Since the early days of ocean color remote sensing (when CZCS images first became available), researchers have been looking for means to extend their reach beyond the surface of the ocean. By recalling Fig. 2.2, it is clear that the only capable means to explore the ocean’s vertical structure from space is through the window of optics in the EM spectrum, where other bands - including microwaves and IR-have negligible penetration beyond the very thin, top layer of the ocean, due to their strong absorption by water.
The most transparent channel, depending on the constituents of the water, is that of the green to blue wavelength. Green wavelengths are especially favored because low-cost, high-power lasers are readily available in the form of frequency-doubled Nd:YAG lasers at 532 nm. This is the basis of the majority of development of active optical sensing from space, or lidar. Lidar is similar to sonar, and is very close to that of range-gated active imaging (discussed in Chapter 4). This chapter focuses on the application of airborne lidar for sensing the vertical structure of the ocean. The most relevant topics involve measurements of temperature T, [chla], altimetry, and water depth (bathymetry or bathy).
In addition to the vertical information active remote sensing provides, lidars are not limited by the availability of light, meaning that night operations are possible. In fact, due to atmospheric scattering and background illumination, lidar performs better at night because of higher SNR. It is also important to keep in mind the limitations of lidar, such as limited channels or spectral information. One could argue that lidar is trading spectral coverage for vertical information, since horizontal coverage can be remedied by a high repetition rate and fast-moving platform, including airplanes and satellites.
In this chapter, the basics of lidar systems are covered first, followed by discussions on their applications in ocean bathymetry, temperature, salinity, CDOM, chlorophyll, and physical/biological layers in the ocean.
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