Without getting into the details, the GSAC helps you locate GPS data files which are archived at different GPS Data Archive Centers from a single user interface.

There are multiple clients available to assist you:

1. The GSAC Wizard is a web-based client which walks you through the steps of locating data.

2. The GSAC command-line client helps you locate and download GPS data files onto your local PC or workstation. You must download the correct client for your platform. A big advantage of the command-line client is that it will actually download the data for you.

3. The GSAC Retailer Service uses a macro-language to pass commands to a server using the HTTP URL. The command-line client uses the Retailer Service to communicate with the Retailer Server. One of the best uses of the GSAC Retailer Service in its native state is to use an HTTP/FTP client such as WGET in a CRONTAB script to download data files. The GSAC Wizard will build a URL for you.

4. SOPAC has modified its Map Interface to work with the GSAC. Using the SOPAC Map Interface, you can locate GPS data files using a spatial context.

DEFINITIONS

The GPS Seamless Archive was created to help people find and access GPS-related data. Rather than having to remember numerous FTP sites around the globe and having to FTP into each of these sites to download data, the GSAC is designed to "know" where the data is and GSAC client software can fetch the data for you.

There are three definitions you should know: Data Provider, Data Wholesaler, and Data Retailer. A Data Provider collects or generates data (probably from a GPS receiver) and then supplies the data to a Data Wholesaler. There are many data providers, and many do not operate archive facilities to distribute data. A Data Wholesaler (archive facility) combines data and meta-data, from one or more data providers. A Data Retailer collects information from one or more wholesalers and runs a service for clients to access the information. The same organization can operate as all three entities.

EXAMPLE

For example, let's say there is a site called GPS1 that generates GPS data. GPS1, in this example, does not belong to a particular "network" and has an arrangement to upload the data to a Wholesaler named ACMEGPS. ACMEGPS then makes the information available to the community through an FTP server. This is where the story usually ends...

...in the case of the GSAC, the story continues. The Wholesaler creates special "GSAC" flat files on the FTP site for Retailers to collect on a regular basis. These special files provide Retailers the means to figure out what data the Wholesaler has in its archive and the Retailer stores this information in a relational database. The Retailer also runs a web-based "service" that allows users to remotely access the database from a client.

Now let's say a someone, Nina in this case, wants to download the data generated yesterday from site called GPS1. Nina uses her gsac-client software to send a query to a Retailer. The Retailer gets the request and looks through the database for information about site GPS1. The Retailer "service" then returns the results to Nina's gsac-client software. The gsac-client software then uses the results to collect the data. Within short order, the files appear on her computer, ready to use.

SOPAC has been funded by UNAVCO to build software to support wholesalers, retailers, and clients. Currently, a command-line version of the GSAC-client is available.

For information about becoming a Retailer or Wholesaler, please see the Participation section of the web site.

Please see the UNAVCO GSAC web site for official GSAC information, including definitions and technical background. You should be familar with this information if you want to become a wholesaler or retailer.