The military air communications monitoring enthusiasts over at milaircomms.com have been using a system involving RTL-SDRs to monitor military air traffic through ADS-B. While military aircraft generally do not transmit GPS position information like commercial aircraft do, they are still able to record live information such as the aircraft’s hex code, registration number, aircraft type, the base station location and a graph of recorded altitudes. They also log all this data showing where military aircraft have been spotted over time.
To receive this information they so far have a network of about 30 volunteers running RTL-SDR based ground stations that use their custom MilAirComms1090 software. If you want to contribute, the software is available for Windows and for Linux/Raspberry Pi.
Modesdeco is a Windows/Linux/OSX/RPi compatible command line ADS-B Mode S decoder built for the RTL-SDR. It natively supports the BaseStation format and so can be used with the BaseStation software without the need for com port converters.
Modesdeco has recently been updated to allow for the simultaneous reception of Mode S and Mode A/C. Mode S provides location data for ADS-B while Mode A provides an identification code and Mode C provides the aircraft’s pressure altitude.
The RTL-SDR can be used as a super cheap real time air radar. Modern planes use something called an ADS-B (Automatic Dependent Surveillance-Broadcast) Mode-S transponder, which periodically broadcasts location and altitude information to air traffic controllers. The RTL-SDR can be used to listen to these ADS-B signals, which can then be used to create your very own home aircraft radar system. Compared to dedicatedcommercialADS-Breceivers which can go for between $200 - $1000, the $20 RTL-SDR is very attractive for the hobbyist in terms of price. However, note that the RTL-SDR probably shouldn't be used for ADS-B navigation in a real aircraft for safety reasons.
ADS-B broadcasts at a frequency of 1090 MHz. It has been discovered by the RTL-SDR community, that the RTL-SDR with R820T tuner has the best sensitivity at this frequency. The E4000 and other tuners perform poorly in comparison. So it is recommended that you obtain an R820T tuner if you want to set up ADS-B decoding with the RTL-SDR. Recently there has also been talk about the R820T2 tuner, which seems to have slightly better performance too. See the Buy RTL-SDR dongles page for more information on where to purchase.
We also now note that recently new higher end SDR's like the $199 Airspy have developed very good ADS-B receivers that are several times more sensitive that the RTL-SDR.
Examples of RTL-SDR used as an ADS-B air radar
In this video, YouTube user Superphish shows a timelapse of air traffic over New Zealand using RTL-SDR, ADSB# and Virtual Radar Server.
ADSB Virtual Air RADAR with RTL SDR (RTL2832), ADSB Sharp and Virtual Radar Server