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.
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.
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.
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!
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.
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.
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.
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.
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:
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!
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.
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."
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.
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.
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.)
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.
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.
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.
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.
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’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.
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.
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.
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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
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.
There's nothing that can't be modulated. Lol.
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).
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What does the RF front end look like? I see the Lattice ECP5, but what are you using to go from bits to waves?
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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.
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.
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.
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> I assume the goal is to do something cooler than that.
Yes. Bounce the signal off the moon. The moon.
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.
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What if we tried more power?
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!
1 reply →
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."
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.
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
This is obviously incorrect. Latency is not the same as bandwidth. EME hobbyists will bounce voice signals off the moon.
No, if you do the math it's about 40 bytes per second (300bps) for Earth-Moon-Earth using their 240-antenna array.
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Thank you so much. Are these rough calculations based on the smallest (quad), mini or the large array?
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Ok so ... moon is slow going.
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.
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.
This person has a full SDR LoRa transceiver stack and the meshtastic client code.
https://gitlab.com/crankylinuxuser/meshtastic_sdr
People use WSJTX software and Q65 mode
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.
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).
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.
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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
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