Table of Contents

Lidar – Activity for educators

LIDAR (for "Light Detection and Ranging" or "Laser Imaging Detection and Ranging")

Once Phoenix operations begin on the surface of Mars, the MET (meteorological station) will record the daily weather of the northern landing site using temperature and pressure sensors, and the lidar instrument. With these instruments, MET plays an important role. It provides critical information on the state of the polar atmosphere and how water is cycled between the solid and gas phases in the arctic.

 

How it works

The MET's lidar is an instrument that operates on the same basic principle as radar, using powerful laser light pulses rather than radio waves. The primary difference between lidar and radar is that with lidar, much shorter wavelengths of the electromagnetic spectrum are used, typically in the ultraviolet, visible, and near infrared. In general, it is possible to image a feature or object only about the same size as the wavelength or larger. Thus lidar is highly sensitive to aerosols and cloud particles and has many applications in atmospheric research and meteorology.

The lidar on the Phoenix Lander will transmit laser light vertically into the atmosphere, which is then reflected off dust and ice particles. The reflections and how long they take to return to the lidar instrument are analyzed for information about the size of atmospheric particles and their location.

From this distribution of dust and ice particles, scientists can infer how energy flows within the polar atmosphere, and other aspects of weather on Mars. These particles also reveal the formation, duration, and movement of clouds, fog, and dust plumes, enhancing our understanding of the planet's atmospheric processes.

 

1. Pulses of light are produced by a laser and directed toward the sky.

2. When the light pulse encounters dust, fog, clouds, or molecules, scattering occurs in all directions.

3. Some of the scattered light is directed back toward the ground.

4. The scattered light is electronically detected and digitized by the telescope.

 

 

Basic types

There are three types of lidar: range finding, DIAL, and Doppler. Range-finding lidar is the simplest. It measures the distance from the lidar instrument to a solid or hard target. DIAL (Differential Absorption Lidar) measures chemical concentrations (such as ozone, water vapor, and pollutants) in the atmosphere. A DIAL instrument uses two different laser wavelengths, selected so that one of the wavelengths is absorbed by the molecule of interest while the other is not. The difference in intensity of the two return signals is used to deduce the concentration of the molecule being studied. Finally, Doppler lidar is used to measure the velocity of a target by measuring a shift in wavelength of the returned signal from that transmitted.

Lidar activity for students

Objectives

• To introduce students to the fundamental principles used in range finding lidar technology
• To have students correlate the function of time to that of distance for scientific observation
• To have students infer scientific findings and information derived from an isolated or remote location
• To have students observe sources of errors as a function of instrumentation versus variation of sampling techniques

Groups: individual or student groups of two or three in the junior high school level (grades 7 to 9).

Procedure

Sampling site, option 1

Glue together a series of ten or more empty cardboard tubes (from wrapping paper), so that their ends all line up. To the top third of each tube at different heights insert a stopper, made, for example of foam, to create a plug. Have one of the end tubes partially opened along its length so that students may use this to calibrate their "lidar" devices. The ends of the tube platform that have been capped off with the various plugs are elevated by a few degrees to create a gentle inclination. Note: this option allows for more control as the sampling is divided into controlled channels or routes for testing.

Sampling site, option 2

Construct a peg board with similar dimensions as Site 1. This board should be covered so that no one can observe its surface arrangement. Insert dowel pegs in the top third portion of the board, and then attach elastic bands between each of these pegs. Try to keep the tension of each band about the same. Like the first site, this board is elevated by a few degrees. Note: this option allows for a greater dynamic (and difficulty) for analysis as the sampling can now be affected by the principle of scattering, since return paths may no longer be direct.

Instrumentation construction

This project uses small marbles, or similar round objects, to represent the laser light pulse that would be emitted by lidar instrumentation. Students design a small device that can launch the marble up into the sampling site at the appropriate speed and force (representing the required wavelengths needed for measurement). What students discover is the need for (1) reliable and (2) reproducible results, where they can accurately predict a uniform speed for every launch. Some type of trigger mechanism is encouraged.

Scientific testing of your lidar-type instrument

Each student, or group of students, takes their constructed "lidar" instrument and calibrates it in the testing tube. Here, each must record the time it takes for a marble to return to the instrument once launched into the test-tube. The function of time is then related to a function of distance, which will later be used for the unknown samples. Students should be encouraged to adjust the tube plugs and test their device at different distances in order to calculate whether the function of time versus distance is a linear or curved relationship.

Then the students can use their constructed and calibrated device to measure the sampling sites, which model the Mars atmosphere with various levels of cloud cover. From their data, groups are to construct a visual representation or graph of what they believe the cloud profile looks like. When all are done, direct comparisons are to be made with the revealed sampling sites. Discussions could cover effective engineering designs and probable sources of error, both in testing and construction.