Terra vs. Aqua
NSIDC distributes MODIS data from both the Terra and Aqua satellites. The Temporal Coverage Tables on the MODIS Data Versions Web page lists the date ranges covered by each satellite. In addition, the differences detailed in the following sections can help you choose which satellite meets your needs and preferences.
The MODIS snow mapping algorithm utilizes a Normalized Difference Snow Index (NDSI) to differentiate between snow and other features in a scene. The NDSI is a band ratio that exploits the spectral differences between snow in MODIS Band 4 (short-wave infrared) and Band 6 (visible).
Because 70 percent of the Band 6 detectors on MODIS Aqua failed shortly after launch, all MODIS Aqua snow products through Version 5 use Band 7 (2.1 µm) instead of Band 6 to calculate the NDSI. In addition, the MODIS test for snow in vegetated areas, which compares a Normalized Difference Vegetation Index (NDVI) with the NDSI, was disabled in these versions because using Band 7 yielded too many false snow decisions.
Differences in Terra's and Aqua's orbits result in different viewing and cloud-cover conditions for a given location. In particular, each satellite has a different local equatorial crossing time:
The local equatorial crossing time is approximately 10:30 a.m. in a descending node with a sun-synchronous, near-polar, circular orbit. Visit the Terra Orbit Tracks Web site.
The local equatorial crossing time is approximately 1:30 p.m. in an ascending node with a sun-synchronous, near-polar, circular orbit. Visit the Aqua Orbit Tracks Web site.
The following examples show differences in coverage between Terra and Aqua over the European Alps on 15 February 2005. The top row shows orbital tracks, the middle row shows visible imagery, and the bottom row shows corresponding Level-2 (swath) snow extent data. Images derived from Aqua data normally appear upside-down due to Aqua's ascending orbit. In this case, the images were rotated 180 degrees for visual comparison with the Terra images. Click the thumbnails for larger images.
Sample Terra Orbital Tracks from 15 February 2005
Sample Aqua Orbital Tracks from 15 February 2005
Image derived from the MODIS/Terra Snow Cover 5-Min L2 Swath 500m product (available from NSIDC), acquired over the European Alps on 15 February 2005, 10:30 local time.
Image derived from the MODIS/Aqua Snow Cover 5-Min L2 Swath 500m product (available from NSIDC), acquired over the European Alps on 15 February 2005, 12:10 local time.