Satnav systems built for Earth used by Blue Ghost lander as it approached the Moon

An experimental module attached to Firefly Aerospace's Blue Ghost Moon lander successfully used Earth's orbiting satnav systems, a feat that suggests a specialized lunar positioning system may not be needed. The module is called the Lunar GNSS Receiver Experiment, aka LuGRE, which landed on the surface of the Moon Sunday …

[the Jim bloke]

GNSS uses signal time-in-transit to calculate distance

.. but probably has a much cooler acronym ..

so extra distance makes the calculation more relaxed, leisurely.. not like the knife fighting range that earth focused operations have to work at.

There is an upper limit - which is probably the 'word' length that is used to measure against synchronisation.

There would also be design issues to overcome - machine built to broadcast down towards earth may not send a strong signal in the other direction - or it could be fully unidirectional, i dont know.

Obvious one is no coverage on the 'dark' far side of the moon.

[MyffyW]

Re: GNSS uses signal time-in-transit to calculate distance

Perhaps a series of prisms could be used to refract the signal to The Dark Side of the Moon?

Gets her coat, the pink one, of course

[cookieMonster]

Re: GNSS uses signal time-in-transit to calculate distance

Very very good.

That deserves a pint.

[Paul Crawford]

Re: GNSS uses signal time-in-transit to calculate distance

Obvious one is no coverage on the 'dark' far side of the moon.

That would be my question, though not quite all of the dark side depending on GNSS orbit height above Earth to take a peek slightly to the side of what we see from Earth itself.

[Anonymous Coward]

Re: GNSS uses signal time-in-transit to calculate distance

My initial thought was that the satellites will be transmitting a directional beam towards earth, so very surprised that they could receive anything 200000 miles off the back of them. But then realised that the satellites on the other side of earth will be pointing in the correct direction (and presumably the beam width is such that they reach around the planet) - the extra ~28000 miles won't make much difference but the signals are likely to be a lot stronger.

And the calculations will be exactly the same regardless of distance from the satellites, but the required accuracy will be a lot looser (greater tolerances) and the angular differences will be smaller (needing higher precision in the calculations)

Overall, I have to say "bravo" to the team that decided to try it.

[Hans Neeson-Bumpsadese]

Re: GNSS uses signal time-in-transit to calculate distance

In cases where signals are reaching around the planet, I wonder what corrections need to me made for refraction, etc. I know from experience that GNSS signals coming straight down to earth can be disrupted/refracted/whatever as they come down through the atmosphere, so don't always travel in a straight line (in a previous life I worked on something which performed calculated the amount of inaccuracy introduced by such things, and did some jiggery pokery to correct for it). In a case where the signal is coming around the planet then that could be a lot more atmosphere to travel through.

[Bill Gray]

Re: GNSS uses signal time-in-transit to calculate distance

I suspect refraction (really, ionospheric issues) won't be an issue, because only satellites that are straight-line visible without passing through the atmosphere would be used. The number of navsats that are that close to the earth's limb, but not so far behind the earth that their signals are blocked anyway, will usually be zero or one.

The larger issue is that the entire navsat constellation, as seen from the moon, will span about eight degrees. You'd have something resembling the "urban canyon" problem, where a navsat receiver surrounded by skyscrapers can only see the navsats near the zenith, resulting in "geometric dilution of precision". (Or, for the urban canyon problem, sometimes not seeing the minimum four navsats required for a position fix. I don't think the lunar navsat units will have that problem. They'll be able to see plenty of navsats; they just won't be all that well spread around the sky.)

[Martin Gregorie]

Re: GNSS uses signal time-in-transit to calculate distance

In this context 'Lunar Far Side' is a more apropriate term to use than 'dark side', because anything that's currently on the Lunar Dark Side will be on the Lunar Bright Side in 14 days time.

[Anonymous Coward]

Re: GNSS uses signal time-in-transit to calculate distance

The dark side of the moon refers to how much we know about it / have studied it, not how much light it receives.

It's dark in the same sense as the "dark web" - it's hidden from our view.

[nematoad]

How?

The Lunar PlanetVac has begun to collect and transfer Moon dirt into sample return containers.

I would like to know how that works.

As the surface of the Moon is already in a vacuum, what system are they using to suck up stuff from the surface and transfer it to the containers?

Enquiring minds need to know.

[Steve K]

Re: How?

From what I have read elsewhere, they are using a blast of compressed gas to persuade the loose regolith to board the PlanetVac once the attachment is flush with the surface

https://www.nasa.gov/missions/artemis/clps/nasa-lander-to-test-vacuum-cleaner-on-moon-for-sample-collection/

[Phil O'Sophical]

Re: How?

I would like to know how that works.

As the surface of the Moon is already in a vacuum, what system are they using to suck up stuff from the surface and transfer it to the containers?

A sort of Lunar Ewbank ?

[MachDiamond]

Re: How?

"I would like to know how that works."

It's a one-shot device that uses a compressed gas cylinder to blast the surface and the debris is guided towards a collection scoop. Some years ago, Masten Space Systems flew a prototype on one of their rocket lander, activated the collector and took off again to simulate a collection mission. Of course it had to be done on Earth in atmosphere, but other static tests were done in a vacuum chamber. I'm sure there's info on that online somewhere.

[John Sager]

Precision?

Looking at the references there's an absence of information about accuracy. The way GPS works is that your position is on a sphere around each sat with radius given by the measured range. With 4 or more sats that problem can be solved to give your position. The accuracy is best if the directions to each sat are widely spaced, which they generally are on Earth.

However on the Moon all those sats are in more or less the same direction. The maximum angle subtended at the Moon by GPS sats at opposite ends of their orbit is only about 8 degrees. I hope that experiment can collect sufficient data before it freezes to death to give some estimate of expected position errors. I expect the time errors will be a lot less though.

[Graham Cobb]

Re: Precision?

I thought I had read that relativistic (gravitational) time dilation effects already have to be allowed for in GNSS calculations. These are also, presumably, much greater at lunar orbit distances.

Does anyone know if that will also be affecting the accuracy in this experiment?

[richardcox13]

Re: Precision?

For the special-relativistic effects that's all about your relative velocity: so no different.

For the general-relativistic effects: the moon is less deep in the earth's gravity well that either the satellites or normal (earth bound) receivers: so the effect will be less.

[Graham Cobb]

Re: Precision?

I was referring to the GR effects. I think I read that the difference in clocks between the satellites and earth-based receivers is enough to be noticeable and has to be allowed for.

My question was whether the difference between the satellites and the moon causes an effect that is larger or smaller than that (although of the opposite sign, of course).

[smudge]

Re: Precision?

If the device receives signals from satellites on the far side of the Earth from it - as others have surmised above - will it have to allow for the signal being bent (and thus elongated) as it passes the Earth? (cf Eddington and the 1919 total solar eclipse.)

I know the effect would be very small, but then the timing has to be very precise.

[John Sager]

Re: Precision?

The GR effects of Earth at the Moon are much less so clocks run slower on the Moon relative to those on Earth. GPS and the other GNSS sats take care of that for Earth by running very slightly slower than the nominal frequency. Also the message includes info to cope with time and position varying effects. A Moon receiver will have to remove or ignore the Earth-related compensations and sort its own out for the much greater change at the Moon.

[DS999]

Re: Precision?

But you can "see" a couple dozen satellites (counting GPS, Galileo, Beidu) only about 1/4 of them would be occluded by Earth at any given time. So the angle between them may be smaller at around 50,000 km (they are in medium Earth orbit, not low orbit like Starlink) but that's still fairly wide dispersion so it should be possible to get fairly high precision.

[Acrimonius]

Signal strength

The signal strength received would be interesting

[DS999]

Re: Signal strength

The distance may be further but there is no atmosphere to worry about. It should be well above the noise floor.

[kmorwath]

Interesting that there's no word LuGRE was designed and built in Italy

What's this, the new MAGA bending by The Register?

"The LuGRE payload is a collaborative effort between NASA’s Goddard Space Flight Center in Greenbelt, Maryland, the Italian Space Agency, their industry partner Qascom, and Politecnico di Torino."

https://www.nasa.gov/general/nasa-successfully-acquires-gps-signals-on-moon/

[frankvw]

I wonder if Garmin will release maps for the moon

Ah, I can see it now:

Braking burn...

Pitch-over...

Final descent...

Almost there...

AAAAAANNNDDD......

"Recalculating. Recalculating."

"When possible, make a U-turn."

[Hans Neeson-Bumpsadese]

Re: I wonder if Garmin will release maps for the moon

You have now reached your dusty nation

[lglethal]

Turn left at the next Crater...

How long before the first Rover is destroyed after trying to follow the Navi's instructions, and ends up falling into a crater that the Navi insists is not there?

"Go straight for 500m"

" I cant, there's a bloody big crater in the way!"

"Go straight."

"I cant! There's a crater."

"Overriding controls. Go Straight!"

"Ahhhh!!!!"

<End Transmission>

[Doctor Syntax]

"That's gotta involve some very cool mathematics to achieve that."

It'll need a few more digits for the altitude.

[AlanSh]

What3words?

It's time they expanded their horizons.

[KittenHuffer]

Re: What3words?

Sample moon locations:

It'sColdOutside

NoKindaAtmosphere

I'mAllAlone

MoreOrLess

[Hans Neeson-Bumpsadese]

Re: What3words?

Got to be OneSmallStep for the Apollo 11 landing site

[ArguablyShrugs]

Re: What3words?

Four King Maps would be better. Unlike proprietary What3Words, it's FOSS.

And panties.gobshite.muffdiver.buttstain really rolls off the tongue!

[fishman]

Accuracy?

GPS satellites orbit about 20.000 km above the earth (12,500 mi). That works out to a maximum range of 12 degrees as seen from the moon. On earth the angle between satellites (as seen by the observer) can be much larger. Does the smaller angle from the moon result in less accurate measurements?

[KittenHuffer]

Re: Accuracy?

Yes, but not due to the the angles. GPS doesn't work by looking at the angle between the recieved signals.

What GPS does is to look at the time delta between the various signals arrival times. On Earth you will be much closer to some satellites than others, so there will be a larger time delta between two signals.

On the Moon, the difference in the time deltas between two locations on the Moon will be much smaller. And will require a faster clock to measure the deltas, and a different set of calculations to calculate your position on the outside of the 'shell' of GPS satelltes, rather than on the inside of the same 'shell'.

[gnasher729]

Re: Accuracy?

With two satellites at a very close angle together, I think calculating your position will have bigger rounding errors. Interesting problem.

And then you get coordinates relative to the Earth center, and instead of transforming them to coordinates close to the earth surface into coordinates relative to the moon surface, so you need the distance earth to moon with extremely high precision.

Or put a few GPS receivers on the moon, which should help a lot with your precision. Like you do on Earth for extremely precise measurements.

[Anonymous Coward]

Re: Accuracy?

You would expect so, like being in the sun or being lit by a candle from a healthy distance.

What puzzles me more is the risk of GPS spoofing. As this is very easy to do, it would be costly and dangerous in space.

NASA mixing up Miles and Kilometers comes to mind, resulting in a somewhat higher landing speed than was intended.

[frankvw]

Re: Accuracy?

"What puzzles me more is the risk of GPS spoofing"

And rightly so. If Musk ever puts his money where his mouth is and rides one of his own rockets, I intend organize enough crowdfunding to hire a bunch of Russian or Chinese hackers, get into the USAF systems and set his controls for the heart of the sun.

[MachDiamond]

It's easy.

I take the tube from Endsville, change at Toricelli, nap rest of way. Getting there is the tube's problem.

[MachDiamond]

Is this an issue

Setting up tall nav beacons isn't much of an issue due to lack of wind and lower G. It's also not like people will be heading out on Sunday drives since a break down will be a major problem until AA (or AAA) set up operations and can be out to fix a puncture before one's air runs out (plus 5 minutes). Initially, marking a site of interest and finding it again shouldn't be a huge issue. As long as rover's aren't driven around willy-nilly, there will be roads of a nature. Junctions can be marked with small transponders that can be pinged giving back a grid coordinate and a battery level indication.

[Adam Foxton]

Ground Control to Major TomTom

10...9....8...7

This will not only be good for positioning, it'll also greatly improve timing precision and accuracy on the moon without having to carry an atomic clock on a payload.