The RTL-SDR combined with SDR# and a POCSAG/Flex capable decoding software application can be used to decode pager messages. With this setup you can receive pager messages from all pager users on the system. If you don’t know what a pager is, since they are now uncommon, here is a brief explanation from Wikipedia:
A pager is a wireless telecommunications device that receives and displays numeric or text messages, or receives and announces voice messages.
Not many people use pagers these days with mobile phone text messaging being more common, but pagers are still popular with doctors, some fire and ambulance agencies and various service companies as they tend to be more reliable and have greater coverage.
In most countries it is perfectly legal to receive messages from pagers, as they are plain text unencrypted. BUT it is illegal to act on the information received. Please respect your local laws especially as sensitive data is often transmitted through pagers.
TUTORIAL
While directed at the RTL-SDR, this tutorial may also be useful for use with other software defined radios such as the Funcube dongle and HackRF, or even traditional hardware radios with a discriminator tap.
Since pager signals are usually transmitted at a very strong power level, usually almost any antenna will work to receive them, even the stock antenna that comes with the dongle. Pager frequencies differ among different countries. Usually they will be anywhere from 137 - 160 MHz, around ~450 MHz, or around 900 MHz. Most commonly they are found at around 150 MHz. Check radioreference.com or Google for pager frequencies in your area.
Pagers typically use POCSAG or Flex encoding and the signal waterfall will look something like the example shown below. They also have a distinctive sound on NFM. For a sound example see the signal identification guide at
www.sigidwiki.com .
For this tutorial, you will need to have an RTL-SDR dongle set up and working with SDR# or another SDR receiver program. We will assume you have this much done already. If you not, visit the Buy RTL-SDR dongles page at
http://www.rtl-sdr.com/store and follow the Quickstart Guide at the beginning of this book.
You will also need to have an audio piping method installed and set up. Audio piping will allow the audio from SDR# or your favorite SDR receiver software to be passed to a decoding program. Either stereo mix, or Virtual Audio Cable, or VBCable may can be used. See Appendix A: Audio Piping if you need more information. Also before starting ensure your audio piping method is set to a sample rate of 44100 Hz. See the setting the audio sample rate section if you need help doing this.
To decode the POCSAG or Flex signals, you need need to download and install a free program called PDW. PDW can be downloaded from http://www.discriminator.nl/pdw/index-en.html . After downloading and installing PDW follow these steps.
1. Open your favorite SDR receiver software such as SDR# and set the output audio to use your preferred audio piping method and then start the radio.
2. Tune to a pager POCSAG/Flex signal. Set the receive mode to NFM, filter bandwidth to 12500 Hz, turn squelch OFF, turn OFF any DSP or noise reduction algorithms and if in SDR# set filter audio to OFF. Adjust the RF gain settings until good reception is achieved. Since pager signals are so strong sometimes you may actually need to reduce the gain to ensure that the signal is not saturating.
3. Open PDW. You may initially receive some errors, but they can be safely ignored. Go to Options -> Options and click “Enable Pocsag Decoding”. Ensure the 512, 1200 and 2400 boxes are all checked. Also, ensure Enable Flex Decoding is enabled and that the 1600, 3200 and 6400 boxes are all checked. Press OK.
4. Go to Interface -> Setup . Enable the Soundcard checkbox, set the Configuration to Custom and choose your audio piping method in the Soundcard drop down box. If you only have one audio piping method enabled in the Windows recording properties, it will automatically choose that method. Press OK.
Now, if everything is set up correctly, the pager audio from SDR# should be being sent to PDW. In the top right hand corner of PDW, there should be a volume gauge. You will need to adjust the AF gain volume settings in your SDR receiver software and/or the Windows volume settings so that the volume meter goes up when a pager signal is sent.
The percentage sign under the meter shows the percentage ratio of good to bad decodes. 100% means all the received messages were decoded properly.
OTHER DECODERS
MultimonNG is also capable of decoding POCSAG and can also be run on Windows, Linux and even on an embedded device running Linux. At the command line in Linux use rtl_fm piped into multimon-ng to decode POCSAG and display it to the terminal. In the example code below replace the frequency of 158.1M and the PPM offset with a pager frequency used in your area and your dongles PPM offset value.
rtl_fm -f 158.1M -s 22050 -p 49 | multimon-ng -t raw -a POCSAG512 -a POCSAG1200 -a POCSAG2400 -f alpha /dev/stdin