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For more information and links on The Planetary Society's LightSail 2 and its corner cube retroreflectors, see the question Planetary Society's LightSail Spacecraft's corner cube reflectors; how large, and corrected for aberration? and the answer there as well.

Like the radio analog use of phase-locked coherent transponders for range-rate measurements on spacecraft, the retroreflectors on spacecraft allow for return of Earth-based (or in principle space-based) signals to "bounce" off of a spacecraft and return to Earth. The round-trip time of flight gives information on distance, and the rate of change of this time (in the case of laser pulses) or in the doppler shift (in the case of radio) gives the radial component of the relative velocity.

Telescopes are used to collimate laser pulses to a narrow beam so that enough light hits the reflector so that enough light is returned to be detected and timed. You could call this the "Tyranny of the Radar Equation", in other words, the $1/r^4$ law.

In order to use this then, the beam has to be tightly collimated which means you have to already know to a high degree of accuracy where to point the ground station's laser telescope in order to hit the spacecraft.

But if you know that, it means you already know the spacecraft's orbit. It seems like a catch-22 scenario.

It's pretty hard to hunt. The corner cubes on this spacecraft are relatively tiny, which means the return signal will be particularly weak compared to something like LAGEOS. Photon counting is arduous, and so a search pattern might require a huge amount of time and be impractical.

Question: So, by 'how will it be used' I'm really asking how will they already know where to point the laser, and if they know, then what new information will they get from these measurements if they already knew the orbit?

Also, what ground station are they planning to use for laser ranging?

edit: Thinking further, I suppose if laser ranging were used sufficiently frequently, it could be the source of the information on the orbit. In other words, regular laser ranging $n$ times gives you enough information to know where to point the ($n+1$)th time.

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This is an answer to "what ground station are they planning to use":

Tracking LightSail 2 will be done by International Laser Ranging Service, or ILRS.

The ILRS consists of about 40 laser ranging observatories around the world. There's no single overseer or funding body; many government entities contribute to the network for the good of everyone involved. NASA, for instance, helps operate eight observatories and manages a laser ranging data center at Goddard.

Noll said it's important to note that laser ranging doesn't supplant the orbital information JSpOC provides; it complements it. Without orbital parameters from an entity like JSpOC, the ILRS wouldn't know where to aim their lasers in the first place! Another interesting complementary use for the technology is GPS satellites; for GPS to be accurate, we have to know precisely where the satellites themselves are, and laser ranging provides a cross-check for that information.

Dave Spencer, the LightSail 2 project manager, said he's hoping the ILRS will be able to laser range the spacecraft both before and after the solar sail is deployed. Before sail deployment, the CubeSat will be tiny. And after deployment, the orbit will be changing rapidly due to the thrust from solar sailing, as well as atmospheric drag acting on the large sail area.

Laser ranging LightSail 2 is definitely doable, but Noll said it won't be easy. "This will be a difficult mission," she said.

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