Conclusions and recommendations

A new Ocean Surveyor 75-kHz shipboard acoustic Doppler current profiler (ADCP), model OS-75, was operated in parallel with an old 150-kHz ADCP, model NB-150, during a transit of R/V Endeavor from Florida to Rhode Island. Partial analysis of the two datasets has yielded several conclusions and questions. Although the first conclusion is positive, four significant concerns are raised.

The most basic conclusion is that the OS-75 works; that is, it can be used to measure ocean currents at ranges up to 800 m under good conditions, with nominal vertical resolution 16 m or better. The OS-75 can transmit and process two types of pings: coded (broad bandwidth, BB) and uncoded (narrow bandwidth, NB). Both types work roughly as expected, with the BB pings yielding higher short term accuracy at the cost of about 15% in range under good conditions. In comparison to the fleet-standard NB-150, the OS-75 in NB mode achieves about twice the range; this additional range is of enormous scientific value (Figure 9).

Figure 9: The northbound section crossing the Gulf Stream (test 27) illustrates the range advantage of the OS-75 (broadband mode, left; narrowband mode, center) over the NB-150 (right). The zonal velocity component is plotted with a percent good threshold of 50%. Weather was moderately rough during the crossing, and improved beyond the north wall of the Stream.

The most significant concern about the OS-75--an a priori concern that was confirmed by the test--relates to its acoustic beam pattern. Beam width and sidelobe response are functions of the effective aperture of a transducer and of its spatial uniformity. Single-ceramic transducers, such as those used in the NB-150, tend to have nearly ideal characteristics for a given aperature (or transducer diameter measured in wavelengths at the operating frequency). Phased arrays, as in the OS-75, are inherently inferior because they are made up of many elements; any differences in properties from one element to the next, or in their respective electronic circuits, degrade the beam pattern. The best indicator of beam pattern from the test cruise is the amplitude of the return as a function of "depth" (actually time) when the ocean bottom is in range. Comparing the NB-150 and the OS-75, we found that the latter has higher sidelobes; its return from the sidelobe pointed straight down is about 12-15 db higher than that from the NB. One expected consequence of this poorer beam pattern is increased severity of artifacts in the velocity profile in the presence of strong acoustic scattering layers. Unfortunately, although such layers are ubiquitous in many ocean regions, they were not encountered during the test cruise, so this prediction could not be confirmed. Beam patterns are expected to be more variable from one transducer to another, and over time as the ceramics age, for phased arrays than for single ceramics. It is not yet clear to us, however, whether the particular transducer on the Endeavor is typical of the OS series; cursory inspection of data from the OS-75 on the Oceanus and from the OS-38 on the Kaiyo suggests that their sidelobes are intermediate between the Endeavor's OS and NB, but a more thorough study is needed.

A second concern about the OS profilers is that even in NB mode they have a smaller ambiguity velocity (4.5 m s$^{-1}$) than that of the NB-150 (5.12 m s$^{-1}$ in the axial velocity component). We believe that this has been a little-known but significant problem with the NB-150, and that it may continue to cause trouble with the OS series. RDI has attempted to reduce the problem by shifting the zero-point of the velocity range. It is not clear how much this will help. The problem, if it occurs, will be manifest in rough weather, and in installations subject to large heave--most likely with the transducer mounted near the bow. We have not found evidence of velocity wraps in the data from this cruise.

A third concern is velocity bias in moderately rough weather, as during the test cruise when pitch maxima exceeded 3$^{\circ}$. There is some evidence of weak bias in the top few NB depth bins, but most of the bias appears to be in the OSN and OSB, and to come from anomalous velocity estimates in the top 100 m or more. Perhaps surprisingly, the anomalies were more prevalent in the OSN than in the OSB. Because the bias coincides with reduced percent good, and with occasional loss of complete pings, the net effect on averaged velocity profiles is very sensitive to the way the averaging is done and to any editing prior to averaging. The bias can easily exceed 5cm s$^{-1}$ and can affect both the profile shape and its vertical average. The bias is towards zero relative to the ship; when earth-relative velocity profiles are calculated, the bias is therefore in the direction of ship's motion.

A fourth concern is the bias toward zero of much of the OS bottom tracking data from this cruise. We suspect it may be related to bottom slope and/or roughness, together with ship's pitch and roll, and that similar biases may occur in NB data. We have not yet studied the BT data carefully, and may be able to learn more by doing so.

The tests also revealed a problem with running the OS synchronously with the NB: if the OS is in bottom tracking mode, then with present software it is almost inevitable that bottom tracking pings from the OS, which have a very long duration, will interfere with the water tracking pings from the NB. Although this can wreck much of the NB data, it is simply an operational advisory, not a criticism of the OS.

Areas requiring additional work include:

1. We need a beam pattern specification from RDI; we need to compare this to the theoretical optimum for the particular transducer design, and to decide whether it is acceptable; and we need to establish, with RDI, a beam pattern acceptance test or test suite. This may include pre-installation tank or lake testing, and it should include some reasonably easy post-installation test that can be repeated periodically to check for degradation due to aging. Analysis of the amplitude of the mainlobe and sidelobe return from the bottom, as we have begun here, may be sufficient.

2. We need to implement improved editing and averaging algorithms in data acquisition and processing software. Improved editing of the original single-ping data is critical. Additional study of single-ping water tracking and bottom tracking data is required for developing these criteria. Algorithms for dealing with velocity ambiguity resolution may be required.

3. Integrated accelerometer measurements, preferably from sensors at the transducer, would be very valuable for detecting and resolving ambiguity problems and for reducing velocity bias associated with the loss of valid velocity data from entire pings or sets of pings in bad weather.

Consistent with reports from other recent OS installations, including the OS-38 on the JAMSTEC ship Kaiyo, the OS-75 on the Endeavor delivered excellent range (in NB mode) that promises a major advance in scientific usefulness compared with the NB-150. We recommend that OS-75 systems, and when possible the OS-38 for maximum range, be acquired promptly by the US research fleet, subject to the satisfactory resolution of questions about the beam pattern. Retaining or adding higher frequency ADCPs may be advisable in many cases for studies of the upper 200 m of the water column.