[HN Gopher] Open-source communications by bouncing signals off t...
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Open-source communications by bouncing signals off the Moon
Author : fortran77
Score : 255 points
Date : 2025-11-02 14:44 UTC (7 days ago)
(HTM) web link (open.space)
(TXT) w3m dump (open.space)
| gus_massa wrote:
| 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...
| cactacea wrote:
| 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...
| dbcurtis wrote:
| 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!
| ErroneousBosh wrote:
| > 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 ;-)
| ErroneousBosh wrote:
| 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 wrote:
| 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."
| cactacea wrote:
| 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 wrote:
| A few hundred Watt at a minimum would be my first guess.
| cactacea wrote:
| 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.
| mschuster91 wrote:
| Y'all can run at 1500W? Here in Germany the legal limit
| (depending on band of course) is 750W.
| dbcurtis wrote:
| 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.
| firesteelrain wrote:
| People use WSJTX software and Q65 mode
| drmpeg wrote:
| 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 wrote:
| 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 wrote:
| Latency?
| _bernd wrote:
| 1 sec up and 1 sec down... more or less. Speed of light and
| distance to the moon, two times.... roughly.
| whatsupdog wrote:
| I'm more interested in bitrate.
| radiorickky wrote:
| 300bps
| wslh wrote:
| Is that right? Only 300bps? It is easy to infer that having
| a bigger antenna you can scale that.
| firesteelrain wrote:
| 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 wrote:
| There's nothing that can't be modulated. Lol.
| zerosummer wrote:
| 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 wrote:
| I'm skeptical, but how can you not cheer for this? Sounds so
| awesome.
| NewJazz wrote:
| Have you read Three Body Problem?
| kieranl wrote:
| 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.
| mrtnmcc wrote:
| Great seeing you at Pacificon!
|
| We're starting with the "Quad" tile -- a 4 Tx x 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!
| drmpeg wrote:
| 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).
| mrtnmcc2 wrote:
| 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.
| baobrien wrote:
| What does the RF front end look like? I see the Lattice ECP5,
| but what are you using to go from bits to waves?
| mrtnmcc wrote:
| There are ten 640 MSPS ADCs (I+Q per channel and a cal
| path, per 4-antenna PCB tile). These are custom MASH SD
| 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
| SD, 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).
| dreamcompiler wrote:
| 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 wrote:
| > I assume the goal is to do something cooler than that.
|
| Yes. Bounce the signal off the moon. The _moon_.
| cwillu wrote:
| What if we tried more power?
| topspin wrote:
| 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.
| ErroneousBosh wrote:
| We've bounced 2.4GHz signals off Venus. _Venus_.
| topspin wrote:
| VHF frequencies, that is.
| kragen wrote:
| 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.
| dreamcompiler wrote:
| 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.
| kragen wrote:
| Hey, thanks!
| dreamcompiler wrote:
| https://en.wikipedia.org/wiki/List_of_retroreflectors_on_
| the...
| kragen wrote:
| 1 degree and 54 minutes, it turns out. Amazing when you
| think about that, isn't it?
| euroderf wrote:
| This was a Cold War thing to surveil Soviet air defense radars.
| whatsupdog wrote:
| For someone not well versed with the terminology, can someone
| please tell what kind of bitrate this can provide? In bytes per
| second.
| shakna wrote:
| 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
| whatsupdog wrote:
| Thank you so much. Are these rough calculations based on the
| smallest (quad), mini or the large array?
| shakna wrote:
| 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.
| radiorickky wrote:
| No, if you do the math it's about 40 bytes per second
| (300bps) for Earth-Moon-Earth using their 240-antenna array.
| shakna wrote:
| 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.
| ant6n wrote:
| I dont get it, How does latency affect bandwidth here?
| shakna wrote:
| Because we're not stationary, and nor is the moon.
| Latency means greater dispersion, and lower successful
| return rates.
| but-vaccum wrote:
| 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.
| shakna wrote:
| ... I'd love to see, this particular project, "throw a
| pair" of tranceivers into deep space.
| shevy-java wrote:
| Ok so ... moon is slow going.
| djexjms wrote:
| This is obviously incorrect. Latency is not the same as
| bandwidth. EME hobbyists will bounce voice signals off the
| moon.
| drmpeg wrote:
| 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-java wrote:
| The moon is so useful.
| fourthark wrote:
| We probably wouldn't be here without it!
| baq wrote:
| 12 V DC ([?]1.5 kW peak)
|
| How thick is the cable powering this holy amps Batman.
| HPsquared wrote:
| Car starter motors and alternators are on a similar power
| level.
| tonyarkles wrote:
| 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 wrote:
| 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 wrote:
| Tomorrow on HN: Polishing the moon surface
| henrikf wrote:
| 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 wrote:
| 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.
| giantg2 wrote:
| The wavelength is suboptimal for that type of
| tracking/intercept.
| jmpman wrote:
| 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 wrote:
| I have no understanding of the physics involved, but could the
| broadcast location be reverse engineered? (With triangulation
| and clever math?)
| Redster wrote:
| Your location is very very visible to any plane or satellite
| passing overhead.
| freedomben wrote:
| 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.
| giantg2 wrote:
| Highly directional antennas on a moving platform can
| perform effectively radio direction finding
| independently.
| hdjrnevehhe wrote:
| Let's remember that there is also a antenna array with
| LOS yo the mooon.....
| wjholden wrote:
| 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.
| giantg2 wrote:
| It's not that untraceable when you have satellites that could
| pick it up before the bounce.
| giantg2 wrote:
| 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.
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