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Satellites play persecution to measure gravity, achieve picometer accuracy

  Image of Two Satellites Above Ground

Spikes of gravity wave detectors are not just new scientific discoveries. The technology also has other applications. A good example of this is the mission to measure gravity, GRACE Follow On, which started last year. The first reports on the operation of the laser rangefinder have been released and this makes for an impressive reading. The space-time excitement causes a slight flicker at that distance, which is then detected by comparing the phase transition between the light that has traveled between the two mirrors and the light that has passed on a path that has not been affected by the gravitational wave. To put it in perspective, a gravitational wave detector measures changes that are much smaller than the diameter of the atom and are more like the diameter of a proton.

Gravity of GRACE

Such technology has found its way into space. to increase the sensitivity of Earth monitoring tools. On Earth we have stationary detectors waiting for gravitational waves to pass through them. In orbit, the detector moves and can measure subtle changes in the Earth's gravitational field. another. Or will they be fixed if Earth's gravity does not change in time and space. GRACE used a radar system to measure the distance between two satellites to track these changes by providing information about the gravitational field through which it passes.

Changes in this distance can be translated to local acceleration measurements due to gravity. This, in turn, is used to measure things like the volume of water in aquifers. In this case, the battery of GRACE-2 failed and the pair was cremated in the atmosphere of the Earth. Grace tracking GRACE-FO started in 201


GRACE-FO is mainly a GRACE copy but scientists have taken the opportunity to include a new distance measurement tool that would outperform the old radar system GRACE. If it succeeds, it would mean that measurement data will not limit the way we interpret the data. Instead, tidal patterns need to be improved.

It also happens that the technology required to perform this type of station measurement is exactly the technology required for LISA (proposed space-based gravity wave observatory). The LISA tracker mission was extremely successful, but as a single companion, it could not test driving technology. So, in many ways, the GRACE-FO satellites are also a light mission of the LISA tracer. And, let's just say it, it absolutely breaks the design specifications. For a period of 5 to 1,000 seconds, the system should be able to detect changes between 2 and 40 billionth of a meter between two satellites that are separated by 220km. However, the team reported a sensitivity as low as 300 trillion a meter . To put it in perspective, the original GRACE radar system is sensitive to changes of about 10 micrometers.

How do you achieve this kind of accuracy? In short, with lasers. More seriously, the laser beam of the main satellite is stabilized to have a very precise frequency. This laser fired into the satellite. This in itself is an achievement because the laser has to be steadily moving in the right direction (the radar system manages this with a fixed antenna). The subordinate companion uses the incoming light (all 25nW) to do two things. First use a small amount to check if the laser points in the right direction. The rest of the light is used to set the phase of its own laser, which is sent back to the main satellite – the resulting laser light is too weak to reflect back.

The light received by the captain gets a phase shift relative to transmitted light that is proportional to the distance between the two satellites. Since the distance is constantly changing, this is measured as an extra frequency in the resulting laser spectrum. Therefore, measuring the frequency of the laser input range is a measure of the distance, which in turn measures the acceleration due to gravity.

In the short term, this means that GRACE-FO data will be even better than having to return to work. In the long run, this means that more than LISA technology will be validated with the added benefit of a long-term sustainability study. In a very short time, it gave me a good reading for the weekend.

Physical Review Letters, 2019, DOI: 10.1103 / PhysRevLett.123.031101
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