Basic terminology and hardware setup description for ATC listening
Welcome to our ATCO2 project site. This is the first blogpost from a short series of “What SDR to buy, where to place it, how to setup it and connect to the OpenSky-Network platform .” We hope these posts help you in receiving clean audio signals from ATC VHF communication and feed the community. As most of us were noobs in SDR we had to learn a lot. And now, we are sharing what we learned to make your life easier. If you are an expert in this area skip this post. If you think there is something missing here, share your thoughts!
You decided to buy, set up and use an ATC receiver. Congratulations for your decision! Now let’s see what you particularly need to do. You must decide on where to place, what HW to buy, and how to set it up. WHAT is discussed in this and the next blog post while the WHERE and HOW answers are discussed in the following posts.
You should select a place with as clear visibility as possible to the airport tower (or a place where the transmitter antennas are). Use some on-line map and make an elevation profile between your position and the airport. There should not be any hills. The better your position is close to an approach route or waiting circuit. You will have a clean signal from the plains above you.
Now the general WHAT answers come. What you buy depends very much on your budget. We have tried two varians:
- cheap (~200EUR) and
- more expensive (~400EUR).
There are four components you need to take into account:
The antenna
You want an 50ohm antenna with the highest “gain” (Well, antenna is a passive thing, so there is not any gain technically. You want to minimize signal loss.). There are many types of antennas so please select the one you can mount easily where you want to. We have tried several types of antenna (J-pole, Discone, Dipole). One important parameter of the antenna is the frequency range (or tuned frequency). Here, you are interested only in Rx (receiving frequency) range. The range should be covering the airband (ATC frequencies) which are in range 108MHz to 137MHz (usually around 122MHz). It is good to have a narrow band antenna tuned just for these frequencies. The narrow band antenna may lower noise coming from other strong sources around you (AM/FM radio stations, TV stations, GSM, ...).
The cable
You need a 50ohm coaxial cable to connect your antenna with an SDR device. Every cable has a signal loss. You want a cable as short as possible (but keep some reserve). We used the LLC category one (low loss). The lower the loss, the higher the price. It is also good to check the technical specifications and find the loss (in dB per m) for the given frequency range (you are not interested in loss in 2GHz but just around 120MHz). The last important property of the coax cable are the connectors. Every connector introduces signal losses. Find a cable which has the right connectors for your antenna (N-Type Female connector on the cable side for example) and SDR device (usually SMA Male connector on the cable side). Adding any adapters increases the signal loss. Warning: the coaxial cable cannot be bended in a small radius (several centimeters / an inch) - the bend may introduce high losses. Check the smallest allowed radius in the cable technical specifications. Note: If you want to use more receivers on one antenna, you will need to buy an active splitter. We have tried it and it works. But we will not go into details here.
The receiver
You need an SDR (Software Defined Radio) receiver. There are other types of receivers but we ignore them for the sake of simplicity here. The SDR means that the receiver just digitalizes the analog signal from the antenna. The voice decoding is done by a software in a computer. The most expensive item in your bill is the SDR. The more expensive, the better quality (means coping better with low quality signals). The SDR usually has some analog circuits (gain controllers, filters, etc), an analog-to-digital converter (ADC), and some communication chips to talk to the computer (handling USB port for example). One of the most important parameters is the dynamic range of the SDR. The range is defined by the ADC. The problem is, that you will face strong and weak signals. If the dynamic range is small, then the strong signals may lead to clipping (signal distortion) while the weak ones are sunk in noise. Also the quality of the analog part is essential to overcome noise coming from your computer, power suppliers and other electronic devices at home. Minimum is 8-bit SDR but if you can afford 12, 14, 16 or more bit SDR it would be better. (some more reading about SDR sensitivity is here: SDR Receiver Performance Overview)
The computer
Here you want something small enough with low consumption, but powerful enough to decode all the channels you want to listen to and share with the community. The computer should also have the internet connection (WiFi, Ethernet, etc.). You can use an old notebook, your desktop or some sort of Raspberry Pi etc. Just take into account that the computer should be always on (if you want to be our data feeder). You connect the SDR to the computer (by USB in most cases) and then the computer to the Internet. We provide you with a description of how to install and configure all the software needed. There are several programs running on the computer. First, there is a radio demodulator. This program takes raw data (digitized signal) from the SDR and extracts the voice. Amplitude modulation is used in the VHF ATC. The program listens to selected frequencies (yes you can tune in and listen to voice communications in parallel), detects communication (when the pilot pushes a button and starts to talk), passes data through the demodulator, and stores the demodulated audio internally. Another program immediately post-processes these files and sends them to our servers. You can then log in to the OpenSky Network web and listen to your recordings.
That is all the compressed basic information about what you need to set up your own data feeder and to start to listen to ATC communication. We will go deeper in the next post. We will share what devices we tried and what results we got.