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Field QC - buoy acquisition

This document describes the quality control measures that are incorporated
into CDIP's field acquisition of data from Datawell directional buoys. There
are two levels at which this QC operates - first as the data is read from the
serial port and archived in data files, second as this data is prepared for
the local display.


Acquisition code: DATAWELL_ACQ

  The datawell_acq code performs the basic data acquisition routines on the
shore station: it reads data from the serial port, which is connected to
the Datawell receiver, and then creates data files for archiving and 
transfer back to CDIP's central processing facility. 

  Quality control in datawell_acq is geared towards insuring that a clean,
continuous signal is being read from the serial port. To this end, the
following steps are taken.

  1) Counter check - the Datawell receiver outputs a counter with every
       vector (10-byte packet) of data it emits. The counters are
       checked to make sure they are continuous. If not, the serial port
       and data acqusition are re-initialized.
  2) Sync check - the Datawell buoy transmits a sync word every 14 seconds.
       The positions of these sync words are checked, and any deviations
       are recorded for reference in an errors file.
  3) File creation - new, time-tagged data files are created every 4 minutes, 
       to insure that timing errors cannot affect large blocks of data.

  Note that the acquistion code does not perform any checks on the measurenments
recorded in the data stream; it is concerned with making a clean record of the
data produced by the sensors, not with checking its validity.
Display code: LOG_FD
  Although all of CDIP's official processing occurs centrally, at SIO, a
limited amount of processing is done at the shore station to produce a 
local display. This display shows basic wave parameters - Hs, Tp, Dp - as
well as displacement and spectral values. The display is intended to allow
local users - lifeguards, harbor masters, etc. - to view the data as soon
as possible.

  All the information in the local display is taken from the most recent
four-minute file acquired by datawell_acq. If this four-minute file is
judged to be of good quality, it is used to update the display; if not,
the previous display values are retained. The judging criteria are as follows:

  1) Counters - all the counters (288) in the file must be sequential.
  2) Sync words - all of the sync words (16) must be properly positioned.
  3) Error bytes - in addition to counters, the Datawell receiver puts out an 
       error byte with each vector. The error byte is coded to indicate 
       whether or not the vector was successfully received from the buoy.
       This byte must indicate that each vector in the four-minute file
       was properly received.

  If the criteria above are all met, the latest file will be used to update
the local display.
       

(Note: The values (288 vectors, 16 syncs, etc.) listed above are all given for
Datawell Mark II buoys. Mark I buoys have slightly different values, but the
principles all remain the same.)

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