They use multilateration, but there's no GPS synced clocks needed. They use ADS-B messages with position as reference and uses the relative arrival times of tho

They use multilateration, but there's no GPS synced clocks needed. They use ADS-B messages with position as reference and uses the relative arrival times of those messages to model the clock characteristics of each receiver. That way all you need is an inexpensive receiver (like a RTL-SDR) and with enough receivers in an area you'll get pretty good position estimates of aircraft only transmitting Mode S messages. This is the most popular mlat server software, used by ADS-B Exchange among others: https://github.com/adsbxchange/mlat-server https://github.com/adsbxchange/mlat-server