The current GPS constellation includes 24 satellites, each traveling in a 12-hour, circular orbit, 20,200 kilometers above the Earth. The satellites are positioned so that six are observable nearly 100 percent of the time from any point on Earth. The current GLONASS constellation includes less than 20 satellites, each traveling in a circular orbit, 19,140 kilometers above the Earth. The satellites are positioned so that four are observable nearly 100 percent of the time from any point on Earth.
	GNSS receivers detect, decode, and process signals from the GNSS satellites (e.g., currently GPS and GLONASS and, in the future, Galileo). The satellites transmit the ranging codes on two radio-frequency carriers, allowing the locations of GNSS receivers to be determined with varying degrees of accuracy, depending on the receiver and post-processing of the data.
	The current global IGS network consists of several hundred permanent GNSS (GPS and GPS+GLONASS) receivers. High-accuracy measurements of the change in receiver locations over time allow researchers to study the motions of tectonic plates, displacements associated with earthquakes, and Earth orientation. This map of the permanent IGS network is also available in PDF form. A map (PDF) of IGS sites providing hourly data is available for viewing. A map (PDF) of IGS sites providing high-rate data is available for viewing.

The International GPS Service (IGS) has developed a global system of tracking stations, data centers, and analysis centers to put high-quality GPS (and GPS+GLONASS) data on-line within one day and data products on line with two to ten days of observations. In early 1992, the CDDIS was selected to serve as a global archive center for the IGS, by supporting the daily GPS (and GPS+GLONASS) data retrieved from identified core observatories. The purpose of this international service is to provide GPS data products and highly accurate ephemerides to the global science community to further understanding in geophysical research. The IGS has demonstrated the near real-time capability of the global GPS community to retrieve data and produce products (e.g., satellite ephemerides and Earth rotation parameters) that are of use to a broader community. Several regional data collection centers forward their data holdings in compressed RINEX format to the CDDIS on a daily basis. These data are summarized, inventoried, and archived on-line for access by an international community of users. At present time, an average of nearly 300 sites are archived in this fashion each day. Data analysis centers located worldwide retrieved these data daily to produce IGS data products. These products, such as the daily and weekly satellite ephemerides and the Earth rotation parameters, are in turn, submitted to the CDDIS for availability to the global science community. In general, a majority of the data delivered to and archived on the CDDIS are available to the user community within a few hours after the observation day.

The GLObal NAvigation Satellite System (GLONASS), managed and deployed by the Russian Federation, is similar to the U. S. Global Positioning System (GPS) in terms of the satellite constellation, orbits, and signal structure. Both systems offer precise, global, and continuous position-fixing capabilities; however, GLONASS does not degrade the signals available for civil use, as with selective availablity applied to the GPS signal. All GLONASS satellites are equipped with arrays of laser reflectors allowing the SLR community to range easily to the GLONASS satellites.

The International GLONASS Service - Pilot Project (IGLOS-PP) is a pilot service of the International GPS Service (IGS) to track and analyze data from the Russian GLONASS satellite constellation. The primary products of the service are dual-frequency observations from the global tracking network and precise orbits computed for each satellite. These are made available to support and encourage other applications and studies. In particular, the products from IGLOS willfacilitate the use of combined GLONASS and GPS observations for scientific and engineering applications, and allow users to experiment with the combined systems as a prototype Global Navigation Satellite System. The pilot service will operate for a period of up to four years, from 2000-2003.

IGLOS-PP is the IGS-sponsored follow-up to the very successful International GLONASS EXperiment (IGEX-98). IGEX-98 conducted the first global GLONASS Observation Campaign for geodetic and geodynamics applications. The results were presented and evaluated in an international workshop in 1999. The campaign duration of three months (and extention) was intended to simulate an operational environment, i.e., observations, data transfer, and data analysis have to be performed in a service-like manner (as opposed to a campaign-oriented strategy). IGEX utilized the existing infrastructure provided by the operational IGS wherever possible; many stations, data centers, and analysis centers currently participating in the IGS also contributed to IGEX.

Some of the scientific uses of GNSS data include:

• Maintenance of global accessibility to, and the improvement of, the International Terrestrial Reference Frame (ITRF)
• Monitoring deformations of the solid Earth
• Monitoring Earth rotation
• Monitoring variations in the liquid Earth (sea level, ice sheets, etc.)
• Precise GPS satellite orbit and clock determinations for analysis of regional GPS campaigns
• Monitoring of the ionosphere and troposphere
• Precise time transfer

Some potential scientific uses of GNSS data include:

• Monitoring deformations of the solid Earth
• Seismic hazards programs (maintenance of regional geodetic reference networks and collocations with seismic stations)
• Support ground-based gravimetry
• Collocation of gravimeters and GPS instruments
• Monitoring variations in the liquid Earth
• Sea level through maintenance of tide gauge benchmarks
• Scientific satellite orbit, clocks, and integrity information
• Real-time dissemination of GPS satellite orbits for wide-area differential GPS navigation applications
• Space-based remote sensing systems (radar altimeters and SAR)
• Support recovery of geopotential data from space systems
• Monitoring of the ionosphere and troposphere for global climate change research
• Ionosphere electron density and ionic-current distribution
• Space-based radio occultation profiles
• Ground-based water vapor distributions for enhanced weather forecasting

Related GPS information and data links:

• An overview of GPS
• International GPS Service (IGS)
• GPS Joint Program Office (JPO)
• U.S. Coast Guard Navigation Center (NAVCEN)
• Federal Geodetic Control Subcommittee (FGCS) and GPS Interagency Advisory Council (GIAC)
• GPS constellation status
• GPS Calendar
• GPS Seamless Archive Center (GSAC)

Related GLONASS information and data links:

• Russian GLONASS home page
• GLONASS constellation status
• IGS International GLONASS Service Pilot Project (IGLOS)

A description of CDDIS GNSS data sets is available.