User manual

Table Of Contents
All three quantities are expressed in mathematical compass coordinates. It may be necessary to
wrap θ
T
into the range -180° < θ
T
+180° by adding or subtracting 360°.
Finally, the eastward and northward velocities in the true north Cartesian earth frame are:
Synchronizing the Data Streams
THNTHE
SVSU
θ
θ
sincos
=
=
The second task for processing and interpreting MMP data is synchronizing the sensor
and engineering records to a common time base (CTD and ACM records are not individually time
tagged). To synchronize these records you:
Use the accurate pressure time tags in the engineering record to align the CTD
record and synchronize it to the RTC.
Align the ACM record using the start and stop transients of the vertical velocity
and pressure rate records.
Sensor and motion start and stop times for each profile are recorded in the engineering
files and in TIMETAGS.TXT. The engineering files also include a subset of the CTD pressure
measurements, each with a time tag that is accurate to 0.5 second. The motion start and stop tags
are accurate to better than one second of the RTC. The motion start time is logged at the
beginning of the velocity ramp. The motion stop time is logged when the brake is set at the end.
The sensors do not start and stop at precisely the same time and do not record
synchronously. As a result, the sensor start is late by approximately 15 to 20 seconds for the CTD
and by approximately 5 to 10 seconds for the ACM (the ACM stops logging first and the CTD
follows).
Reliably aligning the CTD and ACM data streams requires identifying common features
in the two time-series, such as comparing w, the vertical velocity measured by the ACM, with
dP/dt, the pressure rate that can be derived from the CTD pressure measurements.
Begin by matching CTD pressures to time tagged pressures in the engineering record.
Then interpolate linearly between the known points to fill up the central portion of the record.
Take care of the ends of the record by extrapolating at the sample rate measured by your
interpolation. Presumably this will be close to 1.85 Hz. Verify that the result is consistent with
the sensor and motion start and stop times stored in the engineering and time tag files.
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