SPECIAL STUDY GROUP 2.193
FIELD MISSIONS: CALIBRATION AND VALIDATION
Visser1 (chair) and C. Jekeli2 (co-chair)
Delft Institute for Earth-Oriented Space Research, Kluyverweg 1, 2629
HS, Delft, The Netherlands, Email: Pieter.Visser@lr.tudelft.nl
Civil & Environmental Engineering and Geodetic Science, 470
Hitchcock Hall, 2070 Neil Avenue, Columbus, OH 43210, Email: firstname.lastname@example.org
report describes the activities and the achievements of the IAG
Special Study Group (SSG) 2.193 "Gravity field missions:
calibration and validation" in its founding year 2000. The
objective of the study group is to define, evaluate and characterize
possibilities and methods for calibration and validation (Cal/Val) of
observations taken by gravity field missions and the derived gravity
field model products. These activities have become very relevant with
the launch of the German CHAMP satellite in July 2000 and the upcoming
launch of GRACE in the fall of 2001, and GOCE in 2005. These missions
make use of new generations of GPS receivers and new technologies like
ultra-sensitive accelerometers, a gradiometer and drag-free control
system. Many questions in the field of calibrating the several types
of observations are open and need to be addressed. In addition, new
concepts for validation derived gravity field products have to
considered, designed and developed, and old concepts reviewed due to
the unprecedented demands in accuracy and both spatial and temporal
resolution. In 2000, several studies have been carried out to assess
and review many Cal/Val possibilities. In addition, several institutes
and organizations are preparing for ground and airborne campaigns to
support Cal/Val activities. Finally, a joint effort with the Special
Commission "SC7: Gravity Field Determination by Satellites"
has resulted in a first set of simulated observations made available
to the scientific community. This data set can be used for testing
Introduction and background
recent launch of the German CHAMP satellite in July 2000 can be seen
as the starting point of what can be defined as the "Decade of
Geopotential Research", with two more missions to come: the
U.S./German/French GRACE in 2001 and the European Space Agency (ESA)
GOCE in 2005. These satellites make use of advanced technologies that
will enable global gravity field mapping with unprecedented accuracy
and resolution in space and time.
is currently providing observations that will enable the production of
the first consistent long-wavelength mean gravity field model
employing a geodetic-quality GPS receiver for high-low
Satellite-to-Satellite Tracking (SST) in combination with an
ultra-sensitive accelerometer measuring non-conservative forces. GRACE
aims at monitoring long- to medium-wavelength gravity field
variations, but will also enable the mapping of the mean global
gravity field with a resolution significantly surpassing existing
models. To this aim GRACE will consist of two low-flyers enabling
high-accuracy low-low SST tracking in combination with GPS high-low
SST and accelerometers, that are even more sensitive than the one on
board of CHAMP. Finally, GOCE will be the first satellite equipped
with a space-borne gravity gradiometer (SGG) together with a
high-quality GPS/GLONASS receiver and a drag-free control system. GOCE
aims at recovering the global mean gravity field with unprecedented
observations that will be produced by the satellites need to be
properly calibrated and reduced to gravity field model products that
need to be validated. The calibration entails the conversion of the
raw instrument measurements into engineering units within known limits
of accuracy and precision, for example cm and mm/s for the SST
measurements, m/s2 for the accelerometer and Eötvös units (E or 10-9
m/s2/m) for the differential accelerometer measurements by the
gradiometer. The validation concerns the conversion of these
engineering quantities into geophysical units with sufficient
accuracy, for example cm for geoid undulations, mgal for gravity
anomalies and E for the gravity gradients.
satellites will provide new types of observations never used before in
gravity field modeling and the spectral (error) characteristics of
these are not well understood. Several calibration methods need to be
reviewed, assessed, enhanced and/or designed in order to retrieve
these characteristics and guarantee a proper calibration. Due to the
required accuracy and resolution, the same can be said about
validation methods for the gravity field products.
number of different categories of possibilities for Cal/Val are
considered and subject of the activities of this SSG :
pre-flight calibration of the science instruments;
on-the-fly calibration and validation;
use of ground truth data;
comparison with existing state-of-the-art gravity field models;
intercomparison between gravity field products from different
missions, but also based on different instruments
within one mission.
SSG 2.193 - members
2.193 has 20 regular members, including the chair and co-chair and 18
corresponding members. The SGG started its activities in January 2000.
The names of the members of the SGG and countries are given in the
Visser (The Netherlands)/Chris Jekeli (USA)
Aguirre (The Netherlands), Dimitri Arabelos (Greece), Srinivas
Coleman (Australia), Rene Forsberg (Denmark), Cheng Huang (China),
Hwang (Taiwan), Karl-Heinz Ilk (Germany), Steve Kenyon (USA),
Kruizinga (USA), Jürgen Müller (Germany), Felix Perosanz (France),
Sato (Japan), Martijn Smit (The Netherlands), Dru Smith (USA),
Suenkel (TU-Graz/Austria), Peter Schwintzer (Germany), John Wahr (USA)
Anselmi (Italy), Georges Balmino (France), Stefano Cesare (Italy),
Dietrich (Germany), Yoichi Fukuda (Japan), Johnny Johannessen
Oberndorfer (Germany), Christian LeProvost (France), John Manning
Rummel (Germany), Jens Schroeter (Germany), Avri Selig (The
Shum (USA), Christian Tscherning (Denmark), Pierre Touboul (France),
Woodworth (Great-Britain), Changyin Zhao (USA), Yaozhong Zhu (China)
of the members have been involved very actively in the (partly) final
preparation stages for CHAMP and GRACE and preparatory stages of GOCE
leading to activities relevant but not under coordination of this SSG.
Many of the members met informally during several international
conferences and symposia in 2000, e.g. the EGS, AGU and IAG GGG2000
meetings. Many of them also contributed to national and international
studies and research programs that include preparations for Cal/Val
activities, and also acted as consultants to instrument manufacturers
and industry in general.
to changes in positions and obligations, it may become necessary to
revise the membership lists of this SSG.
of the members formed part of a study team that investigated and
assessed a number of Cal/Val methods and possibilities summarized in [Albertella
et al., 2000; Visser et al., 2000]. Moreover, several preparatory
activities took place by members in preparation of gravity field
modeling from future satellite missions that might be beneficial to
Cal/Val [Kusche et al., 2000; Oberndorfer et al., 2000; SID, 2000].
These activities included closed-loop simulations and full-scale
modeling of satellite (sub)systems required to understand instrument
and satellite behavior, helping in establishing realistic error models
and procedures for Cal/Val. Also a study was conducted to assess
gravity field modeling possibilities from arrays of GPS-carrying
low-flying satellites [Hwang, 2000].
launch of CHAMP and the final preparatory phase for GRACE triggered
many efforts and activities related to observation data processing and
quality control in general and Cal/Val of both observations and
gravity field model products in general. Cal/Val possibilities and
capabilities were demonstrated based on both supporting satellite
observations, e.g. SLR observations in case of CHAMP, and ground
support data [AGU, 2000].
members participated in both national and international cooperations
for setting up ground and airborne (gravimetry) campaigns to support
Cal/Val of the future gravity missions GRACE and GOCE. Certain members
also cooperate with members of the related SSG 2.162 "Precise
Orbits Using Multiple Space Techniques", chaired by Remko
Scharroo, and the Working Group " Preparation of Standard
Procedures for Global Gravity Field Validation", chaired by Th.
Gruber, partly in the framework of Science Working Teams (e.g.
JASON-1) and Precise Orbit Determination teams (e.g. ENVISAT).
Finally, this SSG contributed to the generation of a data set of
simulated CHAMP, GRACE and GOCE observations that was made available
to the scientific community. This activity was coordinated by Special
Commission SC7, chaired by Karl-Heinz Ilk.
Conclusions and outlook
activities have been undertaken by colleagues, members of this SSG in
particular, related to Cal/Val of gravity field mission observations
and gravity field model products. Most of the efforts were carried out
in the framework of ongoing programs related to specific satellite
missions, namely CHAMP, GRACE and GOCE, and not specifically in the
framework of this SSG. It is foreseen that these activities will be
extended and intensified in the years to come. It is the aim of this
SSG to serve in the future as a means for exchanging and distributing
information relevant to these efforts, and trigger cooperation between
different colleagues at both national and international level. In
light of this, it is intended to promote and continue cooperation with
SC7, SSG 2.162 and the above mentioned Working Group.
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