filterPPK

Description

filterPPK is a utility designed to process and filter RTK (Real-Time Kinematic) heave data, specifically the RTK_at_RP (RTK height at Reference Point) field stored in OMG-HDCS merged files. It applies a low-pass filter (cosine-squared tapered weighted average) to this RTK height data to extract the long-period heave component.

The tool aims to separate the short-period heave (swell) from the long-period heave (tide and any residual vertical datum issues) for more accurate data processing. The filtered long-period heave is then outputted, and the high-pass filtered component could theoretically represent the swell heave.

Usage 

filterPPK <OMG_HDCS_datafile(s)> [-out <ASCII_filename>] [-longperiod <val>] [-verbose]

Arguments

Option Description Default / Example
<OMG_HDCS_datafile(s)> Required. One or more paths to input OMG-HDCS merged files. survey_data.merged
-out <ASCII_filename> Required. The path for the output ASCII file (contains filtered RTK heights and timestamps). filtered_rtk.txt
-longperiod <val> Specifies the half-width of the cosine-squared tapered low-pass filter (in seconds). This effectively controls the cutoff frequency of the filter. 15.0
-verbose Enable verbose output.  

How It Works

  1. File Processing: The tool iterates through each provided merged file.
  2. Summary Header Reading: Reads the summary header from the merged file.
  3. Memory Allocation: Allocates memory for longperiod_heave, RTK_at_RP, filtRTKatRP, heave, highpassHeave, TtimE (timestamps), and weight arrays.
  4. Data Extraction & Filtering: Loops through each profile (ping) in the merged file:
    • Reads the profile header.
    • Extracts profile.RTK_at_RP (scaled by 100.0) into RTKatRP[k].
    • Extracts the Unix timestamp (TtimE[k]) from the profile.
    • Low-Pass Filter Application: Applies a cosine-squared tapered weighted average filter to RTKatRP values:
      • For each ping k, it calculates a weighted sum of RTKatRP values from surrounding pings within the longperiod (time window) using a cosine-squared taper.
      • This weighted sum, divided by the total weight, gives the filtRTKatRP[k], which represents the low-pass filtered (long-period) heave.
      • The highpassHeave[k] (swell component) is calculated as heave[k] - longperiod_heave[k] (though heave[k] and longperiod_heave[k] aren’t clearly populated from original merged file data in the provided snippet, implying they might be internal calculations or zero-initialized).
  5. Output to ASCII File: For each ping, it prints the Unix timestamp (TtimE[k]) and the low-pass filtered RTK height (filtRTKatRP[k]) to the output ASCII file.
  6. Cleanup: Frees allocated memory and closes files.

Output Files

  • <ASCII_filename>: An ASCII file containing timestamps and filtered RTK heights.

Dependencies

  • OMG_HDCS_jversion.h: For OMG-HDCS data structures.
  • support.h: For general utility functions and error handling.
  • math.h: For mathematical functions.

Notes

The RTK_at_RP field often contains high-frequency heave noise along with the desired long-period tide component. This tool separates these components using a spectral filter, which is crucial for applying accurate vertical datum corrections in hydrographic surveys. The terms heave and highpassHeave in the code are noted as potentially confusing; RTKatRP (from profile.RTK_at_RP) is the primary input.