Scientific Objectives
ACES will conduct the first experiments with cold atoms under microgravity conditions equivalent to a freely falling laboratory; it will perform fundamental physics tests to high resolution, and develop applications in different areas of research.
Scientific objectives
The scientific objectives of the ACES mission are:
 To demonstrate the performances of a new generation of space clocks
 To achieve time and frequency transfer with stability better than 10^{16}
 To perform fundamental physics tests
The combination of PHARAO and SHM will define an onboard frequency reference whose fluctuations are expected to be below 10 ps per day. The ISS orbit will allow ground users to compare their own clock to the ultrastable frequency reference provided by ACES. The results of these comparisons will provide new tests of fundamental laws of physics such as an improved measurement of the Einstein’s gravitational redshift, a search for anisotropies of the speed of light, and a search for spacetime variations of fundamental physical constants.
ACES Mission Objectives
Test of a new generation of space clocks  
Cold atoms physics microgravity  Such studies will be essential for the development of atomic quantum sensors for space apllications (atomic clocks, atom interferometers, sensors based on quantum gases)  
PHARAO frequency stability and accuracy  PHARAO performance: frequency instability better than 1•10^{13}•t^{1/2} for 1s ‹t ‹10day, where t is the integration time expressed in s; frequency inaccuracy at the 10^{16} level.  
SHM frequency stability  SHM performance: frequency instability better than 1.5•10^{13} at 1 s, 2.1•10^{14} at 10 s, 5.1•10^{15} at 100 s, 2.1•10^{15} at 1000 s, and 1.5•10^{15} at 10000 s of integration time.  
Stable and accurate time and frequency transfer via the ACES MWL  
Spacetoground time and frequency transfer  Time transfer stability better than 0.4 ps over one ISS pass, 8 ps after 1 day, and 25 ps after 10 days.  
Comparison of ground clocks  Common view comparisons to an uncertainty level below 1 ps per ISS pass. Noncommon view comparisons to an uncertainty level of 3 ps and 10 ps for spacetoground comparisons separated by a dead time delta t of 1000 s and 10000 s respectively.  
Ground Clocks synchronization  Absolute synchronization of ground clock time scales with an uncertainty of 100 ps.  
Contribution to atomic time scales  Contribution to TAI and comparison of primary frequency standards with accuracy at the 10^{16} level.  
Distribution of the ACES time scale  Distribution to ground users of an atomic time scale with time accuracy at the 100 ps level.  
Fundamental physics tests  
Measurement of the gravitational redshift  Precision measurement of the Einstein’s gravitational redshift to an uncertainty level of 2•10^{6}.  
Time variations of fundamental constants  Search for time variations of the fine structure constant α at an uncertainty level of α^{1}•δα/δt ‹1•10^{16} year^{1} after 1 year, down to 3•10^{17} after 3 years of mission duration.  
Search for Lorentz transformation violations and SME tests  Search for anisotropies of the speed of light at the δc/c ‹1•10^{10} uncertainty level. 
Conclusion
The measurement of time has experienced spectacular progress over the last centuries and ACES will provide the next leap forward. Time really does fly on the International Space Station.
ACES Scientific Applications
Relativistic Geodesy  Demonstration of a new technique for mapping the Earth gravitational potential with a resolution better than 1 m based on measurements of the differential gravitational redshift between clocks on ground. 
Clock comparison and time transfer via ELT 

Optical ranging via ELT 

GNSS applications 

Conclusion
The measurement of time has experienced spectacular progress over the last centuries and ACES will provide the next leap forward. Time really does fly on the International Space Station.
Last update: 3 November 2011
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