Accuracy

With good weather and consequently high percent good (near 100%) in the top 250 m for all instruments, the OSN, OSB, and NB agreement was excellent (Figure 6). No significant biases could be detected. As shallow percent good dropped, however, the velocity profile estimates began to diverge. Relative to the NB, both the OSN and OSB were biased towards zero by about 5cm s$^{-1}$. There are also differences in vertical shear above 70 m, with the NB showing weaker or reversed shear relative to the OSN and OSB. The shallow shear difference may reflect bias in the NB more than in the OS, particularly in view of the low NB percent good in the top few depth bins during rough weather. The offset between the curves, however, appears most likely to reflect biases in the OS.

To investigate the differences between NB and OS during rough weather, we calculated the reference layer velocity difference between matched pings--that is, between simultaneous NB and OSN pings, and between simultaneous NB and OSB pings. We then grouped the pings according to the percentage of good depth bins within the 50-150-m reference layer as a rough indicator of a priori profile reliability. We find that the histogram of NB minus OS difference in the forward velocity component is skewed to negative values; most of the tails, or outliers, are instances when the NB is measuring a larger (more negative) flow of water past the ship (Figure 7). Although this could be a bias of the NB toward large values, it seems more likely that it is a bias of the OS toward zero. The skewed distribution is clear when the NB pings have high percent good (top panels in Figure 7) and the OSB pings have medium or low percent good; but it is found even with high percent good in the OSN pings. This is at least partly a consequence of the normally greater range of the OSN; the reference layer is a small part of the normal range, so a badly impaired ping may still have a high percent good within the reference layer. Nevertheless, we are left with the impression that the OSB algorithm includes more stringent acceptance criteria than the OSN algorithm, and that the latter leaves more outliers (bad estimates) that need to be removed by editing before averaging the pings into ensembles.

There is also a hint of bias toward zero in the low percent good NB pings, the bottom panels in Figure 7. The red curves, showing high percent good OSB pings (left) and OSN pings (right) are skewed to the right.

Figure 6: Comparison of velocity profiles (top panels) and percent good pings (bottom panels) from two-hour intervals of good weather (left; decimal days 51.64-51.74) and rough weather (right; decimal days 52.3-52.4). The velocity profiles were averaged from matched sets of single pings divided into 5-minute ensembles, using a reference layer averaging algorithm as described in the text. Only the forward component of the velocity relative to the ship is shown. NBB and NBN denote NB pings synchronized with the OSB and OSN pings, respectively.

Figure 7: Histograms of ping-by-ping reference-layer velocity differences, NB minus OS, stratified by the percentage of good bins within the 50-150-m reference layer. OSB panels are on the right, OSN on the left. Top panels show the cases with high NB percent good; red curves show cases with high OS percent good. Only the forward velocity component of the water relative to the ship is shown, so negative velocity differences represent a bias of the OS toward zero or of the NB toward large negative values. Note that the histograms use a log scale on their y-axes to emphasize the outliers. NBB and NBN denote NB pings synchronized with the OSB and OSN pings, respectively.

Although the available single-ping data set provides valuable clues about rough-weather bias in the OS and NB data, it is inadequate to answer all our questions. We need integrated accelerometer measurements to resolve the short-term velocity variations of the transducer; that would allow us to correctly fill the gaps left by missing pings. It would also facilitate conclusive recognition and resolution of velocity ambiguities.