A COMPARISON OF PROPAGATION MODELS 



At present, the state of the art of propagation modeling for stratified oceans 

 may be illustrated by two figures compiled by E. Jensen of NUSC. (See figures 

 10a and 11a. ) Both compare FFP, ^^ FACT, ^^ rayMODE 9, ^^ and NISSM 11^^ 

 computer predictions for 50-Hz propagation in the Pacific. The choice of pro- 

 grams included in the comparison was mainly of convenience, since each is 

 available at NUSC, New London, and all but FACT were designed there. 



Briefly, the Fast Field Program (FFP) utilizes Fast Fourier Transforms 

 to evaluate the integral representation. The Fast Asymptotic Coherent Trans- 

 mission Model (FACT) is a constant gradient ray tracing program incorporating 

 sophisticated low frequency modifications. RAYMODE 9, the latest version of 

 the series, uses ray theory to determine which intervals dominate the integral 

 representation, but uses normal modes to compute the acoustic amplitude. The 

 Navy Interim Surface Ship Model (NISSM) II is a continuous gradient ray tracing 

 program designed to predict the performance of active sonar systems. All but 

 FFP have the option to combine multipath contributions incoherently as well as 

 coherently, and all but FFP use alternative procedures for surface duct propa- 

 gation. 



As a result, the first case (figure 10a), which is dominated by surface duct 

 propagation, will show the greater variability. FACT is £Ui order of magnitude 

 faster than NISSM II and RAYMODE 9, while FFP is a good deal slower. 



Upon adding CONGRATS V to the comparison (figure 10b) and invoking the 

 coherent phase option, one sees good agreement with FFP. Had the incoherent 

 phase option been invoked instead, CONGRATS V would have agreed with the 

 others. 



The second case, figure 11a is dominated by convergence zone propagation. 

 The agreement is better than before although running times continue to differby 

 orders of magnitude. 



Upon adding CONGRATS V to this comparison (figure lib), one obtains 

 reasonable agreement with FFP. It is imusual to find agreement among models 

 that are based upon different theories and written by different programmers. 

 Unfortunately, comparisons are not always this good. Hopefully, all discrep 

 ancies will soon be eliminated or at least accounted for. 



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