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Satellites play the chase to measure gravity, achieve the accuracy of the picometer



Image of two satellites on earth.

The derivations of gravitational wave detectors are not just new scientific discoveries. Technology also has other uses. A good example of this is the gravity measurement mission, GRACE Follow On, which was launched last year. The first reports on the performance of your laser rangefinder have been published, and the reading is impressive.

Gravitational wave detectors work by measuring small changes in the distance between two mirrors. Ripples in spacetime cause a small oscillation in that distance, which is then detected by comparing the phase shift between the light that has traveled between the two mirrors and the light that has traveled along a path that was not affected by the gravitational wave. To put it in perspective, a gravitational wave detector measures changes that are much smaller than the diameter of an atom and more closely resemble the diameter of an individual proton.

The gravity of GRACE.

A similar technology found its way into space to increase the sensitivity of Earth monitoring instruments. On Earth, we have stationary detectors that wait for gravitational waves to pass through them. In orbit, the detector is moving and can measure subtle changes in the Earth's gravitational field.

That brings us to GRACE, an acronym for gravity recovery and the climate experiment, a pair of satellites that orbited the Earth at a fixed distance from each other. Or they would be fixed if the Earth's gravity did not change in time and space. GRACE used a radar system to measure the distance between the two satellites to track those changes, providing information on the gravitational field through which it was traveling.

Changes in that distance could translate into local measurements of acceleration due to gravity. This, in turn, is used to measure things like the volume of water in aquifers.

GRACE turned out to be very useful but, like all satellite missions, he finally died. In this case, the GRACE-2 battery failed and the couple was incinerated in Earth's atmosphere. The GRACE follow-up, called GRACE-FO (follow), was launched in 2018.

GRACE-FO is primarily a copy of GRACE, but scientists took the opportunity to include a new distance measurement tool that would exceed the old GRACE radar system. This, if successful, would mean that the measurement data would not put a limit on how we interpret the data. Instead, the tide models would have to be improved.

It also happens that the technology required to make this type of station maintenance measurement is exactly the technology required for LISA (a gravitational wave observatory based on the proposed space). The mission of the LISA search engine was enormously successful but, being a single satellite, it could not test the station maintenance technologies. Therefore, in many ways, GRACE-FO satellites are also a light mission of the LISA pioneer.

Quiet career

Since June 14, when contact was made with satellites, scientists have been testing the laser range search system. And, to put it simply, it absolutely breaks the design specifications. For a period of time between 5 and 1,000 seconds, the system should be able to detect changes in distance of 2 to 40 billionth of a meter between two satellites that are separated by 220 km. However, the team reports sensitivity as low as 300 trillion one meter. To put this in perspective, the radar system in the original GRACE was sensitive to changes in the level of approximately 10 micrometers.

How to achieve this kind of precision? In short, with lasers. More seriously, a laser beam on the master satellite is stabilized to have a very precise frequency. That laser fires at the slave satellite. This, in itself, is an achievement because the laser must be continuously oriented in the right direction (the radar system handles it with a fixed antenna). The slave satellite uses the incoming light (all 25nW of it) to do two things. First use a small amount to verify that the laser is pointing in the right direction. The rest of the light is used to establish the phase of its own laser, which is sent back to the master satellite: the laser light received is too weak to simply be reflected.

The light received in the master acquires a phase change with respect to the transmitted light that is proportional to the distance between the two satellites. Since the distance changes continuously, this is measured as an additional frequency in the received laser spectrum. Therefore, a frequency measurement of the incoming laser spectrum becomes a distance measurement, which, in turn, becomes a measurement of the acceleration due to gravity.

In the short term, this means that GRACE-FO data will be even better than expected, and the modelers will have to return to work. In the long term, it means that most of the technology for LISA will be validated with the additional benefit of a long-term robustness study. In the very short term, he gave me a good weekend reading.

Physical Review Letters, 2019, DOI: 10.1103 / PhysRevLett. 123.031101 (About DOIs)


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