fluoresce, active lasers have been used to excite this phenomena in the 

 oil, v^ith the receiver detecting only the emitted fluorescence. At 

 microwave frequencies, radar has been effectively used to detect oil 

 spills since the oil has a different emissivity than the water and thus 

 gives rise to a different radar return. Such techniques are being rapidly 

 made operational by agencies such as the USCG. 



Remote water temperature measurements have also now become 

 routinely received from aircraft and satellites. The primary remote sen- 

 sor is an infrared radiometer, sensitive to thermal infrared wavelengths 

 of about 8 to 12 microns. NOAA weather satellites routinely produce 

 thermal images where the varying gray scale is proportional to the sur- 

 face water temperature. As noted earlier, the USCG also employs stand- 

 ard infrared temperature sensors in its ART survey system. 



NASA developments have also shown that a passive microwave 

 radiometer sensitive at S-Band frequencies will make surface water 

 temperature measurements to an accuracy of about +1° C. The 

 microwave devices are attractive since they are minimally affected by 

 clouds, whereas the infrared sensor above clouds will give a measure of 

 the cloud-top temperature rather than the water temperature below. 



Many water pollutants are detectable because they cause changes in 

 water color. Images from the LANDSAT satellites have often shown 

 water color changes that have been attributed to such factors as sewage 

 sludge, acid waste dumps, industrial effluents, heavily sedimented 

 rivers, and algal blooms. The LANDSAT images are processed from 

 multispectral scanner data from four spectral regions in the visible and 

 near infrared. These wideband (approximately 100 nm) radiance data 

 can be processed in black and white or color images and are available on 

 Computer Compatible Tapes for automated analyses. 



There are numerous multispectral scanners, including those commer- 

 cially available for aircraft use. For example, NASA has aircraft versions 

 of a Multichannel Ocean Color Sensor (MOCS) and an Ocean Color 

 Scanner (OCS), with the latter very close to an aircraft prototype of the 

 Coastal Zone Color Scanner that will fly on the NASA Nimbus G 

 satellite in 1978. Both the MOCS and OCS are especially suited for the 

 detection of color changes in the water because they have numerous 

 channels with narrow bandwidths (15-20 nm) in the visible region of the 

 spectrum. Such spectral resolution allows separation of the subtle 

 effects of various water constituents such as sediments, chlorophyll, and 

 acid wastes. This discrimination is a result of the "spectral signature" of 

 the pollutants. The instrument not only discriminates between pollu- 

 tants, it infers a quantitative measure of the actual pollutant concentra- 

 tion. Film-filter combinations can be used with cameras to provide 

 multispectral images, and several systems have been developed to ap- 

 proximate the spectral coverage provided by the LANDSAT scanner. 

 However, this device is not as adaptable to automated analysis tech- 



56 



