4. Conclusions and prospective

In this paper, a promising technology: coastal altimetry is described in many facets. The advances in the last decade are reviewed, especially on the coastal waveform processing. In the Hong Kong offshore case study, we find that the coastal sea level trend is about twice as much as the one observed further offshore (which can be inferred as +2.7 mm/yr. from the ESA Sl\_CCI product [41]). It suggests that in the Hong Kong region, the short-term sea level trend significantly increases when approaching the coast. Scientists worldwide have discovered many new features in different coastal regions (e.g., the Mediterranean coast, the Australia coasts, the coasts of the United States, West African coast, etc. [44]), and the technology would be further exploited in the future benefiting from the new conceptual altimeters.

Delay Doppler Altimeter (DDA), or Synthetic Aperture Radar (SAR) altimeter, is one of the most exciting advances in altimetry [45]. The along-track resolution is significantly improved by an order magnitude (from several kilometers to a few hundred meters), which is particularly useful to capture the small scale features. Cryosat-2 is the first satellite to demonstrate the DDA approach, and Sentinel-3 satellites operate in DDA mode all the time [46, 47]. China and other countries are also planning to launch altimeters of this type. A few investigations have been

reported to show the potential of the DDA technology, and solid contribution would be made to the coastal community if more dedicated waveform processing and geographic corrections approaches are developed and validated.
