Those of you familiar with ADSB in/out systems might be interested in this information from a book I just finished:

"The system employs ground based radar to survery the airspace and displays the information on a screen. The radar stations are located about as close together as current VOR stations The radar presentation is overlayed by a detailed low level map and the combined image is tramsmitted to a display in the aircraft cockpit.

Each aircraft in the area shows up as a tear-shaped blip on the display with the tail of the blip indicating direction and speed. A radial line on the display allows the pilot to determine which blip is his own aircraft. The pilot therefore has a low altitude chart in front of him at all times and can see his own progress as he travels along the airways, as well as the location of other trafic around him."

Why is this so interesting? Because author and aircraft designer Fred Weick saw it demonstrated by the RCA corportation in the fall of 1946. Sadly, the system (which was called "Teleran") did not find favor with the CAA who adopted VOR and OMNI systems instead.