ACES Time and Frequency Links
ACES carries both a microwave and optical time and frequency link.
The ACES Microwave Link (MWL)
Primarily the clock signal will be transferred to ground by a specially designed dual frequency bi-directional, 4 channel microwave link (MWL). The MWL is an all weather system that provides the possibility to compare the ACES frequency reference to ground clocks. The system operates continuously with a carrier frequency in the Ku-band. The high carrier frequencies of the up and down links (13.5 GHz and 14.7 GHz respectively) allow for a noticeable reduction of the ionospheric delay. A third frequency in the S-band (2.2 GHz) is used to determine the Total Electron Content (TEC) and correct for the ionosphere time delay. A PN-code modulation (100 Mchip/s) of the carrier removes the phase ambiguity between successive comparison sessions separated by large dead times. The system is designed for multiple access capability, allowing up to 4 simultaneous ground users. The susceptibility of the system to narrowband and broadband interference, as well as to multipath effects has been characterized.
The ACES MWL Ground Terminal
The MWL ground terminal electronics is similar to the MWL flight hardware, symmetry being important in a two-way system to reduce instrumental errors. The ACES MWL Ground Terminal (MWL GT) is a microwave station interfacing the local clock on ground to the ACES payload allowing space-to-ground clock comparisons. To reduce phase instabilities due to the tracking motion, the electronic unit of the MWL GT has been rigidly attached to the antenna unit. The system is housed below a protective radome which also allows to stabilize the temperature of the enclosed volume by an air conditioning system, part of a separate service pallet.
European Laser Timing (ELT)
The European Laser Timing (ELT) link will allow clock comparisons, time transfer, and ranging experiments in the optical domain. The on-board part of ELT consists of a the Instrument Unit comprising an optical detector based on a Single-Photon Avalanche Diode (SPAD), an event timer board connected to the ACES time scale, and a Corner Cube Reflector (CCR). Light pulses fired towards ACES by a laser ranging ground station will be detected by the SPAD diode and time tagged in the ACES time scale. At the same time, the CCR will re-direct the laser pulse towards the ground station providing precise ranging information. The ELT Instrument Unit includes the optical detector and the electronics.
Comparison of distant clocks, both space-to-ground and ground-to-ground, to frequency uncertainty levels well below 1•10-16 after a few days of integration time will be performed. Using the high stability of the ACES clock signal, global view comparisons of clocks across intercontinental distances will be possible.
The optical link also finds interesting applications in the distribution of the ACES time reference and in the synchronization of geodetic observatories. Combined with MWL performance, European Laser Timing will contribute to the characterization and crossed comparison of two different time transfer and ranging systems. Optical versus dual-frequency microwave measurements also provide useful data for the study of atmospheric propagation delays and for the construction of mapping functions at three different wavelengths.
Last update: 3 November 2011