# LidarWhisk2

Keywords: lidar, gmapd

## Summary

This demonstrates a nadir viewing Geiger-mode Avalanche Photo-diode (GmAPD) laser radar system that whisk scans across the scene. The collection shoots 40 overlapping pulses over a scene composed of a tree on a background.

## 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 multi 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): 20,000 Hz (20 kHz)
    Pulse length: 1 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 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]
    ...
        Minimum total photons in a pixel = 0.549208
        Maximum total photons in a pixel = 3.65245
        Pixels with zero total photons = 0/4096 (0%)
        Average background photons per pixel = 6.87829e-07
        Average total photons per pixel = 1.08298

    ...

    Pulse #40:
        Minimum total photons in a pixel = 0.549208
        Maximum total photons in a pixel = 3.80463
        Pixels with zero total photons = 0/4096 (0%)
        Average background photons per pixel = 2.00738e-06
        Average total photons per pixel = 1.00801

    Complete!

### 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:

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