All XBT profiles used in this analysis were originally classified as visually acceptable 

 profiles. In the absence of independently measured surface temperatures, all 736 XBT 

 surface temperatures would have been considered valid. In that event, the statistics would 

 have been markedly altered, with the average difference being -0.1 5°C with a standard 

 deviation of 0.47°C. In addition, the bias in the distribution of differences would have 

 been reversed, with 56.3% of the differences being negative, 40.3% positive, and 3.4% zero. 



TEMPERATURE ACCURACY OF 460-m SYSTEMS 



XBT measured temperatures are compared with average hydrocast and STD/SV 

 measurements, quasisimultaneous STD/SV measurements, and simultaneous thermistor 

 chain measurements. 



Two studies compared the average hydrocast and STD/SV measurements with XBT 

 profile measurements. The first study was based on a set of 826 visually acceptable profiles 

 that reached a minimum depth of 200 m and the second on a set of 528 visually acceptable 

 profiles that all reached 400 m and also had an independent surface temperature measure- 

 ment. For the first study, comparisons were made at 200, 300, and 400 m since, at these 

 depths, vertical temperature gradients are small. The second study made comparisons 

 at the surface and at 200, 300, and 400 m. 



The first study showed that, in the absence of independent temperature 

 measurements to detect XBT profiles made when an XBT system malfunctioned, the XBT 

 system measures, on the average, temperatures that are higher and vertical temperature 

 gradients that are larger than those measured by the hydrocast and STD/SV systems. 

 Eliminating the profiles made when the two XBT systems malfunctioned, a reanalysis of 

 the remaining 559 profiles still supported the above conclusion. Thus, even with the omis- 

 sion of obviously erroneous profiles made when XBT systems malfunction, there remain 

 enough erroneous profiles to bias systematically the average temperature and the average 

 vertical temperature gradient. Using an accuracy criterion based on the average hydrocast 

 and STD/SV measurements, the data set was divided into a subset of 354 profiles that 

 satisfied the criterion and one of 205 profiles that did not satisfy the criterion. The 

 differences at all three depths for the data set that satisfied the criterion were normally 

 distributed with mean near-zero and standard deviations of 0.07°C to 0.1 3°C. Once the 

 biasing profiles were identified and removed from the data set, the remaining profiles 

 accurately measured the temperature. The biasing profiles were detectable only with the 

 aid of independent 200-, 300-, and 400-m temperature measurements. 



The results of the second study were similar to those obtained in the first study. 

 For the set of 528 visually acceptable profiles, the XBT measured a surface temperature 

 that, on the average, was 0.15'^C less than the independently measured surface temperature. 

 This results in an average positive temperature gradient bias in the surface-to-200-m, layer. 

 Analysis of the 351 profiles that remained after the elimination of the profiles made with 

 the two malfunctioning XBT systems showed that, at the surface, the XBT profiles 

 measured a temperature slightly higher than the independently measured temperature. At 

 200, 300, and 400 m, the XBT profile still measured temperatures slightly higher than the 

 average hydrocast and STD/SV measurements. In addition, the vertical temperature gradient 

 from the surface to 200 m was slightly biased. 



A total of 66 pairs of quasisimultaneous XBT and STD/SV profiles were used to 

 examine further the accuracy of the XBT measurements at the surface and at 200, 300, 

 and 400 m. All measurements were made with the vessel hove to and drifting or at anchor. 



128 



