To download AIRS Near-Real Time (NRT) data, users must use authenticated login. The latter consists of userid and password, which can be acquired by visiting http://lance.nasa.gov/, and then following through the “Sign up” at the bottom of the page. Users will then fill up requested information in the secure web form.
After receiving the userid and password, the most convenient thing for Users is to store them in the user’s home directory in a file named .netrc (assuming you work in UNIX environment). Users should have a lines in this file, like:
machine discnrt1.gesdisc.eosdis.nasa.gov login <userid> password <password>
machine discnrt2.gesdisc.eosdis.nasa.gov login <userid> password <password>
where they will substitute <userid> and <password> for the ones they acquired from the LANCE site, above. Having the .netrc file will greatly help in automating all methods of downloading AIRS NRT data: ftp or http. E.g. Users will not need to type their authentication every time they login using ftp or wget from shell command line. But Users will still need to enter them while using a browser to establish connection to AIRS NRT archive.
2. OPeNDAP Service
The OPeNDAP server is technically a http service allowing for simple subsetting and format conversion services. The server receives requests from the user in the form of URLs. These can be typed in the browser URL box, or could be included as input to the command line utility wget, examples provided below.
In the examples below, substitute the file paths with current ones!
Since the NRT archive is rolling, data paths older than 8 days are removed.
Also, since new algorithms are implemented every 2-3 years, the data collection version will also change.
2.1. Drilldown and download using a browser. ASCII download.
The base OPeNDAP URL for all NRT data from the Goddard Earth Sciences Data and Information Services Center (GES DISC) is:
This URL will ask for authentication, and after successful login Users can drill down using a browser, and see the current data paths. For instance, on Jun 14, 2011, a User using some browser drilled down for NRT AIRS Level 1B IR Radiances, AIRIBRAD_NRT, and arrived at the following path:
Further drilling down discovers a list of HDF data files. Clicking on any one of them opens a web form where Users can check the boxes to select desired variables, and row-, column-, and band-subset, as appropriate. Let’s assume variable “radiances” is selected, and nothing else is modified. User can then click on one of the buttons on top of the form:
and initiate download in one of the implied formats. Users may notice that “Get ASCII” produces long text page, with a url:
It is important to note the format of this URL, which is actually a request to the OPeNDAP server, and which can be saved and re-used as a pattern. We see the familiar path, and the HDF data file name, suffixed with
Apparently, this particular URL requests OPeNDAP to return data in “ascii” format, from the variable “radiances”, and commands the [start:stride:end] to subset along each dimension of “radiances”. In this example, full size is requested, in which case the [start:stride:end] could have been omitted.
2.2. Command line download
The above ascii-request URL can be re-used as a pattern for a different download scenario. Let's assume we now want to download the same variable in binary format, from shell command line. We also don't want all IR bands, but only a group of water vapor bands around 1250 cm-1 (see table in Appendix A.). Thus we must only change the suffix from before to:
Note, the format changes to “dods” (means binary), and the bands subset changes to [1263:1:1368]. Thus, the following one-line command line is an example of download of subset of data in binary format. The command contains familiar URL request to OPeNDAP, but this time it is sent from the shell using wget:
wget “http://discnrt2.gesdisc.eosdis.nasa.gov/opendap/Aqua_AIRS_NRT/AIRIBRAD_NRT.005/2011/165/AIRS.2011.06.14.019.L1B.AIRS_Rad.v220.127.116.11.R11164235819.hdf.dods?radiances[0:1:134][0:1:89][1263:1:1368]” –O AIRIBRAD_NRT.subset.2011.06.14.019.dat
Remember, this is a one-line command. The option “-O AIRIBRAD_NRT.subset.2011.06.14.019.dat” says that we want the binary subset to be output to the indicated file name. The example assumes user’s authentication is stored in the .netrc file as indicated in Section 1.
More sophisticated users can automate the wget downloads by exploiting the so-called THREDDS catalog which is a simple XML file cataloging the current content of the particular directory. For that particular day the catalog was in:
This XML file can be parsed, the file names or full paths extracted from there, and script can be build that can be executed by shell, PERL, Python, or other means.
APPENDIX A. Common application of AIRS bands, by groups.