Open-source communications by bouncing signals off the Moon

by fortran77 • 9 months ago

277 82 9 months ago

gus_massa• 9 months ago

I started reading thinking it was impossible but it has been done with other devices https://en.wikipedia.org/wiki/Earth%E2%80%93Moon%E2%80%93Ear...

cactacea3• 9 months ago

Not impossible, just extremely difficult. I'm a ham and getting some contacts over moonbounce is a personal goal of mine. Historically this kind of thing has required some pretty large antenna arrays and very high power though:

https://hamradio.engineering/eme-moonbounce-bouncing-signals...

http://www.g4ztr.co.uk/app/download/13284489/RaCcom_Feb14+EM...

http://www.g4ztr.co.uk/app/download/13300096/Radcom_Mar144+E...

dbcurtis2• 9 months ago

Isn't there a moon bounce mode in WSJT (or one of those digital modes) that provides enough coding gain that 100W and a single large Yagi is enough? I seem to recall hearing something like that... but, yeah, on CW a monster antenna and the legal limit of 1500W seems to be the median system.

A long time ago I started collecting parts for a 432MHz EME system. Life got in the way and I never built it out. Good luck with your endeavor!

ErroneousBosh3• 9 months ago

It's been being done for 70 years or so. You just need to be able to generate quite a lot of power on VHF upwards, and have a very low-noise antenna preamp, and a large array of directional aerials to focus the signal.

If you're reasonably handy with simple hand tools you can build a moonbounce array for a couple of thousand and a month or so of evenings.

Polizeiposaune• 9 months ago

The big dish antenna at Stanford University (visible from I-280) was, among many other things, used to monitor Soviet radar signals from Sary Shagan in Kazakhstan, that bounced off the moon some of the time.

The wikipedia article:

https://en.wikipedia.org/wiki/Stanford_Dish

links to:

https://web.archive.org/web/20201108114110/https://www.cia.g...

which has this tidbit which explains why it works as well as it does when it works:

"Fortunately for us, the moon appears only slightly rough to radio waves; most of the reflected energy comes back from an area at the near point just a few miles in diameter. The bulk of the energy striking farther around on the side is reflected out into space and never returns to earth."

cactacea3• 9 months ago

Expected array gain: ~39.3 dBi / EIRP: ~63.1 dBW

Tx power: 1 W per antenna

Yeah... so free space path loss at legal frequencies for hams this thing can transmit on is ~283dB. Neat idea but consider me skeptical. Having said that I can see some interesting applications for this kind of gear, EME seems overly optimistic though.

jacquesm• 9 months ago

A few hundred Watt at a minimum would be my first guess.

cactacea3• 9 months ago

Yeah that is what is used for moonbounce today (if not full legal power - 1500W for US amateurs) but these little panels won't put out anything remotely close to that. Hence my skepticism.

dbcurtis2• 9 months ago

At those power levels they would have to use some kind of highly error-corrected modulation and coding scheme to provide enough coding gain to overcome the path loss. I agree they are pretty optimistic, but until they detail their modulation scheme, it's hard to tell.

A few years ago I was experimenting with 900 MHz LoRa for a work project -- we had need to communicate a very small data payload from inside elevator cabs, with forgiving latency requirements. So we took a LoRa board to a hotel building 2 city blocks away from our lab and cranked the coding gain up to the max, which gave us about a 1 byte payload every second. Perfectly sufficient for our application. Astoundingly, we had great copy in our lab even when the doors of the elevator cab were closed, inside a building 2 blocks away. I can't remember the power level, 500mW I think, but I may be wrong.

firesteelrain2• 9 months ago

People use WSJTX software and Q65 mode

mystraline• 8 months ago

This person has a full SDR LoRa transceiver stack and the meshtastic client code.

https://gitlab.com/crankylinuxuser/meshtastic_sdr

drmpeg3• 9 months ago

It's theoretically possible.

63.1 dbW = 93.1 dBm (240 watts + 39.3 dB gain)

path loss at 5760 MHz = 283.2 dB (at perigee)

RX gain = 39.3 dB

93.1 - 283.2 + 39.3 = -150.8 dBm

Noise floor at 1.2 dB noise figure and 500 Hz bandwidth = -151.9 dBm

SNR = +1.1 dB (easily detectable by ear with CW).

Neywiny• 9 months ago

It's 1 watt per antenna. They have 240, or 53.8 dbm. So assuming 39.3 and your 283 (which seems to be around what I'm seeing online) that's -283+(39.3*2)+53.8=-150.6 dbm receive power. That should be plenty.

amelius• 9 months ago

Latency?

_bernd• 9 months ago

1 sec up and 1 sec down... more or less. Speed of light and distance to the moon, two times.... roughly.

whatsupdog3• 9 months ago

I'm more interested in bitrate.

radiorickky2• 9 months ago

300bps

firesteelrain2• 9 months ago

KA1GT recently found a $100 “solar cooker” dish on AliExpress. Also available on Amazon. It was tested back in August.

Announced on the EME Facebook Group: https://www.facebook.com/share/p/19zLsGZiE7/?mibextid=wwXIfr

Output power was 500w

NoiseBert69• 9 months ago

There's nothing that can't be modulated. Lol.

zerosummer• 9 months ago

The open.space phased array is a similar power, but an electronically steerable all-digital beamformer.

Was covered on zr: https://www.zeroretries.org/p/zero-retries-0224?#%C2%A7opens... Looks beautiful on a tripod.

skeptrune• 9 months ago

I'm skeptical, but how can you not cheer for this? Sounds so awesome.

NewJazz• 9 months ago

Have you read Three Body Problem?

kieranl• 9 months ago

I got to see this in person at pacificon a few weeks ago. Also the creator is my friend from UIUC who I consider a brilliant rf/DSP engineer.

The demo was able to show and end to end tx chain from gnuradio to a receiver. Really excited to see this! As there are a myriad of other things that this hardware can be used for as well.

mrtnmcc2• 9 months ago

Great seeing you at Pacificon!

We’re starting with the “Quad” tile — a 4 Tx × 4 Rx SDR designed for arraying — and expect to ship the first units toward the end of this year. They're actually quite capable as a standalone SDR. A Quad can interface directly with a Raspberry Pi 5, and we’ve built a combined enclosure for the SDR + Pi setup. You can run SDR software locally on the Pi or stream IQ samples over gigabit Ethernet to a remote PC.

Software support includes GNU Radio, Pothos SDR, and just about any tool compatible with SoapySDR. We’re also doing some fun demos, like visualizing Wi-Fi signal sources in real time ("Wi-Fi camera") and performing mm-scale 3D localization—a prerequisite for the automatic array calibration.

Larger arrays are assembled by simply tiling these Quads into an aluminum/PCB lattice framework, enabling anything from compact 4-antenna MIMO nodes up to 240-element lunar-bounce arrays. The goal is to have full phased-array capability by March 2026.

The broader vision behind open.space is to make advanced RF and space-communications hardware open and accessible—so anyone can experiment with technologies once limited to national labs: moon-bounce (EME) links, satellite reception, terrestrial RF imaging.

Happy to answer questions here.

One thing I'm excited about getting working is mobile moon bounce!

drmpeg3• 9 months ago

Will you have arrays with the opposite antenna polarity for point to point links? That is, LHCP (Tx), RHCP (Rx) instead of RHCP (Tx), LHCP (Rx).

baobrien• 9 months ago

What does the RF front end look like? I see the Lattice ECP5, but what are you using to go from bits to waves?

dreamcompiler4• 9 months ago

If the goal is only to communicate with people on the other side of the world, HF ionosphere skip can do that with cheap 100-year-old technology (although transistors make it easier).

I assume the goal is to do something cooler than that.

lll-o-lll• 9 months ago

I assume the goal is to do something cooler than that.

Yes. Bounce the signal off the moon. The moon.

cwillu• 9 months ago

What if we tried more power?

topspin2• 9 months ago

Hams bounce signals off of everything. Aircraft scatter, for example. Probably the coolest is using the ion trails of meteorites. When meteors punch through the atmosphere, they create ionized trails. These trails can reflect RF signals. Some of these happen to be optimal at low VFH frequencies, and hams make contacts using frequencies that ordinarily don't work beyond line of sight.

kragen5• 9 months ago

The entire HF band, including the parts already used for something, is only 27 MHz of bandwidth, it's full of noise, and at any given time only a fraction of it can propagate to the other side of the world, dependent on time of day and, literally, sunspots. This antenna has 1100 MHz of bandwidth, the analog front end has 40 MHz for any given conversation, and noise levels are much lower. It could conceivably deliver Shannon bit rates one or more orders of magnitude higher. But it only works when the moon is visible to both sides of the connection.

I discussed these possibilities and some more challenging ones in 02013 in https://dercuano.github.io/notes/ultraslow-radio.html, although I was considering laser moonbounce rather than phased-array microwave moonbounce because of the higher antenna gain available.

dreamcompiler4• 9 months ago

I didn't see any discussion on your page of the retroreflectors the Apollo astronauts left on the moon. These were put there for distance measuring but they might be useful for laser-based communication too.

Caveat: Retroreflectors only reflect in the same direction as the incoming beam. But I'd guess that imperfections in their construction together with the roughly 1 degree of arc spanned by 2 stations on opposite sides of the earth might make this idea practical with a better S/N than using only the lunar surface as a reflector. But I don't know. They might be a lot more precise than 1 degree.

euroderf• 9 months ago

This was a Cold War thing to surveil Soviet air defense radars.

whatsupdog3• 9 months ago

For someone not well versed with the terminology, can someone please tell what kind of bitrate this can provide? In bytes per second.

shakna5• 9 months ago

It's not even a byte per second. The latency of the distances involved mean you're looking at around two seconds per round trip, plus a little extra because of the fuzziness of radio in space and absolutely everything that can distort it.

There's a lot of math that goes into selecting the right bit-width for the signal, which I ain't doing here and now [0], but most 24dB things tend to be 32bit for reasons. The arrays here are a bit more, but probably fit that kind of channel.

Assuming 32bit and 30dBi, you'd be sending at roughly 20-30MHz, and receiving at about 1kHz. (Less if you hit bad weather.)

So... 1 bit per second. Not byte. Bit.

[0] https://www.spaceacademy.net.au/spacelink/spcomcalc.htm

whatsupdog3• 9 months ago

Thank you so much. Are these rough calculations based on the smallest (quad), mini or the large array?

radiorickky2• 9 months ago

No, if you do the math it's about 40 bytes per second (300bps) for Earth-Moon-Earth using their 240-antenna array.

shevy-java2• 9 months ago

Ok so ... moon is slow going.

djexjms• 9 months ago

This is obviously incorrect. Latency is not the same as bandwidth. EME hobbyists will bounce voice signals off the moon.

drmpeg3• 9 months ago

A lightly coded (13/15 LDPC) 256QAM OFDM signal at 40 MHz wide could do 250 Mbps.

Or 31.25 million bytes per second if you prefer.

This would be for a point to point terrestrial link. OFDM probably wouldn't work for EME (at any power level).

shevy-java2• 9 months ago

The moon is so useful.

fourthark• 9 months ago

We probably wouldn’t be here without it!

baq• 9 months ago

    12 V DC (≈1.5 kW peak)

How thick is the cable powering this holy amps Batman.

HPsquared• 9 months ago

Car starter motors and alternators are on a similar power level.

tonyarkles• 9 months ago

Definitely. The alternator on a small aviation engine (~100 HP) that I work with is rated for 24V/150A ~= 3.6kW and it’s not really that big in the whole scheme of things. The cables are about the thickness of my thumb.

mikestew• 9 months ago

I’ve wired a 3000W inverter into an RV, and the cables were a bit thicker than my thumbs. Use welding cable, it’s much more flexible.

fouronnes3• 9 months ago

Tomorrow on HN: Polishing the moon surface

henrikf• 9 months ago

That's very impressive and I'm even more impressed if you can manage to sell tiles at that low price.

PA looks suspiciously similar to SE5004L. I just needed some for my own projects but every distributor is out of stock. I wonder if this is where all of them went?

tatjam• 9 months ago

Haha love this "Not intended for radar applications. Core functionality needed for radar not included due to export control restrictions".

Wonder how they prevent usage as radar as this thing could pretty much be a drop-in missile seeker.

giantg24• 9 months ago

The wavelength is suboptimal for that type of tracking/intercept.

jmpman2• 9 months ago

I’d wondered about using moon bouncing in order to distribute video streaming keys (piracy) in a way which would make it impossible to locate the sender. Unlikely to be viable at scale, as the moon isn’t always visible, but it’s an intriguing covert broadcast mechanism.

npsomaratna• 9 months ago

I have no understanding of the physics involved, but could the broadcast location be reverse engineered? (With triangulation and clever math?)

Redster• 9 months ago

Your location is very very visible to any plane or satellite passing overhead.

wjholden• 9 months ago

Hadn't thought of it from this perspective. An untraceable signal coming from the moon would also be useful for military communications. Electronic warfare and signals intelligence have been powerful tools for both sides in the Ukraine-Russia war.

giantg24• 9 months ago

It's not that untraceable when you have satellites that could pick it up before the bounce.

giantg24• 9 months ago

Really cool phased array. However, I don't think this will really make EME more accessible when their EME version is $2499+. Maybe the 4 element version would be fun to play around with.

retrac• 9 months ago

While it isn't exactly communication, the Moon is not the only planetary body of which signals have been bounced off. Radar is workable within the solar system, including the moons and rings of the outer planets.

https://www.jpl.nasa.gov/news/radar-astronomy-used-to-resear... (1991)

Open-source communications by bouncing signals off the Moon

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