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Receiving Weather Satellite Images With An £8 Dongle (mattg.co.uk)
509 points by mmastrac on July 25, 2015 | hide | past | web | favorite | 55 comments

The author mentions that he used "A 2 metre bit of wire jammed in the screwhole at the base of the tiny antenna" to try and get better reception. While the signal he is receiving is in the 2-meter band, a monopole antenna works better if it is 1/4 the wavelength, so he might actually get better reception with a 0.5 meter length of wire. What he created is usually called a "random wire" antenna, and may deliver acceptable performance with a strong signal.

But the NOAA satellites (and quite a few other satellites) use a circularly polarized signal, and for just a tiny bit more money (perhaps the cost of the dongle) you can make a MUCH better antenna for circularly polarized signals. Some designs are just helixes made with coax cable, some are just a couple of wires in an x-shape.

Google is your friend, and building an antenna is a fun cheap weekend project and will greatly enhance your results when hunting for satellite signals.

P.S. if you make a circularly polarized antenna and you discover that you made a LH polarization instead of the RH that you wanted (or vice versa), just turn the antenna upside down. It may behoove you if you are planning on listening to signals with both polarities to construct one that is easily flipped over.

    While the signal he is receiving is in the 2-meter 
    band, a monopole antenna works better if it is 1/4 the 
    wavelength, so he might actually get better reception 
    with a 0.5 meter length of wire. 
I always thought a lambda/4 antenna is used because it's almost as good as a lambda antenna but much more conveniently to handle. I understand that more complicated antenna designs can be better than a simple lambda wire, but how can a lambda/n wire be better? I don't doubt what you are saying, I'm just curious how this can work.

Linear monopole antennas are very easy to reason about.

Imagine (or actually draw) a sine wave graph, and then fit your antenna along the X axis of the graph. Now look at "signal" amplitude at the antenna's extremities. You'll notice that 1/4 wavelength antenna will give you the strongest signal, while 1/2 and full wavelength will apparently give no signal at all. The only reason that full-wavelength piece of wire picks up anything at all are build imperfections - wire length not perfectly matched to wavelength, not perfectly straight, not perfectly aligned with signal source, signal reflections hitting it from different angle, etc. Thats why olympus refers to performance of full-wavelength piece of wire as "random piece of wire"-kind of performance.

On the other hand, linear dipole would be 1/2-wavelength (or 2 times 1/4-wavelength), as it is just two 1/4-monopoles put together.

This makes sense. Thank's a lot.

This is awesome; am ordering a dongle right away!

Did some digging and there's a whole load of things which can potentially be picked up, including dead satellites [0]. RTL-SDR.com has a whole load of awesome looking tutorials [1].

[0] http://www.rtl-sdr.com/receiving-dead-satellites-rtl-sdr/ [1] http://www.rtl-sdr.com/category/tutorial/

This is the kind of cool Hacker News I like to see! Ordered.

Also just ordered one. This is the best HN thread I've seen in ages!

I just ordered one too - this looks awesome.

You can track aircraft (newer airplanes broadcast their locations, ships, etc.

This [0] is the one I ended up ordering, I just wish one day shipping was available.

[0] http://www.amazon.com/dp/B009U7WZCA

$20 is way too expensive for these. Here's the same thing for half the price: http://www.ebay.com/itm/Newsky-TV28T-v2-USB-DVB-T-RTL-SDR-Re...

There are a variety of factories producing these at variable quality. I'd still recommend the Nooelec version if you have the budget to spare.

£4.62 = $7.16 http://www.ebay.com/itm/171749696878 £6.96 = $$10.80 http://www.ebay.com/itm/131532641773 ebay search : RTL2832U R820T -Connector -Adapter -Angle

DX has some cheap RTL2832U based dongles ⌘ http://eud.dx.com/search/RTL2832U

I recommend an [antenna port] to BNC or other adapters as well.

Great, just recently I became passionate about anything relating to radio theory and transmission and I had learnt about these dongles and was ready to get one.

But before that, I decided I wanted a proper "hardware radio", and bought myself a portable wide band radio receiver [1] (great value for the price, I advice it). Then when I was about to get one of these dongles, as usual and as for the above radio, I got sucked into the reviews suggesting this or that other model for better sensitivity or extra features. Now I understand the price range for these cheap sticks is just in the order of a few tenths at maximum, but maybe there is a chance anyone has investigated a bit more on the available choices already?

For example.. the one that has been written about in the article has 4 stars and 159 reviews while the first alternative in the "Amazon recommends" box has some 940 reviews and same 4 star score for just £ +2 (but I'm not even sure they do exactly the same things.. sorry). So, just wondering, maybe someone has already gone through this search and can recommend the best one to get?

[1] http://www.amazon.com/Tecsun-PL-660-Portable-Shortwave-Singl...

> Now I understand the price range for these cheap sticks is just in the order of a few tenths at maximum, but maybe there is a chance anyone has investigated a bit more on the available choices?

Thanks to the popularity of this family of DVB dongles, all the information you need is right here: http://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/

Also look into the Funcube dongle for a sdr dongle that is designed for ham radio and not just an adapted DVB dongle -- they are pretty much the same thing though. If you can find an E4000 dongle on ebay it's probably worth twice as much as the R820T dongles because it gives you a wider frequency range. I have an R820T dongle and it works just fine though.

One thing to keep in mind is that these dongles only have 8 bit ADCs in them, so no matter how great your signal path, you will top out at ~48db of sensitivity (rough formula is sensitivity in db = 6*bits of ADC). A "real" SDR like the USRP B210 has a 12 bit ADC in it, which means that it will top out at around 72db of sensitivity. Plus, spending the big money means that you will get a radio that takes less shortcuts than a cheaply produced dongle, so you are much more likely to get closer to that theoretical peak. But certainly start with a dongle, and play with it and see how far you get before you "absolutely need" some TX capability or have truly maxed out what you can do with only USB 2.0 and 8 bits of resolution (though there is a guy who was passively tracking aircraft with FM radio stations and two synchronized dongles so what you can do with these is pretty crazy).

One thing to keep in mind is that these dongles only have 8 bit ADCs in them, so no matter how great your signal path, you will top out at ~48db of sensitivity (rough formula is sensitivity in db = 6x bits of ADC). A "real" SDR like the USRP B210 has a 12 bit ADC in it, which means that it will top out at around 72db of sensitivity.

Of course, the ~6 dB per bit rule applies only at the ADC's full (Nyquist) bandwidth. If the ADC is followed by a decimation and filter chain it can potentially do a lot better. Do these dongles perform their conversion at the final signal bandwidth, or do they digitize at a wider IF and downconvert to baseband digitally?

Excellent point. I believe that the ADC samples at 28.8 Msps and then decimates to the rate requested by the computer (max ~3.2 Msps). So theoretically it could be doing ~3 bits better, if everything was implemented properly and those extra bits were delivered across the USB to the computer. I think the samples delivered to the computer are still only 8 bits so the ~48 dB theoretical limit to the dynamic range remains.

CamperBob you sound like you know what you're talking about and I'm mostly self taught in this area so if I'm wrong please let me know.

Same here, I (try to) spend most of my time out of my depth. :)

It sounds like the bandwidth of the 137 MHz satellite signal is in the 36 kHz vicinity, going by the article. So one possible strategy would downconvert 137 MHz to somewhere in the first Nyquist zone of the ~30 MSPS ADC (i.e., below about 15 MHz). The ASIC picks up 3 more bits of process gain by decimating to 3 MSPS, then the host CPU could pick up about 5 more bits by further decimation to about 100 kSPS. The 36 kHz signal would then have a dynamic range approaching that of a 16-bit ADC, which is quite adequate for most purposes.

Of course, IMD artifacts and other spurious tonal responses due to the cheap RF front end don't experience any such improvement, but you sure can't complain for $20.

Thanks for all the info! And yes what matters most to me is the possibility to receive the largest possible frequency range.. so I will check out that E4000 (you say to look for it on ebay.. means that it's out of production...? Then a new long search for a more recent equivalent or better thing will start! :-\ ;-) ).

Also I was wondering.. is there a trasmitting dongle? Yes I do know already about transmitting licensing and fines :)

With my Tecsun PL 660 I already receive air band and can listen to air traffic or tower, but I am not sure what you meant by 2 synchronized dongles... maybe you meant that one was listening on tower and another one on air traffic control?

No, the cheap rtl dongles are receive only because they are re-purposed TV receivers, and there is no companion transmitter.

If you want to transmit, I HIGHLY suggest you look into ham radio. It's cheap to get a license, under $50 to test and get your technician's license in the USA. That will give you a lot of legal opportunities to TX and you will learn quite a bit in the process. The ARRL technician study guide is a great book and if you get through it all you are pretty much guaranteed to pass the test (all the test questions are in the book). The test isn't a cakewalk but it's passable for most anyone who studies.

The E4000 is out of production, but I think it gives ~500 MHz more receive frequency on the top end. It's really not game changing and the new R820T2 dongles seem to be better overall, but the E4000 is worth the extra $30 to lots of folks. You can get to higher freqs without an E4000 anyway. A popular "hack" is to use an MMDS downconverter (another $25 Chinese purchase) to get access to the 2.4 GHz bands and sniff bluetooth and wifi packets. This is fairly advanced stuff that goes beyond just listening to voice broadcasts that you are doing now since you will be trudging through some cryptically written tutorials and writing some of your own code to get everything to work. Some people just want to listen to voice, and don't need the extra frequency since you can listen to AM/FM broadcasts, civilian air traffic, ham VHF/UHF, and even military VHF/UHF with just a regular dongle.

As far as synchronized dongles, a guy was running two dongles off of the same clock source (he soldered a wire between the two chips so they could use the same crystal). That way he could compare the differences from signals received at two physically separated antennas and track moving objects (like airplanes) that reflected radio transmissions. This is by far the most advanced project (I feel bad calling it a hack) I've ever seen with an RTL dongle, and most of the licensed hams I know don't have the brains to do it: http://kaira.sgo.fi/2013/09/passive-radar-with-16-dual-coher...

I did this a few years ago with a similar dongle and a homemade antenna. It's a pretty cool thing to see something you cobbled together receiving real-time messages from space.


Last thing I used my R820T for was decoding POCSAG[0] messages here in the UK. It's a paging system, and the messages seem to be broadcast nationwide in the clear. Seems to be mostly used by the NHS and vets. I got kind of depressed reading about dying pets.

[0] https://en.wikipedia.org/wiki/POCSAG

There's a Reddit for software defined radio, using the cheap Realtek RTL2832U devices.


Just be aware that some of the moderators have iron fists and like to go ban happy.

I did something similar using the same libraries and all using a Raspberrypi and DVB-T dongle [1].

Then I configured the tcp server, rtl_tcp to forward the packets to my workstation. This allowed me to put the Pi in a location more suited to receive transmissions, i.e. not next to RF emitting servers and power supplies. Then, using Gqrx with remote server, I analysed the results.

[1] https://www.joechin.com/raspberry-pi-and-sdr-getting-started...

Looks like your SSL certificate has expired.

I would expect the communication to be encrypted, or is it supposed to be open to the outside world?

I think weather satellites were meant to be useful to everyone who could receive the signal.

Especially for marine traffic.

WeatherFax has been used for decades now.

This, however, wouldn't be of much use in a regular cruiser as you do not want to keep your laptop constantly to wait for broadcast. Dedicated devices are quite cheap and reliable.

I think most people are doing this for fun. But if you needed it and couldn't get your hands on the real thing, adding an Rpi and a screen to it makes a functional substitute.

Higher quality images are encrypted, but the basic stuff is open.

There are actually several vastly higher res weather broadcasts that are in the clear - they are just more work to receive. Search for LRPT and HRPT.

True, I was only thinking about the NOAA satellites used in the article.

I have this one: http://www.nooelec.com/store/sdr/nesdr-mini2-rtl2832u-r820t2...

I already received aircraft positions (ADS-B), aircraft radio, ACARS, VDL, HFDL, satellite pictures, ISS pictures, a ISS astronaut speaking, a bunch of other satellites (including radio pirates on US military sats), small outdoor weather stations, a portable radio for children, some AM radio stations, a lot of ham radio and pagers.

There is also huge amount of signals I can't decode yet.

BTW: I'm using an old TV antenna.

I don't understand these signals are not encrypted, for commercial reasons.

It's not a commercial service, but a government one. The NOAA (National Oceanic and Atmospheric Administration) satellites belong to a department of the US government.

In many countries, public transport is run by the government, but still costs money.

Governmental does not have to be orthogonal to commercial.

In Sweden the agency that deals with cartography and land measurements (Lantmäteriet) charged money for their data. An acquaintance who works with the government checked their income and where it same from. Something like 80% came from other government departments and the rest from companies. But the extra 20% was roughly the cost of running the administration to manage the billing. So the data got limited use for no financial gain.

In May they started releasing the maps under Creative Commons [1]. Not only me was really excited about this here in Sweden.

[1] in Swedish http://computersweden.idg.se/2.2683/1.616018/slapp-kartorna-...

With a few exceptions, any content created by the US federal government is in the public domain.


Save for certain privacy concerns, data collected through public funding should belong to the people who funded it.

You can use the same dongle to tap into ADS-B signals to get realtime ATC and display it on a map. Also, it's interesting to just sit and listen to SFO ATC audio channel.

He has a screenshot of this in the article.

Nice hack! I wonder what other useful common digital signals can one record with a dongle? Time and date, teletext (which includes weather forecasts and news) I am aware of. There is also project Lantern & Outernet which might deliver 20 MB of data to all continents through nanosats next year. What other digital signals are transmitted?

Free fast shipping: http://www.ebay.com/itm/251240985308

Most of them seem to have the same two chips in them, RTL2832U and R820T. Does anyone see an issue with this item I linked to? Some people were mentioning quality issues with generics.

There is also OsmocomSDR[1] which is an excellent starting point into software defined radio.

[1] http://sdr.osmocom.org/trac/wiki/rtl-sdr

How does this one differ?

All I have done till now is tune to local FM stations. I would like to tune to AM SW stations all over the world. I think buying a 50 USD RF up converter is the only option ?

You can do a very simple, though somewhat permanent, modification to the dongle and receive DC-14MHz. http://www.rtl-sdr.com/rtl-sdr-direct-sampling-mode/

There's a software modification here which is also interesting:


The whole idea of the community figuring out hacks to use this hardware in ways that were never intended by its original designers, including exploiting bugs/artifacts, reminds me of the demoscene.

I wonder if that would work on a commercial jet @ 30K feet. Would be an interesting test.

- https://twitter.com/grkvlt/status/600998588475375616

This is the output I got on a flight from EDI to LHR at about the half-way point, decoding ADS-B transmissions.

done this few years ago http://imgur.com/a/3GELZ

That's totally awesome! Have to try this myself.

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