Choosing Terra or Aqua MODIS Data
NSIDC distributes MODIS data from both Terra and Aqua satellites, with overlapping dates for select time periods. Refer to the temporal coverage table for a listing of the various dates. Here are some basic facts about both platforms to help you make an educated choice of what data to use, based on your needs and preferences.
Differences in Algorithms
The MODIS snow mapping algorithm is based on a Normalized Difference Snow Index (NDSI). The NDSI is a measure of the difference between the infrared reflectance of snow in visible and shortwave wavelengths. For Terra data, the algorithm uses MODIS Bands 4 (0.55 µm) and 6 (1.6 µm) to calculate the NDSI. MODIS Band 6 detectors failed on Aqua shortly after launch, so Band 7 (2.1 µm) is used to calculate the NDSI for Aqua. Also with Aqua data, the NDSI/NDVI test for snow in vegetated areas was disabled because the use of Band 7 resulted in too much false snow detection.
The sea ice mapping algorithm did not change from Terra to Aqua.
Differences in Terra's and Aqua's orbits result in different viewing and cloud-cover conditions for a given location. The following are some facts about both satellites:
Terra: 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.
Aqua: 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.
10:30 local time
12:10 local time