MODAPS/LAADS Frequently Asked Questions

Last Update: January 18, 2013

This is a FAQ document for MODIS, MAS, VIIRS, and MERIS data users, particularly, those interested in radiance, geolocation, cloud mask and other atmospheric products. The goal of this document is to address questions about using Level 1 and Atmosphere Archive and Distribution System (LAADS) and to direct researchers to the multitude of MODIS, MAS, VIIRS, MERIS resources that are available.

Q: What is LAADS?

A: LAADS is an online archive of MODIS, MAS, VIIRS, MERIS data products. Products are currently available via HTTP, web order, or with web services. Web ordering supports product selection, subsetting and special processing. Get more details at the LAADS Help Overview.

Q: What data are available via LAADS?

A: LAADS is being populated with large volumes of MODIS data from the NASA EOS Terra and Aqua spacecrafts as they are produced. These data include Collection 5 and some earlier data from Collection 3 & 4 such as Aqua/Terra Atmosphere Level 2 & 3 products. New Collection 6 MODIS Aqua Level 1 and Level 2 Atmosphere products are available while all Terra and rest of Aqua products will be included soon. LAADS also provide access to MODIS Airborne Simulator (MAS) data (via HTTP only; not searchable), NPP VIIRS Level 1, Level 2, and Level 3 products, and ENVISAT MERIS Level 1B Full Resolution (FR) and Reduced Resolution (RR) data sets from European Space Agency (ESA). To get an up to date picture of available data, please consult the LAADS Web Data Availability Chart.

Q: How do I acknowledge/cite the use of LAADS DAAC Data?

A: Please read LAADS_Data-Use_Citation_Policies.pdf

We would appreciate receiving a copy of your publication, which can be forwarded to MODAPSUSO@lists.nasa.gov.

Q: Can you start with the big picture and explain this Level 1, Level 2, Level 3 business?

A: These are different levels of products produced and delivered to users, generated from the data collected by an instrument. For example. MODIS data are processed at five levels (Level 0 to Level 4). The Level 0 represent instrument data at original resolution, time order-restored, with duplicate packets removed. The Level 1 products include Level 1A (L1A) scans of Raw Radiance measurements, Level 1 Geolocation, Level 1B (L1B) Calibrated Radiances, Cloud Mask, and Atmospheric Profiles. The Level 2 data are Geophysical parameter data retrieved from the L1B data by application of geophysical parameter algorithms. Level 3 processing produced Earth-gridded geophysical parameter data, which have been averaged, gridded, or otherwise rectified or composited in time and space. The Level 4 MODIS processing generates model output or results of analysis from lower-level data; for example, variables derived from multiple measurements. For more details regarding data level are available at MODIS webpage.

The NPP VIIRS data levels start with Level 0, Raw Data Records (RDR). The RDR is an accumulation of binary data generated by sensors on board the NPP spacecraft and assembled into groups called application packets (APs). The VIIRS Level 1 products are generated from Raw Data Records (RDR). Therefore, NPP VIIRS Level 1 products are Sensor Data Records (SDRs) and are full resolution sensor data that are time referenced, Earth located, and calibrated by applying the ancillary information, including radiometric and geometric calibration coefficients and geo-referencing parameters such as platform ephemeris from RDRs. The Level 2 data products are 5-min L2 Swath Environmental Data Records (EDRs) and Climate Data Records (CDRs) produced from SDRs.

There are three types of processing levels described for MERIS products. The Level 1B MERIS products are images resampled on a path-oriented grid, with pixel values having been calibrated to match the Top Of Atmosphere (TOA) radiance. The Level 2 products are data derived from the Level 1B products, with pixel values having been processed to get geophysical measurements. And, the Level 3 products are a synthesis of more than one MERIS product (and possibly external data) to display geophysical measurements for a time period. However, LAADS is making available Level 1 Full Resolution, Full Swath Geolocated and Calibrated TOA Radiance, Level 1 Reduced Resolution Geolocated and Calibrated TOA Radiance (stripline), and Level 2 Reduced Resolution Geophysical Product for Ocean, Land and Atmosphere (stripline) products.

Q: And what about Level 1? Would I be interested?

A: All Land and Atmosphere products are derived from L1B calibrated radiances (MODIS at 250m, 500m or 1km resolution and VIIRS at 375m or 750m) using L1 geolocation and the (MODIS/VIIR) cloud mask. These products all span the same 5 minute orbital granules as the L2 products. Some researchers do want radiance data for various purposes, including making great pictures. They may also need 1km geolocation files (e.g., in the case of MODIS MOD03/MYD03) in conjunction with the L2 products. MODIS Atmosphere L2 products do contain lower resolution lat/long parameters, so it may not be necessary to acquire the L1 geolocation files.

Q: Where can I get technical details about the Level 1 radiance products?

A: For MODIS products start with the MODIS Specifications page for a complete listing of MODIS spectral bands. The MODIS Characterization Support Team maintains a number of good resources, including information about MODIS Calibration.

For VIIRS products visit the Data Products description page. For a complete listing of spectral bands review the following document:

Joint Polar Satellite System (JPSS) VIIRS Radiometric Calibration Algorithm Theoretical Basis Document (ATBD).

MERIS Product Specifications provides a comprehensive information about MERIS Level 1 products.

Q: I see references to MODIS, Aqua and Terra. Please elaborate.

A: The Moderate Resolution Imaging Spectroradiometer (MODIS), instrument flies on two polar orbiting satellites. The first, Terra, has provided Land, Atmosphere and Ocean data products since late February 2000. The second, Aqua, started delivering science data in late June 2002.

Q: How do I choose between Aqua and Terra?

A: 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 SSEC maintains an excellent resource for both Terra Orbit Tracks and Aqua Orbit Tracks. These sites can be used to determine how closely a daily overpass samples any geographical point and to locate the 5 minute data granule that contains overpass data of interest.

Q: What is the difference between Atmosphere C5 and C5.1?

A: The Atmosphere C5 and C5.1 refers to MODIS atmospheric products produced in collection 5 and 5.1 using different versions of science algorithms.

Q: Where do I go for pictures?

A: Each discipline maintains an image library. MODIS Level 1 and Atmospheric products images can be found at the MODIS-ATMOS Image Gallery, the eMAS images are available at the eMAS Image Gallery, the VIIRS Image Gallery has NPP VIIRS images, and the MERIS product images can be acquired from the MERIS Image Gallery.

Q: Is there a page for up-to-date instrument status and information?

A: The EOS project maintains a Terra Project Page, and an Aqua Project Page. Another resource is MODIS News and Current Status. Suomi NPP provides details regarding VIIRS under Spacecraft and Instruments. For data production status, please check the LAADS Web Data Availability Chart.

Q: Please point me to more MODIS Atmospheric product information.

A: The best starting point is the MODIS Atmosphere Discipline Site. There you will find the Atmospheric Products Guide. The Product Change Summary for Collection 5 was prepared as a guide to product changes between collections 4 and 5. The team records the complete production history of each product in the Detailed Atmosphere Product Availability Calendar. If you want the complete story on how QA bits are used within each product, see QA Product Information for Collection 5.

Q: Where can I find details about the MODIS retrieval algorithms for Atmospheric products?

A: The MODIS Atmosphere Discipline Site is the place to start. Two documents of particular interest are the Known Problems of Atmosphere Products and Validation of Atmosphere Products. You should also be aware of the Atmosphere Team Publications and Presentations. The GES-DISC also maintains a general resource known as the GES-DISC Parameter Information Pages.

Other more detailed algorithm references: Historical ATBDs (somewhat outdated), PGE03 Algorithm Change History, PGE04 Algorithm Change History, PGE05 Algorithm Change History, PGE56 Algorithm Change History, PGE57 Algorithm Change History, PGE70 Algorithm Change History, PGE83 Algorithm Change History

Q: How do I determine which MODIS atmospheric product to order?

A: 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 single 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 higher resolution Atmospheric L2 products, please consult the Atmospheric Products Guide for more detailed product information.

Q: Will I have immediate online access to all of these products?

A: Yes and no. MODIS and VIIRS Geolocation and Atmosphere products as well as MAS various campaigns data and MERIS L1B products make up the LAADS permanent archive. They remain online and accessible once created. However, there is a small amount L2, L2G, and L3 MODIS products that are not stored permanently. They are available as a rolling datapool archive or, after deletion, are recreated via web order.

Q: What is the format of the product data files?

A: The standard formats for MODIS, MAS and VIIRS products are HDF and HDF-EOS while MERIS products are in special ENVISAT format. This format is intended to be self-documenting, that is, with "metadata" imbedded within each product file. The bad news for anyone encountering HDF for the first time is that you are likely to be at an impasse until you acquire a special set of tools to break open and investigate these files. However, there is a lot of good news as well. There are a lot of mature tools that work well. They are provided free of charge and generally run on a range of computer platforms (Windows, Linux, OS X, Irix, etc.). LAADS Web also provides data in non-native formats such as GeoTIFF. HDF-EOS allows us to deliver geographical cut-outs, sub-sampled products, mosaic-ed products, etc, all in a format that is usable by these tools.

Q: The product file names seem a bit arcane. Is there a pattern here?

A: 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. A quick reference guide can be found in the Guide to Atmospheric Product File Names.

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 products file names are similar to MODIS product file names. So, for a detail description of L0 and L1 VIIRS data file naming convention checkout Data Format Control Book and for L2 and L3 product naming convention consult MODIS product file names pattern above.

The MERIS product file names identifie, the mode (when relevant), level, (P) Parent or (C) Child Product, Processing stage, Generating Center, Start day, time, duration(in second) of the acquisition, mission Phase, orbit number and satellite. File naming convention is fully explained in Section 2.2.1 of the MERIS Product Handbook.

Q: How are the product files compressed?

A: Product files as a whole are generally not compressed; but individual SDSs within each product file may be compressed, usually by using the hrepack utility. The same routine can be used to unchunk and decompress the SDSs, by choosing the compression method "NONE" and chunking info of "NONE". If $HDFBIN/hrepack doesn't exist on your system, go to the HDF Tools website to get a copy.

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 can be minimized by what HDF calls "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 the best performance a program can call the HDF library routine SDgetchunkinfo() in C, or sfgichnk() in Fortran, to determine the chunking parameters. Then it can 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. This is 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.

Q: What if I want a large amount of MODIS and/or VIIRS data or a time series of products?

A: Please contact us 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 Web archive.

MODAPS Web Services (MWS) can be used to submit search and order requests through an Application Programming Interface (API). Our services understand both SOAP method calls and REST (CGI) requests. To make SOAP calls, you need to write a script that uses a SOAP library and calls the methods in the API. There are methods for searching for files by product, geolocation, and sample time, and there are methods for ordering files to be staged on our HTTP server, where you can download them (the search queries also return URLs for the files themselves, that you can use for immediate download).

We have equivalent REST methods that you can use with a tool like WGET to do the same thing (or use a REST or CGI library and write your own). The idea is that for placing large complex orders (for example, lots of different sites of interest), it is easier for you to write a script than to place all the orders manually via LAADS Web.

Q: What is the format of the Distribution Notice (DN) files generated by LAADS?

A: LAADS can generate Distribution Notice (DN) files for subscriptions or LAADS Web orders using the FTP Push delivery method. The DN files use the standard SIPS Distribution Notice format that is defined in "Interface Control Document between the EOSDIS Core System (ECS) and the Science Investigator-Led Processing Systems (SIPS), Volume 0 Interface Mechanisms". A standard DN file will list the destination FTP host and directory information, the order ID (REQUESTID) in LAADS, the time at which the order was pushed, and file name, file size, cksum type and cksum value for each product file in the order.

++++++++++

ORDERID: NONE
REQUESTID: 12588352
USERSTRING:
FINISHED: 06/13/2006 09:15:32

MEDIATYPE: FtpPush
FTPHOST: mtvs2
FTPDIR: /MYDATML2
MEDIA 1 of 1
MEDIAID:

        GRANULE:
        ESDT: MYDATML2

                FILENAME: MYDATML2.A2006158.2315.005.2006164085603.hdf
                FILESIZE: 3295302
                FILECKSUMTYPE: CKSUM
                FILECKSUMVALUE: 2538623922

        GRANULE:
        ESDT: MYDATML2

                FILENAME: MYDATML2.A2006158.1825.005.2006164085619.hdf
                FILESIZE: 2278554
                FILECKSUMTYPE: CKSUM
                FILECKSUMVALUE: 3462502739

Q: What tools do you recommend to crack open these HDF-EOS files?

A: Three tools are particularly indispensable for working with MODIS products. The first is "ncdump", available from the HDF4 Tools page. The command "ncdump -h file_name" displays all file metadata. Otherwise, "ncdump" will show you more than you want to see. The second tool is HDFView with the HDFEOS Plugin. This tool was written collaboratively by the HDFGroup (the same folks who produce the HDF library) and the HDFEOS Group (who produce the HDFEOS extension library) and can be used to view both data and metadata inside HDF files. The third recommended tool is HDFLook, which was developed by the Lille team with EOS project support. HDFLook is an interactive visualization tool that can also be run on the (Unix) command line to produce images or reformat HDF file data. (If you have to have binary files, HDFLook can, for example, create binary files containing single-parameter arrays.)

Q: Where do I find more useful tools?

A: A good starting point for Level 1 and Atmosphere products is the list of Atmosphere Discipline Recommended Tools. The GES-DISC also maintains a list of Data Tools. These lists contain a range of tools of varying complexity. Some require licensed software such as IDL.

Q: Does the software that produces Level 1, Atmosphere, and Land products available to the public?

A: Yes, the software is available to the public however you must register and agree to the standard NASA Software Usage Agreement. In order to register for MODIS Software Registration, you can go to the following URL:

/:9500

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. After your fax is received, you will be added to the access group, and will receive an email notification with the URL for the software downloads.

Q: How long after an acquisition of MODIS data are the Level 1B data are available in your archive?

A: The MODIS Science data is available from LAADS at https://ladsweb.modaps.eosdis.nasa.gov/archive.

Terra data are available about 6 - 8 hours and Aqua data are available after about 30 - 32 hours after acquisition. The Terra and Aqua MODIS Near Real Time (NRT) products are available in 3 hours. (NOTE: The NRT products are not of same accuracy as normal MODIS products.). These products are available from LANCE-MODIS. However, you need to register with the EOSDIS User Registration System to access the main LANCE-MODIS HTTP server nrt1.modaps.eosdis.nasa.gov or the backup LANCE-MODIS HTTP server nrt2.modaps.eosdis.nasa.gov.

Q: What are different data processing tools and services available through LAADS system?

A: The LAADS system provides a number of tools and services that perform the following types of transformations to MODIS data sets:

• Parameter Subset
• Geographic Subset
• Mosaic
• Reproject/resample/bin data
• Filter
• Reformat

To order any of these operations, you need to perform the following 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 want and hit "Add files to shopping cart"
3. Select "View your shopping cart"
4. Select "Post process and order data", then hit "Order"
5. Select all Post-processing option(s) you will need, then hit "Order"
6. Select additional constraints for each option. For example, specify your area of interest if you selected "Geographic subset"; specify the desired data sets if you selected "Subset by parameter", etc., then hit "Order"

The LAADS Web system will process your files with the selected operations, and will email you download instructions when it has completed your order.

Q: What are the day/night flags?

A: The Day/Night flag for each scan indicates the mode MODIS was collecting data for that scan. Night mode is when nadir aperture is viewing dark side of the earth. In Day mode, MODIS data are acquired for all 36 bands whereas in Night mode data are acquired only in thermal emmisive bands (bands 20-36). The MODIS specification page provide a list of all MODIS bands.

Q: How do I download my data once I receive my notification from LAADS?

A: If you are using web browser, take the following steps:

1. go to the url: https://ladsweb.modaps.eosdis.nasa.gov/archive/orders
2. then click your order_id provided in your notification
3. then click on the file name you want to download

Q: How do I save my data order parameters that I have entered to reuse at a later date?

A: Please do the following steps to save and retrieve parameters:

1. Click Save, another screen will pop up
2. Click on Save File another screen will pop up
3. Give it a file name and click Save (Only in IE but in Firefox or Chrome filename option is not available)
4. To retrieve saved parameters, go back to LAADS Web and click Browse another screen will pop up
5. Select your saved file and click Open
6. Once the file name is in the Browse box, click Search
7. This should bring up the parameters you have saved.

Q: How long are the data available on the server after I have received my order notification?

A: As indicated in your order notification email, data are available in your order directory for 5 days.

Q: Do I need to notify LAADS after downloading my ordered data?

A: No, you don't need to notify LAADS because data are automatically deleted after 5 days.

Q: How long are science data archived and are available to users?

A: There are 2 types of data archives on LAADS, a permanent archive that is available forever and a rolling archive. The rolling archive, such as data pool, saves products on rolling basis for at least 30 days (may be longer depending on the disk usage) before data go offline whereas, NPP VIIRS data stay in rolling archive for a minimum of 60 days. In most cases, offline data can be ordered through our Processing on Demand (POD) system on LAADS. However, if a data pool data product has already expired it cannot be produced again and cannot be ordered even through POD system.

Q: What products are available in the Near Real Time System?

A: You can visit our web site at http://lance-modis.eosdis.nasa.gov for more information on NRT products and their availability.

Q: Why is my IP address being blocked?

A: We distribute data to a large number (thousands) of users around the global every day. To keep resources available for all of our users, LAADS has implemented a police of allowing 15 connections from one IP address. When there are more than allowed 15 connections from one IP address, the system automatically blocks the access from that IP address. If your IP addressed is blocked, you need to contact MODAPS user support office at modapuso@lists.nasa.gov.com to resolve the issue. You should make sure that you keep number of connections from your system to less than 10 at all times to avoid similar problem in the future.

Q: How do I resubmit my order once it has expired?

A: When your order expires, you can go to our order tracking page at https://ladsweb.modaps.eosdis.nasa.gov/data/track.html and enter your e-mail address. This will give you a list of your orders, when you click on the order Id, details of your order will come up and there will be a button to resubmit your order. But, we strongly encourage you to download your order within the allocated time (5 days) as it adds load on the resources to regenerate orders.

Q: Where can I find information on direct access?

A: Information on LAADS data access options, including direct access methods, may be found on the MODAPS User Services website

Q: Why do I lose Latitude and Longitude data when I transform MODIS Level 1 and 2 products to UTM coordinates using post processing tools available through your website?

A: After gridding MODIS Level 1B or Level2 (swath products), the Latitude and Longitude arrays lose their meaning, and so are not included in the output. This is because in swath products, the pixels are not uniformly sized, nor are they uniformly spaced (refer to the document, titled "MODIS Level 1A Earth Location: Algorithm Theoretical Basis Document Version 3.0" for a more detailed explanation -- see figure 2-12). When these pixels are gridded, they may span multiple grid cells, and when reprojecting and resampling, may not exactly line up with the new grid cells -- thus the original latitude and longitude of the swath pixel lose their meaning in the grid. You can still calculate the longitude and latitude of any pixel in the grid, since the grid specifies the lon/lat of the upper right hand corner and the lower left hand corner, and the number of pixels across and vertically.