Sunday, January 28, 2007

Aerosol and Cloud Study using MISR

Multi-angle Imaging SpectroRadiometer (MISR) is an instrument flown aboard terra satellite by NASA on August 1999. As its official web-site truly claims; no instrument like MISR flown before in the space. The unique feature of MISR is its nine cameras looking in nine different directions simultaneously. This affords us to view a given place on the earth from different angles and made it possible to estimate many physical parameters from space.

Concept of the MISR is based on a fact that if directional dependence of the scattered light is studied carefully it can reveal many features about target. Sunlight falling on the earth is not scattered equally in all directions. Halos around the moon in the presence of cirrus clouds is one of such phenomena.

Ability to view a given location from multiple angle has made it possible to observe many physical quantities for atmospheric aerosols and clouds which otherwise were not possible or difficult to observe from space such as single scattering albedo, aerosol optical depth on land, cloud-snow differentiation, etc. Nine cameras are placed symmetric around nadir direction, four looking in forward direction, four in backward direction. Having multiple cameras allow to view a given location multiple times in relatively small interval. Hence tracking a cloud in field of view one can estimate wind velocity at cloud height. Also viewing a given location simultaneously with angular separation allows to obtain depth information. Our own perception of depth is based on angular separation of light beam arriving at our eyes. However this is easy said than done. To derive meaningful quantity from observation, it requires unprecedented level of precision for collocation of images from each camera.

Few of the important data available from MISR web-site are aerosol optical depth on land and ocean, single scattering albedo, angstrom exponent, cloud height, cloud motion, cloud phase, etc. Data are provided free of charge for academic purpose. MISR uses HDF-EOS stack format for level 1 and level 2 data. Level 1 data are georectified radiances and instrument related raw data, whereas level 2 data are georectified physical parameters. Level 3 data are spatially and temporally averaged physical parameters at various resolution.

The swath data i.e. level 1 and level 2 data are projected on Space Oblique Mercator (SOM) map projection. This can be bit weary for the first time user for getting latitude and longitude. One can take advantage of MISR data ordering tool, which allows customization of data, where one can include lat-lon information for every pixels in data file. However, who are comfortable with programming it is pretty straight forward but a word of caution for IDL users. If you are using MAP_PROJ_INIT function then don’t forget to convert inclination from radians to degrees and to include SOM_PERIOD=98.88 in the function.


Pawan Gupta said...

Hi Harish,

This is very good article you have posted here. I am really big fan of MISR instrument and had used extensively for my current research.


Bruce said...

Can you comment on the proper values (or where to get them) for the other arguments to MAP_PROJ_INIT in IDL?