360radar.co.uk is an independent system for enthusiasts which is available to all for £20 a year. Free if you contribute data. The cost of contributing is about £100 for the hardware, and you need to have an internet connection.
One advantage of contributing is that you get your own local display.
Unlike FR24, this system will show you almost all ADSB traffic out there, including QRA, Tankers, Power FLARM and private jets. The big exceptions (for now) are F22 & F35.
Where aircraft are sending ModeS returns with position info, only one hit is required, but for calculating the position of an aircraft not sending gps co-ords, the system needs five receivers to see the transponder returns to workout where the aircraft is.
This is never going to work for Mode a/c only aircraft, as the older transponders all look the same, but ADSB / Mode S boxes are uniquely identified.