Bottom tracking and calibration

Most of the potential bottom tracking data for the NB was corrupted by the incorrect Peripherals 2 board scale factor setting; only a small data sample (26 5-minute ensembles) from the end of the cruise was available. Based on this, all water and bottom track velocities were multiplied by a scale factor of 0.9976. (Correction for the speed of sound at the transducer was done during ping averaging by DAS 2.48, based on the transducer temperature and a nominal salinity of 35 psu.)

The OS tracked the bottom during more than half the cruise (Figure 2), but the BT velocities were inconsistent. During decimal day 49 the BT velocities were erratic but generally biased towards zero by up to 10%. Scatter and bias were smaller the following day, but the inferred scale calibration factor was still systematically larger than during the first half of decimal day 48. The worst bias coincided with reduced percent good of the bottom track pings, but the reduction was moderate; biased bottom track velocity estimates were made even with greater than 90% ``good'' BT pings. The cause remains to be determined.

Figure 2: Ocean Surveyor bottom tracking (BT) throughout the cruise, as a function of time in decimal days. BT data shown were averaged by VMDAS in 5-minute ensembles. The top panel shows the depth from the BT data; the bottom was tracked to 800 m or more. The second panel is the velocity scale factor from BT: the ratio of speed over the ground from GPS to speed from BT. Most values from dday 48.8 through 51.5 were unreasonable. The third panel shows the transducer alignment estimate (red), together with standard deviations of pitch (black) and roll (cyan) from the Ashtech ADU-2. The variations in alignment angle prior to 48.4 reflect lack of Ashtech heading correction. The bottom panel shows ship's speed based on BT.