Open-source communications by bouncing signals off the Moon | Hacker News Reader

Open-source communications by bouncing signals off the Moon

open.space

277 pointsfortran7783 comments

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.

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

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

Indeed, though it would take some coordination to actually narrow it down precisely. You'd need a few different planes/satellites to detect the signal and share their reading to allow triangulation. With only a single plane or a single satellite that is not in geosynchronous orbit, you could take multiple readings and get a rough idea of location, but the inability to turn from a straight line (not impossible for a plane of course, but it would require intentionality and willingness for the crew/commanders and typically not cheap as it disrupts whatever flight plan they previously had) would be a hindrance. That said, with how many satellites are up there I doubt it would take much extra effort to do that coordination if the satellite operators have motivation to do so.

Highly directional antennas on a moving platform can perform effectively radio direction finding independently.

Let's remember that there is also a antenna array with LOS yo the mooon.....

The moon is visible to ?half? the earth at a time? That’s a huge search area. Certainly the antennas broadcasting to the moon are quite directional, and outside the main beam, would be hard to detect?

I don’t understand how? Wouldn’t the signal be highly directional? Surely it wouldn’t be easily detectable unless the viewer’s POV intersects the path of the beam?

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.

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

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

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

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.

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.

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!

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).

Great question, the latest version has Tx RHCP, and then Rx either LHCP or RHCP controlled with RF switches (in each antenna). This allows point to point links (where the Tx pol and Rx pol should be the same), or "bounce links" where the circular polarization flips with the bounce. I should note RHCP Rx has a bit worse noise figure (LNA is different) but good enough for any line of sight.

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

There are ten 640 MSPS ADCs (I+Q per channel and a cal path, per 4-antenna PCB tile). These are custom MASH ΣΔ designs built from discrete diff pair transistors (cost about $0.08 each) and do noise shaping/decimation to get a clean 50 MHz of baseband bandwidth. The 8x DACs are also ΣΔ, using the LVDS pins of the FPGA and some modulating DSP. Mixers are MAX2850/1, LNA are custom design based on Infineon transistor, and RFPA is a Skyworks part meant for WiFi (have iterated on a few model numbers).

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.

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

    12 V DC (≈1.5 kW peak)

How thick is the cable powering this holy amps Batman.

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

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.

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.

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.

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

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

What if we tried more power?

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.

We've bounced 2.4GHz signals off Venus. Venus.

VHF frequencies, that is.

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.

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.

Hey, thanks!

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

1 degree and 54 minutes, it turns out. Amazing when you think about that, isn't it?

I was thinking earth was about twice the diameter of the moon, hence 1 degree. Turns out it's closer to a factor of 4. Should have looked it up.

I used units(1):

    : yeso; units
    Currency exchange rates from FloatRates (USD base) on 2025-10-06 
    3749 units, 113 prefixes, 120 nonlinear units

    You have: 2 arcsin(earthradius/moondist)
    Unknown unit 'arcsin'
    You have: 2 asin(earthradius/moondist)
    You want: dms
            1 deg + 53 arcmin + 57.540656 arcsec

The APOLLO lunar laser ranging experiment uses a 3.5 meter telescope as a laser turret and manages to get about 2,400 photons back from those retroreflectors every half an hour, and it's a challenge just to find the things as the spot's a few km wide by the time it gets to the moon. Good luck.

Yeah, I was just doing some calculations on this. You'd think that with 3.5 meters you could do better than a few kilometers, wouldn't you? Is something wrong with their telescope?

I don't know what wavelength they're using, but at 555nm, 1.22λ/d would be 0.193 microradians, which, unless I'm doing the math wrong, works out to a 74-meter Airy-spot radius at the distance to the moon. At that sort of size, you'd think the majority of the photons in the desired wavelength band would be from their laser rather than stray Earthshine.

I was doing calculations based on λ = 350nm and a 500-mm reflector, and no retroreflector, and getting rather sad estimates of 3 joules of light transmitted per returned photon (per receiver). While that's clearly a feasible commnications system, it's going to be pretty limited in bandwidth. I'm not sure if C-band radio is better?

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...

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...

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!

> but, yeah, on CW a monster antenna and the legal limit of 1500W seems to be the median system.

A good ten years ago or more, they used Arecibo to transmit CW moonbounce on 70cm. I was able to receive it in my back garden with a handheld and an 11-element Yagi balanced on my clothesline ;-)

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.

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."

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.

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)

Tomorrow on HN: Polishing the moon surface

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

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

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

That was for the large array, assuming you were bouncing from you to moon and back again.

Point-to-point on the Earth would actually be semi-decent, as another comment pointed out.

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

Care to share the math, if you've done it?

It's 1.3 to 1.6 seconds each way to the moon, by radio link.

JPL's much, much, much bigger arrays can only achieve 64kbps.

I dont get it, How does latency affect bandwidth here?

Because we're not stationary, and nor is the moon. Latency means greater dispersion, and lower successful return rates.

Latency itself doesn't mean squat. Throw a pair of trancievers into deep space and no relative motion and all bitrate limiting factors (dispersion, multipath, doppler shift- I'm sure there's more) disappear besides SNR/inverse square and retransmission.

Going beyond intuition- secondary reasoning says since GSO bandwidth & bitrate is acceptable for TV and sat phones by the hundred.

Tercheriary is we have/had a few hundred bps from Voyager II and that's a might bit further out than the moon, and it was called out that using an 80meter dish it could be pushed to a little over 1kbps- which means at some point inverse square becomes the only non-neglegiable factor.

So please, explain with something besides repeatedly saying "latency." Even if I'm wrong, they're strong enough counters to deserve more than a single word.

... I'd love to see, this particular project, "throw a pair" of tranceivers into deep space.

Ok so ... moon is slow going.

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

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).

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.

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

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.

Y'all can run at 1500W? Here in Germany the legal limit (depending on band of course) is 750W.

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.

People use WSJTX software and Q65 mode

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

https://gitlab.com/crankylinuxuser/meshtastic_sdr

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).

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.

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?

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

Have you read Three Body Problem?

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

The moon is so useful.

We probably wouldn’t be here without it!

Latency?

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

I'm more interested in bitrate.

300bps

Is that right? Only 300bps? It is easy to infer that having a bigger antenna you can scale that.

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