What is remote sensing?“the acquisition and measurement of data/information on some property(ies) of a phenomenon, object, or material by a recording device not in physical, intimate contact with the feature(s) under surveillance; techniques involve amassing knowledge pertinent to environments by measuring force fields, electromagnetic radiation, or acoustic energy employing cameras, radiometers and scanners, lasers, radio frequency receivers, radar systems, sonar, thermal devices, seismographs, magnetometers, gravimeters, scintillometers, and other instruments”.

Source: NASA tutorial on remote sensinghttp://rst.gsfc.nasa.gov/Intro/nicktutor_I-1.html

Shorter definition

• Remote sensing is the collection of information about an object or system without coming into direct physical contact with it

How are measurements made?

• Ground-based

• Airborne

• Satellite

Why do we do remote sensing?

• Unobtrusive• Automated• Useful for extreme conditions• Offers excellent spatial and temporal coverage• Provides real time or near-real time

observations• Often cost-effective• Extends our senses

Remote Sensing Systems• Active Sensor - illuminates the subject from an artificial

energy source

• Passive Sensor - uses natural radiation from the Sun or Earth

• Imaging Sensor - creates a “picture” by scanning across a linear array of detectors while the array moves through space

• Non-imaging Sensor - measures along a transect or at a point; or uses a non-photonic approach

REMOTE SENSING DATA TYPES

Visible, infrared, thermal, and microwave are most common

The Remote Sensing Cycle

Sensor

Data

Processing

DataAnalysis

Interpretation

Information

Assessment

Development

Electromagnetic Radiation• Its harmonic wave form can be described

according to the Maxwell equations:

E E0 cos(t kx)Where, E is the electric fieldEo is a constant vector = angular frequency = c/= wavelengthc = speed of light in a vacuum (300,000,000 ms-1)k = wavenumber (x = distance along the x-axist = timeThe value of E at any point depends only on x and t

Electromagnetic Waves

Described by: • Wavelength• Frequency• Amplitude

Frequency vs. Wavelength

= distance of separation between two successive wave peaks

= number of wave peaks passing in a given time

The product of wavelength and frequency is a constant:

c=

c = speed of light in a vacuum = 3.0 108 ms-1

Energy vs. FrequencyWhen considering the particle form of energy,

we call it a photonThe energy of a photon, Q, is proportional to its

frequency, :Q = h = c/

Q = hch = Planck’s constant = 6.63 10-34 Jsc = speed of light = 3.0 108 ms-1

Thus, Q

Spatial Resolution• “A measure of the smallest angular or linear

separation between two objects that can be resolved by the sensor”. (Jensen, 2000)

• Resolving power is the ability to perceive two adjacent objects as being distinct– size– distance– shape– color– contrast with background– sensor characteristics

• Instantaneous field of view (IFOV) is the angular field of view of the sensor, independent of height

• IFOV is a relative measure because it is an angle, not a length

• It can be measured in radians or degrees

sensor

IFOV

GIFOV

• Ground-projected instantaneous field of view (GIFOV) depends on satellite height (H) and the IFOV

GIFOV 2H tanIFOV

2

1 meter resolution 250 meter resolution

IKONOS image of Gunnison River Basin, CO1

kilo

met

er

Digital Number

imaging opticsdetectors

electronics

at-sensorradiance

DN

The DN that is recorded is proportional to the radiance at the sensor

Digital Raster Imager Format

Radiometric Resolution

• Determined by the number of bits of within which the digital information is encoded

21 = 2 levels (0,1)22 = 4 levels (0,1,2,3)28 = 256 levels (0-255)212 = 4096 levels (0-4095)

• Surface components with very distinct spectral differences can be resolved using broad wavelength ranges

Temporal Resolution

• The frequency of data acquisition over an area

• Temporal resolution depends on:– the orbital parameters of the satellite

– latitude of the target

– swath width of the sensor

– pointing ability of the sensor

• Multi-temporal imagery is important for– infrequent observational opportunities (e.g.,

when clouds often obscure the surface)– short-lived phenomenon (floods, oil spills,

etc.)– rapid-response (fires, hurricanes)– detecting changing properties of a feature to

distinguish it from otherwise similar features

Atmospheric Effects

• EMR is attenuated by its passage through the atmosphere

Attenuation = scattering + absorption– Scattering is the redirection of radiation

by reflection and refraction

– Attenuation is wavelength dependent

– scattering by molecules and particles whose diameters are <<

– primarily due to oxygen and nitrogen molecules

– scattering intensity is proportional to -4

– responsible for blue sky

Rayleigh Scattering

Mie Scattering

– Spherical particles that have a mean diameter 0.1 to 10 times the incident wavelength

– Examples for visible light: water vapor, smoke particles, fine dust

– Scattering intensity is proportional to -4 to 0

(depending on particle diameter)

• Clear atmosphere has both Rayleigh and Mie scattering. Their combined influence is between -0.7 to

• Non-selective Scattering– aerosol particles much larger than the

wavelength (> 10x)– examples: water droplets, ice crystals, volcanic

ash, smog

– independent of wavelength: 0

Atmospheric Absorption

Absorption is the process whereby radiant energy is absorbed by atmospheric constituent and converted to thermal energy

Atmospheric absorbers are primarily:• H2O water vapor, water droplets• CO2 carbon dioxide• O2 oxygen• O3 ozone• Dust and soot

Absorption Bands

• An absorption band is a portion of the EM EM spectrum within which radiant energy is spectrum within which radiant energy is absorbed by substances such as water absorbed by substances such as water (H(H22O), carbon dioxide (COO), carbon dioxide (CO22), oxygen (O), oxygen (O22), ),

ozone (Oozone (O33), and nitrous oxide (N), and nitrous oxide (N22O), dust, O), dust,

soot, etc.soot, etc.

spectral absorptance

Spectral Reflectance

Microwave Brightness Temperature

• Microwave radiometers can measure the emitted spectral radiance received (L

• This is called the brightness temperature and is linearly related to the kinetic temperature of the surface

• The Rayleigh-Jeans approximation provides a simple linear relationship between measured spectral radiance temperature and emissivity

At long wavelengths, such as in the microwave region, the relationship between spectral emittance and wavelength can be approximated by a straight line.

Rayleigh-Jeans Approximation

• k is Planck’s constant, c is the speed of light, is emissivity, T is kinetic temperature

• This approximation only holds for >> max

• (e.g. > 2.57mm @300 K)

L 2kcT

4

spectral radiance is a linear function of kinetic temperature

a constant

Brightness Temperature

T is also called the “brightness temperature” typically shown as TB

TB 4

2kcL

Brightness temperature can be related to kinetic temperature through emissivity

Thus, passive microwave brightness temperatures can be used to monitor temperature as well as properties related to emissivity

Tb Tkin

Microwave Radiometers

• Advanced Microwave Sounding Unit (AMSU) 1978-present

• Scanning Multichannel Microwave Radiometer (SMMR) 1981- 1987

• Special Sensor Microwave/Imager (SSM/I) 1987-present

• Tropical Rainfall Measuring Mission (TRMM) 1997-present

• Advanced Microwave Scanning Radiometer (AMSR-E) 2002-present

Comparative Operating Characteristics of SMMR, SSM/I, and AMSR Parameter (Nimbus-7)

SMMR (DMSP-F08,F10, F11,F13) SSM/I

(Aqua) AMSR-E

Time Period

1978 to 1987 1987 to Present 2002 to Present

Frequencies (GHz)

6.6, 10.7, 18, 21, 37 19.3, 22.3, 36.5, 85.5 6.9, 10.7, 18.7, 23.8, 36.5, 89.0

Sample Footprint Sizes (km):

148 x 95 (6.6 GHz) 27 x 18 (37 GHz)

37 x 28 (37 GHz) 15 x 13 (85.5 GHz)

74 x 43 (6.9 GHz) 14 x 8 (36.5 GHz) 6 x 4 (89.0 GHz)

Radar=Radio Detection and Ranging

Radar system components

Radar: How it Works• A directed beam of

microwave pulses are transmitted from an antenna

• The energy interacts with the terrain and is scattered

• The backscattered microwave energy is measured by the antenna

• Radar determines the direction and distance of the target from the instrument as well as the backscattering properties of the target

History of the Landsat series

Currently, Landsat 5 and Landsat 7 (ETM+) are in orbit