PROBLEM 



Develop techniques for predicting characteristics of underwater signals and 

 reverberation fields from environmental inputs. These techniques are inputs to performance 

 prediction models, fleet tactics, sonar design, and force level studies. Specifically, examine 

 the accuracy of expendable bathythermograph (XBT) system temperature profiles. This 

 report contains a suite of accuracy studies based on 1961 460-m XBT profiles and 26 

 1830-m XBT profiles collected between May 1971 and October 1975 during six Naval 

 Undersea Center (now Naval Ocean Systems Center) propagation loss experiments. The 

 measurements were made on 1 1 460-m systems and two 1830-m systems from four ships 

 and two research platforms. 



RESULTS 



• Properly functioning XBT systems may develop malfunctions while making a 

 series of profiles and produce visually acceptable, but erroneous, temperature profiles. 

 Other information, such as independent temperature measurements, is required to detect 

 and identify such malfunctioning 460-m XBT systems. In these studies, independent 

 surface temperatures were measured concurrently with 736 visually acceptable XBT profiles 

 made on seven different 460-m systems. Surface temperature comparisons showed that two 

 of the systems, after making a series of accurate profiles, developed malfunctions. Com- 

 parison of 400-m temperatures showed that the malfunction did not result in a simple 

 temperature displacement of the profile. Consequently, it produced profiles having system- 

 atic errors in the vertical temperature gradients. The profiles made after the system 

 malfunctioned were visually acceptable profiles. Of 736 profiles, 36.5 percent were made 

 by the malfunctioning systems. 



• Of a total of 1961 attempted 460-m profiles, the following percentages apply: 



Visually acceptable to the maximum depth 80.1% 



Partially successful 10.8% 



Catastrophic failures 6.4% 



Miscellaneous failures 2.7%. 



• Of a total of 5 1 8 460-m XBT profiles made when the XBT systems were not 

 malfunctioning, only 37.8 percent of those reaching 400 m satisfied 200-, 300-, and 400-m 

 accuracy criteria at all three depths and 19.9 percent failed to satisfy the accuracy criteria 

 at all three depths. The accuracy criteria were based on average hydrocast and STD/SV 

 temperatures. 



• Comparison of XBT temperatures with average hydrocast and STD/SV 

 temperatures, quasisimultaneous STD/SV temperatures, and thermistor chain temperatures 

 taken underway at 3 knots showed that the 460-m XBT systems measure, on the average, 

 temperatures that were higher and vertical temperature gradients that were larger than 

 those measured by the other systems. Once the profiles associated with the large differences 

 were identified and removed from the data set, the remaining profiles accurately measured 

 the temperature. For this data set, the average differences were near zero with standard 

 deviations of0.07°C to 0.1 3°C. 



