ACES microwave link requirements

ACES, for “Atomic Clock Ensemble in Space”, is a project of the European Space Agency, on-board the future International Space Station (ISS). The payload consists mainly of two atomic frequency standards, one space hydrogen maser (SHM) prepared by the Observatoire de Neuchltel (Switzerland), and one cold atom cesium clock studied by the CNES, the BNM-LPTF, the ENS-LKB, and the CNRS-LHA (Laboratoire de I´horloge atomique du Centre national de la recherche scientifique, Orsay, France), called PHARAO. Because of the anticipated performances of these clocks on-board the ISS, for example an expected stability of PHARAO at the level of 7×10^-14×τ^-1/2 , for τ in seconds up to a few days, the requirements of the links between the payload and the clocks on the Earth are at the limits of the known potential of the optical or microwave techniques. The microwave link (MWL) requirements are described in this paper. Taking into account the chacteristics of the ISS orbit, and fixing a limit to the noise brought by the MWL, the computation of the required stability leads to two kinds of requirements: the first one at the sub-picosecond level over each single continuous pass of the ISS above any Earth station, and the second one at the level of one part in 10¹⁶ over a one day averaging period