Built by Airbus Defence and Space in Friedrichshafen, Germany, under subcontract to JPL, the twin GRACE-FO satellites -- known respectively as GRACE-FO-A and GRACE-FO-B -- are identical in all respects except for transmit and receive frequencies. Each is 10 feet, 3 inches (3.123 meters) long, 2 feet, 6.7 inches (0.78 meters) high, 6 feet, 4.5 inches (1.943 meters) wide at bottom, 2 feet, 3.3 inches (0.69 meters) wide at top, and weighs 1,323.2 pounds (600.2 kilograms), including onboard propellant.
For the accelerometer to measure only non-gravitational forces, it is important that the spacecraft center of gravity be placed at the center of the proof-mass of the accelerometer. The mass-trim mechanism and associated mass-trim electronics serve this function. The six mass-trim mechanisms each consist of a mass moving on a spindle, with each pair providing center-of-gravity trim along one axis.
The mass trim mechanism will be completely operated from the ground. The mechanism is a rebuild from GRACE with slightly increased mass and trim ranges.
All science instruments, fuel tanks and batteries and other satellite subsystems are mounted on a carbon-fiber reinforced plastic platform. This material, which has a very low coefficient of thermal expansion, provides the dimensional stability necessary for precise range change measurements between the two spacecraft.
As with GRACE, the key science instrument for GRACE-FO is the microwave tracking system, known on GRACE-FO as the Microwave Instrument (MWI). The MWI provides precise (1 micron, about the diameter of a blood cell or a small fraction of the width of a human hair) measurements of the distance changes between the two satellites -- and, in turn, fluctuations in Earth’s gravity -- by measuring microwave signals sent between the two satellites. Each satellite transmits signals to the other at two frequencies -- 24 gigahertz (K-band) and 32 gigahertz -- (Ka-band), allowing for ionospheric corrections.
The MWI on each satellite consists of a redundant pair of ultra-stable oscillators, a K/Ka-band ranging assembly and an instrument processing unit.
The Instrument Processing Unit (IPU) is the nerve center for the science instruments for the spacecraft. It provides the digital signal processing functions for the K and Ka band signals, as well as for the GPS signals. It also provides various timing references for the satellite. The IPU includes a Trig Navigation Processor, Trig GPS sampler front end, and GRAIL Radio/Frequency Unit. The GPS component provides navigation information, time tagging/correlation of data products and ancillary Earth limb occultation measurements. The IPU subsystem includes the primary, redundant and occultation GPS antennas. The IPU was manufactured by JPL.
The experimental Laser Ranging Interferometer (LRI) is a technology demonstration that uses laser interferometry instead of microwaves to measure fluctuations in the separation distance between the two GRACE-FO spacecraft. This is the same measurement made by the MWI, but the LRI offers the potential to improve the precision of range fluctuation measurements by a factor of at least 10, largely due to the laser wavelength being 10,000 times shorter than the microwave wavelength. These improvements will enable the satellites to detect gravitational differences at smaller scales. The LRI will demonstrate precision inter-spacecraft laser interferometry for future GRACE-like geodetic missions. GRACE-FO LRI data are for technology demonstration purposes only and will not be the mission’s data of record for use by the science community.
The LRI was developed jointly by the United States and Germany. JPL managed the development of the laser, laser frequency stabilization reference cavity, and interferometer readout and control electronics, and supported spacecraft integration. Germany provided the optical components of the LRI (optical bench assembly and electronics, triple mirror assembly and baffles) and supported spacecraft integration. The German contribution was managed by the Max Planck Institute for Gravitational Physics Albert Einstein Institute (AEI) in Hannover, with implementation by SpaceTech (STI) in Immenstaad.