The payload actually contains two clocks: a caesium laser cooled atomic clock (PHARAO) developed by CNES, France for long-term stability and an active hydrogen maser (SHM) developed by Spectratime, Switzerland for short-term stability.
Further, it will allow to identify the optimal operating conditions for PHARAO and to select a compromise between frequency accuracy and stability.
After earlier plans for launch readiness in 2012,[5] the clock ensemble was expected to travel to the space station aboard a SpaceX Falcon 9 in 2021.
[6] Major delays due to difficulties in the development and test of the active hydrogen maser and the time transfer microwave system have extended the launch to 2025.
[7] The ACES module will be externally mounted to the ESA's Columbus Laboratory [8][9] with an 18-30 month expected operations phase.