respect to the ship track). The longer stern waves seem to be observed 
best when they are azimuth travelling (i.e., for a look direction perpen- 
dicular to the ship track), apparently because velocity bunching is the 
optimum imaging mechanism for these waves. 
Turbulent wakes are observed under moderate to high winds, and appar- 
ently under any stratification conditions, although the role of stratifica- 
tion in the formation of these features has not yet been determined. Tur- 
bulent wakes appear in both X-band and L-band images. The contrast of the 
dark lines associated with the turbulent wake is often larger at L-band, 
but the bright lines which sometimes appear at the edge(s) of the turbulent 
wake are usually more prominent at X-band. These wakes are observed with 
any look direction, but there is some evidence that a look direction normal 
to the ship track produces stronger signatures. 
Ship-generated internal waves appear under moderate winds and require 
the presence of a strong, shallow density gradient. Modulations of the 
image intensity are usually larger at L-band than at X-band, because of the 
longer lifetime of the L-band Bragg waves. Since the induced surface cur- 
rents due to internal waves are approximately in the cross-track direction, 
Bragg waves travelling in this direction are most strongly modulated. 
Thus, the internal wave patterns are strongest for a cross-track look 
direction. 
The status of our understanding of each of the phenomena involved in 
the SAR imaging of ship wakes is summarized below. Wakes caused by ship- 
generated surface waves (including both narrow wakes and classical Kelvin 
wakes) are relatively well understood, although the dependence on environ- 
mental conditions and radar parameters is not resolved. Turbulent wakes 
are perhaps the least understood among the various wake phenomena. 
Although initial modeling results appear encouraging, additional data col- 
lected over a broader range of environmental conditions with more detailed 
surface measurements need to be studied to validate these models. Since 
these are perhaps the most frequently observed wake feature as well as the 
least understood, more research in this area is strongly recommended. 
Finally, ship-generated internal waves are at an intermediate level of 
understanding. Detailed models exist for these phenomena which appear to 
yield reasonable results, at least at L-band, but which need to be further 
evaluated by detailed comparison with actual SAR images. 
Acknowledgements 
  
This work was primarily supported by the Office of Naval Research (ONR) 
under Contract No. N00014-81-C-0692. The technical monitor for this work 
was Mr. Robert Winokur. The authors gratefully acknowledge the contribu- 
tions to this work by Dr. Claude Swanson of Applied Physics Technology, 
Inc., and Mr. Eric Kasischke of ERIM. 
References 
Alpers, W.R. and C.L. Rufenach, The Effect of Orbital Motions on Synthetic 
Aperture Radar Imagery of Ocean Waves, IEEE Trans. Antennas 
Propagat., AP-27, pp. 685-690, 1979. 
  
JASONs, SEASAT III and IV, MITRE Report No. JSR-84-203, McLean, VA, 150 
pp., 1984a. 
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