Moderate Resolution Imaging Spectroradiometer (MODIS) imagery offers a multi-spectral advantage over Geostationary Operational Environmental Satellite (GOES) imagery for viewing volcanic ash plumes and other atmospheric and surface features. The advantage is due to the larger number of spectral bands available on MODIS (36 spectral bands: 19 bands in the visible and near-infrared [reflective], and 17 bands in the longer-wavelength infrared [thermal] portion of the earth's spectrum); compared to 5 bands for the GOES Imager and 19 bands for the GOES Sounder. In addition, some of the MODIS bands are in portions of the spectrum not sensed by GOES. This spectral advantage however is offset by the lack of sufficient temporal resolution and continuity to follow rapidly-changing phenomena that is customary from geostationary orbit.

Principal component analysis is a method that can help in the detection of subtle atmospheric and surface features in multi-band imagery. This technique has been applied to GOES Imager and Sounder data to create Principal Component Images (PCIs). It is an increasingly important technique for analyzing satellite imagery as the number of spectral bands increases. The abundance of bands available on MODIS requires sophisticated band-differencing techniques such as PCIs to determine the best combinations of bands for detection of volcanic ash. It appears that several MODIS bands (bands not available on GOES Imager, but some of which are available on the GOES Sounder) contribute to the detection of ash plumes both day and night. Even though the explained variance of most of the PCIs is generally small, several of them provided sufficient signal above noise to see the ash plume. Differences among the PCIs (from different combinations of the MODIS bands) highlighted variations in the ash plume, where the ash occurs at different levels and has different particle concentrations.