Considerations for an Optical Link for the ACES Mission

The design of an atomic clock ensemble to be operated on the International Space Station (ISS) is currently under development in Europe. This experiment constitutes a high precision timescale in space and offers the great opportunity to study the behavior of ―state of the art‖ atomic clocks in a micro-gravity environment. A key element for this project is a microwave link between one or several ground stations and the ISS clock ensemble. Due to systematic delays on the propagation path of the microwave link (insufficient knowledge of the dielectric number as a function of time in the troposphere and ionosphere and phase center variations) there remains an uncertainty in the time comparison (ie. the time difference of a specific epoch on the ground and in space) of up to several nanoseconds. Optical technologies, based on time of flight measurements of ultra short laser pulses, offer the prospect to reduce this uncertainty in time comparison to approximately 25 ps one way. The successful LTT (Laser Time Transfer) project on the Chinese Compass M1 satellite is based an earlier version of the proposed experiment below. Furthermore we note, that a similar system, T2L2 (Time Transfer by Laser Link) was initially part of the ESA selection in 1997 for ACES, but shifted to the Jason 2 satellite mission following a re-arrangement of the project in 2000. Scientific Objectives Timescale comparisons by GPS and TWSTFT (microwave) techniques are in routine operation for many time laboratories around the world. The comparison of timescales by means of cw optical frequency transfer is the most advanced technique in this respect and has been used to compare optical clocks over distances of several kilometers [1,2]. While this approach essentially works on a narrow bandwidth transmission line, it fails to provide a direct link to an exact epoch (point in time) for the two timescales under investigation with an accuracy of better than several ns. In order to compare the epochs of two widely separated timescales at high precision, one has to apply a broadband technique such as the time of flight measurement of ultra-short laser pulses. Such an approach is characterized by several critical aspects, which are:  Geometrically well defined start point of optical range measurement  A well defined propagation path with clearly modeled path delays (given in the visible)  Geometrically well defined end point of optical range measurement  2way ranging to establish a precise distance and to derive the epoch of arrival at the ISS  Low jitter conversion of laser pulse to time (epoch) The ACES mission provides a unique opportunity for the evaluation of the limits in precise clock comparison investigation. The existing microwave link serves as a reference against Proceedings of the 16th International Workshop on Laser Ranging 583 which the optical technique can be compared. Apart from improving the atmospheric propagation models by comparing the refractive index to the microwave propagation delay (including phase center stability of the microwave antenna), SLR will provide independent precise optically derived orbits of the ISS as well as a link between a timescale on the ground and the ACES timescale with an accuracy of about 100 ps.