[HN Gopher] CHART: Completely Hackable Amateur Radio Telescope
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CHART: Completely Hackable Amateur Radio Telescope
Author : cenazoic
Score : 125 points
Date : 2023-09-11 11:18 UTC (1 days ago)
(HTM) web link (astrochart.github.io)
(TXT) w3m dump (astrochart.github.io)
| lawlessone wrote:
| I really like this , i have an orginal Pi and one of those cheap
| usb SDR's sitting around doing nothing.
| thoop wrote:
| This is really cool. Could you build a few of these and make an
| interferometer?
|
| I remember reading that interferometers are usually all connected
| by physical cables with physical loops to make sure the incoming
| data is combined at exactly the right time. But are we at a point
| now where that can somehow be done intelligently in software? Or
| are these little RTL-SDR's not accurate enough to even begin
| trying that?
| semi-extrinsic wrote:
| You'll want to look at the KrakenSDR. It's basically a circuit
| board integrating 5 RTL-SDR chips driven from a single clock
| source and calibrated for phase coherency, for the express
| purpose of doing radio interferometry amongst other things.
| They're in stock at Mouser.
| thoop wrote:
| Whoa yeah this looks like it would be easier than doing it
| all by hand. $466 at Mouser. Interesting!
|
| Looking around on Google, I don't see anyone that has tried
| using this for interferometry yet but the KrakenSDR team
| explicitly mentions that it can be used for interferometry.
| CamperBob2 wrote:
| They were pitching it for use in bistatic passive radar,
| which is a very similar application, until somebody put the
| fear of ITAR into them. Some of that code is probably still
| lying around.
| ooterness wrote:
| Broadly speaking, the main prerequisite to interferometry is to
| make sure all the RF circuits are phase-coherent, meaning that
| all the oscillators are operating in lockstep within some
| tolerance.
|
| The accuracy needs to be within about 1/10th of a period, give
| or take. I'm not sure the min/max frequency range for this
| system, but I saw 1.4 GHz in a screenshot, which would yield a
| tolerance of about 0.1 * (1 / 1.4 GHz) = 70 picoseconds.
|
| That is achievable with the right hardware, but unfortunately
| the RTL-SDR doesn't include an option to use an external clock
| reference. As a result, each dongle's ADC sample timing and RF
| synthesizer phase will constantly be wandering relative to the
| others. It's not a one-time calibration; it's an ongoing random
| walk that changes on a millisecond-by-millisecond basis.
|
| In theory you _might_ be able to pull it off if you had a
| separate emitter in view of each antenna, calibrate each unit
| based on that signal, and then synchronize everything in
| software. But at some point it 's easier to just use hardware
| that has an external clock input, and avoid the whole problem.
| TigeriusKirk wrote:
| The always entertaining saveitforparts Youtuber tried to make
| a "very small array" from a bunch of used satellite mini-
| dishes. He ran into all sorts of fun issues with his ad hoc
| construction.
|
| I admire the guy for having as much fun with his failures as
| his successes.
|
| https://www.youtube.com/watch?v=dklYG70e7R0
| dekhn wrote:
| The lesson I've learned (I build scientific instruments as
| a hobby) is not to chase the latest, greatest ideas. Many
| of them only work because the implementors have access to a
| huge knowledge base, excellent parts and facilities, and
| really smart people to help debug the inevitable problems.
|
| I focus more on maximizing what i can get out of a simple
| hardware setup, which means skipping anything that involves
| complex digital analysis or extremely sophisticated and
| sensitive equipment; it means more time having fun and less
| time debugging problems where I actually don't know enough
| to debug the problem.
| thoop wrote:
| Very interesting! Thanks for the great response.
|
| Is there a better frequency available to amateur hardware
| that would give tolerance within more reasonable limits?
|
| Without a shared external clock reference, i.e. over longer
| distances, how expensive does the hardware get if you want to
| be able to accurately measure the time/phase wandering to
| correct in software?
|
| Just curious if it's a limit of the low cost RTL-SDR or if
| it's a harder problem than that (or both?).
| gorlilla wrote:
| There are RTL-SDR [0] dongles that have pin headers for a
| common clock source already on the market.
|
| [0]: https://www.rtl-sdr.com/rtl-sdr-blog-v-3-dongles-user-
| guide/...
| ooterness wrote:
| Neat! I hadn't heard about that capability. That definitely
| makes things a lot easier.
| toomuchtodo wrote:
| Can you use GPS as clock reference?
|
| https://www.gps.gov/applications/timing/
|
| https://ilrs.gsfc.nasa.gov/docs/timing/gpsrole.pdf
| civilitty wrote:
| Theoretically yes but using GPS time of flight to
| synchronize sensor triggers to within tens of picoseconds
| is so far outside of "amateur" that you might as well
| incorporate and start replying to DoD RFPs.
|
| When your math starts requiring relativistic physics, it's
| a lot easier and cheaper to just run some fiber.
| ooterness wrote:
| There's simpler options if all your equipment is in the
| same building, but yes, GPS-disciplined oscillators are a
| really great way to sync up clocks anywhere in the world to
| within a few nanoseconds. That said, it doesn't really help
| here because a) nanoseconds not picoseconds and b) the RTL-
| SDR doesn't have anywhere to plug it in.
| toomuchtodo wrote:
| Please forgive the silly question, but can the RTL-SDR
| receive the GPS signals software defined and process for
| timing?
| civilitty wrote:
| No, it's not enough to know when samples from different
| SDRs were taken, they must all be taken within a very
| small interval (tens of picoseconds in OP's example).
| What the SDRs really need is an electrical signal called
| a "trigger" that tells them to read their sensors at that
| exact moment which is what GP meant by plugging them in.
| You can use a second GPS enable device to generate that
| trigger but synchronizing those triggers using time of
| flight is _very_ hard.
| touisteur wrote:
| Maybe you can bruteforce time correlation by shifting
| every source slightly, trying a yuge number of shift
| combinations and seeing which combination gives the
| 'best' (most correlated) output on a known signal? I've
| done that for many out-of-sync systems, and since I've
| started using GPUs in the two Os I've become obsessed
| with brute force methods :-)
| kawfey wrote:
| It is possible - https://www.rtl-sdr.com/rtl-sdr-
| tutorial-gps-decoding-plotti...
| zzbn00 wrote:
| Could try intensity inteferometry? A good mental exercise
|
| https://en.wikipedia.org/wiki/Hanbury_Brown_and_Twiss_effect
| ooterness wrote:
| In this case, the coherent source is the celestial object
| being observed. The problem here is that the "combining"
| step is being performed in software, and the sequence of
| digital samples have time- and phase-offsets that are ever-
| changing.
|
| The two options are to keep those offsets under control
| (i.e., lock everything to a common clock) or to rapidly
| measure the offsets as they change and try to compensate in
| software (difficult).
| zzbn00 wrote:
| In intensity interferometry the phase is not measured,
| and the timing accuracy is I believe only proportional to
| the desired effective bandwidth of the measurement. It
| was done in 1950's with bandwidths ~10 MHz -> 0.1microsec
| accuracy, should be do-able with SDR. Intensity
| interferometry is a bit of mind-twister...
|
| (Another at first surprising thing is that radiation
| received from celestial sources is only coherent because
| of their very small apparent size -- the sources
| themselves are not coherent at all, because their
| physical size is very large)
| ale42 wrote:
| My first understanding of the title: a telescope for amateur
| radio... I was wondering what it was supposed to be exactly ;-)
| vvoid wrote:
| A stacked Yagi array. Seriously!
| fukpaywalls2 wrote:
| The youtube is about 4 years old. There does not seem to be much
| updates. Sounds like fun project overall.
| JKCalhoun wrote:
| I find myself increasingly drawn to SDR. I only played with it a
| bit (got a cheap RTL-SDR and played around with some "waterfall"
| software).
|
| I wish the software was in fact a little slicker. For my brief
| toying with it, the apps felt like either Swiss Army knife apps
| with too many knobs, and/or had kind of janky UI.
|
| Radio astronomy seems like another cool use of SDR that could
| draw me back in to learn more about it.
| dekhn wrote:
| Could you mount this on an alt-az platform and get radio image
| data? I'm not super interested in manual pointing and just
| looking at spectra.
| snats wrote:
| What can you do with the data that you get? It seems like a
| really cool project but I feel that it has been a long time since
| they posted something and the analysis sections seems incomplete.
| I want to do one of this, but I want to know what is the data
| that I get and what cool stuff you can do.
| peter_d_sherman wrote:
| Amazing what one can build out of Aluminum Foil...
| [deleted]
| jejeyyy77 wrote:
| are there any example images showing what can be captured?
| owlmirror wrote:
| You can find a picture of what the data you capture looks like
| in the "Taking Data" page in the tutorials. Unfortunately the
| "Analysis" page is "under construction". But you won't get
| photographs, like you would get from an optical telescope.
| empyrrhicist wrote:
| At least in principal it is indeed possible to process data
| from a collection of radio telescopes into images, and they
| often overlay the data on optical images to see regions of
| radio emission. Whether that's in the reach of amateur
| equipment is another story that I don't have insight into.
|
| It would certainly be cool if it were possible.
|
| Check out the Galaxies tab here:
| https://public.nrao.edu/gallery/
| lawlessone wrote:
| Do they work during the day or does the scattering and
| sunlight also make that impossible?
| empyrrhicist wrote:
| I'm not the right person to ask. I'm guessing the EM
| environment is too noisy, but who knows.
| tomr_stargazer wrote:
| Yes! Radio astronomy at most frequencies is totally
| feasible 24 hours a day, as long as you're not pointing
| too close to the Sun. Source: I'm a radio astronomer.
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(page generated 2023-09-12 23:00 UTC)