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Documentation of the U.S. Geological Survey Oceanographic Time-Series Measurement Database

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Time and Orientation

Time

Times are reported in Universal Time (UT), which is the same as Greenwich Mean Time (GMT). Drift of instrument clocks is determined by comparing instrument times with accurate times (from the Global Positioning System (GPS) or clocks synchronized with the National Institute of Standards and Technology in Boulder, CO; (http://tf.nist.gov/)) before and after deployments. Ordinarily, the observed offsets were small compared to the sample intervals, typically a few seconds; therefore most instrument times in this data set are not adjusted for clock drift.

Time is stored in the netCDF files as two variables named "time" and "time2" in compliance with EPIC conventions. This technique avoided round-off error on older computers that did not have sufficient resolution when time is stored as an integer. "Time" is the time in whole Julian days, where midnight on May 23, 1968 = 2440000 and "time2" is the time in milliseconds since midnight. Thus, the time in Julian days is computed as
time+(time2 / (1000x3600x24)),
where the product in the denominator is the number of milliseconds in a day.

Instrument Orientation

When interpreting the current meter data, processing software must consider two aspects of orientation: (1) whether the instrument's (transducers) face up or down, and (2) the orientation of the instrument's beams relative to the Cartesian system. Each of the ADVs, PCADPs, and ADCPs are equipped with internally mounted flux-gate compasses. These are calibrated prior to deployment according to manufacturer's recommendations. The ADCP velocity data are recorded in raw beam coordinates by instrument firmware and are rotated to geographic coordinates (east, north, up) in post processing using the internal compass and are corrected for local magnetic variation. Data from the ADV and PCADP are recorded in instrument coordinates (referred to as x, y, z coordinates here) and later rotated into geographic coordinates and corrected for local magnetic variation.

In cases where several current meters are mounted on the same tripod, possibly facing different directions, all data from the raw beam coordinate system are initially converted to the east-north-up coordinate system. Because the current is expected to be uniform over small spatial scales, the primary axes of data from similar sampling heights may be aligned and used to determine the actual instrument orientation.

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This page last modified on Monday, 24-Nov-2014 12:28:50 EST