Ray

Description

Ray is a ray tracing utility that takes a sound velocity profile (SVP) and builds Time-Angle (TA) lookup tables. These tables contain the across-track distance and depth for each ray at constant time intervals. The core functionality is based on Snell’s Law, tracing rays through a layered water column where each layer has a constant sound velocity.

The generated lookup tables (.Time_LUT, .ACROSS_LUT) are crucial for converting raw sonar measurements (like one-way travel time and beam angle) into georeferenced positions (across-track and depth). This is fundamental for multibeam data processing and image rectification.

Usage 

Ray -pro <svp_handle> [-fishdep <val>] [-layerthick <val>] [-spacing <step> <start> <finish>] [-verbose]

Arguments

| Option | Description | |—|—| | -pro <svp_handle> | Required. Base name for the sound velocity profile file and output lookup tables. * Input: <svp_handle>.RAWPROF. * Output: <svp_handle>.Time_LUT and <svp_handle>.ACROSS_LUT. | | -fishdep <val> | Depth of the transducer/fish (meters). | 2.5 | | -layerthick <val> | Thickness of the constant velocity layers in the water column model (meters). | 0.1 | | -spacing <step> <start> <finish> | Defines the angular spacing for ray tracing: * step: Angular step (degrees). * start: Starting beam angle (degrees). * finish: Finishing beam angle (degrees). | 0.1 0 75 | | -verbose | Enable verbose output. | |

How It Works

  1. Initialization: Parses command-line arguments, setting various ray tracing and output parameters.
  2. SVP Reading (read_in_raw()):
    • Opens the input SVP file (<svp_handle>.RAWPROF).
    • Reads depth-velocity pairs (rawdep, rawvel) and determines fishdep (transducer depth), maxdepth, and maxrange.
    • Based on maxdepth, it sets timestep (time interval for sampling ray paths) and layerthick.
    • Calculates num_time_samples based on maxslantrange and timestep.
  3. Equi-spaced SVP (make_equisvp()):
    • Interpolates the raw SVP data into an equi-spaced layered model (veldep) based on layerthick.
  4. Ray Tracing (do_raytrace()):
    • Iterates through outangle from startangle to finishangle with anglestep.
    • For each outangle:
      • Initializes posx, posz, ttime, and rangle.
      • Layer-by-Layer Propagation: Traces the ray through the water column layers:
        • Updates posx, posz, and ttime based on the ray’s path through the current layer’s velocity.
        • Stores posx, posz, and ttime for this ray and layer in across[raynum][i], deep[raynum][i], and Ttime[raynum][i].
        • Applies Snell’s Law to calculate the new rangle for the next layer.
        • Handles cases where the ray goes beyond maxrange or becomes horizontal.
  5. Time Sampling (get_time_sampling_positions()):
    • Resamples the ray paths at constant timestep intervals along each ray.
    • Interpolates deep and across values at these constant time intervals and stores them in LUT_Depth and LUT_Across.
  6. Dump Time-Angle LUTs (dump_time_LUT()):
    • Initializes a JHC_header for the output files.
    • Opens two binary files: <svp_handle>.Time_LUT and <svp_handle>.ACROSS_LUT.
    • Writes the JHC_header to each file.
    • Writes the LUT_Depth and LUT_Across arrays to their respective files.
  7. Cleanup: Frees allocated memory and closes all files.

Output Files (JHC .r4 Array Format)

  • <svp_handle>.Time_LUT: A 2D floating-point array (raster) where each cell contains the depth for a given time and outgoing beam angle.
  • <svp_handle>.ACROSS_LUT: A 2D floating-point array (raster) where each cell contains the across-track distance for a given time and outgoing beam angle. Each output file contains a JHC_header followed by the raw float data.

Helper Functions

  • read_in_raw(): Reads SVP data in a raw format (depth, velocity pairs).
  • make_equisvp(): Interpolates raw SVP data to create an equi-spaced layered velocity profile (veldep).

Dependencies

  • support.h: For general utility functions.
  • array.h: For JHC_header structure.
  • j_watercolumn.h: For water column structures and ray tracing functions.
  • math.h: For mathematical functions.

Notes

The lookup tables generated by Ray are essential for correcting multibeam sonar data for refraction effects. By converting measured two-way travel times and beam angles into accurate across-track positions and depths, the tool contributes to the precise georeferencing of bathymetric soundings. The tool creates new files, preserving the original SVP data. The fishdep, layerthick, and spacing options allow for fine-tuning of the ray tracing model.