The purpose of this demo is to show the correct way to configure a Lidar receiver to resolve sub-pixel structure. In this case, the scene is a flat plate that is tilted. The receiver contains a single pixel array with a GSD of 1m (100 micron pixel through a 1 meter focal length optical path from 10,000 meters). When the surface imaged by the pixel is sloped, there is a spread in the range to the surface across the area imaged by the pixel. As a result, the returns from these various ranges within the pixel combine to form a wider return. This effect can only be reproduced in DIRSIG when sub-pixel sampling is enabled, so that the model can measure the variations in sub-pixel ranges. In this simulation, the sub-pixel sampling was 5x5.

Note
 The sub-pixel sampling demonstrated in this single-pixel example can be applied to multiple-pixel arrays.
Note
 This effect is generally more pronounced with larger GSDs where the spread of ranges across the pixel are proportional to the pulse width.

Overview

There are two simulations in this folder: one looking at a flat surface (see flat.sim) and one looking at a surface sloped at 45 degrees (see slope.sim). Both simulations refer to the same platform file, so both simulations output to the same demo.bin file. The command-line LIDAR .bin file analysis tool can be used to easily extract the temporal profile for this one pixel array:

$ bin_analyze --output_profiles=true demo.bin

The --output_profiles=true option triggers the creation of an ASCII\Text file for each pulse that contains the sum of the photons across the focal plane for each time bin. Since this simulation is only a single pixel and only a single pulse is fired, only one file is created named profile_0.txt.

Tutorial

The following is a step-by-step tutorial that will allow the user to reproduce the results provided below.

  • Run the flat.sim simulation.

  • Extract the temporal profile using the bin_analyze tool as described earlier.

  • Rename the resulting profile_0.txt file to flat.txt.

  • Run the slope.sim simulation.

  • Extract the temporal profile using the bin_analyze tool as described earlier.

  • Rename the resulting profile_0.txt file to slope.txt.

  • Plot the two profiles to observe the change in return shape.

A copy of the return profiles for the flat.sim and slope.sim files are included here with the names flat.txt and slope.txt, respectively. The plot (below) of these files is included in the file returns.png.

Tilted Returns