Decoding ADS-B with free QIRX software
In the last two blog entries, I took a look at the DAB capabilities of free software QIRX by Clem Schmidt, DF9GI, from Frankfurt. It directly works with RTL-SDR, Airspy and RSP2 SDRs. I tried this very smart software from my location near Hannover/North Germany now also with ADS-B, mostly with my RSP2.
ADS-B stands for “Automatic Dependant Surveillance – Broadcast” and is an automatic service where aircraft continuously transmits several vital data on around 1.090MHz. Most important part of these data is the 2D location of the aircraft which it gets by GPS plus height by a baromatric altimeter. From this position data, many other data are derived, e.g. climbing/sinking or speed. If matched to databases, you will also see type of aircraft, flight number and many other data.
“The internet” provides many services showing the results of ADS-B and other data, collected from receivers all over the world, among them Flightradar24, OpenSky, FlightAware and AirNavRadarbox. They each provide many additional data, somtimes available at different schemes. Most provide free access to much of their data, with some more specific data behind their paywall. OpenSky as a scientific and non-profit organization offers billions of datasets for free, see Scientific Datasets. QIRX uses an OpenSky data base with about 650’000 entries.
Backbone of all these services is a net of ADS-B receivers, connected via the internet and curated by each company.
QIRX shows some capabilities of such a receiving station, using a proper antenna and a simple SDR. It decodes the I/Q stream of it. ADS-B is transmitted via pulse-position modulation, or ppm. The system is explained in ICAO Annex 10 Volume IV [free download].
With QIRX, you must set the sampling rate of you SDR to 200000[Hz], as other sampling rates won’t work, see screenshot below.
After that, and having started QIRX in ADS-B mode, decoding is done automatically. Release your seatbelts, and simply relax by viewing the activities above your head. Coverage largely depends on the “view” of you antenna and a few other factors like te sensitivity of your SDR and the attenuation of your cable connecting your antenna with your SDR. Some web services, thanks to anticipatory obedience/security reasons/data protection etc., do mute some “special” flights . This is not the case, of course, with this setup. QIRX always provides stable decoding at even low SNRs – great!
Last, but not least, please find below a comparison of FlightRadar24 and QIRX setup with Flight Number TK1554/THY6KG, Hannover->Istanbul, starting from Hannover Airport. One difference between both screenshots is that at my location (Burgdorf), I got the Airbus only after it had climbed to an atlitude of 200m or so, whereas the FR24 receivers are placed at positions allowing for tracking the aircraft from even the runway.
Also small aircraft is equipped with transponders, but not necessarily with ADS-B transponders, broadcasting the position, derived from their GPS. These small aircraft may haveonly Mode-S transponders on board, transmitting identification, height and squawk (transponder code) as assigned by their responsible ATC, or Air Traffic Control.