**2.2 Previous and current satellite altimeter missions**

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) mission and widespread ground segment that enabled:


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

## **Figure 2.**

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

**81**

**Table 1**.

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

At present, several satellites are providing measured altimeter data:

• Sentinel-3 launched in 2015 by ESA and operated by EUMETSAT,

previous TOPEX/Poseidon and Jason 1/2/3 satellite missions.

• Cryogenic Satellite (CryoSat)-2 designed and built by ESA and launched

• Haiyang (HY)-2a approved and led by China National Space Administration

• SARAL launched in 2013 as a cooperative mission between the Indian Space

• Jason-3 designed in collaboration of the NASA and ESA as the successor of

• Haiyang (HY)-2b launched as the second in the series of Chinese Haiyang

Sentinel-6 satellite mission is currently in its commissioning phase, i.e., in the calibration/validation phase. **Figure 3** presents Sentinel-6 sea-level anomaly derived from 'Short Time Critical Level 2 Low Resolution' data, overlaid on a map showing similar products from the other Copernicus altimetry missions: Jason-3, Sentinel-3A, and Sentinel-3B (for details and original research, please see [17]). The background image is a map of sea-level anomalies from satellite altimeter data provided by the Copernicus Marine Environment Monitoring Service for 4 December 2020. The data for this image were taken from the Sentinel-6 products generated on 5 December 2020. Being in its commissioning phase, the measurements obtained by

The characteristics of previous and current satellite missions are given in

to operate in Ka-band with a 0.86 cm radar wavelength [18].

**2.3 Advanced altimeter processing methods – retracking**

Surface Water Ocean Topography (SWOT) mission is planned to be launched primarily to enable terrestrial water monitoring. The mission is a joint project of NASA, CNES, the Canadian Space Agency, and the UK Space Agency. It is expected

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,

• and Sentinel-6 Michael Freilich (previously referred to as Jason CS) launched in late 2020, which continues the EU Copernicus and NASA program and

RMSE (Root Mean Square Error) of 2 cm and orbit accuracy estimated at around 2.5 cm (see [6, 9]). Later improvements of the TOPEX/Poseidon data processing based on its dual-frequency altimeter estimates of sea-surface height resulted in an overall precision expressed with root-sum-of-squares (RMS) of about 4 cm [6], which today is an expected accuracy of altimeter data from different satellite missions and can get up to RMS of 2 cm for open ocean altimetry [9]. The advances in orbit determination were due to the development of the DORIS satellite tracking system. DORIS was developed by CNES to determine the satellite orbits with centimeter accuracy from a network of 60 ground stations settled worldwide [15].

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

in 2010,

(CNSA) launched in 2011,

TOPEX/Poseidon and Jason 1/2,

satellites in 2018,

the Sentinel-6 are promising [17].

Research Organization (ISRO) and CNES,
