Many years ago, Kinetics Avionics Ltd produce the world's first ADSB box the SBS-1. As technology improved so did their range of products the SBS 1ER and the SBS-3. Unfortunately for the end user Kinetics decided to cease production of their SBS ADSB boxes. They had designed a PUCK SDR and a Trilby Hat. I don't think the last two products were as sucessful as their SBS ADSB boxes. Hopefully Kinetics will start production of their boxes but it's highly doubtful. Time will tell. These boxes are being out performed by little RTL SDR 820T2 SDR dongles and RTLSDR software (in various flavours). Eventually the SBS ADSB boxes will become higly sought after. On a good day, all being well you may purchase a decent second hand SBS1 for just over £200 on Ebay .
An alternative to the range of Kinetics ADSB hardware is either SDR dongles tuned in to 1090mhz or acars frequencies or hardware supplied by AirNav®RadarBox.
ADSB is being developed more and more, aircraft will be able to transmit their positions over vast oceans and seas, satellites will relay the information to air traffic controllers and transmit back to them a new altitude or speed. Airspace is becoming heavily congested, dangerous, unsafe so new ways of fitting more aircraft and flights in the same space is a logical assuption and the next step.
ON the left is a screen capture of SBSplotter, it show the distance plotted of each aircraft at it's distance away from my home location. As you can see the best is 153.5 nautical miles from me. That distance was obtained by making a home made collinear antenna and sticking it out of a window because I'm not allowed to have a fixed antenna up. Unfortunately my best distance (which I didn't capture) is 701.4 nautical miles.