Space

GRACE-FO to precisely measure Earth’s gravitational field with lasers

GRACE-FO to precisely measure Earth’s gravitational field with lasers
GRACE-FO will demonstrate the effectiveness of using lasers instead of microwaves to more precisely measure fluctuations in the separation distance between the two spacecraft
GRACE-FO will demonstrate the effectiveness of using lasers instead of microwaves to more precisely measure fluctuations in the separation distance between the two spacecraft
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GRACE-FO will demonstrate the effectiveness of using lasers instead of microwaves to more precisely measure fluctuations in the separation distance between the two spacecraft
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GRACE-FO will demonstrate the effectiveness of using lasers instead of microwaves to more precisely measure fluctuations in the separation distance between the two spacecraft
The Laser Ranging Interferometer instrument
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The Laser Ranging Interferometer instrument
As the satellites orbit Earth, one following the other, these moving masses alter the gravitational pull below them, changing the distance between them very slightly
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 As the satellites orbit Earth, one following the other, these moving masses alter the gravitational pull below them, changing the distance between them very slightly
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When NASA's Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission launches on May 19, it will be able to map the Earth's gravitational field with unprecedented accuracy. Thanks to a new technique called laser ranging interferometry, the two satellites that make up the five-year mission will be able to chart gravitational anomalies with a ten-fold greater precision than the previous GRACE mission.

The original GRACE mission came to a close in October 2017, when the two orbiters shut down operations before making a controlled reentry into the Earth's atmosphere. Launched in 2002, the German/American project consisted of the spacecraft GRACE-1 and GRACE-2, which flew in close formation in low-Earth orbit.

Using a microwave ranging system, the pair could measure the distance between them to a few microns, or a fraction of the width of a human hair. This was combined with GPS tracking data, attitude data from star trackers, and an accelerometer to eliminate such external factors as atmospheric drag, orbital decay, and solar-wind pressure, to provide a precise record of the pair's velocity and separation distance.

As the satellites orbit Earth, one following the other, these moving masses alter the gravitational pull below them, changing the distance between them very slightly
 As the satellites orbit Earth, one following the other, these moving masses alter the gravitational pull below them, changing the distance between them very slightly

By doing this, scientists could measure how much each sped up and slowed down in response to changes in the Earth's mass below them. Such anomalies acted like an X-ray into the planet's interior as the spacecraft mapped out the structure of mountains, the shift of magma beneath the crust, the effects of earthquakes, and the global flow of water and ice sheets.

However, where GRACE relied on microwaves to measure the gap between them, GRACE-FO is upping the game by bringing lasers into play. Just as the GRACE spacecraft used the way in which microwaves interfered with one another to make measurements, GRACE-FO will use the interference of lasers for even more precise results.

Developed by NASA's Jet Propulsion Laboratory (JPL) in California and the Max Planck Institute for Gravitational Physics (Albert-Einstein Institut) in Germany, the new Laser Ranging Interferometer (LRI) uses wavelengths ten times shorter than microwaves to deliver an increase in precision that, until now, has been confined to the laboratory.

The Laser Ranging Interferometer instrument
The Laser Ranging Interferometer instrument

"With GRACE-FO, we're taking something cutting-edge from the lab and making it ready for space flight," says Kirk McKenzie, the LRI instrument manager at JPL. "The reason we spend decades working in the lab is to see our technology enable a new type of measurement and result in scientific discoveries."

NASA says the tricky bit will be for the GRACE-FO satellites to acquire one another while traveling 137 mi (220 km) apart. It's a bit like trying to lock onto a star using a high-powered telescope – not easy unless it has a lower powered sighting scope. In this instance, the lasers will flash signals at one another when first activated and then analyze any received through all the possible configurations of the twin craft, which will take about nine hours. However, it will take only a millisecond to set up the optical link once its acquired and locked.

This new technology will also help future missions with the potential to achieve resolutions of better than 200 mi (300 km) in diameter.

"The laser ranging interferometer on GRACE-FO is potentially an enabling technology for future missions around Earth or even to look at the universe," says Frank Webb, GRACE-FO's project scientist at JPL. "This new, higher precision measurement should enable more efficient missions in the future with lower mass, power and cost. We're eager to see how it performs and what new signals we might be able to tease out of the data."

The video below explains the principle behind GRACE-FO.

Source: NASA

Crazy Engineering: GRACE-FO

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3 comments
3 comments
Knut
The mass is used by the military as replacement to GPS. With this accuracy it is close to being able to replace GPS to identify where you are with say 10 feet accuracy. The mass can be measured by planes, as they move at a know speed through the grid determined by the mass of the earth. A bigger induction, the greater mass, allowing a moving object to know where it is - without emitting any radio signals or listening to anything that can be distorted. They measure the acceleration to more than 2 digits: 9.6xxxxx - the "xxxxx" tells where you are.
notarichman
could be useful for mineral exploration.
F. Tuijn
I remember the time, more than half a century ago, when lasers were a solution looking for a problem.