Entries tagged sdr

Related tags: esp8266, iot, linux, radio.

Decoding 433Mhz-transmissions with software-defined radio

Sunday, 11 February 2018

This blog-post is a bit of a diversion, and builds upon my previous entry of using 433Mhz radio-transmitters and receivers with Arduino and/or ESP8266 devices.

As mentioned in my post I've recently been overhauling my in-house IoT buttons, and I decided to go down the route of using commercially-available buttons which broadcast signals via radio, rather than using IR, or WiFi. The advantage is that I don't need to build any devices, or worry about 3D-printing a case - the commercially available buttons are cheap, water-proof, portable, and reliable, so why not use them? Ultimately I bought around ten buttons, along with a radio-receiver and radio-transmitter modules for my ESP8266 device. I wrote code to run on my device to receive the transmissions, decode the device-ID, and take different actions based upon the specific button pressed.

In the gap between buying the buttons (read: radio transmitters) and waiting for the transmitter/receiver modules I intended to connect to my ESP8266/arduino device(s) I remembered that I'd previously bought a software-defined-radio receiver, and figured I could use it to receive and react to the transmissions directly upon my PC.

The dongle I'd bought in the past was a simple USB-device which identifies itself as follows when inserted into my desktop:

  [17844333.387774] usb 3-9: New USB device found, idVendor=0bda, idProduct=2838
  [17844333.387777] usb 3-9: New USB device strings: Mfr=1, Product=2, SerialNumber=3
  [17844333.387778] usb 3-9: Product: RTL2838UHIDIR
  [17844333.387778] usb 3-9: Manufacturer: Realtek
  [17844333.387779] usb 3-9: SerialNumber: 00000001

At the time I bought it I wrote a brief blog post, which described tracking aircraft, and I said "I know almost nothing about SDR, except that it can be used to let your computer do stuff with radio."

So my first step was finding some suitable software to listen to the right-frequency and ideally decode the transmissions. A brief search lead me to the following repository:

The RTL_433 project is pretty neat as it allows receiving transmissions and decoding them. Of course it can't decode everything, but it has the ability to recognize a bunch of commonly-used hardware, and when it does it outputs the payload in a useful way, rather than just dumping a bitstream/bytestream.

Once you've got your USB-dongle plugged in, and you've built the project you can start receiving and decoding all discovered broadcasts like so:

  skx@deagol ~$ ./build/src/rtl_433 -U -G
  trying device  0:  Realtek, RTL2838UHIDIR, SN: 00000001
  Found Rafael Micro R820T tuner
  Using device 0: Generic RTL2832U OEM
  Exact sample rate is: 250000.000414 Hz
  Sample rate set to 250000.
  Bit detection level set to 0 (Auto).
  Tuner gain set to Auto.
  Reading samples in async mode...
  Tuned to 433920000 Hz.
  ...

Here we've added flags:

  • -G
    • Enable all decoders. So we're not just listening for traffic at 433Mhz, but we're actively trying to decode the payload of the transmissions.
  • -U
    • Timestamps are in UTC

Leaving this running for a few hours I noted that there are several nearby cars which are transmitting data about their tyre-pressure:

  2018-02-10 11:53:33 :      Schrader       :      TPMS       :      25
  ID:          1B747B0
  Pressure:    2.500 bar
  Temperature: 6 C
  Integrity:   CRC

The second log is from running with "-F json" to cause output to be generated in JSON format:

  {"time" : "2018-02-10 09:51:02",
   "model" : "Toyota",
   "type" : "TPMS",
   "id" : "5e7e0637",
   "code" : "63e6026d",
   "mic" : "CRC"}

In both cases we see "TPMS", and according to wikipedia that is Tyre Pressure Monitoring System. I'm a little shocked to receive this data, unencrypted!

Other events also become visible, when I left the scanner running, which is presumably some kind of temperature-sensor a neighbour has running:

  2018-02-10 13:19:08 : RF-tech
     Id:              0
     Battery:         LOW
     Button:          0
     Temperature:     0.0 C

Anyway I have a bunch of remote-controlled sockets, branded "NEXA", which look like this:

Radio-Controlled Sockets

When I press the remote I can see the transmissions and program my PC to react to them:

  2018-02-11 07:31:20 : Nexa
    House Code:  39920705
    Group:  1
    Channel: 3
    State:   ON
    Unit:    2

In conclusion:

  • SDR can be used to easily sniff & decode cheap and commonly available 433Mhz-based devices.
  • "Modern" cars transmit their tyre-pressure, apparently!
  • My neighbours can probably overhear my button presses.

| 2 comments.

 

Tracking aircraft in real-time, via software-defined-radio

Thursday, 5 October 2017

So my last blog-post was about creating a digital-radio, powered by an ESP8266 device, there's a joke there about wireless-control of a wireless. I'm not going to make it.

Sticking with a theme this post is also about radio, software-defined radio. I know almost nothing about SDR, except that it can be used to let your computer "do stuff" with radio. The only application I've ever read about that seemed interesting was tracking aircraft.

This post is about setting up a Debian GNU/Linux system to do exactly that, show aircraft in real-time above your head! This was almost painless to setup.

  • Buy the hardware.
  • Plug in the hardware.
  • Confirm it is detected.
  • Install the appropriate sdr development-package(s).
  • Install the magic software.
    • Written by @antirez, no less, you know it is gonna be good!

So I bought this USB device from AliExpress for the grand total of €8.46. I have no idea if that URL is stable, but I suspect it is probably not. Good luck finding something similar if you're living in the future!

Once I connected the Antenna to the USB stick, and inserted it into a spare slot it showed up in the output of lsusb:

  $ lsusb
  ..
  Bus 003 Device 043: ID 0bda:2838 Realtek Semiconductor Corp. RTL2838 DVB-T
  ..

In more detail I see the major/minor numbers:

  idVendor           0x0bda Realtek Semiconductor Corp.
  idProduct          0x2838 RTL2838 DVB-T

So far, so good. I installed the development headers/library I needed:

  # apt-get install librtlsdr-dev libusb-1.0-0-dev

Once that was done I could clone antirez's repository, and build it:

  $ git clone https://github.com/antirez/dump1090.git
  $ cd dump1090
  $ make

And run it:

  $ sudo ./dump1090 --interactive --net

This failed initially as a kernel-module had claimed the device, but removing that was trivial:

  $ sudo rmmod dvb_usb_rtl28xxu
  $ sudo ./dump1090 --interactive --net

Once it was running I'd see live updates on the console, every second:

  Hex    Flight   Altitude  Speed   Lat       Lon       Track  Messages Seen       .
  --------------------------------------------------------------------------------
  4601fc          14200     0       0.000     0.000     0     11        1 sec
  4601f2          9550      0       0.000     0.000     0     58        0 sec
  45ac52 SAS1716  2650      177     60.252    24.770    47    26        1 sec

And opening a browser pointing at http://localhost:8080/ would show that graphically, like so:

NOTE: In this view I'm in Helsinki, and the airport is at Vantaa, just outside the city.

Of course there are tweaks to be made:

  • With the right udev-rules in place it is possible to run the tool as non-root, and blacklist the default kernel module.
  • There are other forks of the dump1090 software that are more up-to-date to explore.
  • SDR can do more than track planes.

| 2 comments.

 

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