COMPARISON OF SIMULTANEOUS XBT MEASUREMENTS 



From 10-18 December 1972, simultaneous XBT measurements were made during 

 the ORB-3 experiments, in which two independent 460-m XBT systems were used. The 

 probe launchers were located about 30 feet apart. Another set of simultaneous XBT 

 measurements was obtained from 17-28 October 1975 during the RAPLOC/DEEPTOW 

 experiments. Two 460-m and one 1 830-m XBT system were used to make these measure- 

 ments. The two 460-m launchers were located about 5 feet apart, with the 1 830-m launcher 

 located about 45 feet from the 460-m launchers. For both data sets, probes were launched 

 just a few seconds apart — the time it took for the observer to walk from one launcher to 

 the other(s). 



SIMULTANEOUS 460-m XBT PROFILES 



During the ORB-3 experiments, a total of 49 pairs of 460-m XBT profiles were 

 attempted from the ORB - 20 in transit to the experimental area at 5 knots and 29 at 

 anchor during the experiments. The number of visually acceptable pairs (both profiles 

 extending to depths greater than 50 m) was 45 — 18 under tow and 27 at anchor. During 

 the RAPLOC/DEEPTOW experiments, a total of 47 pairs of 460-m profiles were attempted. 

 The number of visually acceptable pairs was 45. All profiles were made from the ORB 

 at anchor. 



In this study, temperature comparisons will be made at standard hydrocast depths 

 from the surface to 400 m. The depths of the near-surface layer, or top of the thermocline, 

 also will be compared. Temperature differences between two profiles are dependently 

 related to differences in measuring temperature and depth. In the near-surface layer, or 

 above the thermocline where vertical temperature gradients are small or non-existent, the 

 differences are due almost solely to the accuracy of the measured temperature. In the 

 thermocline, where vertical temperature gradients are large, the temperature differences 

 result primarily from errors in measuring depth. Below the thermocline, where vertical 

 temperature gradients are smaller than in the thermocline and larger than in the near- 

 surface layer, temperature differences are the result of errors in measuring both temperature 

 and depth. 



The temperature and layer depth differences (arbitrarily system A minus system B) 

 for the ORB-3 under-tow and at-anchor measurements and for the RAPLOC/DEEPTOW 

 measurements are tabulated in appendix H. From the surface to the horizontal line, the 

 differences are below the thermocHne; and for all differences between the horizontal lines, 

 the differences are in the thermocline. An asterisk after the XBT profile pair number 

 indicates pairs which agree to within ±0.34°C for comparisons made above and below 

 the thermocline.* 



The data tabulated in appendix H are summarized in figure 41 . The left-hand pair 

 are for the ORB-3 measurements, and the right-hand pair are for the RAPLOC/DEEPTOW 

 measurements. The upper portion of each pair is a plot of the differences at each standard 



*lf the standard deviation of an XBT temperature measurement is ±0.1 2°C, then the standard deviation 



of the difference between two simultaneous XBT temperature measurements is ±-^0.12- +0.12-^ , or 

 ±0.1 7°C, and two standard deviations is ±0.34°C. 



103 



