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E. R Anderson.

Expendable bathythermograph (XBT) accuracy studies online

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and RAPLOC/DEEPTOW experiments.



105



Some additional observations, based on the data of appendix H and table 44, are
of interest:

1. The ORB-3 differences for 26 (57.8%) of the 45 pairs were less than or equal to
±0.34°C at all above- and below-thermocline depths, and for the RAPLOC/DEEPTOW
differences the comparable number was 36 (80.0%) of the 45 pairs. For the combined
data set of 90 XBT pairs, 62 (68.9%) of the pairs had differences less than or equal to
±0.34°C at all above- and below-thermocline depths. For the remaining 28 (31 .1%) of the
pairs, the differences were greater than ±0.34°C at one or more depths.

2. For the combined data set of 90 pairs, the differences were all less than or equal
to ±0.34°C in the near-surface layer for 79 (87.8%) of the pairs, in the thermocline for 46
(5 1 .7%) of the pairs, and below thermocline for 61 (7 1 .8%) of the pairs.

3. For the combined data set of 90 pairs, the differences at all comparison depths
were less than or equal to ±0.34°C for 43 (47.8%) of the pairs.

4. For the 45 ORB-3 profiles, the layer depth differences were less than ±5 m for
40 (88.9%) of the pairs, and for the 45 RAPLOC/DEEPTOW profiles, the comparable
number was 41 (91 .1%) of the pairs. For the combined data set of 90 pairs, the near-surface
layer depth differences were less than or equal to ±5 m for 8 1 (90.0%) of the pairs and
exceeded ±5 m for nine (10.0%) of the pairs.

Figure 42 is an ogive of the absolute value of the 1116 simultaneous XBT temper-
ature differences measured at standard hydrocast depths from the surface to 400 m during
the ORB-3 and RAPLOC/DEEPTOW experiments. The absolute value is used since the
signs of the differences are arbitrary. This figure shows the percentage of the 1116 compar-
isons of simultaneous XBT measurements that exceeded a given difference. For example,
16.6% exceeded 0.34°C.

The preceding discussion was concerned primarily with the statistics of the
differences between simultaneous pairs of XBT profiles. The following discussion presents
and considers selected specific pairs of profiles, and supports the previous discussion.

An inspection of the ORB-3 data listed in appendix H shows that for XBT pair 47,
the differences were greater than 0.34°C at all depths except the surface and 400 m, and for
XBT pair 12 they were greater than 0.48°C at all depths except the surface. For these two
comparisons, system A recorded a higher temperature than system B. Figure 43 shows
superimposed temperature profiles for these two comparisons. Note the large differences
in the near-surface layer where the temperature is nearly isothermal. In each case at least
one of the profiles must be incorrect.

Average below-thermochne differences were greater than or equal to ±0.50°C for
four XBT pairs-XBT pair 46 (-1 .05°C), XBT pair 54 (-0.58°C), XBT pair 1 2 (0.54°C),
XBT pair 47 (0.50°C). XBT pairs 46 and 54 are presented in figure 44. These pairs show
how near identical agreement from 40 to 1 60 m for XBT pair 46 and from the surface to 1 90 m
for XBT pair 54. However, for depths greater than 160 m and 190 m, respectively, the
differences increase to as much as -1 .74°C at 400 m for XBT pair 46 and -0.84°C at 400 m
for XBT pair 54. In the figure 44 comparisons, system B measured temperatures higher
than system A.

An inspection of the RAPLOC /DEEPTOW comparisons listed in appendix H shows
that some of the largest below-thermocline differences are associated with pairs 33 and 38,
which are presented in figure 45. Pair 33 shows minor differences in the near-surface



106



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ABSOLUTE VALUE OF TEMPERATURE DIFFERENCE, °C



Figure 42. Ogive of the absolute value of 1 116 simultaneous XBT temperature differences measured at standard
hydrocast depths from the surface to 400 meters during the ORB-3 and RAPLOC/DEEPTOW experiments.



107




Figure 43a. Temperature profiles for ORB-3 XBT pair 47.



108




Figure 43b. Temperature profiles for ORB-3 XBT pair 12.



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112



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Figure 45b. Temperature profiles for RAPLOC/DEEPTOW XBT pair 38.



113



layer with identical agreement in the thermocline. However, below the thermocline they
begin to differ, with the difference being an increasing function of depth. At 400 m, the
difference was 3.01°C. Pair 38 shows identical agreement from the surface to 1 15 m. At
115 m, the two profiles begin to differ, with the difference being a variable function of
depth. Figures 45a and 45b exhibit characteristics similar to those shown by two of the
ORB-3 comparisons (figure 44). The major difference is that for XBT pair 38 (figure 45b),
the differences do not diverge with depth.

RAPLOC/DEEPTOW XBT pairs 4 and 39 exhibited a marked difference in the
above-thermocline comparisons as well as in the below-thermocline comparisons. These
two comparisons are shown in figure 46.

Remember, all profile pairs shown in figures 43-46 were simultaneous pairs made
by using two independent XBT systems whose launchers, in the case of the ORB-3
measurements, were located 30 feet apart and, in the case of the RAPLOC/DEEPTOW
measurements, were located 5 feet apart. All of the measurements except the one presented
in figure 43b were made from the ORB at anchor.

Table 45 shows the average temperatures for each XBT system for the 26 ORB-3
and the 36 RAPLOC/DEEPTOW XBT pairs whose differences met the accuracy specifi-
cation at all comparison depths above and below the thermocline ("starred" comparisons
in appendix H). For both data sets, system A, on the average, measured slightly higher
temperatures than system B. All average differences, even the in-thermocline differences,
are well within the ±0.34°C accuracy specification. It is concluded that when the biasing
profdes are detected and removed from a data set, excellent agreement between the
remaining profiles is reached.

Note in figures 43^6 that the left-hand profiles, except possible ORB-3 pairs
47 and 54 (figures 43a and 44b), agree better with the average temperatures listed in
table 45 than the right-hand profiles. This suggests that, in at least three cases, the right-
hand profiles are the incorrect profiles. This is important since the temperature errors
introduced into these sets of XBT profiles are systematic, biasing, and nonrandom. The
implications are obvious.



114









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Figure 46a. Temperature profiles for RAPLOC/DEEPTOW XBT pair 4.



115



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Figure 46b. Temperature profiles for RAPLOC/DEEPTOW XBT pair 39.
116









Temperature, °C






Depth,
m


ORB-3


RAPLOC/DEEPTOW


System A


System B


Difference


System A


System B


Difference





16.16


16.13


0.03


17.10


17.05


0.05


10


16.19


16.13


0.06


17.06


17.01


0.05


20


16.22


16.14


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17.05


16.98


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30


16.23


16.17


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17.01


16.95


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16.15


0.02


15.68


15.68


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75


12.56


12.40


0.16


12.45


12.36


0.09


100


10.99


10.97


0.02


11.08


10.98


0.10


125


10.20


10.15


0.05


10.12


10.11


0.01


150


9.73


9.63


0.10


9.44


9.46


-0.02


200


8.90


8.85


0.05


8.69


8.69


0.00


250


8.23


8.16


0.07


8.04


8.07


-0.03


300


7.58


7.60


-0.02


7.18


7.17


0.01


400


6.43


6.42


0.01


6.12


6.13


-0.01


Layer














depth


57


58


-1


38


37


1



Table 45. Comparison of XBT pairs satisfying the accuracy specification for comparisons
above and below the thermocline.



SIMULTANEOUS 1830-m AND 460-m XBT PROFILES

During the RAPLOC/DEEPTOW experiments, 1830-m XBT profiles were obtained
simultaneously with 460-m profiles taken on two different 460-m systems. This section
compares the 1830-m profiles (system C) with the 460-m profiles (systems A and B).
During the experiment, 18 sets of simultaneous profiles were attempted. Of the 18 system C
profiles, seven failed catastrophically; the calibration correction for one exceeded 0.34°C
and therefore was not used. This left 10 system C visually acceptable XBT profiles taken
simultaneously with 10 system A and 10 system B profiles. All profiles were made from the
ORB at anchor. Comparisons are made at standard hydrocast depths.

The temperature and layer depth differences for system C versus system A and
for system C versus system B are presented in tables 46 and 47, respectively. The asterisk
after the XBT profile pair number indicates pairs which agreed to within ±0.34°C for all
above- and below-thermocline depths. Of a total of 20 comparisons, 15 (75.0%) pairs met
the accuracy criteria at all above and below thermocline depths. For 17 pairs, the layer
depth differences were less than or equal to ±5 m.

Of a total of 238 comparisons, 143 (60.0%) differences were negative and 88 (37.0%)
were positive. This means that system C measured temperatures that were lower than those
measured by systems A and B. Table 48 lists pertinent statistics for the above-, in-, and
below-thermocline subsets of differences, as well as statistics for the set of all differences.


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