GNSS - Global Navigation Satellite Systems
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 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% 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% of the time from any point on Earth.
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.
The International GNSS 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. 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.
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
SLR - Satellite Laser Ranging
SLR targets are satellites equipped with corner cubes or retroreflectors. Currently, the global SLR network tracks over 40 such satellites. The observable is the round-trip pulse time-of-flight to the satellite.
SLR systems are equipped with short-pulse laser transmitters that can range to orbiting satellites. Lunar Laser Ranging (LLR) systems can range to retroreflectors located on the moon.
The International Laser Ranging Service (ILRS) was formed to provide a service to support, through Satellite and Lunar Laser Ranging data and related products, geodetic and geophysical research activities as well as IERS products important to the maintenance of an accurate International Terrestrial Reference Frame (ITRF). The service also develops the necessary standards/specifications and encourages international adherence to its conventions. The ILRS collects, merges, archives and distributes Satellite Laser Ranging (SLR) and Lunar Laser Ranging (LLR) observation datasets of sufficient accuracy to satisfy the objectives of a wide range of scientific, engineering, and operational applications and experimentation.
Some of the scientific results derived from SLR include:
- Detection and monitoring of tectonic plate motion, crustal deformation, Earth rotation, and polar motion
- Modeling of the spatial and temporal variations of the Earth's gravitational field
- Determination of basin-scale ocean tides
- Monitoring of millimeter-level variations in the location of the center of mass of the total Earth system (solid Earth-atmosphere-oceans)
- Establishment and maintenance of the International Terrestrial Reference System (ITRS)
- Detection and monitoring of post-glacial rebound and subsidence
In addition, SLR provides precise orbit determination for spaceborne radar altimeter missions mapping the ocean surface (which are used to model global ocean circulation), for mapping volumetric changes in continental ice masses, and for land topography. It provides a means for subnanosecond global time transfer, and a basis for special tests of the Theory of General Relativity.