LidarSide1

Keywords: lidar

Summary

This demonstrates a side-looking LIDAR system, like one that might be mounted on a vehicle driving down the street and mapping the side of buildings.

Details

This demo models a LIDAR system with a simple transmitter and a 128 x 128 receiver array with 2 us long listening window and 1 ns time resolution (resulting in 2001 time bins). The single pulse collection is then run through both a Linear mode and Geiger mode detector model to produce ASCII/Text XYZ point clouds. The LIDAR detector model has a graphical interface available under Tools → Lidar Tools in the Simulation Editor, as well as a command-line interface.

Important Files

This section highlights key files important to the simulation.

Geometry and Materials

The scene is comprised of a single maple tree (see geometry/Red_Maple_1.gdb) planted at the Scene ENU origin a flat plate. The reflectance properties are spectrally flat and not realistic (see materials/demo.mat and the optical property files in the materials folder).

Platform

This section summarizes the properties of the transmitter and receiver components of the laser radar system used in this demo.

Transmitter

Laser wavelength: 1064 microns
Pulse repetition rate (PRF): 2000 Hz
Pulse length: 2 ns
Pulse energy: 1 mJ

Receiver

Focal length: 400 mm
Array size: 128 x 128 elements @ 50 micron pitch
Range gate width: 2 us
Range gate resolution: 1 ns

Setup

Run the Radiometry Simulation

To run the simulation, perform the following steps:

Run the DIRSIG demo.sim file

This produces the file demo.bin.

Run the Analysis Tool

It is always useful to verify the link budget (the number of photons arriving onto the pixels during a pulse). The BIN Analysis tool can be used to examine and summarize the radiometric values and supporting metadata in a BIN file. This tool can be run via the Analyze button in the detector model interface or via the command-line:

$ bin_analyze demo.bin
...
[header summaries deleted for documentation purposes]
...
Pulse #1:
    Minimum total photons in a pixel = 3.4105e-14
    Maximum total photons in a pixel = 101.604
    Pixels with zero total photons = 0/16384 (0%)
    Average background photons per pixel = 9.37674e-06
    Average total photons per pixel = 4.36375

Complete!

Run the Linear-mode (Lm) Detector Model

To generate a linear mode detector model on the spatial x spatial x time data captured in the demo.bin file, the detector model should be run in linear mode with the following parameters:

A delay time of 2 ns
A dead time of 2 ns

Generating the Linear-mode point cloud product can be accomplished using the graphical interface to the LIDAR Detector Model or via the command-line:

$ simple_processor --input_filenames=demo.bin \
    --model=linear \
    --delay_time=2.0e-09 --dead_time=2.0e-09 --return_count=3 \
    --output_format=ascii --output_filename=demo_linear.xyz

This produces the "demo_linear.xyz" file, which is an ASCII/text point cloud, that can be visualized in most 3D point cloud viewers or generic plotting tools.

Run the Geiger-mode (Gm) Detector Model

The Geiger-mode detector model simulation uses detector specifications commonly found in commercial Geiger-mode array devices:

Photon detection efficiency (PDE) of 0.5
Dark count rate (DCR) of 5 kHz

Generating the Geiger-mode point cloud product can be accomplished using the graphical interface to the general detector model or via the command-line:

$ simple_processor --input_filenames=demo.bin \
    --model=geiger --pde=0.5 --dcr=5.0e+03 \
    --output_format=las --output_filename=demo_geiger.las

Note	The large average link budget (over 4 photons/pixel) is higher than an actual Geiger-mode system would use. Expected link budgets for a Geiger-mode system is usually less than 1 photon/pixel.

Results

Visualizations of Linear-mode and Geiger-mode point clouds in GNUplot (a simple 2D and 3D plotting tool) are shown below:

Figure 1. The linear-mode 3D point cloud visualized in GNUplot

Figure 2. The geiger-mode 3D point cloud visualized in GNUplot