2.1 Principles of satellite altimetry

"Coastal altimetry" might be much more familiar for the altimetry community than for the coastal community. A very brief background introduction of satellite altimetry would be presented here, and interested readers are encouraged to refer to the elaborate books such as [2, 3].

The concept of the satellite altimetry is straightforward. A nadir-pointing spaceborne radar transmits short pulses and receives the echoes from the earth (usually, sea) surface. The sea level parameters (range between the satellite and sea surface, significant wave height, and backscatter coefficient) are extracted from the echoes via a process called "retracking". The altimetric range can be extremely precisely measured in this way. Meanwhile, the satellite orbit can also be precisely determined. After carefully compensating for a variety of error sources, the surface height relative to an absolute datum (usually the reference ellipsoid) can be accurately retrieved, within no more than few centimeters over open ocean surface.

While the satellite altimetry can be dated back to the 1960s [4], one must admit that the Topex-Poseidon satellite launched in 1992 is a benchmark [5]. Thanks to its unprecedented accuracy, it remolded the knowledge scene of many fields in oceanography, such as ocean circulation, ocean tide, El-Nino, and global climate change. Its successors, Jason series satellites, have been extending the high-quality sea level record incessantly [6–8]. Space agencies of Europe, China, and India are also handling altimetry missions, such as ERS-1, ERS-2, Envisat, Saral, and HY-2 [9–12]. Now a constellation of complementary altimetry satellites (with different orbit sampling strategy) have been formed and abundant data are worthy of exploiting. Nowadays, altimetry is not only a fundamental tool for oceanographers and geodesists, but also an attracting resource for those who research into the fields of coastal zone, Cryosphere and inland waters, etc.

#### 2.2 Difficulties in coastal altimetry

Coastal altimetry is not an easy task. There are a couple of difficulties when extending altimetry technology to coastal zone. Firstly, in the coastal band a few kilometers wide (comparable to the altimeter footprint size), radar echoes are severely contaminated by the nearby land surface, leading to complex waveforms significantly departing from that of open ocean. Things are further complicated by the fact that the geographic and environmental characteristics of the coast (e.g., coastline direction, relief, bathymetry, and rain rate) are extremely diverse throughout the world, and altimetric mission (orbit configuration, on-board tracker, flight direction, etc.) are also different. Figure 1 shows two examples of waveforms, one over the open ocean (a) and the other over a coastal zone (b). Therefore, a specific process called 'retracking' is widely employed to extract the sea level parameters from these nonstandard waveforms.

Another difficulty is related to the various corrections applied to the altimeter measurements that are usually less accurate at the coast. The most suffered corrections are wet tropospheric delay, ocean tide correction, dynamic atmospheric correction (DAC), and sea state bias. Consequently, most altimeter data near land are flagged as invalid and eliminated from the standard products.

Coastal Altimetry: A Promising Technology for the Coastal Oceanography Community DOI: http://dx.doi.org/10.5772/intechopen.89373

Figure 1.

Examples of typical open ocean waveform (left, the red line corresponds to the fitted Brown model) and coastal ocean waveform (right).
