Solar Electromagnetic Radiation
Jan 04, 2016
Solar Electromagnetic Radiation
Satellite Remote Sensing—Landsat 7
Sun-synchronous orbit:Satellite always crossed the equator at precisely the same local time.
Temporal Resolution
The shortest time needed to repeat a ground track
Spatial Resolution
185 km
Field of View
175kmscene
Landsat
Satellite ground tra
ck705km
Spatial Resolution
Pixel size=(30x30m)
Data transmission to the ground station, allows faster and efficient data delivery to the ground. As the technology advances, the speed to data transmission increased tremendously allowing us to collect more images from space.
Radiometric Resolution
The number of levels of DN values is determined by the radiometric resolution of the instrument. For example, 8-bit system can differentiate 256 (0-255) levels of radiance
MinimumIntensity
MaximumIntensity
Dig
ital n
umbe
rs (
DN
)
Radiance intensity0
255 Radiance DN (min=2, max=130 ) [0.0,2.0] 0 (2.0, 2.5] 1 (2.5, 3.0] 2 … … [100.0,100.5] 196 … [129.5,130.0] 255
1010 30
25
5
30
10
30 30
1. Space is covered continuously with cells.2. Each cell has one number indicating the amount of energy received from the cell3. The cell is called pixel (picture element)4. The size of the pixel is the spatial resolution
sensor
Digital Images
Landsat 7 ETM+ Spectral Bands
The number of bands and the width of spectrum that each sensor covers are referred to spectral resolution
1010 30
25
10
30
10
20 15
1010 30
25
5
30
10
30 30
301 40
15
30
30
12
20 40
Multispectral remotely sensed data
Each band will generate a layer of remotely sensed data, usually with the same cell (pixel) size. For Landsat satellite, we will have 6 layers of data corresponding to the 6 bands.
Spectral Vegetation IndexSimple Ratio/Normalized Difference Vegetation Index
dNIR
dNIR
RR
RRNDVI
Re
Re
NDVI: [-1.0, 1.0]
The more the leaves of vegetation present, the bigger theContrast in reflectance in the red and near-infrared spectra.
d
NIR
R
RSR
Re
Color Arithmetic
red+green=yellowgreen+blue=cyanred+blue=magenta
red
blue
green
white
yellow
black
cyan
magenta
Color Space
Landsat Images
Landsat 5 TM image on Dec 10,1988 at Shenzhen special econ. zone, China (RGB=432)
Landsat 5 TM image on Dec 30,1995 at Shenzhen special econ. zone, China (RGB=432)
DN to Radiance Conversion
MinimumIntensity
MaximumIntensity
DN
L 0
255
Radiance received at satellite has to be converted DN for effective communication. Users often has to convert the DN back to radiance and further calculate surface reflectance.
DNDN
LLLL
max
λminλmaxλminλ
Where Lλ is the radiance received at satellite sensor, Lλmin is the sensor read of radiance when DN is zero. Lλmax is the maximum radiance read from the sensor producing DNmax. Lλmin, Lλmax, DNmax can be found in the header of the image.
DNGBLλ
Apparent Reflectance
Total Energy: JoulesEnergy Flux: J/sEnergy Flux Density (irradiance): J/m2/sRadiance: J/m2/s/srSurface energy flux density is the integration of radiance from the entire upper hemisphere (2π solid angle). Assuming Lambertian surface, the total amount of irradiance the surface reflected is πLλ. Thus the percent energy reflected at the surface with a particular wavelength range, reflectance, would be:
)cos( zE
L
Where Eλ is the solar constant within the wavelength range. θz is solar zenith angle. This is called the at satellite reflectance, or apparent reflectance.
Surface Reflectance
))cos((
/)(
downzz
vhazesat
EET
TLL
Where Eλ is the solar constant within the wavelength range. θz is solar zenith angle. Edown is the down welling diffuse radiation.
Edown
Eλ
Lg
Lhaze Lsat=Lg+Lhaze
Tv=exp(-τ/cos(θv))Tz=exp(-τ/cos(θz))τ is atmospheric optical depth. It is composed of two parts: atmospheric molecules and aerosols.
sun sensor
Dark Object
Edown
Eλ
Lg=0
LhazeLsat=Lg+Lhaze
sun sensor
Lsat=Lhaz
Image Histogram:
DN
Freq Can you find the DN for dark object in the histogram?
Dark Object Subtraction
DN
Freq
1000
Real histogram often is noisy. Not all pixels with low DN values are valid pixels. One way to avoid this noise is assuming the highest DN with at least 1000 pixel counts on the lower side of the histogram peak has 1% surface reflectance.
/))cos((01.0min downzzhaze EETBDNGL
Transmittance
Tv=exp(-τ/cos(θv))Tz=exp(-τ/cos(θz))τ is atmospheric optical depth. It is composed of two parts: atmospheric molecules and aerosols.
)0.000131130.0(1.0 .0085690 -4-2-4
Aerosol optical depth are difficult to get and it varies from time to time and place to place. However, for a high quality satellite image, aerosol optical depth is relatively low. People used to assume Tv and Tz as 1, Song et al. (2001) showed that we should consider Raleigh atmospheric effect.
Surface Reflectance
))cos((
/)(
downzz
vhazesat
EET
TLL
Where Eλ is the solar constant within the wavelength range. θz is solar zenith angle. θv is viewing zenith angle. It is 0 for Landsat (nadir view). Edown is the downwelling diffuse radiation. Can be obtained through 6S, an atmospheric radiation transfer model.
Lsat=G*DN+BLhaze=G*DNmin+B-0.1*(TzEλcos(θz)+Edown)Tv=exp(-τ/cos(θv))Tz=exp(-τ/cos(θz))
)0.000131130.0(1.0 .0085690 -4-2-4
AVHRR
AVHRR=Advanced Very High Resolution Radiometer
Temporal Resolution: dailySpatial Resolution: 1km at nadirRadiometric Resolution: 8-bitSpectral: 4/5 bands
Where Are the Bands Located ?
NDVI Change in China from 1982 to 2000NDVI Change in China from 1982 to 2000
MODIS (MODerate resolution Imaging Spectroradiometer)
Temporal Resolution: 2 daysSpatial Resolution: 250, 500, 1000Radiometric Resolution: 12-bitSpectral: 36 bands
Leaf Area Index
March 24 - April 8, 2000
Monitoring forest fire
Pre-forest fire
Post-forest fire
Burned area identified from space
Phytoplankton bloom in the Black Sea. MODIS band 1 (red), 4 (green) and 3 (blue)
Sea Surface Temperature
1997/1998 El Nino
Cloud types from MODIS: pink: cold high level snow and ice clouds; neon green: low level water clouds. Different cloud types reflect and emit radiant energy differently.
A massive iceberg, one of the largest ever observed, broke off the Ross Ice Shelf near Roosevelt Island in Antarctica in mid-March 2000. This Iceberg is about 40 miles wide and 300 miles long. The break off of such big iceberg may be related to global climate change.