Sea ice remote sensing from space Remote Sensing I Lecture 10 Summer 2006
Jan 20, 2016
Sea ice remote sensing from space
Remote Sensing I
Lecture 10
Summer 2006
Sea ice remote sensing from space
• Optical (e.g. MODIS)– High resolution (about 100 meters)– Can‘t „see“ through clouds; difficult to distinguish
clouds and sea ice• Passive Microwave
– Can differentiate between open water, first year and multiyear ice
– Can „see“ through clouds– Low spatial resolution (several km)
• Radar– Can look through clouds– High spatial resolution (< 1km)– Images difficult to interprete
MODIS - Antarctica 21 March 2005
Radar Image
ENVISAT ASAR
15 April 2005
Sea ice concentration fromAMSR-E 89 GHz
09 July 2006
courtesy of Lars Kaleschke
www.seaice.de
Sea ice concentration fromAMSR-E 89 GHz
23 April 2006
courtesy of Lars Kaleschke
1. RADAR
Real Aperture Radar
h
S
Opening angle β
Xa
Xr
Viewing angle θ
Antenna beamwidth
D
The opening angle β (beamwidth) of an antenna with aperture Dat a wavelength of λ is given by:
The azimuth resolution for a real aperture radar is then
coscos D
hhX a
Azimuth resolution fro real aperture radar
coscos D
hhX a
Example: h=800km, λ=23cm, D=12m then Xa=16km
This is a coarse resolution!
Synthetic aperture radar: Schematic
L
L D
hL
2
Ground
Radar flight direction
Synthetic aperture radar
D
hL
2
h
D
L 2SAR
2SARSAR,
DhX a
Ground „footprint“ L given by:
Individual measurements can be combined, corresponding to aMeasurement with a virtual or synthetic aperture of size L.
The resolution for such a synthetica aperture radar (SAR) is thengiven by:
2. Passive Microwave Sensors
Passive microwave remote sensing
Emissivity of Sea Ice vs. Frequency
Frequency [GHz]
Em
issi
vity
Water
Multiyear Ice
First year Ice
Summer
Polarization Ratio (PR) and Spectral Gradient (GR)
HTVT
HTVTPR
BB
BB
1919
1919
VTVT
VTVTGR
BB
BB
1937
1937
Clouds
Sea ice concentration
1iceyear Multiiceyear First Open Water CCC
The sea ice surface concentration C can then be calculated from thecontributions of the three base points for open water, first year ice,and multi year ice.
The sum of the three concentrations must sum to unit:
MODIS
645, 555, 469 nm
AMSR-E
89 GHz
courtesy of Lars Kaleschke
Optical tick clouds are still transparent at 89 GHz
courtesy of Lars Kaleschke
courtesy of Lars Kaleschke