Software defined radio (aka SDR), is a way of automating tuning into Audio/Digital radio receptions.

I am using a very simple (un-nervingly sensitive) USB dongle type RTL-SDR.COM

(cost inclusive antenna & mount was < £25).

This is able to tune through a huge spectrum of frequencies 0hz!! > 2.5Ghz

On a basic level it will tune into music radio station.....how ever where is the fun in that......

Also covers all the Radio Amateur bands , Dab radio and also Dab TV....etcetera

I am currently using a neat software package called SDR# (SDR Sharp), below gives an idea for what control you have over the RF goodness. (right view image for closer look).

The above Gui is tuned into the FM Broadcast band spectrum analyzer style, the colour frequency bands beneath it shows a running time graph of the signal levels, its useful when you are searching/tuning into data signals which just blip their short data packets sporadically (make it easier to home in on the signal).

I have been experimenting with tracking aircraft and can recommend RTL1090 by jetvision.de for simple quick data.

If you want a more portable system, its possible to connect the  SDR dongle to an Android device (via OTG connector/cable) .

So far my best recommendation is Avare ADB ,  when running, the receiver picks up the Planes Transponder data, giving various metric's of the flight.

Below you can see the airspace in my locality , this was just a quick test with buildings all around, as you can see it will detect aircraft up to 80kms.

(luckily for me Zurich, Friedrichshaven and Altenrhein airports are all within range .. \ü/ .. ).

Its also possible to pick up the International Space Station and also Weather Satellites, however that will take a bit longer for me to achieve.

The advantage of the RF SDF dongle is that its portable, i.e. no need for internet connections, un-tethers you from the Sky-Net . (some 4G bands are in range of this device, however 5G bands might be safe for a bit at least).

I have already a space for it in my "Zombie Apocalypse" rucksack, buy one today before they are outlawed (thats a serious prediction by me ;-)