How it Works
GRACE-FO is different than most Earth-observing satellite missions. Except for its atmospheric limb sounder GPS measurements, which will provide data to aid weather forecasting, it does not carry a suite of independent science instruments that point down at Earth’s surface to observe some part of the electromagnetic spectrum. Instead, the twin satellites act in unison as the main instrument, pointing their microwave instruments at each other to obtain data on distance changes between the two spacecraft that are used to generate monthly models of Earth’s gravity field.
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) measurements of the distance change 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 ultra-stable oscillators serve as the frequency and clock reference for the GRACE-FO satellites. They are built by the Johns Hopkins University Applied Physics Laboratory and are based on the ultra-stable oscillators flown on NASA’s GRAIL mission.
K/Ka-Band Ranging Assembly
The K/Ka-Band Ranging Assembly is the radio frequency front-end of the GRACE-FO measurement system. It is comprised of a dual-band, dual-linearly polarized horn antenna, waveguide feeds and redundant Microwave Assembly K/Ka-Band transmitter/receivers. The ranging horn transmits and receives K-band (24 gigahertz) and Ka-band (32 gigahertz) carrier signals to and from the other GRACE-FO satellite. The antennas are nearly identical to those flown on GRACE, with a few modifications to the aperture cover and feed components. The MWAs up-convert the ultra-stable oscillator signal to K and Ka-Band for transmission, and down-convert the received K and Ka-Band signals to baseband frequencies of 670 kilohertz and 500 kilohertz. They are based on the original GRACE design, with minor improvements from the design used on NASA’s GRAIL mission on the Moon.
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 clocks for the satellite and performs data processing for the star camera attitude quaternions. 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.
Global Positioning System
A GPS system keeps track of each spacecraft’s position relative to Earth's surface, and onboard accelerometers record forces on the spacecraft other than gravity, such as atmospheric drag and solar radiation. These data are then combined to produce monthly maps of the regional variations in global gravity and the corresponding surface mass variations.
The GPS receivers are used as references to determine the precise location of the two satellites in orbit. The receivers measure changes in the distance of the GRACE-FO satellites to the constellation of GPS satellites circling Earth. Each spacecraft has three antennas. One antenna is used to collect navigation data, one collects the mission’s atmospheric occultation data, and the other is used for backup navigation. It also provides digital signal processing functions for the Microwave Instrument. The GPS receivers were manufactured by JPL.