IV. OCEANS MISSION REQUIREMENTS SUMMARY 



Report Based on NASA Oceanography 



Study Team Workshop 



Washington, D.C. 



April 27-28, 1981 



Over the ocean, a Synthetic Aperture Radar (SAR) is sensitive to short 

 gravity waves or to capillary-gravity waves and the oceanographic phenomena 

 measurable in this way are those that influence the structure or distribution 

 of these short waves. 



The presently demonstrated capabilities of SAR are predominately in the 

 area of mapping of oceanographic (and atmospheric) features that produce 

 contrasts in short surface wave structures over relatively small horizontal 

 scales. Many familiar phenomena have been detected, including swell, internal 

 waves, warm core rings And oceanic fronts, and a number of new (and sometimes 

 unexpected) properties have been discerned, including apparent f ilamentation 

 of large-scale current systems, apparent small-scale "eddies" of 10-50 km, and 

 surface indications of bottom topography produced by tidal flow in relatively 

 shallow water. 



Within the next five years, we hope that much of the pattern information 

 presently available will be enhanced by the ability to interpret 

 quantitatively the modulations or variations in return intensity, in terms of 

 the characteristics of the ocean structures that produce them — wave height, 

 current shear, wind speed, and perhaps temperature contrast across features. 

 These developments will considerably increase the utility of SAR for 

 oceanographic purposes. 



Over a longer time span, it may be possible to use Doppler information 

 to measure the speed of propagation of the surface structures producing the 

 SAR return and thus infer surface current speeds. We do not underestimate the 

 technical difficulty of measuring small velocities from a rapidly moving 

 platform, and to date there has not been a careful study to assess such 

 feasibility. To achieve this will require thorough analytical evaluation of 

 existing data and a substantial development program. Nor do we underestimate 

 the difficulty of interpretation of the velocity so measured — the speed of 

 short surface waves is influenced by the orbital velocities of longer waves if 

 they are freely travelling, and harmonic constituents of longer waves will 

 also be detected which travel at a phase speed appropriate to the basic wave, 

 not to the harmonic detected. The speed of propagation of short surface waves 

 is also influenced by surface wind drift so that, to infer the velocity of the 

 underlying current, corrections would be necessary to subtract out the 

 influence of both longer waves and wind and these corrections may well be 

 larger than the signal sought. Our expectation of the success of such an 

 endeavour is therefore low; nevertheless, if it were successful, the 

 oceanographic returns would be extremely high. Consequently, the expected 

 return, the product of the two, is highly uncertain. 



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