Waverider

Weather stations with transmitters (Figure 1) are used in many households today, including my own. The small transmitter, in this case, is on the window sill, measuring weather data such as temperature, air pressure, and humidity and transmitting the results digitally to a base station, which then displays them. However, a base station is not even necessary for receiving the data. With a little passion for tinkering, and the help of software-defined radio, I can receive, read, and even produce this wireless data with my own computer.

Figure 1: Communication between a commercial weather station (left) and the associated sensor (right) can be evaluated with SDR.

Software-defined radio (SDR) picks up electromagnetic waves almost directly at the antenna and uses software to process them. In the simplest case, an SDR-receiver consists of an antenna and an analog-to-digital converter plus software. Depending on the device, it can thus scan a very large frequency range. Applications for implementing SDR include GNU Radio, GNU Radio Companion, or Gqrx (see the "GNU Radio from a PPA" box).

echo 'SUBSYSTEM=="usb", ATTRS{idVendor}=="Vendor_ID", ATTRS{idProduct}=="Product_ID", GROUP="adm", MODE="0666", SYMLINK+="rtl_sdr"' > /etc/udev/rules.d/20.rtlsdr.rules

service udev restart

sudo add-apt-repository ppa:gqrx/snapshotssudo apt-get update
sudo apt-get install gqrx gnuradio

The software does the majority of the work, so your soldering iron can stay safely in the cabinet. You only need to find the right hardware, which costs only a few dollars, as well as an editor to process the digitized radio data on your computer.

The Hardware

The first step is to find the right reception hardware. Although specially developed hardware is quite expensive, DVB-T sticks at around US$ 20 offer a very convenient entry point into SDR, as long as they use a Realtek chipset (RTL2832U). The whole thing operates under the RTL SDR umbrella and relies on Librtlsdr library.

A look at my weather station shows that it receives data on 868MHz. Online sources list various chipsets and the frequency ranges they cover [1] [2]. On the recommendation of a colleague, I bought a Terratec Cinergy T Stick RC with Elonics chipset that covers the largest frequency range of the listed DVB-T receivers – and that includes the weather sensor's frequency. The hardware cost me US$ 25; a rod antenna is included.

Software

The basis for the installation was a netbook with Mint 15; the GNU Radio Framework [3] helped to analyze the received radio signals. This kind of kit even includes a graphical editor and supports raw data signal processing (optionally also in realtime) to create a target format with the help of various filters. The software automatically generates Python code that processes the data.

To use the DVB-T stick as a receiver, I first need the RTL SDR package. Typing:

git clone git://git.osmocom.org/rtl-sdr.git

installs RTL SDR on my computer. Gentoo and Arch Linux include prebuilt packages; on Ubuntu, you can install the software from a PPA (see the "GNU Radio from a PPA" box).

Building the software from the source works with cmake, but there's a catch: RTL-SDR uses the libusb library to communicate with the stick. However, when certain drivers are loaded, the software fails to correctly talk to the stick. My Mint system loaded the modules from Listing 1. Listing 2 shows a guide from the website [1] for building the software from the source code.

Testing, Testing, 123 …

To prevent the modules from loading, the udev rules need to be modified. To do this, I had to expand the cmake call adding the -DINSTALL_UDEV_RULES = ON parameter and then still issue the following command:

sudo make install-udev-rules

after doing so. You can now read the hardware functions as follows

rtl_test -t

to find out about, for example, which frequency range the DVB-T stick covers. Another practical test would be to receive a local radio station:

rtl_fm -f 98.5M -W -s 200000 -r 48000 - | aplay -r 48k -f S16_LE

The demodulator rtl_fm takes 200,000 samples per second on a frequency of 98.5MHz (the frequency of a radio transmitter). The aplay command plays the data stream on stdout at a rate of 48kHz. Because the raw audio file does not contain header information, aplay reads the data at this rate and encodes it with 16-bit in little-endian format.