![Wettzell Laser Ranging System](https://upload.wikimedia.org/wikipedia/commons/1/13/Wettzell_Laser_Ranging_System.jpg)
Satellite geodesy is
geodesy
Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure (geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equivale ...
by means of
artificial satellites
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...
—the measurement of the form and dimensions of
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, the location of objects on its surface and the
figure of the Earth
Figure of the Earth is a Jargon, term of art in geodesy that refers to the size and shape used to model Earth. The size and shape it refers to depend on context, including the precision needed for the model. A Spherical Earth, sphere is a well-k ...
's gravity field by means of artificial satellite techniques. It belongs to the broader field of
space geodesy
Space geodesy is geodesy by means of sources external to Earth, mainly artificial satellites (in satellite geodesy) but also quasars (in very-long-baseline interferometry, VLBI), visible stars (in stellar triangulation), and the retroreflectors on ...
. Traditional
astronomical geodesy
Geodetic astronomy or astronomical geodesy (astro-geodesy) is the application of astronomical methods into geodetic networks and other technical projects of geodesy.
Applications
The most important applications are:
* Establishment of geodetic d ...
is ''not'' commonly considered a part of satellite geodesy, although there is considerable overlap between the techniques.
The main goals of satellite geodesy are:
# Determination of the figure of the Earth, positioning, and navigation (geometric satellite geodesy)
# Determination of
geoid
The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended ...
,
Earth's gravity
The gravity of Earth, denoted by , is the net acceleration that is imparted to objects due to the combined effect of gravitation (from mass distribution within Earth) and the centrifugal force (from the Earth's rotation).
It is a vector quantity ...
field and its temporal variations (dynamical satellite geodesy or satellite
physical geodesy
Physical may refer to:
*Physical examination
In a physical examination, medical examination, or clinical examination, a medical practitioner examines a patient for any possible medical signs or symptoms of a medical condition. It generally cons ...
)
# Measurement of
geodynamical phenomena, such as
crustal dynamics and
polar motion
Polar motion of the Earth is the motion of the Earth's rotational axis relative to its crust. This is measured with respect to a reference frame in which the solid Earth is fixed (a so-called ''Earth-centered, Earth-fixed'' or ECEF reference f ...
Satellite geodetic data and methods can be applied to diverse fields such as
navigation
Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...
,
hydrography
Hydrography is the branch of applied sciences which deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers, as well as with the prediction of their change over time, for the primary p ...
,
oceanography
Oceanography (), also known as oceanology and ocean science, is the scientific study of the oceans. It is an Earth science, which covers a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dynamic ...
and
geophysics
Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' som ...
. Satellite geodesy relies heavily on
orbital mechanics
Orbital mechanics or astrodynamics is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft. The motion of these objects is usually calculated from Newton's laws of ...
.
History
First steps (1957-1970)
Satellite geodesy began shortly after the launch of
Sputnik
Sputnik 1 (; see § Etymology) was the first artificial Earth satellite. It was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957 as part of the Soviet space program. It sent a radio signal back to Earth for t ...
in 1957. Observations of
Explorer 1
Explorer 1 was the first satellite launched by the United States in 1958 and was part of the U.S. participation in the International Geophysical Year (IGY). The mission followed the first two satellites the previous year; the Soviet Union's ...
and
Sputnik 2
Sputnik 2 (, russian: Спутник-2, ''Satellite 2''), or Prosteyshiy Sputnik 2 (PS-2, russian: Простейший Спутник 2, italic=yes, ''Simplest Satellite 2'') was the second spacecraft launched into Earth orbit, on 3 November 195 ...
in 1958 allowed for an accurate determination of
Earth's flattening. The 1960s saw the launch of the Doppler satellite
Transit-1B and the
balloon satellite
A balloon satellite is inflated with gas after it has been put into orbit. It is also occasionally referred to as a "satelloon", which is a trademarked name owned by Gilmore Schjeldahl's G.T. Schjeldahl Company.
List of balloon satellites
abb ...
s
Echo 1
Project Echo was the first passive communications satellite experiment. Each of the two American spacecraft, launched in 1960 and 1964, were metalized balloon satellites acting as passive reflectors of microwave signals. Communication sign ...
, Echo 2, and
PAGEOS
PAGEOS (PAssive Geodetic Earth Orbiting Satellite) was a balloon satellite which was launched by NASA in June 1966.
Design
PAGEOS had a diameter of exactly , consisted of a thick mylar plastic film coated with vapour deposited aluminium enc ...
. The first dedicated geodetic satellite was ANNA-1B, a collaborative effort between
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
, the
DoD, and other civilian agencies.
ANNA-1B carried the first of the
US Army
The United States Army (USA) is the land service branch of the United States Armed Forces. It is one of the eight U.S. uniformed services, and is designated as the Army of the United States in the U.S. Constitution.Article II, section 2, cla ...
's SECOR (Sequential Collation of Range) instruments. These missions led to the accurate determination of the leading
spherical harmonic
In mathematics and physical science, spherical harmonics are special functions defined on the surface of a sphere. They are often employed in solving partial differential equations in many scientific fields.
Since the spherical harmonics form a ...
coefficients of the geopotential, the general shape of the
geoid
The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended ...
, and linked the world's geodetic datums.
Soviet military satellites undertook geodesic missions to assist in
ICBM
An intercontinental ballistic missile (ICBM) is a ballistic missile with a range greater than , primarily designed for nuclear weapons delivery (delivering one or more thermonuclear warheads). Conventional, chemical, and biological weapons c ...
targeting in the late 1960s and early 1970s.
Toward the World Geodetic System (1970-1990)
The
Transit
Transit may refer to:
Arts and entertainment Film
* ''Transit'' (1979 film), a 1979 Israeli film
* ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world
* ''Transit'' (2006 film), a 2006 ...
satellite system was used extensively for Doppler surveying, navigation, and positioning. Observations of satellites in the 1970s by worldwide triangulation networks allowed for the establishment of the
World Geodetic System
The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also descri ...
. The development of
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
by the United States in the 1980s allowed for precise navigation and positioning and soon became a standard tool in surveying. In the 1980s and 1990s satellite geodesy began to be used for monitoring of
geodynamic
Geodynamics is a subfield of geophysics dealing with dynamics (physics), dynamics of the Earth. It applies physics, chemistry and mathematics to the understanding of how mantle convection leads to plate tectonics and geologic phenomena such as sea ...
phenomena, such as
crustal motion
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
,
Earth rotation
Earth's rotation or Earth's spin is the rotation of planet Earth around its own axis, as well as changes in the orientation of the rotation axis in space. Earth rotates eastward, in prograde motion. As viewed from the northern polar star Polar ...
, and
polar motion
Polar motion of the Earth is the motion of the Earth's rotational axis relative to its crust. This is measured with respect to a reference frame in which the solid Earth is fixed (a so-called ''Earth-centered, Earth-fixed'' or ECEF reference f ...
.
Modern Era (1990-present)
![GRACE artist concept](https://upload.wikimedia.org/wikipedia/commons/e/e6/GRACE_artist_concept.jpg)
The 1990s were focused on the development of permanent geodetic networks and reference frames. Dedicated satellites were launched to measure Earth's gravity field in the 2000s, such as
CHAMP,
GRACE
Grace may refer to:
Places United States
* Grace, Idaho, a city
* Grace (CTA station), Chicago Transit Authority's Howard Line, Illinois
* Little Goose Creek (Kentucky), location of Grace post office
* Grace, Carroll County, Missouri, an uninco ...
, and
GOCE
''Goce'' is an opera composed by Kiril Makedonski (1925–1984), written by Venko Markovski and dedicated to Gotse Delchev. The work was commissioned to be the very first opera performed by the Macedonian National Opera Company. It premiered on Ma ...
.
Measurement techniques
![Jason-1 measurement system](https://upload.wikimedia.org/wikipedia/commons/5/5b/Jason-1_measurement_system.gif)
Techniques of satellite geodesy may be classified by instrument platform: A satellite may
#be observed with ground-based instruments (''Earth-to-space-methods''),
#carry an instrument or sensor as part of its payload to observe the Earth (''space-to-Earth methods''),
#or use its instruments to track or be tracked by another satellite (''space-to-space methods'').
Earth-to-space methods (satellite tracking)
Radio techniques
Global navigation satellite systems are dedicated radio positioning services, which can locate a receiver to within a few meters. The most prominent system,
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
, consists of a constellation of 31 satellites (as of December 2013) in high, 12-hour circular orbits, distributed in six planes with 55°
inclinations
Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object.
For a satellite orbiting the Earth ...
. The principle of location is based on
trilateration Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point of interest, often around Earth (geopositioning).
When more than three distances are involved, it may be called multilateration, for emph ...
. Each satellite transmits a precise
ephemeris
In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly vel ...
with information on its own position and a message containing the exact time of transmission. The receiver compares this time of transmission with its own clock at the time of reception and multiplies the difference by the speed of light to obtain a "
pseudorange
The pseudorange (from pseudo- and range) is the ''pseudo'' distance between a satellite and a navigation satellite receiver (see GNSS positioning calculation), for instance Global Positioning System (GPS) receivers.
To determine its position, a ...
." Four pseudoranges are needed to obtain the precise time and the receiver's position within a few meters. More sophisticated methods, such as
real-time kinematic
Real-time kinematic positioning (RTK) is the application of surveying to correct for common errors in current satellite navigation (GNSS) systems. It uses measurements of the phase of the signal's carrier wave in addition to the information con ...
(RTK) can yield positions to within a few millimeters.
In geodesy, GNSS is used as an economical tool for
surveying
Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ca ...
and
time transfer
Time and frequency transfer is a scheme where multiple sites share a precise reference time or frequency.
The technique is commonly used for creating and distributing standard time scales such as International Atomic Time (TAI).
Time transfer sol ...
. It is also used for monitoring
Earth's rotation
Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in retrograd ...
,
polar motion
Polar motion of the Earth is the motion of the Earth's rotational axis relative to its crust. This is measured with respect to a reference frame in which the solid Earth is fixed (a so-called ''Earth-centered, Earth-fixed'' or ECEF reference f ...
, and
crustal dynamics. The presence of the GPS signal in space also makes it suitable for
orbit determination
Orbit determination is the estimation of orbits of objects such as moons, planets, and spacecraft. One major application is to allow tracking newly observed asteroids and verify that they have not been previously discovered. The basic methods wer ...
and satellite-to-satellite tracking.
:''Examples:
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
,
GLONASS
GLONASS (russian: ГЛОНАСС, label=none, ; rus, links=no, Глобальная навигационная спутниковая система, r=Global'naya Navigatsionnaya Sputnikovaya Sistema, t=Global Navigation Satellite System) is ...
,
Galileo
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
''
=Doppler techniques
=
Doppler positioning involves recording the Doppler shift of a radio signal of stable frequency emitted from a satellite as the satellite approaches and recedes from the observer. The observed frequency depends on the radial velocity of the satellite relative to the observer, which is constrained by
orbital mechanics
Orbital mechanics or astrodynamics is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft. The motion of these objects is usually calculated from Newton's laws of ...
. If the observer knows the orbit of the satellite, then recording the Doppler profile determines the observer's position. Conversely, if the observer's position is precisely known, then the orbit of the satellite can be determined and used to study the Earth's gravity. In
DORIS, the ground station emits the signal and the satellite receives.
:''Examples:
Transit
Transit may refer to:
Arts and entertainment Film
* ''Transit'' (1979 film), a 1979 Israeli film
* ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world
* ''Transit'' (2006 film), a 2006 ...
,
DORIS,
Argos
Argos most often refers to:
* Argos, Peloponnese, a city in Argolis, Greece
** Ancient Argos, the ancient city
* Argos (retailer), a catalogue retailer operating in the United Kingdom and Ireland
Argos or ARGOS may also refer to:
Businesses
...
''
Optical triangulation
In optical triangulation, the satellite can be used as a very high target for
triangulation
In trigonometry and geometry, triangulation is the process of determining the location of a point by forming triangles to the point from known points.
Applications
In surveying
Specifically in surveying, triangulation involves only angle me ...
and can be used to ascertain the geometric relationship between multiple observing stations. Optical triangulation with the BC-4, PC-1000, MOTS, or Baker Nunn cameras consisted of photographic observations of a satellite, or flashing light on the satellite, against a background of stars. The stars, whose positions were accurately determined, provided a framework on the photographic plate or film for a determination of precise directions from camera station to satellite. Geodetic positioning work with cameras was usually performed with one camera observing simultaneously with one or more other cameras. Camera systems are weather dependent and that is one major reason why they fell out of use by the 1980s.
![ANNA 1B track](https://upload.wikimedia.org/wikipedia/commons/d/d3/ANNA_1B_track.png)
:''Examples:
PAGEOS
PAGEOS (PAssive Geodetic Earth Orbiting Satellite) was a balloon satellite which was launched by NASA in June 1966.
Design
PAGEOS had a diameter of exactly , consisted of a thick mylar plastic film coated with vapour deposited aluminium enc ...
,
Project Echo
Project Echo was the first passive communications satellite experiment. Each of the two American spacecraft, launched in 1960 and 1964, were metalized balloon satellites acting as passive reflectors of microwave signals. Communication sign ...
,
ANNA 1B
ANNA 1B (acronym for "United States Army, Army, United States Navy, Navy, National Aeronautics and Space Administration, NASA, United States Air Force, Air Force") was a United States satellite launched on October 31, 1962, from Cape Canaveral Air ...
''
Laser ranging
In satellite laser ranging (SLR) a global network of observation stations measure the round trip time of flight of ultrashort pulses of
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
to satellites equipped with
retroreflectors
A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflection (physics), reflects radiation (usually light) back to its source with minimum scattering. This works at a wide range of angle of incidence (opt ...
. This provides instantaneous range measurements of millimeter level precision which can be accumulated to provide accurate orbit parameters, gravity field parameters (from the orbit perturbations), Earth rotation parameters, tidal Earth's deformations, coordinates and velocities of SLR stations, and other substantial geodetic 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
The Earth-centered, Earth-fixed coordinate system (acronym ECEF), also known as the geocentric coordinate system, is a cartesian spatial reference system that represents locations in the vicinity of the Earth (including its surface, interior, ...
of an Earth satellite, allowing for the precise calibration of radar
altimeter
An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. The m ...
s and separation of long-term instrumentation drift from secular changes in
ocean surface topography
Ocean surface topography or sea surface topography, also called ocean dynamic topography, are highs and lows on the ocean surface, similar to the hills and valleys of Earth's land surface depicted on a topographic map.
These variations are exp ...
.
Satellite laser ranging contributes to the definition of the international terrestrial reference frames by providing the information about the scale and the origin of the reference frame, the so-called geocenter coordinates.
:''Example:
LAGEOS
LAGEOS, Laser Geodynamics Satellite or Laser Geometric Environmental Observation Survey, are a series of two scientific research satellites designed to provide an orbiting laser ranging benchmark for geodynamical studies of the Earth. Each satel ...
''
Space-to-Earth methods
Altimetry
![Sealevel chart](https://upload.wikimedia.org/wikipedia/commons/0/0e/Sealevel_chart.jpg)
Satellites such as
Seasat
Seasat was the first Earth-orbiting satellite designed for remote sensing of the Earth's oceans and had on board one of the first spaceborne synthetic-aperture radar (SAR). The mission was designed to demonstrate the feasibility of global satelli ...
(1978) and
TOPEX/Poseidon
TOPEX/Poseidon was a joint satellite altimeter mission between NASA, the U.S. space agency; and CNES, the French space agency, to map ocean surface topography. Launched on August 10, 1992, it was the first major oceanographic research satellite. ...
(1992-2006) used advanced dual-band
radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
altimeters to measure the height of the Earth's surface (sea, ice, and terrestrial surfaces) from a
spacecraft
A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
.
Jason-1
Jason-1 was a satellite altimeter oceanography mission. It sought to monitor global ocean circulation, study the ties between the ocean and the atmosphere, improve global climate forecasts and predictions, and monitor events such as El Niño ...
began in 2001,
Jason-2
OSTM/Jason-2, or Ocean Surface Topography Mission/Jason-2 satellite, was an international Earth observation satellite altimeter joint mission for sea surface height measurements between NASA and CNES. It was the third satellite in a series st ...
in 2008 and
Jason-3
Jason-3 is a satellite altimeter created by a partnership of the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and National Aeronautic and Space Administration (NASA), and is an international cooperative mis ...
in January 2016.
That measurement, coupled with
orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
al elements (possibly augmented by GPS), enables determination of the
terrain
Terrain or relief (also topographical relief) involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The Latin word ...
. The two different
wavelengths
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
of radio waves used permit the altimeter to automatically correct for varying delays in the
ionosphere
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
.
Spaceborne radar altimeters have proven to be superb tools for mapping
ocean-surface topography
Ocean surface topography or sea surface topography, also called ocean dynamic topography, are highs and lows on the ocean surface, similar to the hills and valleys of Earth's land surface depicted on a topographic map.
These variations are exp ...
, the hills and valleys of the sea surface. These instruments send a microwave pulse to the ocean's surface and record the time it takes to return. A
microwave radiometer
A microwave radiometer (MWR) is a radiometer that measures energy emitted at one millimeter-to-metre wavelengths (frequencies of 0.3–300 GHz) known as microwaves. Microwave radiometers are very sensitive receivers designed to measure thermally- ...
corrects any delay that may be caused by
water vapor
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous pha ...
in the
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
. Other corrections are also required to account for the influence of electrons in the
ionosphere
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
and the dry air mass of the atmosphere. Combining these data with the precise location of the spacecraft makes it possible to determine sea-surface height to within a few centimeters (about one inch). The strength and shape of the returning signal also provides information on wind speed and the height of ocean waves. These data are used in ocean models to calculate the speed and direction of
ocean current
An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, s ...
s and the amount and location of heat stored in the ocean, which in turn reveals global
climate variation
Climate variability includes all the variations in the climate that last longer than individual weather events, whereas the term climate change only refers to those variations that persist for a longer period of time, typically decades or more ...
s.
=Laser altimetry
=
A
laser altimeter
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
uses the round-trip flight-time of a beam of light at optical or infrared wavelengths to determine the spacecraft's altitude or, conversely, the ground topography.
:''Examples:
ICESat
ICESat (Ice, Cloud, and land Elevation Satellite) was a NASA satellite mission for measuring ice sheet mass balance, cloud and aerosol heights, as well as land topography and vegetation characteristics. It operated as part of NASA's Earth Obser ...
,
MOLA.''
=Radar altimetry
=
A
radar altimeter
A radar altimeter (RA), also called a radio altimeter (RALT), electronic altimeter, reflection altimeter, or low-range radio altimeter (LRRA), measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it t ...
uses the round-trip flight-time of a microwave pulse between the satellite and the Earth's surface to determine the distance between the spacecraft and the surface. From this distance or height, the local surface effects such as tides, winds and currents are removed to obtain the satellite height above the geoid. With a precise ephemeris available for the satellite, the geocentric position and ellipsoidal height of the satellite are available for any given observation time. It is then possible to compute the geoid height by subtracting the measured altitude from the ellipsoidal height. This allows direct measurement of the geoid, since the ocean surface closely follows the geoid. The difference between the ocean surface and the actual geoid gives
ocean surface topography
Ocean surface topography or sea surface topography, also called ocean dynamic topography, are highs and lows on the ocean surface, similar to the hills and valleys of Earth's land surface depicted on a topographic map.
These variations are exp ...
.
:''Examples:
Seasat
Seasat was the first Earth-orbiting satellite designed for remote sensing of the Earth's oceans and had on board one of the first spaceborne synthetic-aperture radar (SAR). The mission was designed to demonstrate the feasibility of global satelli ...
,
Geosat
The GEOSAT (GEOdetic SATellite) was a U.S. Navy Earth observation satellite, launched on March 12, 1985 into an 800 km, 108° inclination orbit, with a nodal period of about 6040 seconds. The satellite carried a radar altimeter capable of m ...
,
TOPEX/Poseidon
TOPEX/Poseidon was a joint satellite altimeter mission between NASA, the U.S. space agency; and CNES, the French space agency, to map ocean surface topography. Launched on August 10, 1992, it was the first major oceanographic research satellite. ...
,
ERS-1
European Remote Sensing satellite (ERS) was the European Space Agency's first Earth-observing satellite programme using a polar orbit. It consisted of 2 satellites, ERS-1 and ERS-2.
ERS-1
ERS-1 launched 17 July 1991 from Guiana Space Centre ...
,
ERS-2
European Remote Sensing satellite (ERS) was the European Space Agency's first Earth-observing satellite programme using a polar orbit. It consisted of 2 satellites, ERS-1 and ERS-2.
ERS-1
ERS-1 launched 17 July 1991 from Guiana Space Centre ...
,
Jason-1
Jason-1 was a satellite altimeter oceanography mission. It sought to monitor global ocean circulation, study the ties between the ocean and the atmosphere, improve global climate forecasts and predictions, and monitor events such as El Niño ...
,
Jason-2
OSTM/Jason-2, or Ocean Surface Topography Mission/Jason-2 satellite, was an international Earth observation satellite altimeter joint mission for sea surface height measurements between NASA and CNES. It was the third satellite in a series st ...
,
Envisat
Envisat ("Environmental Satellite") is a large inactive Earth-observing satellite which is still in orbit and now considered space debris. Operated by the European Space Agency (ESA), it was the world's largest civilian Earth observation satell ...
,
SWOT (satellite)
The Surface Water and Ocean Topography (SWOT) mission is a satellite altimeter jointly developed and operated by NASA and CNES, the French space agency, in partnership with the Canadian Space Agency (CSA) and UK Space Agency (UKSA). The object ...
''
Interferometric synthetic aperture radar (InSAR)
Interferometric synthetic aperture radar (InSAR) is a
radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
technique used in
geodesy
Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure (geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equivale ...
and
remote sensing
Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth ...
. This geodetic method uses two or more
synthetic aperture radar
Synthetic-aperture radar (SAR) is a form of radar that is used to create two-dimensional images or three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target region to provide fine ...
(SAR) images to generate maps of surface deformation or
digital elevation, using differences in the phase of the waves returning to the satellite. The technique can potentially measure centimetre-scale changes in deformation over timespans of days to
years. It has applications for geophysical monitoring of natural hazards, for example earthquakes, volcanoes and landslides, and also in structural engineering, in particular monitoring of subsidence and structural stability.
:''Example:
Seasat
Seasat was the first Earth-orbiting satellite designed for remote sensing of the Earth's oceans and had on board one of the first spaceborne synthetic-aperture radar (SAR). The mission was designed to demonstrate the feasibility of global satelli ...
,
TerraSAR-X''
Space-to-space methods
Gravity gradiometry
A gravity gradiometer can independently determine the components of the gravity vector on a real-time basis. A gravity gradient is simply the spatial derivative of the gravity vector. The gradient can be thought of as the rate of change of a component of the gravity
vector
Vector most often refers to:
*Euclidean vector, a quantity with a magnitude and a direction
*Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism
Vector may also refer to:
Mathematic ...
as measured over a small distance. Hence, the gradient can be measured by determining the difference in gravity at two close but distinct points. This principle is embodied in several recent moving-base instruments. The gravity gradient at a point is a
tensor
In mathematics, a tensor is an algebraic object that describes a multilinear relationship between sets of algebraic objects related to a vector space. Tensors may map between different objects such as vectors, scalars, and even other tenso ...
, since it is the derivative of each component of the gravity vector taken in each sensitive axis. Thus, the value of any component of the gravity vector can be known all along the path of the vehicle if gravity gradiometers are included in the system and their outputs are integrated by the system computer. An accurate gravity model will be computed in real-time and a continuous map of normal gravity, elevation, and anomalous gravity will be available.
:''Example:
GOCE
''Goce'' is an opera composed by Kiril Makedonski (1925–1984), written by Venko Markovski and dedicated to Gotse Delchev. The work was commissioned to be the very first opera performed by the Macedonian National Opera Company. It premiered on Ma ...
''
Satellite-to-satellite tracking
This technique uses satellites to track other satellites. There are a number of variations which may be used for specific purposes such as
gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
field investigations and
orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
improvement.
* A
high altitude satellite may act as a relay from ground tracking stations to a
low altitude satellite. In this way, low altitude satellites may be observed when they are not accessible to ground stations. In this type of tracking, a signal generated by a tracking station is received by the relay satellite and then retransmitted to a lower altitude satellite. This signal is then returned to the ground station by the same path.
* Two low altitude satellites can track one another observing mutual orbital variations caused by gravity field irregularities. A prime example of this is
GRACE
Grace may refer to:
Places United States
* Grace, Idaho, a city
* Grace (CTA station), Chicago Transit Authority's Howard Line, Illinois
* Little Goose Creek (Kentucky), location of Grace post office
* Grace, Carroll County, Missouri, an uninco ...
.
* Several high altitude satellites with accurately known orbits, such as
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
satellites, may be used to fix the position of a low altitude satellite.
These examples present a few of the possibilities for the application of satellite-to-satellite tracking. Satellite-to-satellite tracking data was first collected and analyzed in a high-low configuration between
ATS-6
ATS-6 (Applications Technology Satellite-6) was a NASA experimental satellite, built by Fairchild Space and Electronics Division It has been called the world's first educational satellite as well as world's first experimental Direct Broadcast ...
and
GEOS-3
GEOS-3, or Geodynamics Experimental Ocean Satellite 3, or GEOS-C, was the third and final satellite as part of NASA's Geodetic Earth Orbiting Satellite/Geodynamics Experimental Ocean Satellite program (NGSP)Henriksen, S. W. (ed) (1977National Geo ...
. The data was studied to evaluate its potential for both orbit and gravitational model refinement.
:''Example:
GRACE
Grace may refer to:
Places United States
* Grace, Idaho, a city
* Grace (CTA station), Chicago Transit Authority's Howard Line, Illinois
* Little Goose Creek (Kentucky), location of Grace post office
* Grace, Carroll County, Missouri, an uninco ...
''
=
GNSS
A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pre ...
tracking =
:''Examples:
CHAMP,
GOCE
''Goce'' is an opera composed by Kiril Makedonski (1925–1984), written by Venko Markovski and dedicated to Gotse Delchev. The work was commissioned to be the very first opera performed by the Macedonian National Opera Company. It premiered on Ma ...
''
List of geodetic satellites
*
ANNA-1B
*
Beidou
The BeiDou Navigation Satellite System (BDS; ) is a Chinese satellite navigation system. It consists of two separate satellite constellations. The first BeiDou system, officially called the BeiDou Satellite Navigation Experimental System and ...
*
BLITS
BLITS (Ball Lens In The Space) is a Russian satellite launched on September 17, 2009, as a secondary payload on a Soyuz-2.1b/Fregat, from the Baikonur Cosmodrome in Kazakhstan.
The satellite is totally passive and spherical, and is tracked us ...
*
CHAMPDiadème*
Echo
In audio signal processing and acoustics, an echo is a reflection of sound that arrives at the listener with a delay after the direct sound. The delay is directly proportional to the distance of the reflecting surface from the source and the list ...
*
Envisat
Envisat ("Environmental Satellite") is a large inactive Earth-observing satellite which is still in orbit and now considered space debris. Operated by the European Space Agency (ESA), it was the world's largest civilian Earth observation satell ...
*
ERS-1
European Remote Sensing satellite (ERS) was the European Space Agency's first Earth-observing satellite programme using a polar orbit. It consisted of 2 satellites, ERS-1 and ERS-2.
ERS-1
ERS-1 launched 17 July 1991 from Guiana Space Centre ...
*
ERS-2
European Remote Sensing satellite (ERS) was the European Space Agency's first Earth-observing satellite programme using a polar orbit. It consisted of 2 satellites, ERS-1 and ERS-2.
ERS-1
ERS-1 launched 17 July 1991 from Guiana Space Centre ...
*
Etalon
*
Experimental Geodetic Payload
Ajisai (Japanese: あじさい, meaning "Hydrangea") is a Japanese satellite sponsored by NASDA, launched in 1986 on the first flight of the H-I rocket.
It is also known as the Experimental Geodetic Satellite (EGS), as it carried the Experiment ...
"Ajisai"
*
Explorer program
The Explorers program is a NASA exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. Launched in 1958, Explorer 1 was the first spacecraft of the United Stat ...
*
Galileo
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
*
Geo-IK-2 The Geo-IK-2 is a Russian series of new generation military geodesy satellites replacing the Soviet Union's Geo-IK and Sfera constellations. They are intended to be used to create high precision three-dimensional maps of the Earth's surface, and t ...
*
GEOS-3
GEOS-3, or Geodynamics Experimental Ocean Satellite 3, or GEOS-C, was the third and final satellite as part of NASA's Geodetic Earth Orbiting Satellite/Geodynamics Experimental Ocean Satellite program (NGSP)Henriksen, S. W. (ed) (1977National Geo ...
*
Geosat
The GEOSAT (GEOdetic SATellite) was a U.S. Navy Earth observation satellite, launched on March 12, 1985 into an 800 km, 108° inclination orbit, with a nodal period of about 6040 seconds. The satellite carried a radar altimeter capable of m ...
*
Geosat Follow-On
The GEOSAT (GEOdetic SATellite) was a U.S. Navy Earth observation satellite, launched on March 12, 1985 into an 800 km, 108° inclination orbit, with a nodal period of about 6040 seconds. The satellite carried a radar altimeter capable of me ...
*
GFZ-1
*
GLONASS
GLONASS (russian: ГЛОНАСС, label=none, ; rus, links=no, Глобальная навигационная спутниковая система, r=Global'naya Navigatsionnaya Sputnikovaya Sistema, t=Global Navigation Satellite System) is ...
*
GRACE
Grace may refer to:
Places United States
* Grace, Idaho, a city
* Grace (CTA station), Chicago Transit Authority's Howard Line, Illinois
* Little Goose Creek (Kentucky), location of Grace post office
* Grace, Carroll County, Missouri, an uninco ...
*
GOCE
''Goce'' is an opera composed by Kiril Makedonski (1925–1984), written by Venko Markovski and dedicated to Gotse Delchev. The work was commissioned to be the very first opera performed by the Macedonian National Opera Company. It premiered on Ma ...
*
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
*
ICESat-1
*
ICESat-2
ICESat-2 (Ice, Cloud, and land Elevation Satellite 2), part of NASA's Earth Observing System, is a satellite mission for measuring ice sheet elevation and sea ice thickness, as well as land topography, vegetation characteristics, and clouds. ICES ...
*
LAGEOS
LAGEOS, Laser Geodynamics Satellite or Laser Geometric Environmental Observation Survey, are a series of two scientific research satellites designed to provide an orbiting laser ranging benchmark for geodynamical studies of the Earth. Each satel ...
*
LARES
Lares ( , ; archaic , singular ''Lar'') were guardian deities in ancient Roman religion. Their origin is uncertain; they may have been hero-ancestors, guardians of the hearth, fields, boundaries, or fruitfulness, or an amalgam of these.
Lares ...
*
Larets
*
H-IIA
H-IIA (H-2A) is an active expendable launch system operated by Mitsubishi Heavy Industries (MHI) for the Japan Aerospace Exploration Agency. These liquid fuel rockets have been used to launch satellites into geostationary orbit; lunar or ...
-
LRE
*
PAGEOS
PAGEOS (PAssive Geodetic Earth Orbiting Satellite) was a balloon satellite which was launched by NASA in June 1966.
Design
PAGEOS had a diameter of exactly , consisted of a thick mylar plastic film coated with vapour deposited aluminium enc ...
*
Seasat
Seasat was the first Earth-orbiting satellite designed for remote sensing of the Earth's oceans and had on board one of the first spaceborne synthetic-aperture radar (SAR). The mission was designed to demonstrate the feasibility of global satelli ...
*
Starlette and Stella
Starlette (''Satellite de taille adaptée avec réflecteurs laser pour les études de la terre'', or ) and Stella are nearly identical French geodetic and geophysical satellites. Starlette was launched on 6 February 1975 and Stella on 26 Septem ...
*
TOPEX/Poseidon
TOPEX/Poseidon was a joint satellite altimeter mission between NASA, the U.S. space agency; and CNES, the French space agency, to map ocean surface topography. Launched on August 10, 1992, it was the first major oceanographic research satellite. ...
*
TRANSIT
Transit may refer to:
Arts and entertainment Film
* ''Transit'' (1979 film), a 1979 Israeli film
* ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world
* ''Transit'' (2006 film), a 2006 ...
*
WESTPAC
Westpac Banking Corporation, known simply as Westpac, is an Australian multinational banking and financial services company headquartered at Westpac Place in Sydney, New South Wales.
Established in 1817 as the Bank of New South Wales, it ...
See also
*
Geodetic astronomy
Geodetic astronomy or astronomical geodesy (astro-geodesy) is the application of astronomical methods into geodetic networks and other technical projects of geodesy.
Applications
The most important applications are:
* Establishment of geodetic da ...
*
Satellite gravimetry
Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of a gravitational field or the properties of matter responsible for its creation are of interest.
Units of measurement
Gr ...
References
Attribution
Further reading
*
* Smith, David E. and Turcotte, Donald L. (eds.) (1993). Contributions of Space Geodesy to Geodynamics: Crustal Dynamics Vol. 23, Earth Dynamics Vol. 24, Technology Vol. 25, American Geophysical Union Geodynamics Series .
External links
GOCEGRACECHAMP
{{Authority control
*