UNIT 2 MODULE 5 Multispectral Thermal Hyperspectral Sensing

UNIT 2 – MODULE 5: Multispectral, Thermal & Hyperspectral Sensing

MULTISPECTRAL SENSING

* MULTISPECTRAL SCANNERS • Instruments that are designed to collect data in more spectral bands over a wider range of the EM spectrum. • Can also sense in very narrow bands. • Acquired through two primary methods: across -track scanning & alongtrack scanning. Credit: NASA Credit: National Air & Space Museum

ACROSS-TRACK SCANNING • These systems build 2 -D images of the terrain beneath an aircraft. • Uses a rotating or oscillating mirror, these scanning mirror systems scan back-and-forth along the flight line at right angles. • Once incoming energy is reflected off the scanning mirror system, it is separated into several spectral components that are sensed independently. Credit: NASA Credit: Fundamentals of Remote Sensing - Schiewe, 2006

ALONG-TRACK SCANNING • Records multispectral image data beneath an aircraft, just like acrosstrack scanners. • Difference: linear array of detectors are used in replace of a rotating or oscillating mirror. • Each spectral band of sensing requires its own linear array. Credit: NASA Credit: Fundamentals of Remote Sensing - Schiewe, 2006

MIRROR SCANNING VS LINEAR ARRAY • Linear array systems have several advantages: – Acquire Stronger Signals – Smaller, Weigh Less, Use Less Energy – Higher Reliability (No Moving Parts) – Longer Life Expectancy • Disadvantages: the need to calibrate significantly more detectors, and limited range of spectral sensitivity.

ACROSS-TRACK MULTISPECTRAL SCANNER Credit: Remote Sensing & Image Interpretation - Lillesand, Kiefer, Chipman

ACROSS-TRACK IMAGERY Blue Green Red near IR Credit: Remote Sensing & Image Interpretation - Lillesand, Kiefer, Chipman near IR thermal IR

ALONG-TRACK MULTISPECTRAL SCANNER Credit: www. aerialsurvey. com

ALONG-TRACK IMAGERY Credit: Leica Geosystems Credit: Earth. Data International

THERMAL SENSING

THERMAL SCANNING • Multispectral scanning that is limited to thermal portion of the EM spectrum. • Provide for very rapid results. • To maximize sensitivity, detectors need to be kept artificially cooled (i. e. liquid nitrogen) to near absolute zero. Credit: NOAA Credit: www. resourcemappinggis. com/ *

THERMAL RADIATION PRINCIPLES • Radiant vs. Kinetic Temperature • Blackbody Radiation • Radiation from Materials • Atmospheric Effects • Thermal Radiation Interaction w/Terrain Credit: USGS

RADIANT VS KINETIC TEMPERATURE • Radiant Temperature – energy being emitted off of matter. • Kinetic Temperature – energy of moving particles in matter. • Internal temperature of an object may not match external readings. Credit: National Park Service

RADIANT VS KINETIC EXAMPLE • A body of water as a whole will have a cooler temperature than the surface temperature that’s recorded by a thermal sensor. Credit: Dr. Roy Spencer Credit: Remote Sensing & Image Interpretation - Lillesand, Kiefer, Chipman

BLACKBODY RADIATION • Objects that absorb all radiation. • Nothing is reflected. • Can emit their own light if hot enough. • The Sun and other stars are blackbodies. • Real materials do not behave like blackbodies. Credit: glossary. periodni. com

RADIATION FROM MATERIALS • Real materials only emit only a fraction of energy that a blackbody would. • Emitting ability of a material is called emissivity. • Emissivity describes how efficiently an object radiates energy. Based on a scale of zero (low) to one (high). • Can vary with wavelength, viewing angle, soil conditions, etc.

TYPICAL EMISSIVITIES OF COMMON MATERIALS Credit: Remote Sensing & Image Interpretation - Lillesand, Kiefer, Chipman

THERMAL RADIATION: INTERACTION W/ TERRAIN • The lower an object’s reflectance, the higher the emissivity, and viceversa. • Example: water has little -to-no reflectance in thermal part of the spectrum. • Its emissivity is nearly 1. Credit: Department of Energy

ATMOSPHERIC EFFECTS • Has a major impact on intensity & spectral composition of energy acquired by a thermal system. • Atmospheric windows influence the selection of spectral bands for measuring thermal energy. Credit: Penn State University

INTERPRETING THERMAL SCANNER IMAGERY • For many thermal scanning operations, simply studying relative differences in radiant temperatures will suffice. • The time of day is of great importance when analyzing thermal scanner imagery. • Materials warm up and cool down, throughout the day & night, at different rates. Credit: www. outlineglobal. com. au

THERMAL SCANNERS: RADIOMETRIC CALIBRATION • Modern thermal scanners have internal blackbody source referencing, which enables for more accurate readings. • Air-to-ground correlation accounts for atmospheric effects by correlating scanner data with actual surface measurements. Credit: USGS

F. L. I. R. SYSTEMS • Forward Looking Infrared Radar • Instead of acquiring views directly beneath an aircraft, FLIR systems can acquire ahead of an aircraft. • Civilian applications: search & rescue, pollution, fire fighting, nighttime driving, etc. Credit: Massachusetts State Police *

HYPERSPECTRAL SENSING

HYPERSPECTRAL SENSING • Difference between multispectral & hyperspectral sensing is: – Number of Bands – Narrowness of the Bands • Multispectral data: 5 -10 bands of large bandwidths (70 -400 nm). • Hyperspectral data: 100 -200 bands of narrow bandwidths (5 -10 nm). Credit: NASA Credit: www. markelowitz. com *

HYPERSPECTRAL SENSING (Continued) • Hyperspectral sensing combines imaging & spectroscopy into a single system. • Allow us to see the unseen more than a multispectral system. • Limitations: water vapor can have a significant impact on data collection. Credit: Hy. Vista Corporation Credit: Headwall Photonics *