Before further processing is performed, the AST time series is despiked and linearly detrended. These procedures are performed automatically by the processing software upon data import.
Despike
The despiking procedure consists of two independent checks:
- A standard deviation threshold check
- A physical acceleration check
Standard Deviation Check
The standard deviation of the AST time series is first calculated. Samples exceeding ±8 times the standard deviation are identified as outliers and removed.
After removal, the standard deviation is recalculated and the threshold is reduced. This iterative procedure continues until the rejection criterion reaches ±4 times the standard deviation.
Acceleration Check
An additional physical consistency check ensures that the vertical displacement between consecutive samples does not imply an acceleration exceeding gravitational acceleration. This prevents unrealistically steep changes in surface elevation that are not physically possible for gravity waves at the given sampling rate.
Samples that fail either of these checks are removed.
Gaps created by removed samples are filled using linear interpolation.
Bad detects
Samples removed during the despiking procedure are classified as bad detects. The total number of removed samples is reported in the processed dataset as Number of bad detects.
Detrend
After despiking, the AST time series is linearly detrended to remove low-frequency variations
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