The penetration depth of light in the sea is defined for remote sensing purposes as the depth above which 90% of the diffusely reflected irradiance (excluding specular reflectance) originates. It is demonstrated that for a homogeneous ocean, this is the depth at which the downwelling in-water irradiance falls to 1/e of its value at the surface. Penetration depths as a function of wavelength are presented for a variety of water types, and a mean penetration depth z (90) for a broadband sensor is defined and applied to the MSS on ERTS-1. The maximum z (90) expected for ERTS-l is found to be somewhat less than 20 m.

There is obvious value in developing the means for measuring a number of subsuflacs oceanographic parameters using remotely sensed ocean color data. The first step in this effort should be the development of adequate theoretical models relating the desired oceanographic parameters t o the upwelling radiances to be obeerved. A portion of a contributory theoretical model can be described by a modified single scattering approach based upon a simple treatment of multiple scattering.The resulting quasi-single scattering model can be used t o predict the upwelling distribution of spectral radiance emerging from the sea. Tho shape of the radiance spectrum predicted by this model for clear ocean water shows encouraging agreement with measurements mads at the edge of the Sargamo Sea off Cape Hatteris. OCEAN COLOR SPECTRUM CALCULATIONS AYTMDUCTIONP-large number of techniques have bezn developed for sampling the ocean from top to bottom. But horizontal sampling of t h~ ocean has been conducted on a reasonable scale only rarely, and then only at considerate expense of time and money. It is true that a saita.bly equipped research vessel can cover large areas of the ocean and collect a large amount of data in a single cruise. tiowever, with t h e relatively small cruising speeds avaiiable today, the area: extent over which the meaE-!rements can be assumed to be synoptic is severely limited.A s the field of oceanography becomes more and more sophisticated, with many scientists actively involved in large-scale modeling of oceanographic parameters, the need ior synoptic data will continue t o increase. But until the number of available suitably equipped research vessels increases several orders of magnitude and their maximum speeds increase very substantially, neither of which is very likely to happen in the near future, w e w i l l need to rely on data collected by aircraft and satellites in order to obtain the kind of large-scale synoptic data that is needed to build accurate models suitable for yrcJictfon.The need for synoptic data on the ocean, and especially the near-shore a r e a s , is so great that much effort should be expended in this decade aimed at obtaining this data by remote sensing, both actively and passively from aircraft and satellites.Due to t h e essentially opaque nature of seawater outside the visible and near ultra-violet portions of the electromagnetic spectrum, remote measurements of subsurface oceanographic parameters will necessarily be limited to 3 these spectral regions. By "subsurface" i s here meant the region from a few millimeters to a few tens of meters depth, the region of penetration of sunlight and skylight into the sea. Until the maximum weight and power limitations imposed on present-day scientific satellites is permitted to expand significantly, most of the subsurface information collected frow spacecraft w i l l necessarily oe limited to the passive mode, wherein one monitors the incoming radiation from the sun and sky scattered upward at subsurface depths.Of the many subsurface...

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