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 24 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. 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.
	A global network of several hundred permanent GNSS (GPS and GPS+GLONASS) receivers provide data for archive in the CDDIS. 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. An interactive mapping tool is available illustrating the GNSS sites with data in the CDDIS archive; a high-resolution PDF map is also available.

The CDDIS archive of GNSS data and derived products primarily supports NASA programs and the International GNSS Service (IGS).

Types of GNSS

• U.S. GPS operational constellation
  The constellation consists of 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. Two of the original GPS satellites flew laser retroreflector arrays for SLR tracking
• GLObal NAvigation Satellite System (GLONASS)
  GLONASS is 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. The current GLONASS constellation includes 24 satellites, each traveling in a circular orbit, 19,140 kilometers above the Earth. All GLONASS satellites are equipped with arrays of laser reflectors allowing the SLR community to range easily to the GLONASS satellites.
• European Galileo GNSS
  When fully deployed, the European Galileo GNSS will consist of 30 satellites (27 operational and 3 spares) in three orbital planes at an altitude of 23,222 kilometers. Two operational satellites were launched in October 2011 followed by the launch of two additional satellites in October 2012 to be used for In-Orbit Validation (IOV) of the system. All Galileo satellites will also be equipped with laser retroreflector arrays for SLR.
• China's BeiDou/Compass
  This is currently under construction and will consist of 35 satellites in both GEO and MEO. The system is operational in China with 10 satellites in use and provides services to customers in the Asia-Pacific region. BeiDou satellites are also equipped with laser retroreflectors.
• Japan's Quasi-Zenith Satellite System (QZSS)
  QZSS will consist of three satellites in multiple orbital planes for regional positioning and time transfer. QZSS will enable a high accuracy satellite positioning service for nearly all of Japan, including urban areas and mountainous terrain.
• The Indian Regional Navigation Satellite System (IRNSS)
  This is a planned seven satellite constellation. The first satellite, IRNSS-1A was launched in July 2013 in a geosynchronous orbit and an apogee of 20,650 kilometers. When operational the IRNSS will provide accurate position information to user in and around India.

CDDIS Support of the IGS

The IGS has developed a global system of tracking stations, data centers, and analysis centers to put high-quality GNSS (operationally, GPS and GPS+GLONASS) data on-line in a timely fashion. In early 1992, the CDDIS was selected to serve as a global archive center for the IGS, by supporting the archive of daily GNSS data retrieved from identified core observatories. The purpose of this international service is to provide GNSS data products and highly accurate ephemerides to the global science community to further understanding in geophysical research. Several regional data collection centers forward their data holdings in compressed RINEX format to the CDDIS on a daily, hourly, and sub-hourly basis. These data are summarized, inventoried, and archived on-line for access by an international community of users. At present time, data from an average of nearly 475 daily sites, 300 hourly sites, and 160 sub-hourly, high-rate sites are archived in this fashion each day. Data analysis centers located worldwide retrieved these data 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.

Laser Retroreflectors on GNSS

Many current and future GNSS satellites are equipped with laser retroreflectors, allowing SLR tracking of these satellites. GNSS satellite tracking with SLR can provide improved GNSS satellite orbit information, modeling of satellite orbits and clocks, and improvement in the long-term stability of the ITRF. All GLONASS satellites have retroreflector arrays; two previously operational GPS satellites carried arrays. Galileo, BeiDou, QZSS, and IRNSS satellites have (or will have) these arrays. The International Laser Ranging Servic (ILRS) actively supports laser ranging to many, if not all, GNSS satellites equipped with laser retroreflector arrays.