OPERATIONAL WAVE FORECASTING WITH SPACEBORNE SAR: 
PROSPECTS AND PITFALLS 
R. C. Beal 
Applied Physics Laboratory 
The Johns Hopkins University 
Johns Hopkins Road 
Laurel, Maryland 20707 
1. Introduction 
In April 1965, now more than 20 years ago, a small number of radio 
scientists and oceanographers congregated at the Woods Hole Oceanographic 
Institute to consider how the new tools of remote sensing might enhance our 
understanding of the global ocean environment. Among the techniques consid- 
ered were radar altimetry, radar scatterometry, and synthetic aperture ra- 
dar. Of course, at that time, good data sets were scarce, and many of the 
recommendations were by necessity somewhat speculative in nature. Neverthe- 
less, the compliment of active microwave instruments that was identified was 
very similar to the suite later flown on SEASAT in 1978. 
Of particular interest to the topic of ocean wave forecasting is the 
following quotation from Gifford Ewing, the conference editor. With respect 
to the value of SAR in monitoring the global wave spectrum, he said: 
"What is needed is the directional energy spectrum of the waves on 
a two dimensional surface, and for this, the vantage point offered 
by a satellite is ideal .... It is within the capability of pre- 
sent day radar technology to give a complete description of the 
sea surface". 
That statement was made over 21 years ago. Since then, we have had 
SEASAT, SIR-A, and SIR-B, all containing high resolution synthetic aperture 
radars, and all collecting varying "descriptions of the sea surface". The 
descriptions, however, are only more or less complete, and we are just re- 
cently beginning to accumulate the evidence necessary to assess the true 
value of SAR for obtaining useful estimates of the global directional energy 
spectrum. 
II. The Problem of Wave Prediction 
  
Ocean wave prediction over global scales has been a fond hope of ocean- 
ographers for several decades. Significant progress was made during and soon 
after World War II, particularly when the concept of a "wave spectrum" was 
advanced to describe the statistical properties of the wave field. The idea 
of an "equilibrium spectrum" gained support from both theoretical and exper- 
imental perspectives in the late 1950's, and has served as a framework for 
departure even up to the present. Moreover, theories of wave growth are now 
well developed in terms of the source wind field properties, such as fetch 
and duration, and to some extent even in terms of the "background" wave 
field. And yet, in the perspective of the global wave prediction problem, 
there are still several fundamental unanswered questions. These questions 
center on both the models and on the means for verifying them. For example: 
0 How often must we measure the wave field in order to 
predict it? 
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