Application of phase screens to ray chaos.  Martin A. Mazur and 
Kenneth E. Gilbert (Applied Research Laboratory and the Graduate 
Program in Acoustics, The Pennsylvania State University. P.O. Box 
30, State College, PA 16804.)

In wave propagation in complex media, phase changes along the 
propagation path are often approximated by a series of abrupt phase 
changes at discrete planes called ``phase screens." We show here how 
phase screens can be used in an analogous way in ray tracing 
problems. We formulate the total travel time of each ray as a sum 
over phase screen and free space contributions. Fermat's principle is 
then applied to the travel time, yielding a discrete mapping. The 
mapping connects the ray position at one phase screen to that at each 
succeeding screen. Examples of the method are given for both non-
chaotic and chaotic ray tracing problems. We compare the ray tracing 
solution to the wave solution calculated by the parabolic equation. By 
letting the separation between phase screens go to zero, we show the 
connection between continuous propagation and propagation with 
discrete transitions at phase screens. 
[Work supported by the Pennsylvania State University Applied 
Research Laboratory.]