Landsat, a joint program of the USGS and NASA, has been observing the
Earth continuously from 1972 through the present day. Today the Landsat
satellites image the entire Earth's surface at a 30-meter resolution
about once every two weeks, including multispectral and thermal data.
The USGS produces data in 3 categories for each satellite
(Tier 1, Tier 2 and RT).
Read more about the different tiers.
These composites are created from all the scenes in each 8-day/32-day/1 year period beginning from the first day of the year and continuing to the 360th day of the year. The last composite of the year, beginning on day 361, will overlap the first composite of the following year by 3 days. All the images from each 8-day period are included in the composite, with the most recent pixel as the composite value.
The Burn Area Index (BAI) is generated from the Red and Near-IR bands,
and measures the spectral distance of each pixel from a reference
spectral point (the measured reflectance of charcoal). This index is
intended to emphasize the charcoal signal in post-fire images.
The Normalized Difference Snow Index is used to identify snow, based
on its characteristically higher reflectance in the visible portion of
the spectrum compared to the mid-IR. NDSI is computed using the Green
and Mid-IR bands, and has a range of -1.0 to 1.0. See Riggs et al.
(1994) for details.
The Normalized Difference Water Index (NDWI) is sensitive to changes
in liquid water content of vegetation canopies. It is derived from the
Near-IR band and a second IR band, ≈1.24μm when available and the
nearest available IR band otherwise. It ranges in value from -1.0 to
1.0. See Gao (1996) for details.