [49]) along with the extensive analysis of vertical land movements, GNSS measurements, gravity estimations, etc. For such purposes, further

One of the basic geodetic tasks is determining the Earth's shape and size. The satellite altimetry gave an insight into the topography of the oceans, which later enabled the reconstruction of the Earth's gravity field over the oceans through gravity recovery. Gravity recovery stands for the geodetic operations and procedures of fitting the (altimeter) data to a gravity field that allows for the determination of the gravity information at any location [6]. Three standard procedures can be used to compute the gravity field from the altimetry: (1) employing the least-squares collocation on the altimeter measurements with the computed slopes of the sea surfaces along the satellite tracks or (2) along with the computed deflections of the vertical (e.g. [50, 51]), and (3) using the Vening Meinesz formula for the computations of the gravity field from the deflections of the vertical derived from satellite altimetry

Today, the global gravity field models are usually derived from gravity satellite mission(s) only or from combined observations (both ground and satellite data). When using combined data, satellite altimetry is most often included in

**86**

**Figure 6.**

*Altimetry-derived global ocean gravity map (data downloaded from [53]).*

Due to the expenses of the traditional bathymetric measuring methods (e.g., weighted lines/poles), the information about the water depths and topography of the seafloor remained mainly unexplored over the open ocean until the utilization of satellite altimetry. Today, with the global and uniform coverage, satellite altimetry is crucial in computations of the global bathymetric models fulfilling the in-situ data gaps.

Predicting the bathymetry from the altimetry relies on the method developed in 1983 by [59], who have shown the potential of such modeling using the Seasat altimetry data. Over the years, the methods were further developed (e.g., [58]). Today most of the bathymetric models integrate the same altimetry-derived bathymetry. **Table 2** presents some of the most common global bathymetric models starting from the most recently updated: (1) GEBCO\_2019 (The General Bathymetric Chart of the Oceans) [60], (2) SRTM15+ (Shuttle Radar Topography Mission: Global Bathymetry and Topography at 15 arcseconds) [61], (3) EMODnet (European Marine Observation and Data Network) [62], (4) SRTM30\_PLUS [63], (5) S&S V19.1 (Smith & Sandwell) [59], (6) DTU10BAT (Technical University of Denmark) [57], and (7) ETOPO1 (National Oceanic and Atmospheric Administration's dataset) [64, 65].

Bathymetric models derived from satellite altimetry are not reliable enough for underwater navigation, construction works, or similar, as the errors of the bathymetric estimates sometimes exceeds 100 m but do offer general insight onto the seafloor topography and make the best available bathymetric data for many areas (see e.g., [60, 66]). **Figure 7** presents an example of the global bathymetric model.

#### **3.4 Altimeter data with the other technologies and potential studies**

As mentioned above, the satellite altimeter data for geodetic purposes can be integrated with tide gauges when estimating the sea-level change, with shipborne bathymetry obtained by echo sounders when modeling the bathymetry, and with discrete gravity measurements or satellite gravity when computing Earth's gravitational field. Furthermore, the satellite altimetry can be used to access the vertical land motion over the coastal area by comparing the sea level change trends from


#### **Table 2.**

*Basic details on the most common global bathymetric models derived from satellite altimetry and shipborne data.*

**Figure 7.** *Altimetry-derived global ocean bathymetric map (data downloaded from [53]).*

satellite altimetry and from tide gauges where the latter obtain the trend accounted for the vertical land change (e.g., [10, 67, 68]). The altimetry can further be employed in multidiscipline-based early warning systems such as those forecasting the floods [69], or tsunamis [70], and the other climate-related forecasting systems that lead towards the operational oceanography, i.e., to the forecasting system of the sea-related variables such as sea level, temperature, and currents, based on the long-term routine measurements and real-time observations of the oceans and atmosphere (see e.g., [71]).
