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LAADS FAQs

This document provides answers to Frequently-Asked Questions (FAQs) that pertain to MODIS (Moderate Resolution Imaging Spectroradiometer), VIIRS (Visible Infrared Imaging Radiometer Suite), MAS (MODIS Airborne Simulator), and MERIS (Medium Resolution Imaging Spectrometer) data and products that are offered by the Level-1 and Atmosphere Archive & Distribution System (LAADS) Distributed Active Archive Center at the Goddard Space Flight Center. If you seek an answer to a question that doesn't exist in this list, please send your question to the following address: MODAPSUSO@lists.nasa.gov
The questions and answers are compiled under the following four categories:

LAADS is an online archive that serves remote sensing data products from two major satellite missions: EOS MODIS and SNPP VIIRS. In addition, it also hosts the MODIS Airborne Simulator (MAS) and the European Space Agency’s (ESA) Medium Resolution Imaging Spectrometer (MERIS) data products. Users may search, order, and acquire products via the LAADS Web interface, directly download desired data products via HyperText Transfer Protocol (HTTP), or acquire them through Web services. LAADS Web orders support product selection, and a number of on-demand processing options. Consult the LAADS Help Overview for additional details.

MODAPS (MODIS Adaptive Processing System) is the primary production facility that generates the majority of products that are served by LAADS. LAADS is collocated with MODAPS at the Goddard Space Flight Center in Greenbelt, Maryland, and the two systems share common software, hardware, network infrastructure, and support staff. MODAPS is one of many Science Investigator-led Processing Systems (SIPS) that produces NASA EOS standard products for its stakeholders.

The LAADS archive contains large volumes of data derived from both MODIS instruments aboard NASA EOS Terra and Aqua platforms. Collection-6 (C6) provides the latest Terra and Aqua MODIS Level-0 (L0) and L1 products. C6 also provides the MODIS L2 and L3 atmosphere products. In case of the MODIS land domain, we offer C6 products in two reprocessing batches that are called Tier-1 and Tier-2. Each Tier includes a grouping of particular products that were selected based on the availability of the updated product algorithms (PGEs), their product dependencies, and an efficient reprocessing plan. All Tier-1 products have been reprocessed and released to the public except for BRDF-Albedo and Land Surface Temperature suites. The Tier-2 products are expected to release later in 2016.

Certain previous MODIS collection versions (for instance, C5) are also currently available. Please consult the Data Availability Chart for your product of interest (either from the LAADS Mission & Measurements page or the Find Products page) to get a full picture of data availability, data retention, and decommissioning plans, etc.

Currently, LAADS hosts the Land-PEATE NPP VIIRS products from L1 through L3. In the near future, it will offer the NASA standard versions of the SNPP VIIRS atmosphere and land products.

In addition, LAADS also provides access to the following data products:

  • MODIS Airborne Simulator (MAS) data, which is only available via the LAADS Archive directory, and not searchable
  • ESA Envisat MERIS L1B Full Resolution (FR) and Reduced Resolution (RR) data sets from European Space Agency (ESA)
  • A variety of dynamic and static ancillary datasets that include the following:
    • Dynamic Ancillary datasets
      • Terra MODIS 2-hr definitive attitude files from Flight Dynamics
      • Terra MODIS 2-hr attitude files from L0
      • Terra MODIS
      • Aqua MODIS 2-hr refined spacecraft attitude
      • Aqua MODIS 24 hours spacecraft ephemeris/orbit files
      • NCEP GFC [3 hour – 24 hour] Forecast Files
      • NCEP Daily TOVS Ozone
      • NCEP One Degree Global Data Assimilation System data
      • NCEP TOVS and SBUV2 Column Ozone
      • NCEP Ice Surface Concentration
      • NCEP Reynolds weekly Sea Surface Temperature
      • TOMS Column Ozone Earth Probe
      • DAO V4 Late look special subset daily global climatology
      • DAO V5 Late look special subset daily global climatology
      • NAAPS AOT [3-hour – 24-hour] Forecast Files
      • NSIDC Near-real-time global snow and ice extent
    • Static Ancillary datasets
      • MODIS-derived white sky albedo [0.66 – 2.13 micron] (Jan – Dec 2004)

Terra and Aqua refer to the first and second EOS platforms that were launched in December 1999 and February 2002, respectively. Each platform carries a complement of Earth observing instruments; they include a MODIS instrument on each. Besides their platform identity, Terra and Aqua are also referred to as missions within the EOS program. The third and final EOS mission, launched in 2004, is called Aura.

Generally, the same list of retrieved geophysical parameters is available from both missions. However, orbits and instrument calibration differences will often dictate which MODIS data source is most appropriate. The Univ. of Wisconsin’s Space Science and Engineering Center maintains an excellent resource for both Terra Orbit Tracks and Aqua Orbit Tracks. Those interested can use these sites to determine how closely a daily overpass samples a particular geographical point, and locate the 5-minute data granule that contains overpass data of interest.

NASA’s standard science data products provide an internally consistent, well-calibrated record of Earth’s geophysical properties to support science. Near-real-time (NRT) products are produced in an expedited manner to ensure they are available within three hours of observation time. Please note that this latency requirement applies only to the L2 swath products, and not to higher-level products. This necessitates using a different set of satellite attitude and ephemeris data compared to the operational system inputs used in standard processing. In addition, production rules within certain L2 codes have been modified to relax the requirements to accommodate ancillary data products. The Principal Investigator (PI) for Land Surface Reflectance product developed a special version of the L2 code; this code uses NOAA’s Global Forecast System (GFS) ancillary inputs as opposed to Global Data Assimilation System (GDAS) inputs used in the standard processing version. The effects of these changes affect different product types (particularly land vs. atmosphere products) to varying degrees. For more detailed information, see the following two comparisons on the LANCE-MODIS page:

  1. Comparison of C6 Operational and NRT Atmospheric products

  2. Comparison of C6 Operational and NRT Land products

Levels (0 through 4) refer to the processing stages to which the data products are generated and released to users. MODIS data are processed to L0 (instrument packet data) and L1 (calibrated radiance and geolocation), and data at this level are domain-neutral. MODIS atmosphere products are produced at L2 and L3, while MODIS land products are produced at L2, L2G, L3, and L4.

L2 products involve retrieving specific geophysical parameters through applied algorithms. For instance, geophysical retrieval algorithms are used to derive the L2 MODIS atmosphere products, which include aerosol, cloud mask, atmospheric profiles, cloud, and precipitable water. L2 gridded (L2G) products are derived from L2 data and stored in a uniform gridded system that forms the basis for the tiled MODIS land products. L3 processing produces Earth-gridded and averaged geophysical parameter data that are mapped as space-time composites. L4 MODIS processing relates to model output or results of analysis from lower-level data (for instance, the MODIS land Gross Primary Productivity). See definitions of the different data processing levels.

The SNPP VIIRS data levels start with L0 Raw Data Records (RDR). (Please note that RDRs derive from NOAA terminology. Under Land SIPS processing, LAADS will offer (in the near-future), data records from L0 through L4). RDR is an accumulation of binary data generated by sensors on board the SNPP platform and assembled into groups called application packets (APs). The VIIRS L1 products are generated from RDRs. Therefore, NPP VIIRS L1 products are Sensor Data Records (SDRs) and are full resolution sensor data that are time-referenced, Earth-located, and calibrated by applying ancillary information, including radiometric and geometric calibration coefficients and geo-referencing parameters such as platform ephemeris from RDRs. The L2 data products are 5-minute Swath Environmental Data Records (EDRs) and Climate Data Records (CDRs) produced from SDRs.

MERIS products are processed at three different levels. The L1B MERIS products are images that are resampled on a path-oriented grid, with pixel values calibrated to match the Top-of-Atmosphere (TOA) radiance. The L2 products are data derived from the L1B products, with pixel values processed to derive geophysical measurements. The L3 products are a synthesis of more than one MERIS product (and possibly external data) to display geophysical measurements for a given time period. However, LAADS offers L1 Full Resolution, Full Swath Geolocated and Calibrated TOA Radiance, L1 Reduced Resolution Geolocated and Calibrated TOA Radiance (stripline), and L2 Reduced Resolution Geophysical Product for Ocean, Land and Atmosphere (stripline) products.

All land and atmosphere products are derived from L1B calibrated radiances (MODIS at 250 m, 500 m or 1 km resolution and VIIRS at 375 m or 750 m) using L1 geolocation and the (MODIS/VIIRS) cloud mask. These products all span the same 5-minute orbital granules as the L2 products. Users may also need the 1 km geolocation files (e.g., in case of MODIS MOD03/MYD03) in conjunction with the L2 products.

The MODIS Specifications page is a good start for those seeking a full list of the MODIS spectral bands. The MODIS Characterization Support Team offers a number of good resources, including information about MODIS calibration.

For VIIRS Level-1 products, consult the Operational Algorithm Description documents. The MERIS User Guide provides comprehensive information about MERIS Level-1 products.

Science algorithms that produce different data products improve and evolve over time in response to a number of factors. They may include better quality data inputs, improvements in geometry, radiometric calibration, and an improved understanding, and therefore characterization of the problem algorithm itself. Ever since the first version of MODIS products was produced, they were reprocessed in their entirety roughly every two years using the best set of science algorithms available for each product. Each processing run, called a collection, is assigned a new version number. Currently, Collection-6 (C6) is the latest version of MODIS atmosphere and land products. Collection number is formally defined as a 3-digit identifier that is included as part of the HDF filename. Previous collections include 001, 003, 004, and 005. Interim collections (for instance, 041, 051) exist as well to facilitate processing intermediate product versions (to address immediate corrections and adjustments) between main collections. For instance, certain MODIS atmosphere products were processed and released as C51, and C6 versions of these products will eventually supersede and replace them after an overlap period.

Once a new collection is released, the previous one is retained in parallel for about a year before it is decommissioned. C6 MODIS atmosphere L2 products were released in late 2012, and their higher-level products were released in 2014. C6 MODIS land Tier-1 block of products were released in 2015. Tier-2 products are slated to release in 2016.

[See earlier question: What data are available via LAADS? for a definition of Tier-1 and Tier-2]

The MODIS Atmosphere products Web site provides a wealth of information on the complete products portfolio. It offers product guides, product change summaries for C6, file specifications, and a host of other related information. It also provides a detailed atmosphere product availability calendar for available collections on a monthly basis.

First, you must decide whether global L3 summaries at 1-degree resolution will meet your needs. If so, you will find all MODIS Atmosphere parameters together in daily (MOD08_D3/MYD08_D3), 8-day (MOD08_E3/MYD08_E3) or monthly (MOD08_M3/MYD08_M3) product files. However, you do have the option of ordering (via LAADS Web) a subset of one of these files containing only the parameters you select. If you decide to order L2 atmosphere products, please consult additional information on each product in the MODIS Atmosphere site. The Algorithm Theoretical Basis Documents (ATBD), QA Plan, Validation Plan, and User Guides (for Cloud Optical Properties and Cloud Mask) are available from the reference page on the MODIS Atmosphere site.

All MODIS, VIIRS, and MAS products are provided in Hierarchical Data Format (HDF) and HDF-EOS while MERIS products are in special Envisat format. HDF is deemed self-describing with embedded metadata that allows applications to interpret its structure and contents without any external information. A number of proprietary as well as open-source tools to handle HDF and HDF-EOS exist. LAADS Web has the ability to provide data in non-native formats such as GeoTIFF.

MODIS product file names contain the product ID, a date/time ID (usually the beginning of data acquisition), a collection ID, and finally a processing date/time stamp, and they follow specific naming conventions for swath and tiled products.

---

Example of Level-2 MODIS Atmosphere – Aerosol product
MOD04_3K.A2016003.0000.006.2016008022620.hdf

A quick reference guide is available in the Guide to Atmospheric product file names.

Within the file names, MOD refers to Terra MODIS, MYD refers to Aqua MODIS, and MCD refers to Combined Terra and Aqua MODIS.

MOD04_3K: Product Short-name (Aerosol)

A: Acquisition date

2016: Year

003: Day of Year

0000: Hour and Minute (Data acquisition start time)

006: Collection version number

2016: Product date – Year

008: Production date – Day of Year

022620: Production time – Hour : Minutes : Seconds

hdf: Format Suffix

---

Example of Level-3 MODIS Land – BRDF-Albedo product

MCD43A4.A2011017.h13v08.005.2011039133053.hdf

MCD43A4: Product Short-name (Nadir BRDF-Adjusted Reflectance)

A: Acquisition date

2011: Year

017: Day of Year

h13: Horizontal tile number

v08: Vertical tile number

005: Collection version number

2011: Production date – Year

039: Production date – Day of Year

133053: Production time – Hour : Minutes : Seconds

hdf: Format Suffix

---

Example of Level-3 Terra MODIS Land Surface Reflectance Daily Climate Modeling Grid

MOD09CMG.A2006151.005.2008116224047.hdf

MOD09CMG: Product Short-name (Land Surface Reflectance CMG)

A: Acquisition date

2006: Year

151: Day of Year

005: Collection version number

2008: Production date – Year

116: Production date – Day of Year

224047: Production time – Hour : Minutes : Seconds

hdf: Format Suffix

---

The basic VIIRS file naming convention consists of an ~80 character title, comprised of Data Product ID, Spacecraft ID, Data Start Date, Data Start Time, Data Stop Time, Orbit Number, Creation Date, Origin, Domain Description, and the Extension. However, higher-level (L2G and L3) VIIRS product file names are similar to MODIS product file names. So, for a detailed description of L0 and L1 VIIRS data file naming convention check the Common Data Format Control Book; consult the MODIS land product’s file naming convention for L2 and L3 VIIRS products.

The MERIS product file name identifies the mode (when relevant), level, (P) parent or (C) child product, processing stage, generating center, start day, time, duration of acquisition (in seconds), mission phase, orbit number and satellite. Section 2.2.1 of the MERIS Product Handbook fully explains the file naming convention.

All HDF product files, in general, are not compressed in their entirety; but we may have individual Science Data Sets (SDS) that are compressed, usually through the hrepack utility. This same routine also helps to unchunk and decompress the SDSs by selecting compression method NONE and chunking info NONE. If $HDFBIN/Hrepack doesn't exist on your system, you need to acquire a copy from the HDF Tools site.

MODAPS uses a lossless compression method for compressing the SDSs. Therefore, compressing an HDF SDS will have no effect on the contents of the SDS as read by HDF library routines. Those routines check whether or not the SDS is compressed, and decompress the data, if necessary, without requiring any special action on the part of the user.

Reading a compressed SDS is slower than reading an uncompressed SDS, but that effect is somewhat minimized by HDF chunking, and we do chunk many of the larger SDSs that are compressed. Most programs that read a small number of scans at a time, or a large fraction of a scan each time, will not need any re-writing to achieve reasonable performance with chunked data. However, for best performance, a program can call the HDF library routine SDgetchunkinfo() in C, or sfgichnk() in Fortran, to determine the chunking parameters. It can then call SDreadchunk() in C, or sfrchnk() in Fortran, to read (and decompress, if necessary) an entire chunk at a time.

However, any program that attempts to modify part of a compressed SDS may fail, without producing any error indications, essentially because the HDF library only permits modification of the entire compressed SDS in a single write. Exception: if it calls SDchunkwrite() in C or sfwchnk() in Fortran, it can update one chunk at a time, rather than the entire SDS.

The Day/Night flag for each scan indicates the particular mode in which the MODIS instrument was collecting data for that scan. Night mode occurs when nadir aperture is viewing the dark side of Earth. In Day mode, MODIS data are acquired for all 36 bands whereas in Night mode, data are acquired only for thermal emissive bands (bands 20–36). Check the MODIS specifications page for a complete list of all MODIS bands and their details. The Day/Night flag information is available in the granule-level metadata in addition to the scan-level flag in L1 data.

Please visit our LANCE–MODIS web site at for more information on NRT products and their availability.

The availability of products varies with the product-level. Daily L2 swath data is generally available within a day. L3 daily products are available in 2 days. The n-day composite products generally take that many days, i.e., 8-day products in 8 days, and 16-day products in 16 days. Scheduled maneuvers also affect these timeframes. Only NRT L2 swath products are available within 3 hours of observation time.

We distribute data to thousands of users around the globe every day. To ensure optimal resource availability for all our users, LAADS had originally implemented a policy of allowing a certain number of connections from each IP address. Our system was designed to automatically block access from that IP address once that threshold was crossed. Currently, we do not have that threshold in place, but we reserve the right to implement a limit if we experience any abuse from the user community.

Please contact MODAPSUSO@lists.nasa.gov
if you would like to arrange a data subscription, or have special requirements. Alternately, you may use a web client like GNU's wget to download files from the LAADS Download site.

You may also use MODAPS Web Services (MWS) to submit search and order requests through an Application Programming Interface (API). Our services understand both Simple Object Access Protocol (SOAP) calls and Representational State Transfer (REST) Common Gateway Interface (CGI) requests. To make SOAP calls, you need to write a script that uses a SOAP library and calls the methods in the API.

Methods exist to search for files by product, geolocation, and sample-time, and there also are methods for ordering files to get staged on our Download site, and you can download them (the search queries also return URLs for the files themselves, that you can use for immediate download). We can also copy very large volumes of data on user-supplied disks and ship them back to the user.

Several proprietary as well as open-source tools exist to support HDF data files. The HDF Group is an excellent source for tools, libraries, and documentation for all levels of users. For instance, HDFView is a practical Java-based visual tool to browse and edit HDF files. Refer to the Tools & Services section on the LAADS Web page that identifies a number of other open-source tools that handle MODIS HDF products.

LAADS provides a number of tools and services to perform the following types of data transformations to MODIS products:

  • Parameter Subsetting
  • Geographic Subsetting
  • Mosaicking
  • Reprojection and Resampling
  • Reformatting

To order any of the above, follow these steps:

  1. On the LAADS Web Search page, select the data, time-period, and area of interest, then hit Search
  2. Select the granules you seek, and click Add files to shopping cart
  3. Select View your shopping cart
  4. Select Post-process and order data, then click Order
  5. Select all Post-processing option(s) you seek, and then click Order
  6. Select additional constraints for each option. For example, specify your area of interest if you selected Geographic subset; specify the desired datasets if you selected Subset by parameter, etc., then hit Order

LAADS Web will process your files with the selected operations, and upon completion, will e-mail download instructions to retrieve your order.

LAADS also offers additional Web Services and links to a selection of open-source tools. Please consult the Tools & Services section of the LAADS Web page.

As indicated in your order notification email, data are available in your order directory for 5 days, after which they expire.

No, you don't need to notify LAADS because ordered data are automatically deleted after 5 days. However, your order consumes disk space on our server. If you want to free up your disk space to make room for new orders, you can go to our Track Orders page and select "Cancel/Release".

LAADS maintains two type of data archives: a permanent archive that is available forever versus a rolling archive. Certain L2, L2G, and L3 MODIS products comprise the rolling archive that is deleted after a 30-day period. The Processing-On-Demand system offers the ability for users to submit requests to re-generate those deleted products.

When your order expires, you can go to our Track Orders page and enter your e-mail address. Click the Expired Order ID, review details and select the button to resubmit your order. We strongly encourage you to download your order within the allocated time (5 days) as it adds load on the resources to regenerate orders.

You can access all the Atmosphere product’s Algorithm Theoretical Basis Documents (ATBD).

Users may also consult each product’s User Guide, which often provides collection-specific changes as well.

Yes, the software is available to the public however you must register and agree to the standard NASA Software Usage Agreement.

Once you register, you will need to print and sign a copy of the Software Usage Agreement, and fax the form back to us at 301-552-6411. Subsequently, we will add your name to the access group, and you will receive an e-mail notification with the URL for the software download.

Contact LAADS User Services through any of the following channels:

E-mail: MODAPSUSO@lists.nasa.gov
Telephone: 1-800-596-8132

Mail
LAADS DAAC
NASA Goddard Space Flight Center, Code 619, Building 32
8800 Greenbelt Road, Greenbelt, MD 20771, USA

Please read the LAADS Data Use Citation Policies.

In addition, we would also appreciate receiving a copy of your publication; please forward it to MODAPSUSO@lists.nasa.gov

A Digital Object Identifier (DOI) is a digital identifier in the form of a character string, which is used to uniquely identify an object that either exists in electronic or physical form. Such objects may include, for instance, a published document, a data collection, or several other forms of artifacts. The metadata associated with a given object is stored within the DOI, and includes the Universal Resource Locator (URL) where that object is available. The primary advantage of a DOI is its permanence, while its location (URL) and related metadata may change. In other words, a DOI is a persistent data identifier whose object location (URL) may change over time. In the context of a large data center, DOIs enable a consistent mechanism to document what data products were used in a particular study, provide persistent links to archived data, and allow us to collect metrics based on cited datasets. Overall, they contribute towards facilitating the data provenance and reproducibility requirements.

The MODIS Science Team implemented DOIs for the C6 Level-1 MODIS products that were released in late-2012. Based on that implementation, a similar framework is used for all C6 L2 and higher-level MODIS atmosphere and land products. The MODIS Science team decided to use the product short name and the collection version [short_name.version_id] as the unique identifier within the DOI. The DOI URL is hard-coded in the Earth Science Data Type (ESDT) for each product. Two Product-Specific Attributes (PSA) are included as Additional Attributes in the ESDT that facilitate publishing the DOI information as part of the granule-level and collection-level metadata. They include the following:

  1. Digital object identifier that uniquely identifies each data product

    identifier_product_doi [Example: 10.5067/MODIS/MOD04_L2.006]

    In the “10.5067” string, which is the prefix component, “10” identifies the DOI registry, and “5067” identifies the registrant (which in this case is the NASA ESDIS project). The “MODIS/MOD04_L2.006” comprises the suffix component (which in this case identifies the satellite instrument and the specific product along with its collection version number).

  2. URL of the digital object identifier resolving authority

    identifier_product_doi_authority [http://dx.doi.org]

Resolution is the process through which a DOI is submitted to a network service (resolving authority), which returns a specific piece of information related to the identified entity – for example, a URL address of a landing page. In the above case, if we bring up http://dx.doi.org and plug in “10.5067/MODIS/MOD04_L2.006” (assuming it has been registered), it will connect to the specific product’s landing page. A landing page is the location where a user first arrives at, following the DOI hyperlink via the resolving page. This page provides the full breadth of information related to the specific product whose source stems from the DOI. The association between the landing page and the DOI happens after the DOI is registered.

netCDF, an acronym for Network Common Data Form, is defined based on three facets:

  1. netCDF is a set of software libraries to create, access, and share scientific data sets, such as time-series, matrices, and satellite observation-derived gridded multidimensional data.
  2. netCDF provides an interface to a library of data access functions that facilitate storage and retrieval of data as arrays.
  3. netCDF conforms to CF–1.6 conventions. CF or Climate and Forecast conventions pertain to metadata that are designed to support the processing and sharing of data generated using the netCDF API (Application Programming Interface).

The netCDF libraries support a binary data format that is extensively used in the climate science and modelling communities, globally. netCDF’s primary advantages include its machine-independence and self-describing nature.

Consult the following URLs for additional information on netCDF: http://www.unidata.ucar.edu/software/netcdf/

http://www.unidata.ucar.edu/software/netcdf/docs/

http://www.unidata.ucar.edu/software/netcdf/docs/faq.html

Hierarchical Data Format (HDF) is an open-source, extensible, and self-describing file data format that was created independently from netCDF by the National Center for Supercomputing Applications (NCSA), University of Illinois at Urbana–Champaign. HDF4 is the first generation of this file format created by NCSA, and is extensively used in NASA’s Earth Observing System Data and Information System (EOSDIS).

HDF5 was developed and supported by the HDF Group, a non-profit corporation that spun off from NCSA, and continues to help support and develop HDF as a viable technology and file storage format. Built to improve upon the HD4 library, HDF5 is a truly hierarchical data format that supports a variety of complex data types, high file volumes, and an efficient I/O.  HDF5’s primary advantages include its rich data model with efficient access, parallel I/O [1], and its support for high-performance computing. All Level-2G and higher-level NASA standard version of the Suomi National Polar-orbiting Partnership’s (SNPP) Visible Infrared Imaging Radiometer Suite (VIIRS) land products are provided in HDF5 format with EOS conventions that have .h5 as their file-ending suffix.

Consult the following URLs for additional information on HDF:

https://en.wikipedia.org/wiki/Hierarchical_Data_Format

https://support.hdfgroup.org/products/hdf4/

https://support.hdfgroup.org/HDF5/

https://support.hdfgroup.org/h5h4-diff.html

The netCDF4–HDF5 connection

netCDF4–HDF5 is a hybridized approach that leverages several individual as well as collective strengths of the two data/file storage formats.  The current version 4.0 netCDF API supports the use of HDF5 data format; layering an enhanced netCDF access interface atop the HDF5 format helps provide the following advantages:

  • Facilitates creating HDF5 files whose sizes are limited only by the constraints of the underlying file system.
  • Supports multiple unlimited dimensions, per-variable compression, more complex data types, chunking, endianness control [2], and better performance.
  • Supports parallel read/write access to netCDF4/HDF5 files through the HDF5 library.
  • Improves the interoperability between netCDF4 and HDF5, and hence better integration of science observations and model outputs.

The netCDF4–HDF5 hybrid format leverages the common advantages of both formats (extensibility, platform independence, self-describing nature, etc.), while also benefitting from their separate strengths (extensive use across Earth science disciplines, performance strengths, etc.).  This format is used to manage all Level-1 and Level-2 data products in the NASA standard version of the SNPP VIIRS mission that have .nc as their file-ending suffix.

For additional information regarding NASA’s adoption of the netCDF4/HDF5 file format as a community standard, and to review its specification document, consult the following EarthData page: https://earthdata.nasa.gov/standards/netcdf-4hdf5-file-format

[1] The ability to execute multiple input/output operations simultaneously.

[2] https://en.wikipedia.org/wiki/Endianness

The conventions for CF (Climate and Forecast) metadata are designed to promote the processing and sharing of files created with the NetCDF API.  Developed through an international collaboration, CF conventions are increasingly gaining acceptance, and have been adopted by a number of projects and groups as a primary standard.  The conventions define metadata that provide a definitive description of what the data in each variable represents, and the spatial and temporal properties of the data.  This enables users of data from different sources to decide which quantities are comparable, and facilitates building applications with powerful extraction, re-gridding, and display capabilities.  The VIIRS Level-1 and Level-2 products (in netCDF4.2/HDF5) are CF-compliant.

Consult the following URL for more information on CF Conventions and Metadata:

http://cfconventions.org/

Data Product Level

Data Format

SNPP/VIIRS L0 [EDOS PDS*]

CCSDS**

SNPP/VIIRS L1

netCDF4.2/HDF5

SNPP/VIIRS L2***

netCDF4.2/HDF5

SNPP/VIIRS L2G

HDF5-EOS

SNPP/VIIRS L3 (Daily and n-Day)

HDF5-EOS

SNPP/VIIRS L3 CMG

HDF5-EOS

* EOS Data and Operations System, Production Data Set

** Consultative Committee on Space Data Systems [https://public.ccsds.org/default.aspx]

*** The only exception to this is the L2 VIIRS Surface Reflectance product, which is in HDF4, & served from LAADS

SNPP/VIIRS Level-1 and Level-2 products

Syntax: ESDT.AYYYYDDD.HHMM.VID.YYYYDDDHHMMSS.Format

Example: VNP01.A2016202.2200.001.2016203071541.nc

ESDT

Earth Science Data Type or Short Name

A

Stands for Acquisition

YYYYDDD

Data acquisition year & Day-of-year per the Julian Calendar

HHMM

Data acquisition time in Coordinated Universal Time (UTC)

VID

Version ID of the data collection

YYYYDDDHHMMSS

Processing year, Day-of-year (Julian), UTC time (hour, minutes, seconds)

Format

File format suffix, which in the above case represents netCDF

SNPP/VIIRS Level-2G, Level-3, and Level-4 products

Syntax: ESDT.AYYYYDDD.hHHvVV.VID.YYYYDDDHHMMSS.Format

Example: VNP09GA.A2016224.h17v03.001.2016228140615.h5

ESDT

Earth Science Data Type or Short Name

A

Stands for Acquisition

YYYYDDD

Data acquisition year & Day-of-year per the Julian Calendar

hHHvVV

Horizontal tile number (0-35) & Vertical tile number (0-17)

VID

Version ID of the data collection

YYYYDDDHHMMSS

Processing year, Day-of-year (Julian), UTC time (hour, minutes, seconds)

Format

File format suffix, which in the above case represents HDF5

All SNPP/VIIRS land products deemed ready for public release are in version 001; this number is part of the file name for all levels of NASA standard VIIRS products.

All levels of the NASA standard VIIRS products conform to a Stage-1 validation; this entails that their accuracies have been estimated using a small number of independent measurements obtained from selected locations, time periods, and ground-truth/field validation efforts.  Consult the SNPP VIIRS Validation page for more information.

Science Domain

DAAC

URL

VIIRS Level-1 Calibrated Radiances

LAADS

https://ladsweb.modaps.eosdis.nasa.gov/

VIIRS Land (L2, L2G, L3, L4)

LP DAAC

https://lpdaac.usgs.gov/

VIIRS Snow & Ice (L2, L2G, L3)

NSIDC

http://nsidc.org/daac/

VIIRS Atmosphere (All levels)

LAADS

https://ladsweb.modaps.eosdis.nasa.gov/

VIIRS Ocean (All levels)

Ocean Biology

http://oceancolor.gsfc.nasa.gov/

Originally, NPOESS was a partnership involving NASA, National Oceanic and Atmospheric Administration (NOAA), and Department of Defense (DoD) to build and launch a next-generation satellite system to monitor Earth’s weather, atmosphere, oceans, land, and near-space environment.  The NPOESS Preparatory Project’s (NPP) was the planned pathfinder mission for NPOESS, which was later renamed the Suomi National Polar-orbiting Partnership (SNPP).  SNPP was designed to serve as a bridge between existing Earth observing missions (NOAA’s polar satellite missions and NASA’s Earth Observing System) and the Joint Polar Satellite System (JPSS) constellation.  SNPP, with a payload of five instruments, was launched on 28 October, 2011.  Subsequently, DOD’s role was descoped, and NPOESS was restructured, and the JPSS mission was created as a partnership between NASA and NOAA to build and launch four satellite platforms (JPSS-1 through JPSS-4) between 2017 and 2031.  All four platforms are slated to include a VIIRS instrument.

Land SIPS (Science Investigator-led Processing System) is the entity, collocated with the MODIS Adaptive Processing System (MODAPS) at the Goddard Space Flight Center (GSFC), which is responsible for creating NASA standard and experimental versions of VIIRS land products.  Land SIPS produces, archives, and distributes all L1 products via LAADS; it produces and transmits all land and cryospheric products to LP DAAC and NSIDC DAAC, respectively.  The Land SIPS-produced Version-001 NASA standard VIIRS land product suite is the first collection version released to the global science community deemed ready for scientific analysis and applications.  All L1 and higher-level products generated prior to Version-001 were deemed beta quality with known problems, and hence not recommended for scientific use.

Land SIPS has adopted the same approach used for MODIS land quality assessment to represent and document QA information in NASA standard VIIRS products. The Land Data Operational Product

Evaluation (LDOPE) facility, collocated with MODAPS at GSFC, coordinates the quality assessment activities for all VIIRS products. It involves the evaluation and documentation of the science quality of all VIIRS products, which is incorporated within the operational code, and this information is carried within the products (at the pixel-level) and their metadata (at the file-level).

Based on nearly two decades of experience with users attempting to process and parse MODIS QA information and metadata, it is fair to say that the complex nature of QA information generally tends to discourage users from attempting to use them. A four-set tutorial explains how QA information is structured including aspects of its usefulness and usability, which is aimed to educate beginning and intermediate users. Even though this set of tutorials was designed to address MODIS land products, VIIRS land users may find it useful to peruse them, given their common methodology.

VIIRS land products map to their MODIS counterparts except for the Level-4, model-based products that currently don’t exist for VIIRS. Here are the MODIS and VIIRS land product portfolios:

MODIS

VIIRS

Land Surface Reflectance

Land Surface Reflectance

Land Surface Temperature/Emissivity

Land Surface Temperature/Emissivity

BRDF [3] /Albedo/NBAR [4]

BRDF/Albedo/NBAR

MAIAC [5]

MAIAC

 

 

Vegetation Indices

Vegetation Indices

LAI & fPAR [6]

LAI & fPAR

GPP & NPP [7]

Evapotranspiration

 

 

Thermal Anomalies/Fire

Thermal Anomalies/Fire

Burned Area

Burned Area

Land Cover & Phenology

Phenology

Vegetation Continuous Fields

 

 

Snow Cover

Snow Cover

Sea Ice Extent

Sea Ice Extent

Ice Surface Temperature

Ice Surface Temperature

[3] BRDF: Bi-directional Reflectance Distribution Function

[4] NBAR: Nadir BRDF-Adjusted Reflectance

[5] MAIAC: Multi-Angle Implementation of Atmospheric Correction

[6] LAI & fPAR: Leaf-Area Index & Fraction of absorbed Photosynthetically Active Radiation

[7] GPP & NPP: Gross Primary Production & Net Primary Productivity

Once complete suites of NASA standard VIIRS land products have been released, you can expect to see those product suites supported and available as part of the OPeNDAP catalog.  Monitor the alerts posted on the LAADS Web opening page for information regarding the availability of VIIRS land products via OPeNDAP.

VIIRS products produced by NOAA and disseminated via the CLASS (Comprehensive Large Array-data Stewardship System) differ from those produced by Land SIPS in a number of respects.  The differences include the following:

  • NOAA’s operational VIIRS products use different algorithms than those used in production at the NASA Land SIPS. One of the primary goals for the NASA algorithms is to extend the derived biogeophysical parameters to provide a continuous time-series of observations from the MODIS instruments through VIIRS.
  • Currently, NOAA-produced operational VIIRS data are not reprocessed to maintain consistent collections. NASA products are generally reprocessed every 18 to 24 months to address changes in product algorithm, radiometric sensor responses, geolocation, etc. to create a new collection version.
  • NOAA produces swath-based VIIRS products only; NASA Land SIPS produces swath, gridded, and climate modeling grid (CMG) products.

The LANCE near-real-time (NRT) processing system produces and distributes NRT VIIRS products.  Announcements of new NRT products are posted on the LAADS Web home page and are also announced through the LANCE mailing lists .

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Last updated: March 16, 2018