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RTL-SDR Tutorial: POCSAG Pager Decoding

May 25, 2013 admin

The RTL-SDR software defined radio combined with SDRSharp, and a POCSAG/Flex capable decoding 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 used more, 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 these messages, as they are plain text unencrypted, but it is illegal to act on the information received. Please respect your local laws.

Examples

Here YouTube user Superphish shows 5 minutes of pager decoding using HDSDR, PDW and an RTL-SDR.

YouTube Video

Here YouTube user nerdymark shows 18 minutes of pager decoding using SDRSharp, PDW and an rtl-sdr.

YouTube Video

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, Airspy and HackRF, or even traditional hardware radios with a discriminator tap.

Since pager signals are usually transmitted at a very strong power, 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. Check radioreference.com or Google for frequencies in your area, or just search for them manually – they are usually quite easy to spot.

Pagers use POCSAG or Flex encoding, and the signals waterfall will look on a waterfall something like the signal shown below. They also have a distinctive sound when played with NFM mode. A sound sample is also shown below.

POCSAG Waterfall Image
POCSAG Waterfall Image

For this tutorial, you will need to have an RTL-SDR dongle set up and working with SDRSharp. We will assume you have this much done already. If you do not, visit the Buy RTL-SDR page, and then the Quickstart guide.

You will also need to have an audio piping method installed and set up. Audio piping will allow the audio from SDRSharp to be passed to a decoding program. You can use either windows stereo mixVB-cable (free) or Virtual Audio Cable (paid with trial version).

The sampling rate of your audio piping method must be set to at least 48000 samples/sec. To set this in Windows, right click your device in the Windows sound recording tab, go to properties and under the advanced tab, set the sample rate to 48000 Hz. Do the same to the same device under the Playback tab as well. In Windows XP this setting may not be available, but it should be okay without setting it.

Sound Properties

Now, to decode the POCSAG or Flex signals, you need need to download and install a free program called PDW, which can be downloaded from this page, then follow these steps.

  1. Open SDRSharp and set the audio piping method to the one you will use under the Audio Output drop down box and then press Play.
  1. Tune to a pager POCSAG/Flex signal. Set the receive mode to NFM, filter bandwidth to 12500 Hz, filter order to 10, turn squelch OFF and filter audio OFF. Adjust the RF gain settings under the configure menu until good reception is achieved.
  1. Open PDW. You may initially receive some errors upon first opening it, but they can be safely ignored. Go to Options -> Options and Click Enable Pocsag Decoding, and 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.

PDW Enable POCSAG

  1. 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.

PDW Soundcard Interface Setup

  1. Go to Monitor, and ensure POCSAG/FLEX is ticked.
  1. Now, if everything is set up correctly, the pager audio from SDRSharp 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 volume settings in SDRSharp, and/or the Windows volume settings so that the volume meter goes up when a pager signal is sent. The percentage shown below the gauge shows the decode error rate. If you are receiving good signals the error rate should be very low and the percentage should be at or near 100%.

PDW Decoding

Other Decoding Software

MultimonNG is a Linux based decoder which is lightweight enough to run on a Raspberry Pi using rtl_fm.

Some Tips

  • Pager signals are generally very strong, and so almost any antenna can pick them up – even the stock antenna included with many dongle packages. However, if you live far away from the transmitter a better antenna matched to the pager frequency you want to monitor may be required.
  • If reception is very poor, you may get some garbled messages in the PDW window.
  • Since pagers can be so strong, you may actually need to reduce the RF gain to clearly discern between a real pager and an image. Reducing the gain may also help decoding if it is so strong that it begins clipping in the RF spectrum.
  • Sometimes setting the volume too loud can cause the pager audio signal to become distorted. Make sure you do not have the audio set too loud.

If you enjoyed this tutorial you may like our ebook available on Amazon.

The Hobbyist’s Guide to the RTL-SDR: Really Cheap Software Defined radio.

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