The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) twin satellite mission is scheduled for launch on Monday, 21st May, from the Vandenberg Air Force Base in California. It will be aboard the SpaceX Falcon 9 rocket, alongside a number of commercial satellites.
As its name suggests, GRACE-FO is a follow on to the original GRACE mission which was launched in 2002 and reached end of life in November 2017. Both missions use innovative microwave measurement techniques to map deviations in the Earthâs gravity fields, which in turn enable changes in the oceans, ice sheets and land masses to be monitored. Both the original, and follow on, are joint missions between NASA and the German Research Centre for Geosciences (GFZ).
Each of GRACE-FOâs identical satellites weigh around 600 kg and measure approximately 3 m x 2 m x 0.8 m. Theyâll be launched into low Earth circular polar orbits at around 490 km altitude, separated by between 170 and 270 km and to maintain this distance will require regular orbital moves during the mission life.
The essence of the mission is to constantly measure how far apart the two satellites are by sending microwave signals between them. This distance changes under the influence of the Earthâs gravity, by pulling the lead satellite away from its partner. Precisely measuring this, makes it possible map the changes in the Earthâs gravitation field. These changes are caused by the movements in water, ice and earth, and some examples with the GRACE datasets include:
- Loss of ice sheets in Antarctica and Greenland
- Seasonal changes in the Amazon basic
- Loss of groundwater in California.
The key instruments on board GRACE-FO include:
- Microwave Instrument (MWI): this sends K and Ka band microwave signals between the two satellites to precisely measure their distance apart. Its reported that they are accurate to one micron â the diameter of a blood vessel!
- Laser ranging interferometer (LRI): This experimental instrument is an addition to the original GRACE mission and makes the same measurement as the MWI, but using lasers instead. Itâs estimated that this could improve accuracy by at least a factor of 10.
- Accelerometer: Measures the factors other than gravity affecting the satellite orbits.
- GPS Receivers: Determine the exact position over the Earth to within a centimetre.
GRACE-FO will operate on a decaying orbit and so will not have constant repeating ground-track, but it should map the globe every thirty days. In addition, it will produce hundreds of daily profiles of temperature and water vapour.
GRACE produced some fascinating insights during its fifteen year mission; it had been hoped that the two missions would be in space together to allow for calibration between them, but this was not possible. Scientists are itching to get new data from GRACE-FO following the six month gap and it’s hoped that the new pair will provide further insights into how the worldâs climate is changing, the loss of the ice sheets and monitoring how much water weâre currently using â all of which could have major impacts on our future!