Satellite laser ranging
Encyclopedia
In satellite laser ranging (SLR) a global network of observation stations measure the round trip time of flight of ultrashort pulses of light
to satellite
s equipped with retroreflector
s. This provides instantaneous range measurements of millimeter level precision which can be accumulated to provide accurate measurement of orbits and a host of important scientific data.
Satellite laser ranging is a proven geodetic
technique with significant potential for important contributions to scientific studies of the Earth/Atmosphere/Oceans system. It is the most accurate technique currently available to determine the geocentric position of an Earth satellite, allowing for the precise calibration of radar altimeters and separation of long-term instrumentation drift from secular changes in ocean topography.
Its ability to measure the variations over time in the Earth’s gravity field and to monitor motion of the station network with respect to the geocenter, together with the capability to monitor vertical motion in an absolute system, makes it unique for modeling and evaluating long-term climate change by:
SLR provides a unique capability for verification of the predictions of the theory of General Relativity
, such as the frame-dragging
effect.
SLR stations form an important part of the international network of space geodetic observatories, which include VLBI, GPS, DORIS
and PRARE systems. On several critical missions, SLR has provided failsafe redundancy when other radiometric tracking systems have failed.
Several sets of retroreflectors were installed on the Earth's moon as part of the American Apollo
and Soviet Lunokhod space programs. These retroreflectors are also ranged on a regular basis, providing a highly accurate measurement of the dynamics of the Earth/Moon system.
During the subsequent decades, the global satellite laser ranging network has evolved into a powerful source of data for studies of the solid Earth and its ocean and atmospheric systems. In addition, SLR provides precise orbit determination
for spaceborne radar altimetre 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.
The International Laser Ranging Service was formed in 1998 by the global SLR community to enhance geophysical and geodetic research activities, replacing the previous CSTG Satellite and Laser Ranging Subcommission.
has been determined to less than ten centimeters at long wavelengths less than 1500 km.
SLR provides mm/year accurate determinations of tectonic drift station motion on a global scale in a geocentric reference frame. Combined with gravity models and decadal changes in Earth rotation, these results contribute to modeling of convection in the Earth’s mantle by providing constraints on related Earth interior processes. The velocity of the fiducial station in Hawaii is 70 mm/year and closely matches the rate of the background geophysical model.
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
to satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s equipped with retroreflector
Retroreflector
A retroreflector is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The device or surface's angle of incidence is...
s. This provides instantaneous range measurements of millimeter level precision which can be accumulated to provide accurate measurement of orbits and a host of important scientific data.
Satellite laser ranging is a proven geodetic
Geodesy
Geodesy , also named geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal...
technique with significant potential for important contributions to scientific studies of the Earth/Atmosphere/Oceans system. It is the most accurate technique currently available to determine the geocentric position of an Earth satellite, allowing for the precise calibration of radar altimeters and separation of long-term instrumentation drift from secular changes in ocean topography.
Its ability to measure the variations over time in the Earth’s gravity field and to monitor motion of the station network with respect to the geocenter, together with the capability to monitor vertical motion in an absolute system, makes it unique for modeling and evaluating long-term climate change by:
- Providing a reference system for post-glacial reboundPost-glacial reboundPost-glacial rebound is the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period, through a process known as isostasy...
, sea level and ice volume change - Determining the temporal mass redistribution of the solid Earth, ocean, and atmosphere system
- Monitoring the response of the atmosphere to seasonal variations in solar heating.
SLR provides a unique capability for verification of the predictions of the theory of General Relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
, such as the frame-dragging
Frame-dragging
Einstein's general theory of relativity predicts that non-static, stationary mass-energy distributions affect spacetime in a peculiar way giving rise to a phenomenon usually known as frame-dragging...
effect.
SLR stations form an important part of the international network of space geodetic observatories, which include VLBI, GPS, DORIS
DORIS (geodesy)
Doppler Orbitography and Radiopositioning Integrated by Satellite is a French satellite system used for the determination of satellite orbits and for positioning.-Principle:...
and PRARE systems. On several critical missions, SLR has provided failsafe redundancy when other radiometric tracking systems have failed.
History
Laser ranging to a near-Earth satellite was first carried out by NASA in 1964 with the launch of the Beacon-B satellite. Since that time, ranging precision, spurred by scientific requirements, has improved by a factor of a thousand from a few metres to a few millimetres, and more satellites equipped with retroreflectors have been launched.Several sets of retroreflectors were installed on the Earth's moon as part of the American Apollo
Project Apollo
The Apollo program was the spaceflight effort carried out by the United States' National Aeronautics and Space Administration , that landed the first humans on Earth's Moon. Conceived during the Presidency of Dwight D. Eisenhower, Apollo began in earnest after President John F...
and Soviet Lunokhod space programs. These retroreflectors are also ranged on a regular basis, providing a highly accurate measurement of the dynamics of the Earth/Moon system.
During the subsequent decades, the global satellite laser ranging network has evolved into a powerful source of data for studies of the solid Earth and its ocean and atmospheric systems. In addition, SLR provides precise orbit determination
Orbit determination
Orbit determination is a branch of astronomy specialised in calculating, and hence predicting, the orbits of objects such as moons, planets, and spacecraft . These orbits could be orbiting the Earth, or other bodies...
for spaceborne radar altimetre 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.
The International Laser Ranging Service was formed in 1998 by the global SLR community to enhance geophysical and geodetic research activities, replacing the previous CSTG Satellite and Laser Ranging Subcommission.
Uses of SLR data
SLR data has provided the standard, highly accurate, long wavelength gravity field reference model which supports all precision orbit determination and provides the basis for studying temporal gravitational variations due to mass redistribution. The height of the geoidGeoid
The geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest , and extended through the continents . According to C.F...
has been determined to less than ten centimeters at long wavelengths less than 1500 km.
SLR provides mm/year accurate determinations of tectonic drift station motion on a global scale in a geocentric reference frame. Combined with gravity models and decadal changes in Earth rotation, these results contribute to modeling of convection in the Earth’s mantle by providing constraints on related Earth interior processes. The velocity of the fiducial station in Hawaii is 70 mm/year and closely matches the rate of the background geophysical model.