Does anyone have any suggestions for what to do with amateur radio astronomy? I recently looked into this a bit and came to the conclusion that to do anything actually interesting would require rather substantial investment in a big dish and having a place in the middle of nowhere away from interference. I had a hard time getting a list of interesting things you could do with more modest investments. Looking into Radio Jove and the Itty Bitty Telescope seemed like they would just be educational projects that wouldn't yield fruitful, long-term projects. Most projects seemed like just listening/watching to the Sun and Jupiter for various emission pulses, which seems like it would get boring pretty quickly after the initial learning exercise.
Although the more interesting of the two, radio astronomy, for the amateur, seemed like it wouldn't yield as interesting results as visible light astronomy.
The effort-to-results ratio is definitely better on the visible light side. Pictures are exciting, and much easier to explain.
A lot of smart amateurs discuss all things astro at the Cloudy Night forums; the subforum where topics such as radio astronomy are discussed is here: https://www.cloudynights.com/forum/88-scientific-amateur-ast... Could be worth a look if you're seeking ways to contribute to the area.
Take a look at the youtube channel "saveitforparts", he has been doing various satellite comms and recently radio telescope experiments with usually just saved or recovered parts. Much of it is him learning on the way, which for myself is neat to watch
1. Small apertures' one advantage is their being able to see a large amount of the sky at once. For any coherent aperture the larger it's span in a direction the smaller the beam pattern is in the perpendicular direction upon the sky. The bigger your collector the smaller your beamwidth the less you can see at once. Big dishes have small FOV and aren't good for monitoring the entire sky.
There are ways of getting around this: physically scanning the big antenna, multi-apertures for receive across the focal plane, or by abandoning actual imaging and measuring the correlations between arrays of lower gain (smaller) apertures.
But the best way (if the signal is loud enough) is just to have a smaller aperture. Real science monitoring for transient radio signals can and is being done with literally just using the feedhorn from a large dish by itself (with some precise clocks, expensive lna, receiver, and time stamping) to detect FRB (and other high power transients),
Multiple stare2 type instruments distributed around the world are likely to do real radio astronomy in the near'ish future. This is amateur scale possible. In the "Completely Hackable Amateur Radio Telescope" they're using a pyramidal dish because it's easier to construct but for trying to contribute to "real" radio astronomy you'd want a choke ring feed horn or at the very least a conical horn. The idea is to minimize sidelobes which otherwise prevent you from knowing for sure the signal you received came from the direction the horn is pointing.
2. Another low hanging amateur accessible "real science" is actually doing something when you're receiving solar radio emissions. Calculate the velocity of a CME shock by the frequency drift as it travels upwards through decreasing magnetic field. Or even more audacious, use a hackrf (or ettus usrp b200 or something) to frequency hop at 8 GH/s over a few hundred MHz bandwidth and record the fine structure present at milli and microsecond timescales precent in many types of solar radio bursts. Things like spectrogram zebra patterns and fiber bursts and other unexplained highly coherent fine timescale patterns which still lack accepted physical explaination today. Particularly in that their very short timescales indicate the emission regions have to be only ~a handful of kilometers in scale but outshine the rest of the sun. Whatever "small" solar features are making these short intense radio emissions are open to even amateur interpretation at this point.
Regarding your point 2 I don't think there is anything "amateur accessible" there. There are several groups looking into this using LOFAR which is a large radio telescope array with 52 sites all across Europe. E.g. the ref. below.
You don't need LOFAR scale aperture to receive solar radio bursts. I literally do it with a ~meter size antenna sitting outside on my porch. Seeing the frequency drift in solar radio bursts is quite easy all things considered. The lowest hanging fruit there is.
As for the fine timescale structure of solar radio bursts, depending on frquency, and for this you're probably wanting to cover all of l-band and up, you only need about a meter^2 of aperture. And to see the fine-structure you only need a fast enough receiver with wide bandwidth. That's a few hundred to a thousand bucks. And you definitely don't need to do any sort of imaging or array stuff. All you need is a single receiver and to look at the spectrogram with a time precision of milliseconds. It is very amateur accessible despite the fact that even the largest, most complex, radio telescopes are looking at it too.
I remember a fairly sketchy bit of gear at my old university that an amateur could have definitely made for themselves (if I recall correctly the dish was made with chicken wire). They used it for observing the Vela pulsar for a very long time (like years and years) and looked at how the pulses changed with time.
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.
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?
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.
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.
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.
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).
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)
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?).
Over local distances, all you need is a shared clock reference. As others have pointed out, the RTL-SDR dongle can do this with mods.
Over longer distances, this is an active area of research with many different approaches for various applications.
Two recent well-known examples are the "Event Horizon Telescope" (the network of radio telescopes that has been generating images of black holes) and optical frequency combs (a recent demo published in Nature achieved time-transfer accuracy of a few femtoseconds).
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.
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.
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 :-)
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.
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.
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.
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.
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.
this exactly was super interested and then clicked and the page that shows what to actually do after getting data collected being blank feels like... ya this is a dead project where they couldnt explain wtf to do with it lol
I ended up going into a bit of a rabbit hole. I learned that there is this telescope you can use[1] and there is a small guide[2] in there to check out how to use it. It is a bit more information from that.
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.
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.
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.
Although the more interesting of the two, radio astronomy, for the amateur, seemed like it wouldn't yield as interesting results as visible light astronomy.