Corrected Gamma Spectra (CGS)

Raw spectra cannot be compared or summed because differences in the spacecraft's temperature, voltages, or other factors may cause variations in the energy scale from spectra to spectra. Using gain, offset, and linearity engineering information from the Odyssey spacecraft and associated correcting algorithms, the raw gamma spectra are corrected to the same energy scale, which allows direct comparisons of individual spectra and the summing of multiple spectra. The Mars Odyssey Gamma Ray Spectrometer Corrected Gamma Spectra (CGS) data set is a time series collection of these corrected gamma spectra measurements from the Gamma Sensor Head.

Corrected Gamma Spectra data are composed of corrected gamma spectra and the associated timing, spatial and engineering information. The CGS consists of the cumulative counts of gamma rays at the detector over one collection interval binned into 16384 channels based on energy. The collection interval is approximately 19.7 seconds, but may vary over the course of mapping. The counts in Channel 0 are all the counts that would have been shifted to 0 or less than 0 by the spectrum shifting algorithm. The counts in Channel 16383 are all the counts with nergies greater than 10MeV, and counts in Channel 16382 are all the counts that belong in either 16382 or would have been shifted to 16383. The timing and spatial data provided with the spectra includes spacecraft clock values and spacecraft geometry data. The sc_ev_time, utc time and spatial fields are all recorded at the center of the collection interval.

The CGS is intended to be the first intermediate data product available for the gamma portion of the GRS. These data should be useful to those scientists who are experienced in gamma spectroscopy.

Data Collection, Ingest, and Processing

The three Gamma-Ray Spectrometer instruments collects a new spectrum (pixel) approximately every 20 seconds, 360 times per orbit. Approximately 4200 spectra are received every day. The data (both science and engineering) are downloaded from the spacecraft by the Jet Propulsion Laboratory (JPL) into the Telemetry Data System (TDS). The TDS sends data to a process that translates data packets and examines instrument health via messages. Data are output to a spooler that passes it to the University of Arizona (UA) database ingest process. The ingest process inputs raw data into the UA database. Gamma data are processed through a number of programs to yield corrected gamma spectra.

Data Reprocessing

During the mission, data may be periodically reprocessed. For more information on reprocessing of data, see the:
Both of the above documents are in the section of the GRS Data Interface pages.

How to Use the GRS Data Interface, Step by Step

For new users, this web page provides help in a step-by-step way, and will walk you through a typical session using the GRS Data Interface to obtain Gamma-Ray Spectrometer data from Mars Odyssey.

When you first access the GRS data node, you'll notice that the web page is divided into two vertical sections.

The area on the left-hand side of the GRS Data Interface, as shown at left, is a selection "tree." Entries titled "Introduction," "Calibrated Products," "Uncalibrated products," etc. are listed. Note that the tree contains folders with a symbol next to it. The symbol indicates that there are items under that category that are not displayed. Clicking on a symbol changes it to a symbol and all the items under that category will be displayed.

The area on the right-hand side of the GRS Data Interface changes, depending on which part of the decision tree is currently active. It is used to enter dates for database queries, provide informaton on downloaded file names, or to display information.

Before You Download...

Before you execute a query using the GRS Data Interface, please do the following to prepare:

  1. Decide which type of data you wish to download. Remember that Corrected Data (also called "Calibrated Data") is an intermediate product that has been processed. Uncalibrated data, on the other hand, is raw data that has had timing and spatial data added. For more detailed information on the data products, see the data product links on the left menu bar.


  2. Consider your connection speed to the internet and your computer's system resources such as disk space and RAM when deciding how many days of data to download per file. It is best to choose time spans on the scale of a week or several weeks at most for Corrected Gamma spectra, for example, due to the large output file sizes. On any type of data it is best to choose one month or less time spans to maximize the download time and prevent overloading your system's resources.


  3. Decide on the beginning and ending dates and times for the data you wish to download. Keep in mind the flight phases of the spacecraft, and when the desired data was created in the spacecraft's timeline.


  4. Decide on the geographical area of interest, delimited by Martian latitude and longitude, if applicable.

Once you have performed the above four steps, you are ready to begin downloading data.

Mouse clicking instructions are included for the PC-style, two-button mouse. (Left-click) means to click on an icon or text with the left mouse button. Macintosh users have a one-button mouse typically, and should click once with their mouse when (Left-click) is in the instructions. UNIX users with three-button mouses should also use their left mouse button to (Left-click).


Using the GRS Data Node to Query The GRS PDS Database

  1. Choose the category of data you wish to download, either Calibrated or Uncalibrated: (Left-click) once on the data category to expand the selections. For example, if you wished to choose a Calibrated Product, you would (Left-click) once on . It would then show all the calibrated products available and appear as:




  2. After the selections are displayed and open, (Left-click) once on the data type desired. For example, if you wished to download Corrected Gamma Spectra data, you would (Left-click) once on . The type of data you have selected will then be highlighted with the darker grey-purple:

    In the right-hand side of the GRS Data Node window, a description of the chosen data type will appear, including descriptions of all field (column) types in that particular data, the units of each field (column), and other information.


  3. If you would like to print the column-field information about this type of data, (Right-Click) in the Column Descriptions area, and use your web browser's ability to "Open this Frame in a New Window." You may then print from that new window. This will give you a hard-copy description of all of the column descriptions, their units of measure, etc. Return to the main GRS Data Node window before proceeding.


  4. To enter the dates or other database query criteria, click on the button to continue.

    Note that data entry areas appear that allow you to constrain your database query by time, or constrain it by spatial region (if available). Also, an option to exclude data marked bad is offered.


  5. Enter your latitude and longitude constraints. The Data Interface allows you to Constrain by Spatial Region. Note the following area for changing the latitude and longitude:



    If you do not wish to select the entire planet, proceed to the next step. If you wish to select the entire planet, go to step 9.

  6. Click on the link to activate the Latitude-Longitude Chooser. The following new window will appear:




  7. Now you may enter the latitude and longitude in one of two ways:

    1. By pointing and clicking. You will draw an imaginary box, from the upper-left corner of the region you wish to specify, to the lower-right corner. This is identical to drawing a box in a drawing program on your computer.

      Place the mouse cursor over the MOLA image of Mars. Notice that the mouse cursor turns into a crosshair ().

      Locate the cursor on the upper-left corner of the region box you wish to draw.

      Click and hold the (Left-Mouse) button. Pull down and to the right, until you reach the lower-right corner of your desired rectangular area. The corresponding coordinates will appear in the latitude and longitude data entry area below the MOLA map.

    2. By entering the latitude and longitude manually in the data entry area below the map:

      Click in the area. Type in the starting latitude of your region of interest, from +90 (Martian North Pole) to -90 (Martian south pole).

      Click on the area. Type in the ending latitude of your region of interest.

      Click on the area. Type in the starting longitude in degrees East around Mars, from 0 degrees to 360 degrees.

      Click on the area. Type in the ending longitude of your region of interest.

    In either case, note that you may not specify a region that spans the 360-degree longitude mark. As an example, you may not specify a region from 350 degrees to 20 degrees in longitude. This area of interest would have to be split into two regions, one from 350 degrees to 360 degrees, and the other from 0 degrees to 20 degrees.

  8. Click on the button after the correct longitude and latitude of your region of interest are entered.

    Continue entering constraints and processing your query.

  9. Enter your starting constraints for the date and time. For example, if you wished to obtain Corrected Gamma Spectra starting on May 2, 2002 at 09:00:18 GMT you would (Left-click) on the year, month, date, and time fields in the UTC Stop Time and choose the correct values. Submenus will appear when you Left-click on a column value. For example, if you (Left-Click) on the Year, the following submenu will appear:



    Simply pull down the mouse to "2002" or the value that you have chosen. Then (Left-click) again on the desired value. (Note: Your browser and mouse may work slightly differently, but the submenus will still appear.)

  10. Now you will enter the end date and time (or second set of spatial constraints). If you wish to collect 10 days of data from the start time entered above, you may enter "10" in the range box: and press (Enter). The query tool will automatically enter the correct end date.

    OR

    You may enter the end date and time under UTC Stop Time by (Left-Clicking) as above and choosing the correct values. An example end time would be May 4, 2002, at 00:00:00 GMT. The image below shows how your query tool would look after entering these example values:




  11. Decide whether to exclude bad data, or not. If you leave the box checked as shown:



    Data marked as bad will NOT be included in your data set. If you wish to include bad data, (Left-click) once on the box to un-check it. It is almost always recommended that bad data be excluded.

    For an explanation of bad codes, please see the bad_code.txt document.


  12. (Left-click) on the button to start compiling your query. Then an output like the following will appear:



    If no data matching your selection criteria is found, you will get a message that says: In those cases, you need to go back and modify your query.

    Note that in the example above the data is compiled but no data files are downloaded yet. You still may go back and change your criteria. If you wished to modify this query, pressing the button will allow you to go back and redo it.

    All files are downloaded in zip (PC compressed) file format. The above gives you the number of records that matched your query, and the estimated file size of your data file once it has been unzipped.

  13. To download the file, click on the link to it (on the estimated file size) under "Download." In our example above, you would left-click on .

    A status window, generated by your web browser, will then appear and inform you of the progress of the download. Below is an example window from Netscape, version 7:



  14. Click on the "Save to File" in your browser's download window. Then a window similar to the following will appear, prompting you to choose a folder to download the file to:

    Make certain you have adequate disk space on your computer before you save the file.

  15. Click on the button (or equivalent, depending on your browser) to save the file. Depending on the size of the file to be downloaded, this can take a few minutes or many minutes, depending on your internet connection speed, your computer's speed, and the size of the file.

  16. Use the Data Viewer software provided by the GRS PDS Data Node to quickly peruse the data, or write custom programs in the language of your choice to read and utilize the data. A library of source code that will parse the uncalibrated (EDR) and calibrated (CGS) data product files is included in the folder. This library allows a programmer to build applications that display or manipulate the GRS data. This source is written in the Java language, and requires version 1.4 of the Java Runtime Environment (JRE) or Java Software Development Kit (SDK). For those who wish to use other languages, consult the Labels files to create structures to read in the files for each type of data.