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