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 from AMSR-E 89 GHz 09 July 2006 www.seaice.de courtesy of Lars Kaleschke
Sea ice concentration from AMSR-E 89 GHz 23 April 2006 courtesy of Lars Kaleschke
1. RADAR
Real Aperture Radar Viewing angle θ h S Opening angle β Xa Xr
Antenna beamwidth The opening angle β (beamwidth) of an antenna with aperture D at a wavelength of λ is given by: The azimuth resolution for a real aperture radar is then
Azimuth resolution fro real aperture radar Example: h=800km, λ=23cm, D=12m then Xa=16km This is a coarse resolution!
Synthetic aperture radar: Schematic L Radar flight direction Ground L
Synthetic aperture radar Ground „footprint“ L given by: Individual measurements can be combined, corresponding to a Measurement with a virtual or synthetic aperture of size L. The resolution for such a synthetica aperture radar (SAR) is then given by:
2. Passive Microwave Sensors
Passive microwave remote sensing
Emissivity of Sea Ice vs. Frequency Summer First year Ice Multiyear Ice Emissivity Water Frequency [GHz]
Polarization Ratio (PR) and Spectral Gradient (GR) Clouds
Sea ice concentration The sea ice surface concentration C can then be calculated from the contributions 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