Keywords: lidar, gmapd

Summary

This demonstrates a nadir viewing (down-looking) Geiger-mode Avalanche Photo-diode (GmAPD) laser radar system. In this case, we shoot a single pulse at a tree object.

Details

This demo models a LIDAR system with a simple transmitter and a 32 x 128 GmAPD 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 the advanced GmAPD detector model to produce an ASCII/Text XYZ point cloud. The advanced GmAPD 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: 0.5 mJ

Receiver

Focal length: 1 m
Array size: 32 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:

  1. 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_analyze 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 = 0.549215
    Maximum total photons in a pixel = 3.7571
    Pixels with zero total photons = 0/4096 (0%)
    Average background photons per pixel = 3.51892e-06
    Average total photons per pixel = 2.17662

Complete!
Note
 The average link budget of over 2 photons/pixel is higher than an ideal Geiger-mode system configuration would use. Ideal link budgets for a Geiger-mode system is usually less than 1 photon/pixel.

The Geiger-mode (Gm) Detector Model Database

The advanced Geiger-mode APD (GmAPD) detector model simulation uses detector specifications that mirror the commercial available products from Princeton Lightwave, Inc. (see http://www.princetonlightwave.com/mm-products/3d-ladar/128-x-32-gmapd-camera#specifications):

Photon detection efficiency (PDE)
    Mean = 30 (typical)
    Stddev = 5 (typical)
Dark Count Rate (DCR)
    Mean = 5 (typical)
    Stddev = 3 (typical)

This detector performance characteristics are captured in the GmAPD database file (see demo.gmapd). Consult the Advanced GmAPD modeling manual for details about creating these databases.

Run the Geiger-mode (Gm) Detector Model

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

$ gmapd_processor --parameters demo.xyz.params

Results

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

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