Geoscience Reference
In-Depth Information
amount of this decrease depends on the solar activity cycle and may accumu-
late in the mission lifetime to some 50 km on low activity, and up to 200 km
on high activity, see http://op.gfz-potsdam.de/grace.
The range between the two satellites must be determined extremely ac-
curately. Its range rate must be known to better than 1
µ
ms
−
1
,whichis
achieved by intersatellite microwave measurements. The basic idea is that
variations in the gravity field cause variations in the range between the
two satellites; areas of stronger gravity will affect the lead satellite first and,
therefore, accelerate it away from the following satellite (Seeber 2003: p. 479).
GRACE will not only provide a static global gravity field but also its
temporal variations.
To achieve the mission goals, the following payload is on board of the
two satellites:
•
The K-band ranging system is the key instrument of GRACE to mea-
sure the range changes between both satellites using dual-band mi-
crowave signals (i.e., two one-way ranges) with a precision of about
1
µ
ms
−
1
. The ranges are obtained at a sampling rate of 10 Hz.
•
The GPS receiver serves for the precise orbit determination of the
GRACE spacecraft and provides data for atmospheric and ionospheric
profiling. To achieve this, satellite-to-satellite tracking between the
GRACE satellites and the GPS satellites is realized. A navigation solu-
tion comprising position, velocity, and a time mark is derived on board.
The navigation solution is required for the attitude control system. The
precise orbit based on code and carrier pseudoranges is determined on
ground.
•
The attitude and orbit control system comprises a cold gas propulsion
system, three magnetic torque rods, star trackers, a three-axis inertial
reference unit to measure angular rates, and a three-axis magnetome-
ter.
•
The accelerometer measures all nongravitational accelerations on the
GRACE spacecraft, e.g., due to air drag or solar radiation pressure.
•
The laser retroreflector is a passive payload instrument used to reflect
short laser pulses transmitted by ground stations. The distance be-
tween a ground station and a GRACE satellite can be measured with
an accuracy of 1-2 cm. The laser retroreflector data are primarily used
together with the GPS receiver data for the precise orbit determina-
tion.
In 2004, the GRACE science team released to the public a first version
of a new earth gravity field model complete to degree and order 150. The
resulting improved geoid together with satellite altimetry will advance the
