Launch
GRACE will be launched from the Plesetsk Cosmodrome, a former ICBM site in northern Russia. This site has been one of the most active launch sites in the entire world. EurROCKOT GmbH is providing the launch vehicle. The booster unit is an adaptation of the highly reliable SS-19 ICBM tested in flight over 140 times. A newly developed multi-ignitable and highly maneuverable third stage BREEZE was added. The complete ROCKOT system including BREEZE has been proven in flight three times with a 100 percent success rate. The payload is assembled on site at Plesetsk and then transported to a launch pad that has been modified to accommodate the ROCKOT launch vehicle. Plesetsk also provides telemetry and tracking services via the existing ground measurement infrastructure.

The two GRACE spacecraft will fly in co-planar orbits between 186 and 310 miles (300 and 500 km) above the surface at an inclination between 89 and 90 degrees. They will be separated along track by between 62 and 310 miles (100 and 500 km)—distance varies over the life of the mission. Orbit maneuvers will be required every 30-60 days in order to maintain the separation between the satellites in addition to occasional calibration and altitude "make-up" maneuvers. The mission is designed for a five-year lifetime.

Key Spacecraft Components

A sneak peak at the innards of a spacecraft! For the GRACE mission, the spacecraft themselves are the main instruments.

The two identical spacecraft in the environmental test facility at IABG in Ottobrun, Germany. Photo credit: Astrium GmbH.

K-band Ranging System. Provides precise (within 10 µm) measurements of the distance change between the two satellites and hence measures the fluctuations in gravity.

Ultra Stable Oscillator. Provides frequency generation for the K-band ranging system. S-band Boom. Allows the satellite to transmit and receive data from surface tracking stations.

SuperSTAR Accelorometers. Precisely measures the non-gravitational accelerations acting on the satellite.

Star Camera Assembly. Precisely determines two satellites' orientation by tracking them relative to the position of the stars.

Coarse Earth and Sun Sensor. Provides omnidirectional, reliable and robust, but fairly coarse Earth and Sun tracking. To be used during initial acquisition and when GRACE is operating in "safe mode".

Center of Mass Trim Assembly. Precisely measures offset between the satellite's center of mass and the "acceleration-proof" mass and adjusts center of mass as needed during flight.

Black-Jack GPS Receiver and Instrument Processing Unit. Provides digital signal processing; measures the distance change relative to the GPS satellite constellation; and provides secondary atmospheric occultation experiments.

Laser Retro-Reflective Assembly. Provides measurements of the GRACE satellite orbits relative to terrestrial tracking networks.

Globalstar Silicon Solar Cell Arrays. Cover the outer shell of the spacecraft and generate power.

Three-axis Stabilized Attitude Control System. Uses star camera and gyro sensors and a cold-gas Nitrogen thruster system, with magnetorquers for fine corrections of spacecraft position.

1750-A Microprocessor for Flight Computer. Performs calculations for attitude adjustment and telemetry processing.

Key Spacecraft Systems
The GRACE Project is divided into the following five systems:

Satellite System (SAT). Jet Propulsion Laboratory (JPL) leads the development of the Satellite System in partnership with Space Systems/Loral (SS/L) and Astrium GmBH (AGmbH). Engineers at JPL developed the GPS receiver and the laser retro-reflective assembly. AGmbH provides major elements of two flight satellites based on an existing small satellite designed for the Challenging Minisatellite Payload (CHAMP) mission. SS/L provides the attitude control system, microwave instrument electronics and system and environmental testing.

Science Instrument System (SIS). The SIS is managed by JPL and includes all elements of the inter-satellite ranging system, the GPS receivers, and associated sensors such as the star cameras and accelerometers. This system also coordinates the integration activities of all sensors, assuring their compatibility with each other and the satellite.

Launch Vehicle System (LVS). The LVS includes the three-stage ROCKOT launch vehicle, multi-satellite dispenser, and the personnel, test equipment and facilities for preparation, integration and launch of the satellites. The LVS is managed by the Launch Vehicle System Manager at Deutsches Zentrum fŸr Luft und Raumfahrt (DLR) and supported by the JPL Project and its contractors.

Mission Operations System (MOS). The MOS consists of facilities and resources of the German Space Operations Center (GSOC), tracking antennas at Weilheim and Neustrelitz, and other stations and facilities needed for supporting launch and early orbit procedures (LEOP) and contingency operations. These facilities are used to monitor and control the satellite, perform initial processing of the telemetry data, and deliver all data to the SDS for further processing and generating science products. In addition to real-time operations, the MOS function provides the Central Checkout System for ground testing using command and data interfaces. The operations team also monitors satellite performance and health throughout the duration of the mission. Mission operations are conducted at the GSOC control center in Oberpaffenhofen, Germany.

Science Data System (SDS). The SDS functions include science data processing, distribution, archiving and product verification. The SDS is a distributed entity and managed in a cooperative approach by JPL and the University of Texas Center for Space Research (UTCSR) in the US and GeoForschungZentrum Potsdam (GFZ) in Germany. The cooperative approach includes sharing of processing tasks, harmonization of product archives and validation/comparison of products. Data and products to be processed and archived by the SDS include corrected inter-satellite range and accelerometer measurements, GPS orbit and occultation data, orbit, gravity field and GPS occultation products. The SDS also receives, processes and archives ancillary data (e.g. meteorological fields) necessary for data processing and verification.

next: Management and Future
back: A Scientific Geodesy

Remote Sensing
Introduction
A Scientific Geodesy
Launch, Components, and Systems
Management and Future
Spacecraft Diagrams