What fundamental geometric and mathematical principle is used by Multilateration (MLAT) and Wide Area Multilateration (WAM) systems to calculate an aircraft's position?

1. Measuring the temperature change of the air column around the radome
2. Measuring the optical reflectivity of the aircraft livery using laser sensors
3. Measuring the Doppler shift of engine acoustic noise
4. Measuring the Time Difference of Arrival (TDOA) of transponder signals received at multiple time-synchronized ground receiving stations