The ACES GNSS subsystem and its applications

The ESA mission Atomic Clock Ensemble in Space (ACES) will operate a new generation of atomic clocks on board the International Space Station (ISS) in 2013-2015 timeframe. The ACES payload will be attached externally to the European Columbus module. The ACES clock signal will reach fractional frequency stability and accuracy of 1 part in 10 -16 . A GNSS receiver will be connected to the ACES clock signal. Primarily, the GNSS receiver will ensure orbit determination of the ACES clocks using GPS, GALILEO/GIOVE, and possibly GLONASS satellite signals in the L1, L2, and L5/E5a bands. Orbit determination is important for the correct evaluation of relativistic corrections in the space-to-ground comparison of clocks. Secondarily, the receiver offers the potential to support additional functionality for remote sensing applications in the field of GNSS radio-occultation and GNSS reflectometry, exploiting opportunities arising from the new GPS and GALILEO/GIOVE signals. The ACES GNSS instrument consists of a state-of-the-art commercial-of-the-shelf JAVAD GNSS Triumph TRE-G3T receiver board. The receiver is connected to a GNSS antenna which will be directly mounted at the corner of the ACES payload. Antenna boresight is pointing +50° off the ISS flight direction and is tilted 30° toward the zenith direction. This offers ideal conditions to receive coherent reflected GNSS signals and improves radio occultation measurements. Within the ACES project the receiver will be ruggedized and tested for space environment. Initial tests performed by DLR with the Co-60 source in Euskirchen, Germany, indicate a high tolerance to total ionizing dose. The receiver sensitivity to harmful single event effects of ionizing radiation including single event upset (SEU) and latch-up (LU) has been characterized in SEE testing using the radiation test facility of Groningen, NL. The results will be used to design the protection system counteracting these effects. In addition the receiver will be accommodated in a double redundant architecture. Under simulated low Earth orbit (LEO) conditions the JAVAD Triumph receiver firmware demonstrated fast acquisition of GPS signals and respectable orbit accuracy/ performance. Current status and test results of the ACES GNSS instrument will be presented in this paper.