Metrology’s Reach Demonstrated With Inter-Satellite Link

Article By : Bill Schweber

The idea of linking two orbing satellites via an interferometer to continuously measure their separation with ultraprecision may seem incredible, but it is being successfully done now.

It’s easy to get jaded about technology advances. All of us – and especially the non-technical public – are so inundated with truly impressive feats that soon many, if not all, become “so what, that’s no big deal” events. When you create technical “miracles” on a regular basis, it’s easy for the audience to have little regard for the work and persistence it takes to make them happen. There’s an old engineering maxim that the last 10% of a project takes 90% of the effort, and that’s largely true (those percentages do vary among projects, of course).

Think about basic and-based measurement and surveying. Back when George Washington was a surveyor, it required dragging and stretching chains across the terrain plus laborious calculations (and yet many of their results are amazingly accurate) but now it is done quickly and painlessly using GPS-guided, laser-based rangefinders plus complex algorithms executed nearly effortlessly. This simultaneous improvement of many orders of magnitude in both accuracy and speed has not come quickly or easily, of course, even if it seems that way.

Despite these truly incredible advances, scientists and engineers are striving for more and better, and improvements require almost unimaginably sophisticated and complex instrumentation. This is demonstrated, for example, by the Gravity Recovery and Climate Experiment (GRACE) which implements laser-based interferometry between orbiting satellites and operated continuously without interruption for over 55 day (about 850 orbits) during its first months of operation. I’ll be clear: this is not an Earth-based link to an orbiting spacecraft. Instead, its laser-ranging interferometer (LRI) directly links two satellites about 220 km apart to allow precise and real-time measurements. The LRI weighs just 25 kg and requires 35 W, an impressive pair of “big picture” specifications for these two common parameters.

Why do this project? Among other reasons, it allows for ultra-precise assessment of orbital changes which, in turn, are largely due to variations in the Earth’s gravitational field, which is far from uniform and in fact can change (earthquakes, for example, result in shifts of mass). Another use will be as Laser Interferometer Space Antenna (LISA), which will detect gravitational waves at much lower frequencies and higher sensitivity than the existing ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO) which achieved such stunning success in recent years.

The five-degrees of freedom two-way laser link between the spacecraft succeeded in linking and synchronizing on the first attempt. Every element of this system represents truly cutting-edge and beyond, technology. The LRI’s laser output power is a mere 25 mW at 1064.5 nm, and both satellites carry identical optical cavities with one of them stabilizing the frequency of the laser on the “master” satellite. The beam-steering mirror steers a beam with a 140 μrad half-cone angle and has a range of several milliradians in two axes and a speed of greater than 100 Hz. The LRI transmit beam must point to the other distant spacecraft with better than 100 μrad accuracy to ensure that enough light – just a few nanowatts are needed – arrives at the distant receiver’s aperture.

The high-level block diagram of the core of the LRI design (one in each satellite) is complicated, (Figure 1); I can’t imagine what a detailed system block diagram or electronic, optical, and mechanical schematic shows. The software must execute a large number of procedures ranging from basic beam management to high-level data corrections based on “distortion” as described by general relativity.

GRACE project

Figure 1 Functional overview of the LRI units on both spacecraft. The LRI units include the laser, cavity, laser ranging processor (LRP),optical bench electronics (OBE), triple mirror assembly (TMA), and optical bench assembly (OBA) with a fast steering mirror (FSM). (Image source: Physics Review Letters)

I won’t repeat details of the design, implementation, operation, and test results, including noise and error analysis or confidence level in the results. It is all discussed in their detailed paper “In-Orbit Performance of the GRACE Follow-on Laser Ranging Interferometer” published in the prestigious Physical Review Letters of the American Physical Society, with over 50 authors from 13 universities and commercial organizations). Just managing this project must have been a challenge in addition to the technology and implementation.

Advances like this are extremely impressive but don’t get much attention due to their complexity, esoteric nature, and hard-to-describe impact. In contrast, the earth-based Laser Interferometer Gravitational-Wave Observatory (LIGO) project did get a lot of attention – plus a well-deserved Nobel Prize for the project’s lead physicists (see “https://www.laserfocusworld.com/test-measurement/research/article/16569615/ligo-scientists-receive-2017-nobel-prize-in-physics”) ; I think the widespread attention was partially due to the crisp headlines it spurred such as “Experiment finally confirms Einstein’s prediction of gravity waves” – you can’t beat that for keyword hits!

Do you follow any of these extreme-precision, sophisticated physics and electro-optical projects? Are there some that have impressed you, or ones that you feel are overrated? Are they too far removed from your area of interest?

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