Introduction
Another branch of the hobby I have enjoyed for many years now has been detecting and decoding digital data and messages that are all over the airwaves. I thought it would be a good idea to list some of the excellent free software available on the internet, and share a few hints and tips. (I have included a list at the end of this article with links) It can be very satisfying knowing that you have tuned into a signal and decoded it when you were never meant to!
RTL-SDR
By far the cheapest, easiest and most convenient way for anyone to enter into the world of radio frequency scanning is to use a USB dongle plugged into your computer. Usually referred to as RTL-SDR dongles. These cheap devices were first manufactured to receive free-to-air TV broadcasts for a computer. Some bright spark realised that by employing software to 'hack' the tuner inside, they could be repurposed as a handy radio receiver. Since then, manufacturers have cottoned-on to the idea and now they are modified and sold as RTL-SDR instead of DVB-T sticks. This means that the prices went up, but they can still be found for a modest sum. In my experiences the best ones have the RTL2832U chip. On the down side, these radios cannot compare with a dedicated scanner for sensitivity and reception, but they are less expensive.
Uniden make a good range of dedicated scanners both home-base and portable.
Antennas
The frequencies that you wish to tune into will determine the type of antenna you will need. Alternatively, you could go with a broadband scanning antenna which will cover many frequencies, but these tend to be more expensive to purchase. like the one pictured here. However, for more specific decoding the antenna will have to be more specialised. You can always 'home brew' an antenna, and it's surprisingly easy, the simplest being a wire dipole. There are many articles on making your own antenna on line, so I won't go into that here. The main requirement is that, if possible, get the antenna erected outside and as high as possible for best reception. Antennas do work quite well in an attic space, but it will fare much better outside.
AIRBAND & VHF
Where to start, well the list is virtually endless. Let's start with the VHF (Very High Frequency) band that everyone should be familiar with. We are all used to listening to radio stations on FM (Frequency Modulation) between the frequencies of 88 and 108 MHz. But if your radio can tune in just above this range, you will find all kinds of radio traffic. For a start, you can hear the Air Band chatter between aircraft and air traffic control on AM. Search around on this band and you will also hear some brief screeching sounds at 131.525, 131.625,131.725 and 131.825 MHz AM (Amplitude Modulation) These are the sounds of ACARS (Aircraft Communication Addressing and Reporting System) messages sent from aircraft transponders. With the correct decoder you can position aircraft on a map to do some plane spotting, if that's your thing. There are also lots of Amateur Radio operators chatting on this 2m band on FM. Also on the 2m band you will find pager communications, weather satellite transmissions, taxis and a whole lot of other stuff. Moving down to the lower limits of the rtl-sdr dongle, CB transmissions can be received between 27 and 28 MHz. This is the 10m band so a longer antenna will be required. At around 137 to 138 MHz you can hear APT (Automatic Picture Transmission) satellite transmissions from weather satellites at certain times during the day. These can be decoded into pictures that are really impressive using WXtoIMG. Your receiver must be set to receive FM at a bandwidth of about 40KHz though. Your SDR, when set up, will be able to manage this easily. This program also lets you know when they are flying over.
HF
If you have a radio receiver capable of getting the HF (High Frequency) bands, this is where the fun really starts. An SDR dongle is not capable of tuning down to lower frequencies below about 24MHz, unless it is modified in some way with a converter. On these bands you will find radio hams exchanging digital and voice communications between each other 24/7. Morse code, FT4, FT8, RTTY and many others. HFDL (High Frequency Data Link) from aircraft can be decoded giving positions of aircraft all over the world and not just within your own country. Using HF bands increases your range for scanning. It is possible to find Stanag 4285 transmissions from NATO stations, however this is usually encrypted, except for when a test transmission is sent. The French Navy are favourites for this.
This is a decoded test signal using Sorcerer.
WEFAX (Weather Facsimile Picture Transmission) signals can be found on several frequencies. 7.878 MHz USB finds the Deutscher Wetter Dienst WEFAX transmission. I used FLDIGI to decode it into picture format.
There really is too much to mention here because each mode is worthy of an article on its own. But its all there to be listened to with the right decoders. Some of the decoders listed here are capable of many modes. A lot of research is required to find information on how to do it, but it all depends on what you want.
Software
Here you can find links to download your free software. All the decoders here can use your sound card as a modem for decoding. If you are serious about decoding, there are many excellent non-free packages for sale that you may want to consider.
You will need to install a different driver for your dongle, otherwise your computer will still think that it is a DVB-T stick. Use Zadig to install the correct driver.
You'll be needing a software defined radio. HDSDR is the simplest and quickest way to get going. For HDSDR to work with your dongle you will need an ExtIO.dll file. It can be difficult to find this file and you may have to install a different package just to extract the dll. After you have installed HDSDR, find the HDSDR program folder on your computer and copy the ExtIO.dll file there or it will not work. There is a good set-up guide on this page with links HDSDR Set-Up
Another free SDR to consider is Sigmira which has a selection of decoders built-in.
If your sound card doesn't support Stereo Mix or similar, no need to worry. Just install this virtual sound card software and it will pipe the sound from HDSDR to your decoder when selected VB-Audio
When you finally get up and running and are ready for some decoding, it helps greatly knowing what a particular signal actually sounds like. Signal ID Wiki can help you here.
wxtoimg is the best software for decoding satellite images
FLDIGI has a wealth of decoders built into it including WEFAX, Morse and RTTY.
If you want to decode ADS-B 1090 MHz signals for aircraft tracking try this RTL1090 . This is a stand-alone program which connects directly to the RTL-SDR dongle, so no sound card set-up required.
You can then feed the output of RTL1090 straight into your very own Aircraft Radar with VRS
If you do want to work HF signals, Sorcerer is a good decoder for HFDL, Stanag and many others.
Other software you may be interested in:
WSJT-X for ham band decoding of FT4, FT8 etc.
PDW for ACARS and pager decoding.
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