The Measurement of Earths Gravity Field from Spaceborne Satellites

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The Measurement of Earths Gravity Field from
Spaceborne Satellites
Presentation By Nathan Ovans Senior Student, FSU
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Questions

• What is the Earths Gravity Field?
• How does measuring the gravity field help
  surveyors and the height modernization project?

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Earths Gravity Field

• The Earths mass exerts an attractive force
  around the Earth called gravity.
• The gravity field of the Earth is an
  equipotential surface.
• We call this equipotential surface the geoid.
• For general purposes, the mean sea level is
  assumed as the surface of the geoid.

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Earths Gravity Field
When we level a surveying instrument, the
instrument becomes perpendicular to the local
gravity vector, F.
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Earths Gravity Field

• There are two ways to measure gravity
• Absolute measurement
• Relative measurement
• The units of gravity are Gal, or 1 cm / sec2
• Earths gravity is often expressed in terms of
  milliGals (1000th of a Gal)
• Using Stokes formula, one can calculate geoid
  undulation using gravity values.

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Earths Gravity Field
Geoid by Africa
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Earths Gravity Field

• A gravity anomaly is the difference between the
  observed reduced gravity and the normal gravity.
• Normal gravity perpendicular of the Earths
  gravitational force and the centrifugal force
  caused by the rotation of the Earth, theoretical
  value.
• Observed reduced gravity gravity measurement
  reduced to sea level.
• The difference is caused by thecomposition and
  topography of the Earth, the Earths rotation
  (flattening of the poles), and the dynamic
  features of the Earth.

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Earths Gravity Field
Free Air Gravity Anomalies
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CHAMP
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CHAMP

• CHAllenging Minisatellite Payload
• German small satellite mission for geoscientific
  and atmospheric research and applications.
• Designed and built by the Daimler Chrysler
  Aerospace Jena Optronik GmbH.
• Managed by GeoForschungsZentrum Potsdam (GFZ),
  Germany's National Research Center for
  Geosciences.
• Generate for the first time simultaneously highly
  precise gravity and magnetic field measurements
  over a 5 years period.

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CHAMP

• Primary scientific objectives
• highly precise global long-wavelength features of
  the static Earth gravity field and the temporal
  variation of this field,
• with unprecedented accuracy of global estimates
  of the main and crustal magnetic field of the
  Earth and the space/time variability of these
  field components,
• with good global distribution of a large number
  of GPS signal refraction data, caused by the
  atmosphere and ionosphere, which can be converted
  into temperature, water vapor, and electron
  content.

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Components of the CHAMP Satellite
Using precise location and an accelerometer,
recorded changes in the satellites direction are
used to measure gravity.

• 27 feet long including its 13 foot boom, 5 feet
  wide, and 2.5 feet high
• Weighs 1,151 lbs

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GRACE
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GRACE

• Joint satellite mission between the National
  Aeronautics and Space Administration (NASA) and
  the Deutsches Zentrum für Luft- und Raumfahrt
  (DLR) (German Center for Air and Space Travel).
• Successor of the CHAMP mission.
• Mission to provide, with unprecedented accuracy,
  estimates of the global high-resolution models of
  the Earth's gravity field for a period of up to
  five years.
• Uses two satellites on the same orbital track,
  separated by about 200 kilometers.
• Satellites are interconnected by a K-band
  microwave link that will measure the separation
  distance and its rate of change to an accuracy of
  1 µm/s .

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GRACE
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GRACE
Uses the change in distance between the
satellites to determine the gravity.
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GRACE
Improvement in resolution in gravity anomalies
computed from GGM02S (right) compared to GGM01S
(left) in the Tonga-Kermadec region. With the
increased accuracy of the GGM02 model, less
smoothing is required to remove artifacts and
more detail is revealed. Units are milliGals.
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EIGEN-CG01C geoid with Free Air Anomaly
CG03C - 3 cm and 0.4 mgal in terms of geoid
heights and gravity anomalies, respectively. The
overall accuracy of the full 360 model down to
spatial features of 100 km is estimated to be 30
cm and 8 mgal, respectively.
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Results

• Currently the National Geospatial-Intelligence
  Agency is working on a new world geoid model with
  an overall accuracy of 15 cm.
• The accurate measurement of the Earths gravity
  field produces a highly accurate geoid which
  provides higher accuracy for Global Positioning
  and the determination of mean sea level.
• Questions?