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www.sea-technology.com February 2015 / st 35 I n the previous issue of Sea Technology, we wrote an article advocating for in- situ acoustic measurement of transmission loss (TL) to improve anti-submarine war- fare (ASW) sonar search. Here we provide quantitative rationale. The Problem ASW ships' sonar search plans are not fully effective, especially in the acoustically chaotic waters of the littoral, due to the in- accuracy of oceanographic parameters that feed TL models. Predicted sonar detection ranges depend on Figure of Merit (FOM) calculations derived from the sonar equa- tion, in which TL is a central parameter. There can be, and usually are, signifcant differences between the predicted and ob- served detection ranges, many attributable to uncertainties in TL and resulting in a re- duced probability of detection and ineff- cient use of assets. The frst graph of this article, "TL versus Range," depicts the variability of TL and the typical disparity between pre- dicted and measured TL values in shallow water. The squares in the fgure show TLs from a U.S. Navy SQS-26/53 sonar system that operated in the Sea of Japan during summer at four different hydrophone depths along a cross-slope track, parallel to the bathymetric contours. Two model predictions for each of the four hydrophone depths are also plotted: one based on a feet-approved CASS/GRAB TL model; the other based on the Marine Geophysical Survey MGS-1 bottom, using locally dropped BTs and the feet's historical database. The second set of graphs depicts the effects of TL tempo- ral variability in shallow water. TL measurements are shown over a 10-day period from a summer Shelf Break PRIMER experiment between a fxed source at 400 hertz, 300 me- ters depth and a receiver at 30 meters depth, 42 kilometers distance. Extensive TL measurements were used to evaluate temporal oceanographic effects at a fxed range over a se- lected geography. The frst graph in this set shows the plot of measured TLs, and the horizontal line represents the cli- matological mean from historical data. The second graph in this set presents the histogram calculated from the data in the frst graph of this set. The second graph shows a 4-deci- bel difference between the 92-decibel measured mean of the histogram and the 88-decibel mean of the climatologi- cal model, and a one-sigma variability of 4.2 decibels due to semidiurnal internal tides and high-frequency internal waves. Obviously, an error of 4 decibels in the mean with an approximately 8.4 decibel (2 x 4.2 decibels) window of fuctuations of the one-way TL would introduce noticeable errors in predicted sonar detection ranges—which could be reduced by using in-situ measurement of TLs. The third graph describes the sensitivity of sonar de- tection range to predicted TL error. A family of sensitivity curves has been derived so that quantitative values in prob- ability of detection, search time, fuel and naval assets can Measured Transmission Loss: A Key to Improved Sonar Performance Quantitative Rationale for In-Situ Acoustic Measurement By Charles H. Wiseman • U.S. Navy (ret.) RAdm. Richard Pittenger The plot depicts the typical disparity between predicted and measured values of TL in shallow water.