Spaceflight Insider

GRACE-FO uses ‘Range Finder’ to see mountain effects

Artist's rendering of the twin spacecraft of the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission. Image Credit: NASA/JPL-Caltech

Artist’s rendering of the twin spacecraft of the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission. Image Credit: NASA/JPL-Caltech

NASA’s Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission has successfully completed its first mission phase and demonstrated the performance of the precise microwave ranging system that allows it to measure how mass migrates around the Earth. GRACE-FO is a successor to the original GRACE mission, which began orbiting the Earth on March 17, 2002. The twin satellites launched atop a SpaceX Falcon 9 rocket from Vandenberg Air Force Base in California on May 22 of this year (2018). Engineers and mission controllers from NASA and the German Research Centre for Geosciences (GFZ) spent the first few days following the launch completing the mission’s early operations phase. The mission then moved into an 85-day in-orbit checkout period. If everything continues to go as planned, the twin spacecraft will begin science operations once the checkout period has been successfully completed.

The two GRACE-FO spacecraft were maneuvered into their operational orbit formation approximately 137 miles (220 kilometers) during the mission’s initial phase. Spacecraft systems were powered on and checked out. The mission team then activated both primary instruments aboard the spacecraft: the accelerometers, which measure forces other than gravity that affect the spacecraft, such as solar radiation pressure or atmospheric drag; and the microwave ranging instruments that measure the distance between the two spacecraft as they orbit the Earth. The ranging instruments can measure distances with a precision greater than one micron, smaller than the diameter a blood cell, or one-tenth the width of a human hair.

Along the satellites’ ground track (top), the inter-spacecraft distance between them changes as the mass distribution underneath (i.e., from mountains, etc.) varies. The small changes measured by the Microwave Ranging Instrument (middle) agree well with topographic features along the orbit (bottom). Image Credit: NASA/JPL-Caltech/GFZ

Along the satellites’ ground track (top), the inter-spacecraft distance between them changes as the mass distribution underneath (i.e., from mountains, etc.) varies. The small changes measured by the Microwave Ranging Instrument (middle) agree well with topographic features along the orbit (bottom).
Image Credit: NASA/JPL-Caltech/GFZ

The largest gravity variations observed by the satellites during a single orbit are due to massive mountain ranges. To demonstrate the capabilities of GRACE-FO’s microwave ranging system, the mission team analyzed its measurements of changes in the distance between the twin satellites as they flew over the Himalayas. The results are shown in a figure that team members called “The Himalaya Plot.” The plot’s wavy lines show how the distance between the satellites varies as they fly over oceans, land masses and mountains as they orbit the Earth. The observed distance changes between the satellites, which can be as large as hundreds of microns, fit well with expectations. Based on these results, the team is confident that the microwave ranging system is performing well.

By measuring small changes in the distance between the twin satellites, GRACE-FO can detect month-to month variations in Earth’s gravity field caused by the movement of mass as small as about a half-inch (1-centimeter) of water over an area of approximately 200 miles (320 kilometers) in diameter. The data from the satellites will be used to make monthly maps of the Earth’s average gravity field, providing details of how mass, mostly water, is moving around the planet. GRACE-FO data will provide insights into large-scale changes in our planet’s glaciers and ice sheets, Earth systems processes that alter our environment, such as earthquakes and droughts, and some impacts of human activities, such as changes in the levels of aquifers as the result of pumping underground water for use in agriculture.

Video courtesy of NASA/JPL-Caltech

 

 

 

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Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.

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