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A Global Navigation Satellite System (GNSS) is a space geodesy technique that provides autonomous geospatial positioning with global coverage. The U.S. Global Positioning System (GPS) is one type of GNSS.

The current GPS constellation includes 24 satellites, each traveling in a 12-hour, circular orbit 20,200 kilometers above Earth. The satellites are positioned so that 6 are observable nearly 100% of the time from any point on Earth. The current GLObal NAvigation Satellite System (GLONASS) constellation includes 24 satellites, each traveling in a circular orbit 19,140 kilometers above Earth. The satellites are positioned so that four are observable nearly 100% 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. 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.

GNSS Overview

NASA's archive of GNSS data and derived products, held by NASA's Crustal Dynamics Data Information System (CDDIS), primarily supports NASA programs and the International GNSS Service (IGS). 

Learn more about GNSS data holdings and product holdings.

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, 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 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 CDDIS for availability to the global science community.

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 satellite laser ranging (SLR) tracking.

GLObal NAvigation Satellite System (GLONASS)

GLONASS is managed and deployed by the Russian Federation, and is similar to GPS in terms of the satellite constellation, orbits, and signal structure. The current GLONASS constellation includes 26 satellites, 24 of which are in operation and 2 are in flight tests phase. The satellites each travel 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

Galileo is Europe’s global navigation satellite system, and has been operational since December 2016, when it started offering services to public authorities, businesses, and citizens. Europe launched its first two operational satellites for Galileo on 21 October 2011. In total, the Galileo constellation will consist of 30 satellites (27 operational and 3 spares) in three orbital planes at an altitude of 23,222 kilometers, while the ground segment contains control centers located in Europe, and a network of sensor stations and uplink stations installed around the globe. All Galileo satellites are equipped with laser retroreflector arrays for SLR.

China's BeiDou/Compass

China’s BeiDou Navigation Satellite System (BDS), also known as BeiDou, is China’s second-generation satellite navigation system. The nominal constellation consists of 35 satellites, including 5 Geostationary Earth Orbit (GEO), 3 Inclined Geo-Synchronous Orbit (IGSO) satellites (with 1 IGSO per plane), and 27 Medium Earth Orbit (MEO) satellites. BeiDou satellites are equipped with laser retroreflectors. The MEOs are deployed as a Walker constellation (24 MEOs in 3 planes plus 3 spares).

Japan's Quasi-Zenith Satellite System (QZSS)

QZSS, also known as Michibiki, is a four-satellite regional satellite navigation system and a satellite-based augmentation system developed by the Japanese government to enhance the US-operated GPS in the Asia-Oceania regions, with a focus on Japan. QZSS enables a high-accuracy satellite positioning service for nearly all of Japan, including urban areas and mountainous terrain.

The Indian Regional Navigation Satellite System (IRNSS)

The IRNSS, with an operational name of NavIC, is an autonomous regional satellite navigation system that provides accurate real-time positioning and timing services. It is a five-satellite constellation in a geosynchronous orbit and an apogee of 20,650 kilometers. ISRO launched the first of five next-generation satellites on May 29, 2023, featuring new payloads and an extended lifespan of 12 years. The five new satellites – the NVS system, will supplement and augment the current constellation of satellites.

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 Service (ILRS) actively supports laser ranging to many, if not all, GNSS satellites equipped with laser retroreflector arrays.

CDDIS has supported the IGS as a global data center since its inception in 1992, including active participation in several IGS working groups, experiments, and pilot projects. These experiments and projects include the IGS Multi-GNSS Experiment (MGEX), the Real-Time Pilot Project (RTPP), the International GLONASS Service Pilot Project (IGLOS-PP), and the GPS Tide Gauge Benchmark Monitoring Pilot Project (TIGA-PP).

IGS working groups include the Ionosphere Working Group, the Troposphere Working Group, the GPS Tide Gauge Benchmark Monitoring Working Group, and the Low Earth Orbiters Working Group.

International GNSS Service
NASA's CDDIS serves as a Global Data Center for the International GNSS Service (IGS), supporting the archive and distribution of data from the global network of permanent receivers and the IGS products derived from these data. IGS provides the highest quality GNSS data, products, and services in support of the terrestrial reference frame, Earth observations and research, positioning, navigation, and timing (PNT), and other applications that benefit the scientific community and society.
Learn more about IGS
The IGS logo features blue and black squares in a row across the top, evoking the shape of a satellite, with the text below reading IGS International GNSS Service

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