There's a side-effect of having a big array of receivers; if you get the basic Mode S data that all aircraft (including military) transmit from their transponde

There's a side-effect of having a big array of receivers; if you get the basic Mode S data that all aircraft (including military) transmit from their transponders, a little math lets you quickly plot their location using a technique called multilateration (MLAT) using time of flight data and a bit of trigonometry.