ISS ACES - Atomic Clock Ensemble in Space
The core of ACES is a laser cooled Caesium atomic clock known as "Pharao", that will exploit the microgravity conditions onboard the ISS. In addition to the Pharao clock, there are two other elements : a hydrogen maser (Microwave Amplification by Stimulated Emission of Radiation), and a microwave link (used for time and frequency transfer).