Space clocks and fundamental tests: The ACES experiment

In this article we give an overview of the applications of ultrastable clocks in space. We focus on the case of the ESA space mission ACES, which is scheduled for flight onboard the international space station in 2013. With a laser cooled cesium clock, PHARAO, a space hydrogen maser, SHM, and a precise time and frequency transfer system, MWL, several precision tests in fundamental physics can be performed such as a measurement of Einstein's gravitational frequency shift with 2 ppm sensitivity and a search for time variations of the fundamental physical constants at 10(-17)/year. We present the advancement of the various mission instruments and present briefly applications in geodesy and Global Navigation Satellite Systems (GNSS).