Lasers tell the distance between spacecraft

Print edition : August 30, 2019

As each spacecraft passes over, say, a large mountain, it is drawn a bit downward by the extra gravitational force. This leads to a sequence of increases and decreases in the separation between the two satellites. Photo: D. Schütze and G. Heinzel/AEI, Hannover

Researchers have measured the distance between a pair of satellites 200 km apart with an accuracy of one-fifth of a nanometre (1 nm is a billionth of a metre)—about the size of an atom—by bouncing laser beams between them. This will be useful to both map out the earth’s gravitational field with ultra-precision and detect gravitational waves produced by violent astrophysical events.

Large geophysical changes, such as large earth movements, massive ocean currents or glacial and ice-sheet transformations due to climate change, can, in principle, be detected by measuring tiny changes in the earth’s gravitational field. The collaboration behind GRACE Follow-On (GFO), a space mission run by NASA that was placed in orbit around the earth in May 2018, and the German Aerospace Center (DLR) has now demonstrated a technique for making such measurements with unprecedented precision. The team detected gravity-induced changes in the distance between two earth-orbiting satellites by bouncing laser beams between them.

GFO is a successor mission to the GRACE (Gravitational Recovery and Climate Experiment) mission, which was run by NASA and the DLR and flew from 2002 to 2017. In the earlier experiment, GRACE included two spacecraft on a 450 km circumpolar orbit. Their separation of around 200 km was measured by bouncing microwaves between them combining with GPS measurements of orbit parameters.

When the satellites passed over a region of the earth with slightly stronger or weaker gravity, the spacecraft experienced a small change in their separation. The follow-on mission has now measured gravity-induced changes by measuring the inter-satellite distance using interference of laser beams, and thus to far greater precision.

The major challenges of such measurements in space include maintaining a stable frequency for the laser, correcting for distance variations not caused by gravity, say by spacecraft drag due to atmospheric turbulence, and cutting out the intrinsic noise arising from laser operation. The direction of the laser beam must be continually adjusted to keep it on target. According to the project leader Gerhard Heinzel of the Max Planck Institute for Gravitational Physics (AEI), Hannover, for two satellites that are about 200 km apart, the beam tracking keeps the laser locked on to a spot that moves by no more than a metre.

Changes in distance as tiny as 200 picometres (1 pm is a trillionth of a metre) between two satellites about 220 km apart could be measured accurately with the system, according to the first results that have been published in a recent issue of “Physical Review Letters”.

“The laser link is kept alive without any interruption for hundreds of orbits around the earth, while the spacecraft chase each other from pole to pole at speeds of more than 25,000 km per hour,” Heinzel said. The capability points to laser ranging becoming the future technique for space-based earth measurements.

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