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 FTP, web order, or with web services. The FTP portal is via anonymous
FTP to machine "ladsweb.nascom.nasa.gov". 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 FTP 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
LAADS Web Data Availability Chart.
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
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
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
page for a complete listing of MODIS spectral bands. The
MODIS Characterization Support Team
maintains a number of good resources, including information about
For VIIRS products visit the
description page. For a complete listing of spectral bands review the
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.
Moderate Resolution Imaging Spectroradiometer (MODIS),
instrument flies on two polar orbiting satellites. The first,
has provided Land, Atmosphere and Ocean data products since late
February 2000. The second,
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
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,
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,
Aqua Project Page.
Another resource is
MODIS News and Current Status.
provides details regarding
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.
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,
QA Product Information for Collection 5.
Q: Where can I find details about the MODIS retrieval algorithms for
MODIS Atmosphere Discipline Site
is the place to start. Two documents of particular interest are the
Known Problems of Atmosphere Products
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?
if you would like to arrange a data subscription or have special
requirements. Alternately, you may use a web client like
to download files from the
LAADS Web FTP Site.
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 at our ftp site,
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
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.
FINISHED: 06/13/2006 09:15:32
MEDIA 1 of 1
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
page. The command "ncdump -h file_name" displays all file metadata.
Otherwise, "ncdump" will show you more than you want to see. The second
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
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
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
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:
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 Fax # 301-557-6622. 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 on your FTP site?
A: The MODIS Science data is available from
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
However, you need to register with the EOSDIS
User Registration System to
access the main LANCE-MODIS FTP server
or the backup LANCE-MODIS FTP server
Q: What are different data processing tools and services available through
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
- Reproject/resample/bin data
To order any of these operations, you need to perform the following steps:
On the LAADS Web
page, select the data, time period, and area of interest, then hit
- Select the granules you want and hit "Add files to shopping cart"
- Select "View your shopping cart"
- Select "Post process and order data", then hit "Order"
- Select all Post-processing option(s) you will need, then hit "Order"
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:
go to the url:
- click on Data
- then click Ftp Site
- then click Browse LAADS FTP site
- Then click orders
- then click your order_id provided in your notification
- then click on the file name
If you are using command line (Unix/Linux) system for accessing ftp, take
the following steps.
- cd to the directory on your computer you want to put your data
- ftp ladsweb.nascom.nasa.gov
- sign in as anonymous
- password: your email address
- type bin
- type prompt
type (one of PASV, passive, PASSIVE, or pasv to switch to passive mode)
- cd orders/Order_id is provided in your notification email
- mget <files you ordered>
Q: I am unable to download my data because I cannot use PORT?
A: Once you ftp to our site, type either passive, PASV or pass, one of the
three should work for you and allow you to download your data. This may be
necessary because some firewalls only permit FTP in passive mode.
Q: How do I save my data order parameters that I have entered to reuse at a
A: Please do the following steps to save and retrieve parameters:
- Click Save, another screen will pop up
- Click on Save File another screen will pop up
Give it a file name and click Save (Only in IE but in Firefox or Chrome
filename option is not available)
To retrieve saved parameters, go back to LAADS Web and click Browse
another screen will pop up
- Select your saved file and click Open
- Once the file name is in the Browse box, click Search
- This should bring up the parameters you have saved.
Q: How long are the data available on the FTP 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
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
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
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
Q: How do I acknowledge the use of MODAPS Data?
A: The distribution of the MODAPS data sets are funded by NASA's Earth-Sun
System Division (ESSD). The data are not copyrighted; however, in the event
that you publish data or results using these data, we request that you
include the following acknowledgment:
"The data used in this study were acquired as part of the NASA's Earth-Sun
System Division and archived and distributed by the MODIS Adaptive
Processing System (MODAPS)."
We would appreciate receiving a copy of your publication, which can be