**2.3 Advanced altimeter processing methods – retracking**

Pulse-limited altimetry, often referred to as low resolution mode (LRM) altimetry, or traditional altimetry, is limited by the size of the radar surface footprint,

*Geodetic Sciences - Theory, Applications and Recent Developments*

**2.2 Previous and current satellite altimeter missions**

sion and widespread ground segment that enabled:

retroreflector,

• real-time data acquisition,

Overall, the development of the satellite altimetry can be divided into three phases – (1) experimental, (2) modern, and (3) future phase (following [6, 9]). **Figure 2** present the timeline overview of the altimeter satellite missions launched during all three phases along with the origin of the satellite missions and their period of orbit repeating. The modern (current) era can be defined from the launch of the ERS-1 and TOPEX/Poseidon missions in 1991 and 1992 onwards. European ERS-1 was launched on July 17, 1991, into a sun-synchronous polar orbit (Francis, 1984) with three setups of repetitivity: 3-day, 35-day (the most used), and 336-day repeat cycle. The mission lasted till March 2000, exceeding its expected lifespan by far. To support ERS-1, ESA (European Space Agency) developed a satellite-based tracking system within Precision Range and Range-Rate Equipment (PRARE) mis-

• calibration of the radar altimeter to 10 cm using the ground-based laser

Data were disseminated as low-rate fast-delivery products and high-rate products via the Broadband Data Dissemination Network. At the same time, the efforts by NASA (National Aeronautics and Space Administration) and CNES (French National Centre for Space Studies) resulted in TOPEX/Poseidon mission, being the product of 20 years of technological and engineering development [14]. That satellite mission has revolutionized satellite altimetry by introducing the second altimeter frequency (C-band, 5.3 GHz) and the third frequency on the microwave radiometer (18 GHz), which enabled computations of ionospheric delay corrections, and removing of the effects of wind speed on measurements, respectively [14]. The mission provided high measurement precision of measured data with an

*Altimeter satellite missions' timeline overview divided into an experimental era (yellow), modern era (green), and future altimetry era (blue) along with the missions' orbit reportativity and information about their* 

• data processing and generation of fast-delivery products [13].

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**Figure 2.**

*countries of origin (following and updating from [9]).*

**Figure 3.**

*Early Sentinel-6 measurements validation comparing to Jason-3, sentinel-3A, and sentinel-3B [16].*


#### **Table 1.**

*An overview of the basic characteristics of satellite altimetry missions.*

i.e., the size of the area illuminated by the radar from the satellite [6]. Depending on the SWH, the radius of the altimeter footprint can range from 1 km up to 7 km (e.g. for Jason missions), which enables high accuracy of the altimetry in open ocean areas, and on the other side, due to the contamination in the reflected radar altimeter signal caused by the land [19], lower accuracy in the coastal and inland areas (see e.g., [14]).

Significant efforts were done to overcome coastal altimetry issues through different projects, e.g., for the Mediterranean Sea projects were conducted such as ALBICOCCA (Altimeter-Based Investigations in Corsica, Capraia and Contiguous Areas), ALTICORE (Altimetry for Coastal Regions), COASTALT (Development of

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

Radar Altimetry Data Processing in the Coastal Zone), SAMOSA (SAR Altimetry Mode Studies and Applications), and the PISTACH (Coastal and Hydrology Altimetry product) [20]. The projects resulted in improvements of the onboard trackers and developments of the waveform retrackers. On-board trackers are devices used for the prediction of surface measurements thus enabling outlier detection and easier surface tracking [21]. The waveform retrackers work on the ground after the waveform data are downloaded from a satellite. The retrackers most often attempt to fit the model or function to the measured waveform to provide as accurate as possible results [21]. The retrackers integrate physical functions (such as the Brown ocean retracker) or empirical functions. Altimeter retrackers are

Different retrackers process different satellite mission data for different areas. For instance, ALES (Adaptive Leading Edge Subwaveform) is designed to be applied to Jason 1/2 and Envisat in both open ocean and coastal zones [27], X-TRACK retracker was designed particularly for coastal areas, ALES+ was later designed for the sea ice leads, coastal and inland waters [28], Goddard Space Flight Center (GSFC) designed several retrackers for ice areas [29], etc. Such retrackers nowadays enable utilizing of satellite altimetry in the coastal zones, and inland water areas. All the retracked data

is available through the Coastal altimetry community [30].

**2.4 Advanced altimeter processing methods – Delay-Doppler altimetry**

One of the most significant recent developments in satellite altimetry technology was the introduction of the Delay-Doppler (DD) or SAR-mode altimetry that enables better observations of the small-scale features (below 50 km) and improved spatial resolution along the satellite track compared to conventional pulse-limited altimeters (see [31]). DD satellite altimeters employ the Doppler effect caused by the movement of the satellite in the along-track direction to improve the spatial resolution in the same direction [31] enabling the data sampling along-track e.g., up to 300 m for Sentinel-3. In other words, the altimeter footprint of the DD altimeters is reduced by an order of magnitude with respect to conventional altimeters – from a few kilometers up to a few hundreds of meters [32]. Hence, DD altimeters, such as those on the CryoSat-2 (SIRAL, SAR Interferometric Radar Altimeter), Sentinel-3 (SRAL, Synthetic Aperture Radar Altimeter), deliver more and/or improved data over the ocean, and, especially, in sea ice areas and coastal areas in general.

The SAR altimetry is based on the coherent processing of multiple echoes (e.g., 64 Ku-band pulses emitted by CryoSat-2 and Sentinel-3) within each altimeter burst (aperture duration of approx. 3.5 ms for CryoSat-2 and Sentinel-3), which enables resolving the reflected signals for along-track cells rather than the large footprints generated by the pulse limited altimeters. That naturally results in an improved resolution in the along-track (azimuth) direction of the satellite with the pulse-limited form that depends on the altimeter footprint maintained in the

**Figure 4** presents the SAR technology and processing compared to the conventional satellite altimeters. The SAR processing includes counting for the along-track phase shift within each echo obtained from different radar burst, which depends on the geometry of the observation [31]. That produces the multiple echoes gathered at the same ground cell, which allows for the subsequent averaging (i.e., summing coherently) that increases the signal-to-noise ratio, i.e., it results in improved

In [33] different studies on satellite radar altimetry pointed out that the SAR altimetry already performs better over the coastal zones than the conventional altimetry (see also [34]). They also emphasized the potential of the SAR technology

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

further discussed in [22–27].

across-track direction (see [31, 32]).

observations of the sea surface.

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

Radar Altimetry Data Processing in the Coastal Zone), SAMOSA (SAR Altimetry Mode Studies and Applications), and the PISTACH (Coastal and Hydrology Altimetry product) [20]. The projects resulted in improvements of the onboard trackers and developments of the waveform retrackers. On-board trackers are devices used for the prediction of surface measurements thus enabling outlier detection and easier surface tracking [21]. The waveform retrackers work on the ground after the waveform data are downloaded from a satellite. The retrackers most often attempt to fit the model or function to the measured waveform to provide as accurate as possible results [21]. The retrackers integrate physical functions (such as the Brown ocean retracker) or empirical functions. Altimeter retrackers are further discussed in [22–27].

Different retrackers process different satellite mission data for different areas. For instance, ALES (Adaptive Leading Edge Subwaveform) is designed to be applied to Jason 1/2 and Envisat in both open ocean and coastal zones [27], X-TRACK retracker was designed particularly for coastal areas, ALES+ was later designed for the sea ice leads, coastal and inland waters [28], Goddard Space Flight Center (GSFC) designed several retrackers for ice areas [29], etc. Such retrackers nowadays enable utilizing of satellite altimetry in the coastal zones, and inland water areas. All the retracked data is available through the Coastal altimetry community [30].

#### **2.4 Advanced altimeter processing methods – Delay-Doppler altimetry**

One of the most significant recent developments in satellite altimetry technology was the introduction of the Delay-Doppler (DD) or SAR-mode altimetry that enables better observations of the small-scale features (below 50 km) and improved spatial resolution along the satellite track compared to conventional pulse-limited altimeters (see [31]). DD satellite altimeters employ the Doppler effect caused by the movement of the satellite in the along-track direction to improve the spatial resolution in the same direction [31] enabling the data sampling along-track e.g., up to 300 m for Sentinel-3. In other words, the altimeter footprint of the DD altimeters is reduced by an order of magnitude with respect to conventional altimeters – from a few kilometers up to a few hundreds of meters [32]. Hence, DD altimeters, such as those on the CryoSat-2 (SIRAL, SAR Interferometric Radar Altimeter), Sentinel-3 (SRAL, Synthetic Aperture Radar Altimeter), deliver more and/or improved data over the ocean, and, especially, in sea ice areas and coastal areas in general.

The SAR altimetry is based on the coherent processing of multiple echoes (e.g., 64 Ku-band pulses emitted by CryoSat-2 and Sentinel-3) within each altimeter burst (aperture duration of approx. 3.5 ms for CryoSat-2 and Sentinel-3), which enables resolving the reflected signals for along-track cells rather than the large footprints generated by the pulse limited altimeters. That naturally results in an improved resolution in the along-track (azimuth) direction of the satellite with the pulse-limited form that depends on the altimeter footprint maintained in the across-track direction (see [31, 32]).

**Figure 4** presents the SAR technology and processing compared to the conventional satellite altimeters. The SAR processing includes counting for the along-track phase shift within each echo obtained from different radar burst, which depends on the geometry of the observation [31]. That produces the multiple echoes gathered at the same ground cell, which allows for the subsequent averaging (i.e., summing coherently) that increases the signal-to-noise ratio, i.e., it results in improved observations of the sea surface.

In [33] different studies on satellite radar altimetry pointed out that the SAR altimetry already performs better over the coastal zones than the conventional altimetry (see also [34]). They also emphasized the potential of the SAR technology

*Geodetic Sciences - Theory, Applications and Recent Developments*

i.e., the size of the area illuminated by the radar from the satellite [6]. Depending on the SWH, the radius of the altimeter footprint can range from 1 km up to 7 km (e.g. for Jason missions), which enables high accuracy of the altimetry in open ocean areas, and on the other side, due to the contamination in the reflected radar altimeter signal caused by the land [19], lower accuracy in the coastal and inland areas

**Inclination Latitude** 

*Early Sentinel-6 measurements validation comparing to Jason-3, sentinel-3A, and sentinel-3B [16].*

GEOSAT 785 108° 72° 163 Ku 13.5 ERS-1/2 785 98° 81° 80 Ku 13.8

GFO 785 108° 72° 163 Ku 13.5 Envisat 785 98° 81° 163 Ku/S 13.6/3.2 CryoSat-2 717 92° 88° 7 Ku 13.6 HY-2A/2B 964 99° 60° 90 Ku/C 13.6/5.3 SARAL/ALTIKA 800 98° 81° 90 Ka 35

**coverage**

1336 66° 66° 315 Ku/C 13.6/5.3

**Equator track distance (km)**

**Band Frequency** 

**(GHz)**

Significant efforts were done to overcome coastal altimetry issues through different projects, e.g., for the Mediterranean Sea projects were conducted such as ALBICOCCA (Altimeter-Based Investigations in Corsica, Capraia and Contiguous Areas), ALTICORE (Altimetry for Coastal Regions), COASTALT (Development of

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(see e.g., [14]).

**Table 1.**

**Figure 3.**

**Mission Orbit** 

TOPEX/POSEIDON Jason-1/2/3 Sentinel-6

**height (km)**

*An overview of the basic characteristics of satellite altimetry missions.*

**Figure 4.**

*Comparison of the principles of the conventional and SAR altimetry (adapted from the [7]).*

for applications to inland water monitoring as well as the applications in cryosphere studies, such as measuring the ice sheet elevation change and sea ice freeboard.

#### **2.5 Altimeter data download and processing**

Altimeter data are available at different levels of complexity through different platforms and for various purposes. AVISO (Archiving, Validation, and Interpretation of Satellite Oceanographic Data) for instance offers gridded and along-track multi-mission altimeter data products (not) corrected for the geophysical effects and for different purposes [35]. Besides, AVISO offers access to Basic Radar Altimetry Toolbox (BRAT) software as well as the tools such as Marine Geospatial Ecology Tools (MGET). On top of that, through the Live Access Server (LAS), AVISO offers on-the-fly data visualization, metadata access, and quick comparisons of the measurements. For geodetic purposes, AVISO's most valuable products are related to the SSH, often upgraded to show ocean variability or cryosphere changes.

Near-real-time along-track satellite altimeter data are available also through the Jet Propulsion Laboratory (JPL) PO.DAAC Drive system (Physical Oceanography Distributed Active Archive Center). The data are delivered as a map or digital data, focusing on the SSH, wind speed, wave heights, and geostrophic velocity vectors [36].

Different products are also available from Copernicus Marine Service [16], which offers complete studies on sea-related topics. That includes original measurements, sea-level-related maps, and sea-level forecasts.

Finally, all the georeferenced source altimeter measurements and many corrections for the measurements are available through the RADS (Radar Altimeter Database System) [37]. RADS provides harmonized, validated, and cross-calibrated sea level altimeter data for the desired area and period of the observations, and it is probably the best place to start with the altimetry for the geodetic studies. Also, RADS offers data preprocessing and processing steps integrated within the system and available through the additional tools.

**85**

**Figure 5.**

*Global Sea level trends (data downloaded from [38]).*

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

A wide variety of satellite altimetry products cover many research fields. In the following section, we focus on presenting the application of altimetry in geodesy.

Sea level change is studied as the global and local phenomena (**Figure 5**). Today, the global sea-level change is routinely computed from the altimetry for the period from 1992 onwards by AVISO, Commonwealth Scientific and Industrial Research Organization (CSIRO), University of Colorado Boulder (CU), NASA - Goddard Space Flight Center (GSFC), The National Oceanic and Atmospheric Administration (NOAA), and others. All the global research studies agree on the current sea level linear trend of approx. 3.2 mm/yr. although the processing methods could differ slightly. The estimates on the global sea-level change trends from satellite altimetry are regularly reported within the IPCC (The Intergovernmental Panel on Climate Change) reports that provide policymakers with regular scientific assessments on climate change. Several studies reported on the regional and local sea-level change, e.g., [39] consolidated the trends and expected sea-level change globally and for the ocean regions, [40] reported on the projections of the regional sea level for the 21st century, [41, 42] recomputed all satellite altimeter data to get more pronounced sea-level change estimates and a better perspective on the impact

The satellite altimetry enabled finer detection of the current acceleration of global and regional sea-level rise. E.g., [38] reported on the climate-change-driven acceleration in sea level rise over the altimeter era, [43] investigated the regional sea-level rise during the altimeter era with previous studies done on uncovering the anthropogenic influence on the sea level rise in some regions [44]. With the climate change acceleration, monitoring of the sea-level change and its variation is going to be even more important. A future perspective on gauging the sea-level change and the needed improvements, both for the satellite observations and the terrestrial

(tide gauge and other) measurements, is summarized in [45].

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

**3. Altimeter products and study cases**

**3.1 Sea-level change**

of future sea-level rise.

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