radiometers. Moreover, the resolution and spatial scales of these air- 

 borne sensors can provide ground truth and calibration data for the satel- 

 lite sensors over wide areas, thus potentially extending the utility of the 

 present generation of satellite instrumentation. 



Laser fluorosensors are in a class known as "active" remote sensors because 

 they provide their own source of energy. Laser pulses are fired into the 

 water column from low flying aircraft and the induced emission spectrum is 

 sensed in narrow spectral bands. The returning laser light varies as a 

 function of backscatter from particulate matter within the ocean and as a 

 function of absorption. Red shifted Raman backscatter from the water 

 molecule itself is proportional to the number of water molecules accessed, 

 or equivalently, to the penetration depth of the laser beam into the water. 

 The Raman backscatter provides a direct measure of water clarity or tur- 

 bidity. Further, the strength of the Raman signal can be used to correct 

 fluorescent signals received from photopigments for spatial variations in 

 optical transmission properties of water (Bristow et al., 1979; Hoge and 

 Swift, 1981), thus eliminating the need for extensive surface truthing of 

 water transmissivity. As with shipboard or moored f luorometers, the cor- 

 rected chlorophyll a fluorescence signal centered at 685 nm is used to 

 guage chlorophyll a concentration. 



A measure of the relative abundance of different pigment classes or color 

 groups of phytoplankton, such as the golden-brown species (diatoms and 

 dinoflagellates), can also be made by using laser light of different fre- 

 quencies to excite the photopigments (Figure 2-4). Through "time gating," 

 the entire return emission spectra can be sampled separately in layers. 

 Alternatively, a single band can be temporally measured providing detailed 

 vertical distribution of a particular constituent. Unfortunately, chloro- 

 phyll a^, the most useful photopigment in estimating phytoplankton dynamics, 

 fluoresces at 685 nm in the red spectral region where the transmissivity in 

 water is comparatively low. Therefore, the depth measurement of the 685 nm 

 chlorophyll a fluorescence signal itself is especially limited to upper 

 surface layer observations. Differential absorption of two or more laser 

 wavelengths, however, has a good potential for allowing chlorophyll a^ 

 measurements at depth throughout the optical layer of the ocean. Recently 



2-12 



