Fifty Things you can do with a Software Defined Radio | Hacker News Reader

Fifty Things you can do with a Software Defined Radio

blinry.org

341 pointsTimothee40 comments

SDR is great. I'm using an RTL-SDR dongle together with the rtlamr software to read my gas meter. It provides measurements about every 30 seconds, and my gas company doesn't give any real-time data. Quite handy.

(and I run an adsb receiver, which incidentally provides free access to FlightAware and other services if you share with them.)

What frequency is your gas meter transmitting at?

The 900Mhz ISM band, specifically: (dump from rtlamr output)

07:12:02.921590 decode.go:45: CenterFreq: 912380000

07:12:02.921659 decode.go:46: SampleRate: 2359296

07:12:02.921663 decode.go:47: DataRate: 32768

07:12:02.921666 decode.go:48: ChipLength: 72

07:12:02.921669 decode.go:49: PreambleSymbols: 32

07:12:02.921672 decode.go:50: PreambleLength: 4608

07:12:02.921674 decode.go:51: PacketSymbols: 736

07:12:02.921677 decode.go:52: PacketLength: 105984

07:12:02.921681 decode.go:59: Protocols: scm,scm+,idm,r900

Well, it is truly amazing what you can do with a cheap RTLsdr dongle and some time and energy. I used GNU Radio to build a receiver flowgraph that decodes and displays the signal from a nearby VOR aircraft navigation transmitter. It was cool to drive around it, and see the pointer update in real time.

The sensitivity of these things is on the order of a normal receiver, (0.1 microVolts) which surprised me, knowing the samples are 8 bits.

ADC sensitivity is influenced by quite a few things, some of them pretty unintuitive (e.g., some interference from unwanted signals in the RF passband can be a good thing if it provides a source of dithering that allows weaker signals to cross bit boundaries).

Bit depth is surprisingly far down the list in overall importance. To understand why, look up "FFT processing gain." Consider that sigma-delta converters are 1-bit ADCs, yet the S-D topology is used for many applications that require wide dynamic range.

The interface between analog and digital signals is weird science

On hunting weather baloons:

> And he was right: Someone had been faster than us! The status was changed. So in the end, I didn’t find the sonde. But something that might be even better – a friend!

Other than all the other fun packed into SDR this story just put me in a good mood today.

Off-topic, web performance: compress your images, use caching headers, and/or serve them via a CDN (ideally one that supports HTTP/2). The current server is not powerful enough to handle the current set of images on this post in a reasonable time.

It seemed to work fine for me, maybe I came in a little late.

Here’s more stuff to add I did before:

- Scan for drones that are using RC link

- Scan nearby cell operators and their metrics.

- Scan and pentest cellular networks

- Create your own LTE/5G tower.

- Transmitting FM/AM radio

- Communicate with other pilots by tuning on airband

Among others, for this you need a full duplex SDR, so rtl or hackrf won’t do, limeSDR or bladRF would.

For the author: I did have a realtime voice over SDRangel, it allows you to have the sdr in a physical location away from you, if I got some time I will write how to do it.

Have any links or sources for tutorials on getting into those several are intersting

Tutorials? No I don’t, I’m guilty of being bad in documentation, but the plan is there to do full write up. Some are straightforward like AM/FM station as long as you have an SDR capable of doing so. The communicate with pilots one I had some write up being part of a project (https://tamim.io/professional_projects/nerds-heavy-lift-dron...) might give some insight, the scan cellular network one, I have an intro in here (the first project not the phone one) but planning to update it with more details once I get an approval since the client was the government (https://tamim.io/professional_projects/cellular-5g-lte-cover...). There are even other use cases for an SDR I didn’t mention but did try: using it as a GPS receiver, scan for drones in the vicinity if they are equipped with remote ID, program the SDR to broadcast voice messages based on written inputs, either manually or fed from another software, useful in cases when you want to automate the broadcast.

I have a soft spot for SDR. I've only done a bit of it, but the mathematical techniques I learned from it helped kick start my career. While I worked in a different field, having made friends with frequency and phase analysis was hugely powerful.

Images still very slow, but:

https://web.archive.org/web/20240316173921/https://blinry.or...

(there are also two snapshots from today (2024-03-17), but yesterday's is the one I didn't give up on waiting to load)

https://archive.is/ZHnfV

Saving with SinglePage, it's around 125 MB (original a bit less, archived copy a bit more).

For about signal bending at 41 (transport tires monitoring pressure). From my experience with ax-25 I was initially wonder, why transmission usually includes large delay on turning On and large delay on turning off (each about 300ms, some people recommend even 1500ms).

So, idea is, when transmitter turned on, it spend some time to become stable, for modern high-tech equipment this could be faster, I measured less then 50ms on modern Japanese desktop radios, but for older it could really be more than second.

For example, GSM solves this with brute force way - all GSM have high power load, CB guys usually call it "equivalent" and switch, and when GSM 2.0 make call, it turn transmitter on at begin of call and connect it to antenna only when need to transmit, all other time power spent on load, and transmitter turned off after call finished.

When GSM MT working with GPRS or 3G, things are more complicated, but it is usually "half-duplex" communication, most traffic received from network, upload usually limited, so MT could save power, turning on transmitter only when have large batch of data in buffers and turning it off when pause appear. But "equivalent" used anyway.

Returning to tires, their transmitters usually in very harsh environment, and batteries are not always give enough power (I think, batteries are weakest tech in this case), and yes, this could be reason, why transmitter frequency become unstable.

Other may possible reason, but probability is not high, if signal reflected from plane or if some interference happen.

What I mean on interference, I still sometimes see old radios, which don't have good enough filters, and they could retransmit distorted signals with their heterodyne circuit.

Is there a very low bandwith equivalent of Internet via SDR? Like BBS via radio packet thingy?

In this case what is a good SDR device that does TX? The one in the article does only RX

> In this case what is a good SDR device that does TX?

The hackRF is a popular one. If you want to do anything at range, note that you'll need an RF amplifier and an antenna.

Also note that TX is heavily regulated, and you should get licenced before you start blasting.

> Is there a very low bandwith equivalent of Internet via SDR?

There's open source software to build your own LTE tower using an SDR. That's not really low bandwidth, though...

Most of the SDRs are receive-only. The ones that do transmit have really low power. The LimeSDR is 10mW and most are in that range. That means need amplifier or two to make it usable.

Packet radio is done with amateur radio. There is both VHF/UHF ones that use normal radios, and 2.4GHz ones which usually use Wifi with dishes. There aren't BBSes because there isn't much point with Internet, and there isn't much point to Internet access when have mobile. Plus, it isn't allowed to provide non-amateur Internet access.

There's no BBSes now, no. There once were, I often used F6FBB's amazing BBS software though I never ran one full time. But many people did. We had all night chat sessions at a time where phone BBSes cost serious money to use.

These days packet is boring as hell, nobody chats anymore. It's just aprs position messages that nobody bothers to look at :(

The stratux.me project was so much fun. Started off as a sub-reddit and just expanded. Take a simple raspberry pi, add a couple of RLT-SRD dongles, Bluetooth, maybe a USB GPS - and it replaced a $1k bit of kit. The ADSB information that people see on flightaware is available to us flying bug smashers. The stratux will tie into a tablet/moving map and provide tracking of nearby airplanes and weather. From a situational awareness, this is just huge.

ACARS is pretty cool https://www.rtl-sdr.com/rtl-sdr-radio-scanner-tutorial-recei...

I set up one to pull readings from my utility meters. Now I've got the exact figures the power company uses to calculate my bill, and can track it throughout the month, entirely offline

Do you happen to have a write-up of this? I'd be interested in building the same capability, although I doubt we have the same provider.

I'll get one written up post haste

I've been batting around a few blog posts on various home automation projects and will probably sit down and write this one this afternoon

I'll reply with an link when it's up

Thanks! After I asked this I googled around and found on GitHub bemasher/rtlamr, but discovered it doesn't work on my meter's network (FlexNet/Icon/Census). Hopefully someone figures out FlexNet, I'd like to add my meters to my home data collection!

Here's my writeup:

https://pdx.su/blog/2024-03-17-reading-my-electric-meter-wit...

It's based on rtlamr, which might still be a non-starter for you, but it should point you in the right direction.

Most of what I did was just turn-key assembly of existing code, but you might be able to make something work by digging through how rtlamr works

Thanks!

Item #17 - 865-868 MHz - Could that possibly be LoRa-based communication?

Seems so idiotic that you're not allowed to receive maritime traffic in Germany (item #49).

Don't transmit any, if you don't want people to receive it.

I just wanted to post that I can't find anything about it being forbidden, but there is actually a link in the post[1]. So basically the situation is:

Mit einer Funkanlage dürfen nur solche Nachrichten abgehört oder in vergleichbarer Weise zur Kenntnis genommen werden, die für den Betreiber der Funkanlage, für Funkamateure im Sinne des § 2 Nummer 1 des Amateurfunkgesetzes, für die Allgemeinheit oder für einen unbestimmten Personenkreis bestimmt sind.

Which in English would read something like this:

With radio equipment you are only allowed to listen to, or take note of in some other way, those messages that are addressed to the operator of the equipment, those messages that are addressed to ham radio operators in general, those messages that are addressed to the general public or for an undefined group of people.

This is of course absolutely unenforceable, untraceable and generally ridicoulus, but it is what it is. Imagine your neighbours having very loud arguments (or very loud 'horizontal discussions') in a clairaudient flat, and then telling you that you cannot listen to it. And being correct legally ;)

[1] https://www.buzer.de/5_TTDSG.htm

> "Wir haben unsere Daten gesichert, indem wir nicht autorisierte Benutzer freundlich gebeten haben, nicht darauf zu schauen."

:^)

You all may appreciate (a short browse away from TFA): Antragsformular für den Passierschein A38

https://blinry.org/passierschein-a38/

German regulations looks like ahead of technology. - You could affect quantum communications by listen them.

Also I remember, we in Kyiv have interest place - in zone around broadcast tower, signals once was so strong, people made simple spirals with light bulb and wire, around cup, and drink "tea with flame". To be honest, when lot of such devices used close enough to radio station, it could significantly drain power from radio station, so apparently this was prohibited, but I still don't think it is enough to prohibit listening in whole country.

https://en.wikipedia.org/wiki/Kyiv_TV_Tower

Using the electric field was indeed possible when megawatt TV stations were still a thing. You could light a tiny bulb or ccfl tube with it. Though at those frequencies you'd need a seriously long loop to heat water. I've been told farmers next to the TV tower would do it with their perimeter fence.

But these days with DVB the big several megawatt VHF towers covering a whole region have been replaced with smaller base stations running only 10-20kW and usually covering the local city area only.

> when megawatt TV stations were still a thing

In my country it was just few years ago with this tower.

I don't know sum of power of all transmitters (looks like it was secret information), but I knew few engineers, worked at radio stations and TV channels, who have their equipment installed there, and they talked about tens Kilowatts each one.

So, I think, when analog TV used, total transmission power exceed megawatt, now, yes, they use DVB, and I hear moderate numbers, about 20kW on each base station.

Must say, digital changed world, it give much clearer reception, but lost magic.

But maritime and air traffic is not personal, it's factual professional traffic. You're invading no one's privacy by listening to it, and if you're allowed to monitor ADS-B and AIS why not the voice version which conveys similar information?

And even messages to other traffic isn't officially allowed? That doesn't make sense. In fact pilots are always encouraged to monitor all radio traffic even the messages not meant for them as it builds a valuable picture of the traffic around them and its intentions. This saves lives on a regular basis.

Besides it being unenforceable it also would seem a bad idea to discourage it.

There are also FCC regulations in the US that relate to receiving signals under certain circumstances: https://www.fcc.gov/consumers/guides/interception-and-divulg...

I recall Finland had a rule that you can listen (how else would you hear the next message meant for you on maritime VHF), but you can't repeat things publicly or act on knowledge gained from that. Now, why would that be? Well, it's mostly to minimize sensationalist news reporting of people's accidents, misfortunes and such, whether maritime or over old school police radio (before it was encrypted). Discourage ambulance chasing, that sort of thing. It's just another part of the European attitude toward privacy.

I think a number of countries have this sort of thing. It's more, as you say, not banning the listening but if it can be shown you acted on it then that's the no-no. This was probably more relevant in the times of unencrypted analogue cordless telephones and cellphones. The sensationalist news thing is a good thought too.

Reminds me of Germany in WW2 where citizens would get locked up or worse for listening to the BBC to get non-government approved news.

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