Remote Sensing Test 2

Thermal infrared energy is emitted by?

all objects that have a temperature

greater than absolute zero.

How do humans sense thermal energy?

the sense of touch. Our eyes cannot detect differences in thermal infrared energy because they are primarily sensitive to short wavelength visible light from 0.4 μm to 0.7 μm.

Sir Frederick William Herschel

discovered the infrared portion of the electromagnetic spectrum in 1800

bolometer

obtains measurable temperature variations of 1/10,000 ˚C

most important development in infrared technology

detector element by nations at war during

World War II. Early infrared detectors were lead salt photodetectors.

first declassified satellite remote sensor data were collected by

U. S. Television IR Operational Satellite (TIROS) launched in 1960

Heat Capacity Mapping Mission (HCCM)

Launched in 1978, obtained 600 x 600 m spatial resolution thermal infrared data (10.5 - 12.6 μm) both day (1:30 pm) and night (2:30 am).

This was one of the first scientifically oriented (geology) thermal infrared systems.

NOAA Advanced Very High Resolution Radiometer

(AVHRR)

collects thermal infrared local area coverage (LAC)

data at 1.1 x 1.1 km and global area coverage (GAC) at 4 x 4 km.

kinetic heat

energy of particles of matter in random motion

All objects having a temperature above absolute zero exhibit this random motion.

thermometer

can measure the true kinetic temperature (Tkin)

Emissivity

The ratio between the radiant flux exiting a real-world selective radiating body (Mr) and a blackbody at the same temperature (Mb)

E = Mr/Mb

Kelvin

A temperature scale in which zero occurs at absolute zero and each degree equals one kelvin

Water in kelvin

freezes at 273.15 °K and boils at 373.15 °K

emissivity of an object may be influenced by

color, surface roughness, moisture content, compaction ,field-of-view, wavelength, and viewing angle

Emissivity: color

darker colored objects are usually better absorbers and emitters (i.e. they have a higher emissivity) than lighter colored objects which tend to reflect more of the incident energy.

Emissivity: Surface roughness

the greater the surface roughness of an object

relative to the size of the incident wavelength, the greater the surface area of the object and potential for absorption and re-emission of energy

Emissivity: moisture content

the more moisture an object contains, the greater its ability to absorb energy and become a good emitter. Wet soil particles have a high emissivity similar to water

Emissivity: COMPACTION

the degree of soil compaction can effect emissivity

Emissivity: field-of-view

the emissivity of a single leaf measured with a very high-resolution thermal radiometer will have a different emissivity than an entire tree crown viewed using a coarse spatial resolution radiometer.

Emissivity: wavelength

the emissivity of an object is wavelength dependent. For example, the emissivity of an object in the 8 - 14 μm region and the 3 -5 μm region may be different

Emissivity: viewing angle

the emissivity of an object can vary with sensor

viewing angle

Kirchoff’s Radiation Law

the infrared portion of the spectrum the spectral emissivity of an object generally equals its spectral absorptance

If reflectivity increases then emissivity must decrease. If emissivity increases then reflectivity must decrease

What is beyond the visible region of the electromagnetic spectrum

the reflective infrared region from 0.7 - 3.0 μm and the

thermal infrared region from 3 - 14 μm

atmospheric windows

Regions that pass energy

absorption bands.

Regions that absorb most of the infrared energy

responsible for most of the absorption of infrared energy

Water vapor (H2O), carbon dioxide (CO2), and ozone (O3)

Earth ozone absorption

absorbs much of the thermal energy exiting the terrain in an absorption band from approximately 9 - 10 μm

blackbody

a theoretical construct that absorbs all the radiant

energy striking it and radiates energy at the maximum possible rate per unit area at each wavelength for any given temperature

The Sun and Earth as blackbodies

No objects in nature are true blackbodies, however, we may think of the Sun as approximating a 6,000 K blackbody and the Earth as a 300 K blackbody.

Stefan-Boltzmann law

The total emitted radiation (Mλ) from a blackbody is proportional to the fourth power of its absolute temperature

Wein’s Displacement Law

The relationship between the true temperature of a blackbody (T) in degrees Kelvin and its peak spectral exitance or dominant wavelength

thermal conductivity

the ability to conduct heat directly through

them

thermal capacity

heat storage

thermal inertia

resistance of a material to temperature change

Diurnal Temperature Cycle of Typical Materials

If all the curves for soils and rocks, water, vegetation, moist soil, and metal objects lie exactly on top of one another, then remote sensing in the thermal infrared portion of the spectrum would be of no value because all the phenomena would have the same apparent radiant temperature. There would be no contrast in the imagery between the different phenomena. Fortunately, there are only two times during the day (after sunrise and near sunset) when some materials like soils and rocks and water have exactly the same radiant temperature. During this crossover time period it is not wise to acquire thermal infrared remotely sensed data.

Thermal Infrared Data Collection

across-track thermal scanners

push-broom linear and area array charge-coupled

device (CCD) detectors.

Thermal Infrared Detectors composition

In:Sb (indium antimonide) with a peak sensitivity near 5μm;

• Ge:Hg (mercury-doped germanium) with a peak sensitivity near 10 μm, or

• Hg:Cd:Te (mercury-cadmium-telluride) sensitive over the range from 8 - 14 μm

Thermal Infrared Multispectral Scanners

These scanners have provided some of the useful high spatial and spectral resolution thermal infrared data for monitoring the environment

geometric error in thermal infrared scanning systems impact?

the quality of the imagery for visual or digital

image processing and analysis, and the creation of planimetric maps from the thermal infrared data

Considerations in geometric error

• ground swath width;

• spatial resolution cell size;

• tangential scale distortion, and

• one-dimensional relief displacement.

Radiometric Calibration of Thermal Scanner Data

To use the thermal infrared remote sensor data for practical purposes such as temperature mapping, it is necessary to calibrate the brightness values stored on the digital tape to temperature values.

Calibration can be done using

internal blackbody source referencing

external empirical referencing based on in situ data collection

Forward-Looking Infrared (FLIR) Systems

collect the infrared energy based on the same

principles as an across-track scanner previously discussed, except that the mirror points forward about 45˚ and projects terrain energy during a single sweep of the mirror onto a linear array of thermal infrared detectors

Passive remote sensing systems

record electromagnetic energy that is reflected (e.g., blue, green, red, and near-infrared light) or emitted (e.g. thermal infrared energy) from the surface of the Earth

Active remote sensors

create their own electromagnetic energy that 1) is transmitted from the sensor toward the terrain (and is largely unaffected by the atmosphere), 2) interacts with the terrain producing a backscatter of energy, and 3) is recorded by the remote sensor’s receiver.

active microwave (RADAR)

based on the transmission of long-wavelength microwaves (e.g., 3 – 25 cm) through the atmosphere and then recording the amount of energy back-scattered from the terrain;

LIDAR

which is based on the transmission of relatively short-wavelength laser light (e.g., 1040 nm) and then recording the amount of light back-scattered from the terrain

SONAR

is based on the transmission of sound waves through a water column and then recording the amount of energy back-scattered, from the bottom or from objects within the water column

pulse of electromagnetic radiation

sent out by the transmitter through the antenna is of a specific wavelength and duration

microwave energy is usually measured in

centimeters

names associated with the radar wavelengths

K, Ka, Ku, X, C, S, L, and P

Primary Advantages of RADAR Remote Sensing of the Environment

Active microwave energy penetrates clouds and can be an all-weather remote sensing system.

Synoptic views of large areas, for mapping at 1:25,000 to 1:400,000; cloud-shrouded countries may be imaged.

Coverage can be obtained at user-specified times, even at night

Permits imaging at shallow look angles, resulting in different perspectives that cannot always be obtained using aerial photography.

Senses in wavelengths outside the visible and infrared regions of the electromagnetic spectrum, providing information on surface roughness, dielectric properties, and moisture content

Secondary Advantages of RADAR Remote Sensing of the Environment

Active microwave energy penetrates clouds and can be an all-weather remote sensing system.

• Synoptic views of large areas, for mapping at 1:25,000 to 1:400,000; cloud-shrouded countries may be imaged.

• Coverage can be obtained at user-specified times, even at night.

• Permits imaging at shallow look angles, resulting in different perspectives that cannot always be obtained using aerial photography.

• Senses in wavelengths outside the visible and infrared regions of the electromagnetic spectrum, providing information on surface roughness, dielectric properties, and moisture content

RADAR and Vegetation

May penetrate vegetation, sand, and surface layers of snow

Azimuth Direction

The aircraft travels in a straight line that is called the azimuth flight direction

range or look direction

Pulses of active microwave electromagnetic energy illuminate strips of the terrain at right angles (orthogonal) to the aircraft’s direction of travel,

near-range

The terrain illuminated nearest the aircraft in the line of sight

far-range

The farthest point of terrain illuminated by the pulse of energy

depression angle

the angle between a horizontal plane extending out from the aircraft fuselage and the electromagnetic pulse of energy from the antenna to a specific point on the ground.

average depression angle of a radar image

computed by selecting a point midway between the near and far-range in the image strip.

incident angle

the angle between the radar pulse of EMR and a line perpendicular to the Earth’s surface where it makes contact

Polarization

send vertically polarized energy and receive only vertically polarized energy (designated VV),

• send horizontal and receive horizontally polarized energy (HH),

• send horizontal and receive vertically polarized energy (HV)

• send vertical and receive horizontally polarized energy (VH)

like-polarized radar imagery

Produced by HH and VV configurations

Cross polarized imagery

Produced by HV and VH configurations

slant-range geometry

Uncorrected radar imagery

it is based on the actual distance from the radar to each of the respective features in the scene

It is possible to convert the slant-range display into the true ground-range display on the x-axis so that features in the scene are in their proper planimetric (x,y) position relative to one another in the final radar image

Why does Slant-range scale distortion occur?

the radar is measuring the distance to features in slant-range rather than the true horizontal distance along the ground.

This results in a varying image scale, moving from near to far range

Determine spatial resolution

compute the resolution in two dimensions: the range and azimuth resolutions.

Range Resolution

can be computed if the depression angle of the sensor at that location and the pulse length are known

Azimuth Resolution

determined by computing the width of the terrain strip that is illuminated by the radar beam.

Real aperture active microwave radars

produce a lobe-shaped beam which is narrower in the near-range and spreads out in the far-range

the longer the radar antenna

The narrower the beam width and the higher the azimuth resolution

Radar relief displacement

Occurs when trees, tall buildings, or mountains are present in the scene,

Flat terrain means

it is a easy to use the appropriate equation to convert a slant-range radar image into a ground-range radar image that is planimetrically correct in x,y

Foreshortening

All terrain that has a slope inclined toward the radar will appear compressed or foreshortened relative to slopes inclined away from the radar.

RADAR Foreshortening is Influenced by

• object height: The greater the height of the object above local datum, the greater the foreshortening.

• depression angle (or incident angle): The greater the depression angle or smaller the incident angle, the greater the foreshortening.

• location of objects in the across-track range: Features in the near-range portion of the swath are generally foreshortened more than identical features in the far-range. Foreshortening causes features to appear to have steeper slopes than they actually have in the near-range of the radar image and to have shallower slopes than they actually have in the image far-range

Image Layover

is an extreme case of image foreshortening. It occurs when the incident angle (θ) is smaller than the foreslope

RADAR Shadows

enhance the geomorphology and texture of the terrain. Shadows can also obscure the most important features in a radar image, such as the information behind tall buildings or land use in deep valleys. If certain conditions are met, any feature protruding above the local datum can cause the incident pulse of microwave energy to reflect all of its energy on the foreslope of the object and produce a black shadow for the backslope

backslope

in radar shadow when its angle α- is steeper than the depression angle

grazing illumination

If the backslope equals the depression angle, then the backslope is just barely illuminated by the incident energy

RADAR Shadow Characteristics

there is no information within the radar shadow area.

It is black

Two terrain features (e.g., mountains) with identical heights and fore- and backslopes may be recorded with entirely different shadows

Radar shadows occur only in the cross-track dimension

Speckle

a grainy salt-and-pepper pattern in radar imagery present due to the coherent nature of the radar wave, which causes random constructive and destructive interference, and hence random bright and dark areas in a radar image.

Reducing Speckle (Multiple Looks)

processing separate portions of an aperture and recombining these portions so that interference does not occur

produces a more pleasing appearance, and in some cases may aid in interpretation of the image but at a cost of degraded resolution

Synthetic Aperture Radar Systems

Like a brute force or real aperture radar, a synthetic aperture radar also uses a relatively small antenna that sends out a relatively broad beam perpendicular to the aircraft. The major difference is that a greater number of additional beams are sent toward the object

Doppler principle

states that the frequency (pitch) of a sound changes if the listener and/or source are in motion relative to one another.

digital elevation model (DEM)

a file or database containing elevation points over a contiguous area.

Two types of DEM

digital surface models (DSM)

digital terrain models (DTM)

digital surface models (DSM)

that contain elevation information about all features in the landscape, such as vegetation, buildings, and other structures.

digital terrain models (DTM)

that contain elevation information about the bare-Earth surface without the presence of vegetation or man-made structures

4 technologies used to obtain elevation information

in situ surveying;

photogrammetry

Interferometric Synthetic Aperture Radar (IFSAR),

Light Detection and Ranging (LIDAR)

Photogrammetric techniques

can map large areas during the leaf-off season and yield a dense collection of points

can be used to obtain dense elevation information in inhospitable terrain

Breaklines

n analyst can selectively obtain a greater density of x,y,z observations associated with critical features such as ridgelines or steep changes of slope

Why LIDAR is a good alternative

offers an alternative to in situ field surveying, photogrammetric, and interferometric mapping techniques for the collection of elevation data

The LIDAR instrument consists of

a system controller

transmitter

receiver.

Light used by LIDAR for topographic mapping

eye-safe near-infrared laser light in the region from 1,040 to 1,060 nm.

Light used by LIDAR in water penetration

Blue-green lasers centered at approximately 532 nm

Pulse repetition Frequency (PRF)

LIDAR systems can emit pulses at rates >100,000 pulses per second

pulse travel at speed of light

masspoints

The range measurement process results in the collection of elevation data points

The greater the scan angle off-nadir

the more vegetation that will have to be penetrated to receive a pulse from the ground assuming a uniform canopy.

important variables used in the processing laser range information into georeferenced masspoints

x,y,z location of laser in 3-dimensional space at the time of the laser pulse;

• attitude (roll, pitch, and heading) of the laser at the time of the laser pulse;

• scan angle of the LIDAR at the time of the laser pulse;

• effect of atmospheric refraction on the speed of light;

• laser pulse travel time from the LIDAR instrument to the target (ground) and back.

Differential Global Positioning System (DGPS)

use of two GPS receivers that simultaneously record positional information.

knows the exact location of the LIDAR laser at all times

during data collection