downHill

Description

downHill is a utility designed to model and generate synthetic beam footprint files (.bf_file) for various multibeam sonar systems (e.g., EM300, SeaBeam Classic, RESON 8101, ELAC Mk II, FSWP20, ISIS, TOWSS). It simulates how sonar beams intersect a sloping seafloor, producing across-track and along-track distances for each beam at increasing depths.

This tool is useful for understanding sonar coverage, designing survey parameters, and generating ground truth data for beam footprint analysis, especially in areas with varying bathymetry. It can model different beam spacing types (equiangular or equidistant) and account for across-track slope.

Usage 

downHill (-em3000 | -fswp20 | -isis | -towss | -elacII_120 | -elacII_150 | -reson_8101 | -em300 | -sb_classic) -outfile <bf_file> [OPTIONS]

Arguments

Option Description  
(-em3000 | -fswp20 | -isis | -towss | -elacII_120 | -elacII_150 | -reson_8101 | -em300 | -sb_classic) Required. Specifies the sonar model for which to generate the beam footprint.  
-outfile <bf_file> Required. Path for the output beam footprint file.  
-infile <param_file> (Optional) Path to an input parameter file.  
-reprate <val> Repetition rate (ping interval) in seconds.  
-fixed_sector <val> Forces a fixed angular sector width.  
-distance_to_go <val> Total along-track distance to simulate. 1000.0
-desired_swath <val> Desired across-track swath width. 2000.0
-across_slope <val> Specifies the across-track slope in degrees. 0.0
-depthrange <start_depth> <stop_depth> Specifies the starting and stopping depths for the simulation. 250.0 to 3000.0
-ssp <val> Surface sound speed. 1500.0
-sect_vary (Not fully implemented) Intended to vary the angular sector.  
-canyon (Not fully implemented) Intended to simulate a canyon-like bathymetry.  

How It Works

  1. Initialization: Parses command-line arguments to set the sonar model, output filename, and various simulation parameters (e.g., Start_Depth, Stop_Depth, across_slope, Desired_Swath).
  2. Sonar Model Configuration: Based on the chosen sonar model, it populates a BeamFoot structure (bf) with sonar-specific characteristics:
    • toolType, operatingMode, shot_interval, spacing_type, nobeams, nopings, interleave, ping_offset.
    • beamangle, beamheight, beamwidth, offset_dirn for each beam.
  3. Simulation Loop: It simulates the sonar moving along-track, increasing along_dist in steps determined by ping_interval. In each step:
    • Depth Calculation: Determines the current Depth based on Start_Depth, Stop_Depth, along_dist, and across_slope (if canyon or sect_vary are used, more complex depth profiles are simulated).
    • Beam Footprint Calculation: For each beam i defined by the sonar model:
      • It determines full_angle (the beam’s angular position).
      • Calls depth_intersect() to calculate the actual_depth at which the beam intersects the seafloor, considering the current Depth (nadir depth) and across_slope.
      • Calculates across_dist from the actual_depth and full_angle.
      • Writes along_dist and across_dist to the output file.
    • Ping Interval Update: Calculates the ping_interval based on the deepest point of the simulated swath to ensure adequate along-track coverage.
  4. Helper Function depth_intersect(): This function calculates the depth at which a beam, at a given angle, intersects a seafloor with a specified nadirdepth and across_slope. It uses basic trigonometry to account for the slope.
  5. Output: The output file (bf_file) contains pairs of along_dist and across_dist values for each beam at each simulated ping interval, representing the synthetic beam footprint.

Output Files

  • <bf_file>: A binary file containing synthetic beam footprint data.

Dependencies

  • BeamFoot.h: For BeamFoot structure and dump_BF_characteristics function.
  • support.h: For general utility functions.
  • math.h: For mathematical functions.

Notes

downHill is a valuable tool for understanding and visualizing sonar coverage, particularly how it changes with varying seafloor topography and sonar parameters. The synthetic beam footprint data can be used for survey planning, quality control, and as ground truth for testing processing algorithms. The tool creates a new file, ensuring the integrity of the original configuration. The canyon and sect_vary options are listed but not fully implemented.