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I understand that Earth orbiting satellites are tracked using technologies such as radar, but due to the distance between the Earth and the Moon, they aren't really useful for Moon sats.

Is the problem of tracking lunar satellites still an unresolved one?

I've only found information about autonomous navigation (such as this paper on LUMIO which has a precision of ~30km, or this paper on using TRN in moon orbit, which has a much higher precision in the order of the tens of meters, but I'm not sure if I'm missing some obvious problem with it), but haven't found much on tracking lunar satellites.

Is it possible to do so?

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  • $\begingroup$ Isn't it true that if tracking objects at lunar range was a real problem then tracking the moon itself, or anything either natural or man-made travelling near it, would share that problem? $\endgroup$ Sep 20 at 0:11
  • $\begingroup$ Tracking a satellite orbiting the Moon via Earth-based radar is a (mostly) solved problem. NASA and other organizations track spacecraft with Earth-based radar to very high precision much, much further from the Earth than the Moon. $\endgroup$ Sep 20 at 11:45
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From Why was the 100m Green Bank dish needed together with DSN's 70m Goldstone dish to detect Chandrayaan-1 in lunar orbit?

"This computer-generated image depicts the Chandrayaan-1's location at time it was detected by the Goldstone Solar System radar on July 2, 2016. The 120-mile (200-kilometer) wide purple circle represents the width of the Goldstone radar beam at lunar distance. The white box in the upper-right corner of the animation depicts the strength of echo. Inside the radar beam (purple circle), the echo from the spacecraft alternated between being very strong and very weak, as the radar beam scattered from the flat metal surfaces."

above: "This computer-generated image depicts the Chandrayaan-1's location at time it was detected by the Goldstone Solar System radar on July 2, 2016. The 120-mile (200-kilometer) wide purple circle represents the width of the Goldstone radar beam at lunar distance. The white box in the upper-right corner of the animation depicts the strength of echo. Inside the radar beam (purple circle), the echo from the spacecraft alternated between being very strong and very weak, as the radar beam scattered from the flat metal surfaces." Credit: NASA/JPL-Caltech. From here

...Is the problem of tracking lunar satellites still an unresolved one?...

tl;dr: On July 2, 2016 the location of the at-the-time-dead Chandrayaan-1 spacecraft orbiting the Moon was determined by transmitting radar pulses from the Goldstone radar dish and receiving reflections with the Green Bank observatory.

This demonstration shows that it's possible, and that it is non-trivial to do radar detection and tracking of small spacecraft at the distance of the Moon.

There are now proposals both in China and the West to build dedicated deep-space radar networks.

Tracking is much, much easier and more accurate if the spacecraft is "not quite dead" and has at least a functioning coherent transponder.


If the spacecraft is dead

Tracking is something usually done from Earth. If the spacecraft is dead or has only passive radar reflectors, then radar or lidar active tracking will be needed.

Note that for optical tracking, a "passive reflector" would be a corner-cube retroreflector, or array thereof. There are several old ones on the Moon for tracking the Moon itself left by Soviet and US missions, and Beresheet had one as well though it is probably not operational.

It's possible future lunar satellites will cary retroreflectors as well. A solar sail put in LEO had one, as do some large communications satellites

If the spacecraft's coherent transponder is still working

If the spacecraft is still at least partially alive and it's coherent transponder is still operational, then it can receive signals from a tracking station (radio or optical) and turn them around and send them back to the same, or a different tracking station.

Then Earth can measure the total light time of the signal and its doppler shift to measure distance and radial velocity. If you make several measurements like this for something orbiting the Moon you can rapidly converge on its position and orbit with great accuracy. (The position of the Moon itself relative to Earth is known down to centimeters!)

When separate transmit and receive stations, or better yet two simultaneous receive stations are used, you can also use VLBI (very long baseline interferometry) along with the range-rate data to pinpoint the location.

VLBI tracking in cis-lunar space was tested and refined using lunar rovers and the ALSEP telemetry radio transmitters left on Moon by Apollo lander missions.

VLBI will be used to measure the location of the JWST in its halo orbit 1.5 million kilometers from Earth down to centimeters of accuracy. This is absolutely necessary to drive its delta-v budget for station keeping in its orbit to just a few meters per second per year. Part of the trick is also to use the solar shield as a solar sail (the Sun's photon pressure helps hold it in place just to one side of it's most stable halo orbit distance), but there's not much flexibility since it can't be steered completely independently from the telescope. (There is one axis of rotation possible; for some observations the telescope doesn't care how it is rotated around its optical axis).

But what about GPS?

...or near-future technologies?

Beacons or even coherent transponders could also work using light pulses rather than radio waves. In 10 to 100 Gbit/sec optical telecommunications the light pulses are only of order a centimeter long. The light beam itself does not necessarily have to be coherent, only the pulse train.

...or not-so-near-future technologies?

Beacons or even coherent transponders could also work using X-ray pulses rather than light pulses or radio waves.

Those pulses could be from artificial X-ray beacons, or they could be pulsars!

The role of deep space atomic clocks

For single beacons to be particularly helpful the spacecraft needs to know what time it is to nanosecond accuracy. Luckily there's a solution for that coming up.

related questions (the OP already has some familiarity with the topic!)

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    $\begingroup$ Thank you SO MUCH for this answer!! I am indeed on an arduous journey of learning and this helps a lot! Do you have an opinion on the TRN approach for autonomous positioning using an onboard camera? $\endgroup$
    – Bensas
    Sep 18 at 21:51
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    $\begingroup$ @Bensas it doesn't work until you're in orbit, and it doesn't work in the dark :-) $\endgroup$
    – uhoh
    Sep 19 at 1:05
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    $\begingroup$ @Bensas Also you probably need an additional fix on one or more other objects (e.g star trackers) so you know the exact direction your terrain camera is pointing which in some cases may be challenging. I think that for any given mission or spacecraft you just evaluate all the options and choose what's going to be the best set of capabilities to satisfy your requirements. It won't be a one-size-fits-all situation. $\endgroup$
    – uhoh
    Sep 19 at 1:13
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Radar may be used with a transponder in the satellite. A special range measurement signal is send from Earth to the Moon satellite and echoed back by the transponder of the satellite. The round trip delay time may be measured with meters resolution and precision.

This method was used for the Apollo mission. See How to get an initial setting of the range gate for a Lunar Laser Ranging using a new Retro Reflector for the first time?

The necessary power for conventional radar increases proportional to the fourth power of distance. When a transponder is used for radar, the power increases proportional to only the square of distance.

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