RADAR and LIDAR

RADAR and LIDAR
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

Our Need:
- A way to detect "distant" objects, determine their range (i.e., distance away), and determine their velocity
Other Technologies with "Similar/Related" Objectives:
- Determine "our" static latitude and longitude
- Determine "our" dynamic location
- Determine "our" heading and acceleration

Recall:
- When an electromagnetic wave contacts an object it will be absorbed, scattered, or reflected
- Electromagnetic waves travel at (essentially) the speed of light
Using this Information:
- Transmit an electromagnetic wave, receive the reflection, and use the transit time to determine the distance

Determining Velocity

Recall:
- The Doppler Effect - When the source of a wave is moving towards (away from) and observer, the observed frequency of the wave increases (decreases)
Using this Information:
- Transmit an electromagnetic wave, receive the reflection, and use the difference in frequency to determine the relative velocity

RADAR

The Acronym:
- RAdio Detection And Ranging
The Waves:
- Radio waves
The History:
- Used for detection as early as 1904
- Used for range determination in the 1930s

RADAR (cont.)

Transmitter:
- The signal can be generated in a variety of different ways depending on the application
Receiver:
- Is typically at the same location as the transmitter
Antenna:
- The same antenna is sometimes used for both the transmitter and the receiver, so must switch between the two
- Can be omnidirectional or not (e.g., parabolic)
- Can move (e.g., rotate) or not

RADAR (cont.)

Advantages:
- Little attenuation in the atmosphere (even with bad weather) so works at long ranges
Disadvantages:
- The reflected signal is weak (the power declines with the 4th power of the range) and must be amplified or processed
- The amount of reflection depends on the wavelength (i.e., wavelengths that are much longer than the target may not be detected)

LIDAR

Acronyms:
- LIght raDAR
- LIght Detection and Ranging
- Laser Imaging, Detection and Ranging
The Waves:
- Can use ultraviolet, visible, or near infrared (varying to suit the target)
- Eye-safety is an issue (e.g., 1550nm lasers are eye-safe even at high power)
The History:
- COherent Light Detecting and Ranging used lasers for range finding in the 1960s

LIDAR (cont.)

Type of Reflection:
- Diffuse (i.e., scattered at multiple angles) as opposed to specular (i.e., all of the light reflected at one angle) in one
- Often referred to as backscatter
- The specific kind of scattering depends on the wavelength (so is application dependent)
Photosensors:
- Charge-Coupled Devices (CCDs)
- CMOS Image Sensors
Illumination Methods:
- Scanning - a single point is illuminated at a time (and a mirror, often MEMS, is used to direct the light)
- Flash - the entire field of view is illuminated with a single pulse

SONAR

The Acronym:
- SOund Navigation And Randing
The Basics:
- Uses sound waves (which are mechanical waves)
- Most frequently used underwater