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Choosing Terra or Aqua MODIS Data

NSIDC distributes MODIS data from both Terra and Aqua satellites, with overlapping dates for select time periods. (See the temporal coverage table.) Here, we provide 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.

Orbits

Differences in Terra's and Aqua's orbits result in different viewing and cloud-cover conditions for a given location. 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. View Terra orbital tracks.

Aqua: The local equatorial crossing time is approximately 1:30 p.m. in an ascending node with a sun-synchronous, near-polar, circular orbit. View Aqua orbital tracks.

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.

Terra			Aqua
2005-02-15 10:30 local time			2005-02-15 12:10 local time