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Help for Data Users

By following the links at the left you will come to a PDS archive containing your data of interest. All PDS archives have a similar structure. Understanding this structure will help you get the most from the data.

Data archived from NASA missions that were confirmed for flight after November 1, 2011, are required to use the PDS4 standard (see About PDS4). Missions that got their start before that date continue to use the PDS3 standard. Individual data providers are also required to use PDS4 now. Eventually the PDS3 archives will be "migrated" to PDS4 (more about that below), but for now you'll find both PDS3 and PDS4 archives here at the Geosciences Node.

What You'll Find in a PDS4 Archive Bundle
(or skip down the page to read about PDS3)

Bundles, collections, products, and LIDs

These are the building blocks of PDS4 archives.

Data products, document products, and several other kinds of products are the basic components of an archive. A typical product consists of one file with the contents of an observation, document, etc., and another file with its metadata, its PDS4 label. Both files have the same base name, but the PDS4 label is written in XML and has the file name extension ".xml".

A collection is a group of related products, such as a group of documents or a group of calibrated data products from a particular instrument.

A bundle is a group of related collections, such as the raw data products, calibrated data products, and documents from a particular instrument.

Each product has a Logical Identifier (LID) that is guaranteed to be unique PDS-wide. Collections and bundles are actually special kinds of products, and they also have unique LIDs.

Where to look first

In the top-level directory you'll find the bundle label, a file named "bundle_<something>.xml". This label contains overall information about the bundle, references to publications, and a list of the collections that make up the bundle.

Most bundles have a file named "readme.txt" in the top-level directory that explains the contents of the bundle.

Most bundles have a document collection, usually in a subdirectory named "document". For mission-acquired data, look for a document with "SIS" (Software Interface Specification) in the name; this will be the primary documentation.

Where to find the data products

In the top-level directory you'll find one or more subdirectories whose names start with "data", usually one for each data collection in the bundle. (Not always; it's possible to have more than one collection in the same subdirectory.) They may be further subdivided if there are many products in the collection.

Where is the list of data products? (i.e., what happened to the old PDS3 index table?)

In each collection you'll see a  file named "collection_<something>_inventory.csv" and its label, "collection_<something>.xml". The inventory file is a comma-separated-value text file that lists the LIDs of all the products in the collection. Each LID is preceded by a "P" or an "S" indicating whether the product is a primary or a secondary member of the collection.  Primary members reside in the collection directory or one of its subdirectories. Secondary members reside in some other PDS collection where they are primary members, and they may or may not be duplicated in this collection.

How to read PDS4 labels

PDS4 labels are written using XML (eXtended Markup Language). They are text files that are both human- and machine-readable. Labels are best viewed in a text editor that can display XML with formatting that makes them easier to read. (Notepad++ and UltraEdit are two such editors.) The labels may also be used with software that can manipulate XML documents.

How to read PDS4 data

Tools are available in the PDS Tool Registry for working with PDS4 archives. The registry includes the PDS4 Viewer, a program that displays text tables, binary tables, images and arrays by reading their PDS4 labels.  The Python library from which the PDS4 Viewer is built is also available.

 

What You'll Find on a PDS3 Archive Volume

Under the PDS3 standard, data products are grouped into data sets along with related documents and other material. Data sets are stored on volumes, a term that goes back to the days before the World-Wide Web when users could request data on physical media such as CDs. Typically one data set is stored on one volume, but there may be multiple data sets on a volume. A large data set accumulated over time may be stored on multiple volumes.

General Hints

All files with extensions .txt, .lbl, .cat, .tab, and .asc are ASCII text files. Files with other extensions are binary files that may not be viewable in your web browser.

Each subdirectory contains a file with a name ending in info.txt that describes the contents of that directory.

Top-level directory

  • aareadme.txt - An introduction to the data volume. Read this first.

  • errata.txt - An optional file containing notes and errata about the volume.

  • voldesc.cat - A short text file that serves as a PDS label for the volume.

Catalog directory

Don't overlook this valuable source of information about the data set. These files are copies of the PDS Catalog descriptions of the data set, the instrument that collected the data, the spacecraft, the mission, personnel involved in making the archive, and a list of references to published literature.

Document directory

This directory contains documentation such as the data product Software Interface Specification that most missions are required to provide.

Index directory

The file index.tab (or a file with a similar name) lists every data product on the archive volume, its directory and file name, and other information that varies depending on the data set. Its contents are described by the associated PDS label index.lbl. If the data set is spread over more than one volume, there will also be a cumulative index table (cumindex.tab) that lists the data products on all volumes created to date. See the last volume in the data set for the complete cumulative index.

Data directories

The data products themselves are under a directory that may be named "data" or may have a data-set-specific name. There may be more than one data directory. For instance, on some archive volumes the data are organized by year under the directories 2002, 2003, etc.

Every data product is described by a PDS label. The label may either be embedded (attached) at the beginning of the data file, or in a separate file (detached) with the same name, extension .lbl. PDS labels are ASCII text in a keyword = value format that can be read both by humans and by software. In some cases a data product is stored in multiple files which are described by a single label. The thing to remember is when you download a data file, download the label too.

Other directories

Other optional directories include Software, Calib (calibration), Extras, Geometry, Browse, and others. Read the info.txt fi file in each directory for more information.

Migration of archives from PDS3 to PDS4

The Geosciences Node has a plan for the migration of most of its PDS3 archives to PDS4 over the next few years. (The plan excludes currently active missions that are still delivering PDS3 data. That data will be migrated soon after each mission is complete.) The approach is to leave the existing data products and PDS3 labels untouched, and to add PDS4 labels in the same data directories, so that each product has both a PDS3 and a PDS4 label. In cases where the data product is not in a PDS4-compliant format (the rules are stricter in PDS4), then the data product itself may be converted to a compliant format, but both old and new versions will remain in the archive.

For examples of archives that have been migrated from PDS3 to PDS4, see the MESSENGER archives.


If you have questions about Geosciences Node data sets

One place to look for help is the Frequently Asked Questions page.

If you don't see your question there, check the Geosciences Node Forums to see if it's been discussed there. There are forums for announcements, data users, data providers, Analyst's Notebook users, and Orbital Data Explorer users. You may post your question in one of the forums (after signing up for a free account).

If you still need help, you may email your question to geosci@wunder.wustl.edu

 

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