ACES microwave link requirements

Atomic Clock Ensemble in Space (ACES) is a project of the European Space Ag ency on-board the future International Space Station (ISS). The payload con sists mainly of two atomic frequency standards, one space hydrogen maser (S HM) prepared by the Observatoire de Neuchatel (Switzerland), and one cold a tom caesium clock called PHARAO prepared by the CNES (France), with the par ticipation of the BNM-LPTF, the ENS-LKB, and the CNRS-LHA. Because of the anticipated performances of these clocks on-board the ISS, t he requirements of the links between the payload and the clocks on the Eart h are at the limits of the known potential of the optical or microwave tech niques. The microwave link (MWL) requirements are described in this paper. Taking into account the characteristics of the ISS orbit, and fixing an arb itrary limit to the additional noise brought to the clock readings by the M WL, the computation of the required stability leads to two kinds of require ments: the first one at the subpicosecond level over each single continuous pass of the ISS above any Earth station, and the second one at the level o f one part in 10(16) and below over a one day or more averaging period. Mor eover, the ISS orbit parameters should lead to a knowledge of the AGES cloc k position at the m level, and of the AGES clock speed at the mm/s level.