Download - 7.Principles of Remote Sensing Overview
Principles of Satellite Principles of Satellite Remote SensingRemote Sensing
Sundar A. ChristopherDepartment of Atmospheric ScienceUAHuntsville, AL
DefinitionDefinition
Remote sensing is the measurement of an object by a device that is not in physical contact with the object.
Types of Remote SensingTypes of Remote Sensing
Passive Sensors: Satellites
Active Sensors: Radar
Electromagnetic energyElectromagnetic energy
The frequency of EM radiation is directly proportional to the speed of light and Inversely proportional to the wavelength
Commonly used satellite sensors sense EM radiation from visible to infrared part of the spectrum
Why Satellite Remote Sensing?Why Satellite Remote Sensing?
Advantages: Repeated reliable measurements
Disadvantages: Expensive and need expertise to convert measurements to geophysical values such as temperature.
Numerous satellites are now in orbit
NASA Observing Spacecraft for Earth System Research
Moderate Resolution Imaging Spectroradiometer (MODIS)
Common types of orbitsCommon types of orbits
Geostationary orbitAn orbit that has the same Earth’s rotational periodAppears ‘fixed’ above earth Satellite on equator at ~36,000km
Polar orbiting orbitfixed circular orbit above the earth, ~1000km in sun synchronous orbit with orbital pass at about same local time each day
Example of satellite imageryExample of satellite imagery
Shortwave and Longwave RadiationShortwave and Longwave Radiation
Shortwave radiation from the sun is reflected from earth atmosphere that is seen by satellite (< 4 micron)
Longwave radiation is earth emitted radiation (> 4 um)
Fate of EM RadiationFate of EM Radiation
Incident solar energy can either be reflected, transmitted or absorbed
Satellite imagerySatellite imagery
In visible imagery water is dark because it absorbs most of the energy.
Clouds are white because most of the incoming energy is reflected
Pollution is hazy depending upon its absorptive properties
Atmospheric WindowsAtmospheric Windows
Portions of EM spectrum where absorption and scattering is minimal is called Atmospheric Windows
Remote Sensing - ResolutionsRemote Sensing - Resolutions
4 major resolutions
– Spatial resolution
– Spectral resolution
– Temporal resolution
– Radiometric resolution
Spatial ResolutionsSpatial Resolutions
Spatial Resolution : A simple definition is the pixel size that satellite images cover.
Satellite images are organized in rows and column called raster imagery and each pixel has a certain spatial resolution.
Nadirpixel sizeOff-nadir
pixel size
FOVFOV IFOVIFOV
SatelliteSatelliteheightheight
Spectral ResolutionSpectral Resolution
The number of bands is sometimes referred to as spectral resolution
A better definition is the width is the spectral band
Temporal ResolutionTemporal Resolution
How often is data obtained for the same area
Twice daily for polar orbiting satellites
Hourly or sub hourly for geostationary satellites
Radiometric ResolutionRadiometric Resolution
The ability to separate small differences in energy striking a sensor
For example 8 bits per pixel means the image is quantized as 256 gray levels.
Examples, AVHRR : 10 bit, MODIS : 12 bit
Trade Off’sTrade Off’s
A sensor cannot extremely high spectral, spatial and radiometric resolutions.
We discuss this in the Critical Thinking module.
Spectral SignaturesSpectral Signatures
The unique signature of various classes (e.g. Vegetation, Water, bare Soil) in the exam
above allows multi-spectral satellite imagery for identification
What does satellite see?What does satellite see?
Visible spectrum-Satellite measure radiance and not geophysical quantities such as temperature these radiance values must be converted to parameters of interest.
Radiance - DefinitionRadiance - Definition
RadianceI =flux per unit area
per unit solid angle normal to the direction of propagation
[Wm-2sr-1]
The concept ofStera-radian (sr)
From pretty pictures to numbersFrom pretty pictures to numbers
Radiance is converted to reflectance and temperature
Multi-spectral Image must be separated into various features (clouds, aerosols, ocean, land etc.)
This must now be converted to geophysical parameter
For example: Cloud top
temperature
For a known satelliteMeasured radiance theCloud top temperature Can be calculated if theWavelength is given. H, c, and k are all constants