While the GPS satellite has atomic clocks, the GPS device also requires an accurate oscillator that is synced with the satellites' clocks — it uses the satellit

While the GPS satellite has atomic clocks, the GPS device also requires an accurate oscillator that is synced with the satellites' clocks — it uses the satellite signals to approximate the actual time and uses that approximation to measure the time delay to each satellite's clock to estimate distance, using an iterative algorithm.