Gravity and Magnetic Fields
Gravity, Magnetic and Geodynamic
Measurements
Essential Climate Variables: Sea
Level
Not all near-Earth measurements undertaken
by satellite observations are discussed in
this document, since the focus here is on
land, sea and air parameters. Many others
are observed on a routine basis, including
measurements of the space environment and
solar activity. Of particular note are
measurements of Earth’s gravity field,
magnetic field and geodynamic activity.
Gravity-field measurements from space
provide the most promising advances for
improved measurement of the ‘geoid’ and its
time variations. The geoid is the surface of
equal gravitational potential at mean sea
level, and reflects the irregularities in
Earth’s gravity field at the planet’s
surface caused by the inhomogeneous mass and
density distribution in the interior. Such
measurements are vital for quantitative
determination – in combination with
satellite altimetry – of ocean currents,
improved global height references, estimates
of the thickness of the polar ice sheets and
its variations, and estimates of the
mass/volume redistribution of fresh water in
order to understand the hydrological cycle
better.
Gravity-field measurement packages on
satellites often utilise combinations of
different instrument types in order to
derive the necessary information: single or
multiple accelerometers; precise satellite
orbit determination systems; and
satellite-to-satellite tracking systems.
DLR’s CHAMP gravity package (2000-10) and
the NASA/DLR twin satellite GRACE mission
(since 2002) have been providing new
information that has resulted in new and
unique models of Earth’s gravity field and
its variability over time, and determination
of the geoid to centimetre accuracy at
length scales of several hundred kilometres.
GRACE has demonstrated that satellites can
detect groundwater variations by measuring
subtle temporal variations in gravity.
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From 2009, these data have been supplemented
by ESA’s GOCE satellite (ended October 2013),
which is designed to make significant advances
in our understanding of ocean circulation and
the crucial role it plays in regulating the
climate, as well as sea-level rise and
processes occurring in Earth’s interior. GOCE
data also have a broad range of applications
in the field of geodesy and surveying.
A number of missions, including Argentina’s
SAC-C, launched in 2000, and Australia’s
Fedsat, launched in 2002, have carried sensors
to study the electromagnetic environment of
spacecraft. Satellite-borne magnetometers
provide information on the strength and
direction of Earth’s internal and external
magnetic field and its time variations. Such
instruments are on the Ørsted satellite, which
is Denmark’s first satellite dedicated to the
magnetic field, launched in 1999.
The CHAMP mission also provided these
measurements, which are of value in a range of
applications, including navigation systems,
resource exploration drilling, spacecraft
attitude control systems and assessments of
the impact of ‘space weather’.
Further missions are under way or planned for
more in-depth, dedicated studies of magnetic
field. They include Demeter (2004-11), which
investigated links between earthquakes and
magnetic field variations, and Swarm (from
late 2013), which aims to provide the
best-ever survey of the geomagnetic field and
its temporal evolution, providing new insights
by improving our knowledge of Earth’s interior
and climate. NASA is also considering GRACE-II
as a (far future) follow-on to the success of
the GRACE mission.
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