MODIS Terra/Aqua Science Products

Purpose

Understand NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra and Aqua spacecraft, its standard science product families, and how a GeoTIFF-formatted MODIS product maps to the NetCDF data model through NEP's GeoTIFF User Defined Format (UDF) reader.

MODIS products are most commonly distributed from the USGS/NASA Land Processes Distributed Active Archive Center (LP DAAC) in HDF4. NEP does not provide an HDF4 UDF reader, so a MODIS example must use a GeoTIFF rendering of a MODIS product (for example from NASA GIBS or AppEEARS). Once such a sample is obtained, NEP exposes each GeoTIFF band as a 2-D NetCDF variable with CF-1.8 coordinate variables and a crs grid-mapping variable.

When to Use

Use this skill when adding, reviewing, or debugging any NEP example, handler, or workflow that touches MODIS Terra/Aqua data. It covers the spacecraft, the MODIS instrument, standard product families, file naming conventions, and the GeoTIFF-to-NetCDF mapping.

Required Context

When a user asks about a specific MODIS product, identify these items:

| Item | Typical Value | Notes | |------|---------------|-------| | Mission/spacecraft | Terra, Aqua | EOS AM-1 (Terra), EOS PM-1 (Aqua) | | Instrument | MODIS | 36-band whisk-broom scanning radiometer | | Common products | MOD09, MYD09, MOD13, MYD13, MOD11, MYD11, MCD43 | Surface reflectance, vegetation indices, LST, albedo | | Standard format | HDF4 from LP DAAC | Not directly readable by NEP | | NEP-compatible format | GeoTIFF from GIBS / AppEEARS | Required for NEP GeoTIFF UDF reader | | NEP path | GeoTIFF UDF reader | examples/viz/GeoTIFF/ or a custom GeoTIFF script | | Primary variables | Band-dependent, e.g. sur_refl_b01, 250m_16_days_NDVI, LST_Day_1km | Product-specific | | Coordinates | Sinusoidal x/y, or geographic lon/lat after reprojection | Depends on GeoTIFF source | | Quality/flag masks | QA bit-packed rasters, pixel_reliability, etc. | Product-specific | | Data access | LP DAAC, NASA Earthdata, NASA GIBS, AppEEARS | Free; Earthdata login often required |

Spacecraft and Orbit

• Agency: NASA
• Spacecraft: Terra (EOS AM-1), Aqua (EOS PM-1)
• Launch: 1999-12-18 (Terra), 2002-05-04 (Aqua)
• Orbit: Sun-synchronous, near-polar, ~705 km mean altitude
• Equator crossing time: ~10:30 local time descending (Terra), ~13:30 local time ascending (Aqua)
• Revisit: Near-daily global coverage from each spacecraft; combined Terra+ Aqua coverage roughly twice daily for most locations.

Instrument

MODIS is a whisk-broom scanning radiometer. It measures in 36 spectral bands at resolutions from 250 m to 1 km.

| Band | Wavelength (µm) | Ground resolution | Primary use | |------|-----------------|-------------------|-------------| | 1 | 0.620 - 0.670 | 250 m | Red, NDVI | | 2 | 0.841 - 0.876 | 250 m | NIR, NDVI | | 3 | 0.459 - 0.479 | 500 m | Blue | | 4 | 0.545 - 0.565 | 500 m | Green | | 5 | 1.230 - 1.250 | 500 m | Snow/ice | | 6 | 1.628 - 1.652 | 500 m | Land/cloud | | 7 | 2.105 - 2.155 | 500 m | SWIR, burning | | 8-15 | 0.405 - 0.877 | 1 km | Ocean color, aerosol | | 16-19 | 0.866 - 0.936 | 1 km | Atmospheric water vapor, ocean | | 20-25 | 3.660 - 4.549 | 1 km | Surface/cloud temperature | | 26 | 1.360 - 1.390 | 1 km | Cirrus clouds | | 27-36 | 6.535 - 14.385 | 1 km | Water vapor, ozone, temperature |

Standard MODIS Product Families

| Family | Terra | Aqua | Description | |--------|-------|------|-------------| | Surface Reflectance | MOD09 | MYD09 | Daily atmospherically corrected surface reflectance | | Vegetation Indices | MOD13 | MYD13 | 16-day NDVI / EVI composite | | Land Surface Temperature | MOD11 | MYD11 | Daily land-surface temperature and emissivity | | BRDF/Albedo | MCD43 | MCD43 | 16-day nadir BRDF-adjusted reflectance and albedo | | Snow Cover | MOD10 | MYD10 | Daily global snow cover | | Burned Area | MCD64 | MCD64 | Monthly burned-area maps |

For NEP, the key constraint is that the chosen product must be obtained as a GeoTIFF (for example, via NASA GIBS or AppEEARS) because the standard LP DAAC science products are in HDF4, which NEP does not read.

File Naming

A typical MODIS tile file name has the form:

PRODUCT.AYYYYDDD.h##v##.VVV.YYYYDDDHHMMSS.hdf

| Field | Meaning | |-------|---------| | PRODUCT | Product short name, e.g. MOD09GA, MYD13Q1 | | AYYYYDDD | Acquisition year and Julian day | | h##v## | Horizontal and vertical tile index in the MODIS Sinusoidal grid | | VVV | Collection version, e.g. 061 | | YYYYDDDHHMMSS | Production date/time | | hdf | HDF4-EOS format (standard LP DAAC distribution) |

When exported as GeoTIFF from AppEEARS or GIBS, the file will typically have a .tif extension and may include the product, tile, date, and band information.

NetCDF Mapping via NEP GeoTIFF Reader

When NEP is built with -DNEP_ENABLE_GEOTIFF=ON, a GeoTIFF MODIS product is opened through the standard NetCDF API.

• Dimensions: x/y (or lon/lat) matching the raster columns and rows.
• Data variable: data[y, x] containing the band values. The variable name may be configurable through the NEP reader; default behavior exposes the raster as data.
• Coordinate variables: 1-D x[x] and y[y] in projected coordinates, or lon[lon] and lat[lat] in geographic coordinates.
• Coordinate bounds: For pixel-as-area rasters, x_bnds/y_bnds or lon_bnds/lat_bnds are created and referenced via bounds attributes.
• Grid mapping: A scalar crs variable stores the CRS attributes (grid_mapping_name, semi_major_axis, inverse_flattening, and projection parameters). The data variable carries grid_mapping = "crs".
• Multi-band files: A GeoTIFF with multiple bands maps to a 3-D variable data[band, y, x]. Many MODIS GeoTIFF exports are single-band per file, so each band is opened as a separate 2-D dataset.

Data Access

Public sources for MODIS data:

• USGS/NASA LP DAAC: https://lpdaac.usgs.gov/ (standard HDF4 products)
• NASA Earthdata Search: https://search.earthdata.nasa.gov/
• NASA GIBS: https://gibs.earthdata.nasa.gov/ (GeoTIFF browse and tile products)
• NASA AppEEARS: https://appeears.earthdatacloud.nasa.gov/ (subset and export as GeoTIFF)

A free Earthdata login is usually required.

Citation

Please cite MODIS datasets in the following manner:

MODIS/Terra+Aqua [Product Name] [Data Set]. NASA EOSDIS Land Processes Distributed Active Archive Center (LP DAAC), USGS/EROS Center, Sioux Falls, South Dakota. https://lpdaac.usgs.gov

A product-specific DOI (for example, https://doi.org/10.5067/MODIS/MOD09GA.061) should be added once the exact MODIS product used in the example has been selected.

References

1. NASA MODIS Web Site. https://modis.gsfc.nasa.gov/
2. USGS/NASA Land Processes Distributed Active Archive Center (LP DAAC). https://lpdaac.usgs.gov/
3. NASA Earthdata Search. https://search.earthdata.nasa.gov/
4. NASA Global Imagery Browse Services (GIBS). https://gibs.earthdata.nasa.gov/
5. NASA AppEEARS. https://appeears.earthdatacloud.nasa.gov/