Acoustic interference

When the NB and the OS were pinging asynchronously (no ping triggering), acoustic interference was evident in the single-ping amplitude data of both instruments as isolated high amplitudes (Figure 3). More spikes showed up in NB data than in either OSN or OSB. This may reflect the difference in frequencies; the first harmonic of the OS coincides with the operating frequency of the NB. Some amplitude spikes yield random velocity estimates, others are rejected. In broadband mode, the OS rejects most of the high-amplitude samples, presumably because of low correlation. In all virtual instruments, some spikes were probably eliminated by the error velocity screening criterion, but we do not know how many. Although the affected velocity samples that pass the editing criteria are biased towards zero velocity relative to the ship, there are so few of them (less than 1%) that their effect on a 15-minute average profile was too small to see in our limited testing (Figure 4).

Figure 3: Beam-1 amplitude on an instrument scale of 0-255 counts; each count is approximately 0.45 db. Black lines are the means, green dots are the individual values within two standard deviations of the mean, and blue dots are the high-amplitude outliers. Black circles show the outliers for which velocity estimates passed all screening criteria and were incorporated in ensemble-averaged velocity profiles. The top two panels show data from the NB-150 as affected by asynchronous pinging of the OS-75 in narrowband mode (left) and in broadband mode (right). The bottom panels show data from the OS-75 in narrowband mode (left) and broadband mode (right), with asynchronous pinging of the NB-150. Each panel shows 600 pings.

Figure 4: Vertical profiles of the alongtrack component of velocity relative to the ship for the same test periods shown in Figure 3. Red circles show the high-amplitude outliers. A bias toward zero in these outliers is evident except for the OS-75 in broadband mode. Because there are so few outliers, however, there is no obvious bias in the mean velocity profiles; the differences between the black and red curves probably reflect time and space variability in the ocean rather than bias.

Although most of the testing was done with synchronous pinging in an attempt to eliminate any possibility of interference, it was discovered that very serious interference could still occur if bottom tracking was in use. The problem is that bottom tracking pings are usually very long compared to water tracking pings, particularly in the OS and in deep water, and the ping synchronization mechanism does not distinguish between the two types of pings. After each 5-minute averaging interval, the NB DAS pauses pinging for a few seconds to calculate the average, write it to disk, and plot it on the screen. When pinging resumes, the NB starts with the first trigger it receives from the OS, which could correspond to either a bottom track or a water track ping. For example, if NB bottom tracking is on, then in about half of the NB ensembles, the top of every profile is corrupted. If NB bottom tracking is off, the top of every other ping is corrupted in each ensemble. For example, during test 18, OS bottom tracking was on and NB bottom tracking was off, so the top 200 m of every second NB ping was corrupted.

Tests of asynchronous pinging with bottom tracking were not conducted, but we expect the effect of the OS bottom track pings on the averaged NB velocity in this case might be significant; the longer duration of the bottom-track pings would affect a larger number of depth bins in each affected ping.