The MODIS sensor contains a system whereby visible light from Earth passes through a scan aperture and into a scan cavity to a scan mirror. The double-sided scan mirror reflects incoming light onto an internal telescope, which in turn focuses the light onto four different detector assemblies. Before the light reaches the detector assemblies, it passes through beam splitters and spectral filters that divide the light into four broad wavelength ranges. Each time a photon strikes a detector assembly, an electron is generated. Electrons are collected in a capacitor where they are eventually transferred into the preamplifier. Electrons are converted from an analog signal to digital data, and downlinked to ground receiving stations. The EOS Ground System (EGS) consists of facilities, networks, and systems that archive, process, and distribute EOS and other NASA Earth science data to the science and user community.

MOD10A1, the input to this data set, is generated by mapping a best observation of the day from the MODIS/Terra 5 minute swath data set (MOD10_L2) into each cell of the MODIS 500 m Sinusoidal Grid (see the MOD10A1 documentation for details). Maximum snow extent for the period is mapped by compositing eight days of MOD10A1 snow cover. The algorithm reads eight days of MOD10A1 tiles and if snow cover with a value of NDSI > 10 is found in a cell for any day in the period, the cell is mapped as snow in the Maximum_Snow_Extent SDS. To reduce snow commission errors and produce a more spatially consistent snow map, values in the range of 0 < NDSI ≤ 10 are interpreted as uncertain and not counted toward maximum snow cover.

If no snow is found in the period, the algorithm maps the cell with the clear-view observation that occurred most often. Cells are only mapped to cloud if all eight days in the period are cloud-obscured. Although this logic minimizes cloud cover extent, it biases the output toward clear views because only clear views are used to determine composite observations. To catch any unexpected conditions, cells are mapped to no decision if a composite observation is not determined in the algorithm.

The snow/no snow chronology for each cell is written to the Eight_Day_Snow_Cover SDS. Each of the eight bits in this variable represents one day in the compositing period. On days that snow is observed the cell, the bit corresponding to that day is set to on (1).  Days with no snow, cloud cover, and missing data are left unset (0). Days are ordered chronologically across the byte from right to left, from day 1 (bit 0) to day 8 (bit 7).

Because eight days of observations may not be available due to problems acquiring data or generating the product, the algorithm was designed to run with as few as two days of input. No product is generated if only a single day is available. Three global attributes—Number of input days; Days input; and Eight day period—are are written to the metadata that specify how many and which days were used to produce the tile.

Version History

See the MODIS | Data Versions page for the history of MODIS snow and sea ice data versions.

Error Sources

The NDSI technique has proven to be a robust indicator of snow cover. Numerous investigators have utilized MODIS snow cover data sets and reported accuracy in the range of 88% to 93%. The eight day snow cover extent data set is intended to provide users with a map of maximum snow cover extent during the eight-day window and a record of days on which snow cover was observed. Although this data set's accuracy is typically similar to its MOD10A1 input, compositing snow commission errors over eight days into a single tile can increase the error percentage spatially and temporally, even after uncertain snow detections are filtered as previously described.

Accuracy and errors in MOD10A1 originate in the MOD10_L2 swath product. In the eight-day snow maps, errors associated with MOD10_L2 snow/cloud confusion typically manifest as snow in locations and seasons where snow is impossible or very unlikely. These errors accumulate from each day and occur in different locations on different days, thus increasing the spatial extent of errors when composited into the eight-day snow cover maps.

Finally, anomalies in the input data to MOD10_L2 propagate to the output. Table 3 in the MOD10_L2 documentation lists the products that are used as input to the snow detection algorithm. For a more detailed discussion of potential sources of error, including examples, consult the MODIS Snow Products Collection 6 User Guide.

The MODIS instrument provides 12-bit radiometric sensitivity in 36 spectral bands ranging in wavelength from 0.4 µm to 14.4 µm. Two bands are imaged at a nominal resolution of 250 m at nadir, five bands at 500 m, and the remaining bands at 1000 m. A ±55 degree scanning pattern at an altitude of 705 km achieves a 2330 km swath with global coverage every one to two days.

The scan mirror assembly uses a continuously rotating, double-sided scan mirror to scan ±55 degrees, and is driven by a motor encoder built to operate 100 percent of the time throughout the six year instrument design life. The optical system consists of a two-mirror, off-axis afocal telescope which directs energy to four refractive objective assemblies, one each for the visible, near-infrared, short- and mid-wavelength infrared, and long wavelength infrared spectral regions.

The MODIS instruments on the Terra and Aqua space vehicles were built to NASA specifications by Santa Barbara Remote Sensing, a division of Raytheon Electronics Systems. Table 6 contains the instruments' technical specifications:

Calibration

MODIS has a series of on-board calibrators that provide radiometric, spectral, and spatial calibration of the MODIS instrument. The blackbody calibrator is the primary calibration source for thermal bands between 3.5 µm and 14.4 µm, while the Solar Diffuser (SD) provides a diffuse, solar-illuminated calibration source for visible, near-infrared, and short wave infrared bands. The Solar Diffuser Stability Monitor tracks changes in the reflectance of the SD with reference to the sun so that potential instrument changes are not incorrectly attributed to changes in this calibration source. The Spectroradiometric Calibration Assembly provides additional spectral, radiometric, and spatial calibration.

MODIS uses the moon as an additional calibration technique and for tracking degradation of the SD by referencing the illumination of the moon since the moon's brightness is approximately the same as that of the Earth. Finally, MODIS deep space views provide a photon input signal of zero, which is used as a point of reference for calibration.

For additional details about the MODIS instruments, see NASA's MODIS | About Web page.