The Control Segment (cont.)
Global Positioning System

For simplicity, a ground tracking station consists of a dual frequency GPS receiver, a GPS antenna, a PC with sufficient disk space, an uninterrupted power supply, a high speed modem, a network connection, and communication software packages. Some stations are equipped with hydrogen maser frequency standards and high precision barometers. This type of setup provides a remote operation monitoring capability from monitoring stations using PCs and modems. The common monitoring features include examining receiver tracking status, modifying data download procedure, changing tracking configuration, performing troubleshooting as well as rebooting the on-site PC or receiver. Barometer measurements are continuously recorded onto the PC's hard disk every 10 minutes. The real-time, raw orbit and tracking data is collected and stored inside the GPS receiver and then is downloaded onto the PC hard disk after UTC mid-night via a direct connected RS232 cable :

This data is then transferred to the operational data centers (ODCs) via the Internet. After all data transfers are completed the ODC starts the following tasks of data decompression, file format conversion (binary to RINEX), quality control, UNIX compression and finally distribution. The formatted data is then distributed to the Regional or Global Data Centers. To reduce electronic network traffic, the Regional Data Centers are used to collect data from several ODCs before transmitting them to the Global Data Centers. Data not used for global analysis is archived and available for online access at the Regional Data Centers. The Global Data Centers archive and provide online access to tracking data and data products. The online data is employed by the Analysis Centers to create a range of products which are then transmitted to the Global Data Centers for public use. These products include : high accuracy GPS satellite ephemerides, earth rotation parameters, coordinates and velocities of the IGS tracking stations, GPS satellite and tracking station clock information, ionospheric information and tropospheric information. The accuracies of these products are sufficient to support current scientific objectives including : realization of global accessibility to and the improvement of the International Terrrestrial Reference Frame (ITRF), monitoring deformations of the solid earth, monitoring earth rotation, monitoring variations in the liquid earth (sea level, ice-sheets, etc.), scientific satellite orbit determinations, ionosphere monitoring, climatological research and eventually weather prediction. This information, accessible on the Internet, provides a gateway to all the global data and data product holdings, as well as other valuable information.