ACES is a new generation of atomic clock taking advantage of the micro-gravity environment of the ISS. The ACES payload will distribute a stable and accurate time base that will be used for space-to-ground as well as ground-to-ground clock comparisons. The direct comparison of ultra-precise atomic clocks is crucial for the exploitation of ACES potential in different research areas: fundamental physics (General Relativity and String Theory tests), time and frequency metrology, but also geodesy and gravimetry, precise orbit determination, Earth monitoring, Very Long Baseline Interferometry, global positioning and navigation.
ACES is a complex payload, involving both state-of-the-art instruments and High Tech subsystems. The heart of the payload consists of an atomic clock based on laser cooled Caesium atoms. The performances of the Caesium frequency standard PHARAO are combined with the characteristics of a Space Hydrogen Maser (SHM). The ACES clock signal merges together the excellent short and medium term frequency stability of SHM with reference long term stability and accuracy of a primary frequency standard based on Caesium cold atoms. The on-board clock-to-clock comparison (PHARAO-SHM) and the distribution of the clock signal are ensured by the so called Frequency Comparison and Distribution Package (FCDP), while all data handling processes are controlled by the eXternal PayLoad Computer (XPLC). One of the main objectives of the ACES mission consists in maintaining a stable and accurate on board timescale that can be used for space-to-ground as well as ground-to-ground comparisons of frequency standards. Stable and accurate time and frequency transfer is achieved by using a specially developed state-of-the-art microwave link (MWL), which is necessary not only to characterize the ACES clocks ensemble, but also to perform general relativity tests of high scientific relevance.
ACES is also equipped with a GPS receiver, which will supply a very precise position of the clocks, as well as an optical link (ELT) which also ensures a high performance exchange of time with the ground.
In 1967, the second—as one of the base units in the International System of Units (SI)—was defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the Cesium-133 atom.
The extremely high accuracy in the determination of the second using the measurement of atomic frequencies gives to the International System's time unit a privileged role among all the other units, and justifies its use in the definition of many secondary SI units.
Accommodation & Transport
ACES will be mounted on a specific mechanical interface called CEPA and launched by the Dragon transfer vehicle. The Station's robotic arm will then transfer ACES and clip it to the nadir (Earth-pointing) position on the Columbus External Payload Facility.
The stable and accurate time delivered by ACES will be used to perform space-to-ground as well as ground-to-ground clock comparisons. The information, together with payload telemetry, ISS ancillary data and precise orbit determination data will be collected by the facility responsible centre (FRC), elaborated and made available for fundamental physics studies and other applications, e.g. time and frequency metrology or geodesy.
The ACES mission consists of three operational phases: commissioning and in-orbit validation, payload characterization, and operational use.
The first phase lasts several weeks and is concluded by the successful validation of the payload.
During the second phase, which will last 6 months, PHARAO's performances will be evaluated. The Caesium Clock will be characterized through the on-board comparison with SHM (short term) and space-to-ground clock comparison (long term).
In the operational use phase, the PHARAO parameters will be set at their optimal values and the links will be synchronized in order to have an ACES time base with the best performances both on the short/medium and long-term. During this phase the ACES time scale will be compared with the one of ground clocks.
Unique to ACES is the possibility of worldwide participation to the data exploitation programme through a global array of ground users interested in comparing their ground-based atomic clocks with ACES clock signal and in analyzing the scientific data. This is organized by the ACES International Working Group.
The validation and operational use phases are planned to last at least 18 months. The first 6 months will be dedicated to characterizing and evaluating the clocks and the Time & Frequency (T&F) link, followed by its phase of operational use, including a demonstration of T&F distribution involving users around the world. ACES operation can be extended to 30 months.
ACES will be ready for launch by mid 2018.