Gravity Changes over Russian River Basins from GRACE - Planetary Exploration and Science: Recent Results and Advances - page 45

3.1 Introduction

Space-based Earth Observing Systems provided a substantially large amount of
information to the scientific community in the recent decades. One of the most
important contributions of these data sets is their use to study climate change.
Cumulative effects of redistribution of masses in the Earth system can be seen in
the changes of the gravity field of the Earth. Gravimetry is a science with a long
history. Gravity measurement techniques for land and ocean have been developing
all over the twentieth century. But only the space era opened the possibility to study
global gravity field and its changes over a planetary scale, including inaccessible
distant regions.
Technological achievements of our epoch - the NASA/DLR Gravity Recov-
ery and Climate Experiment (GRACE) twin satellites mission was launched on
17.03.2002 from Plesetsk kosmodrom. It allows the observations of monthly
changes in Earth's gravity field with unprecedented accuracy, working already for
11 years by the time of our study, which is twice more than expected. Though the
battery power is ten times less than at a launch time, there is a possibility that the
mission period could be extended till 2017, when the GRACE Follow-on Mission is
anticipated to be launched.
GRACE satellites fly in a near-polar orbit at 500 km altitude following one
another at a distance of 220 km. Accelerations of each satellite, occurring during
the flight above the Earth's mass anomalies, influence the range between the two
GRACE satellites. Microwave K-band range measurements represent the fundamen-
tal observational data, containing information about the gravity field. Data centers
located at GeoForschungsZentrum (GFZ) (Potsdam), Center for Space Research
(CSR) (Austin), and Jet Propulsion Laboratory (JPL) (Pasadena) process these data,
taking into account onboard GPS, accelerometers, star cameras, etc, to produce the
level one (L1) data products (Case et al. 2004 ). Then through sophisticated gravity
field inversion techniques with regularization (Tikhonov et al. 1998 ;Wangetal.
2012 ), correcting the aliasing effects of the atmospheric pressure changes over land
and over ocean, applying solid Earth, ocean and pole tides, and other corrections,
level 2 (L2) data product (Bettadpur 2007 ) is obtained, representing monthly gravity
field in form of Stokes coefficients ( 3.1 ) of spherical decomposition on the surface
of Earth's mean radius (Panteleev 2000 ; Sagitov 1979 ).
Modeling of the mean gravitational field is the primary goal of the space gravity
missions (Kenyon et al. 2007 ). Contemporary models incorporate information,
obtained from CHAllenging Minisatellite Payload (CHAMP), GRACE, and Gravity
field and steady-state Ocean Circulation Explorer (GOCE) satellites. But GRACE
also provides monthly anomalies (1 month is required to cover the Earth). If
the mean model is subtracted from the GRACE monthly Stokes coefficient, it is
possible to see month-to-month changes with several microgals accuracy (1 Gal D
0:01 m=s 2 ) and spatial resolution of 300 km. Monthly L2 files are accessible from
GFZ, CSR, and JPL archives, but one needs to mitigate, for example, removing
or filtering, the meridional correlated high-frequency noise patterns, called stripes.