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

The ACES GNSS Subsystem and its Potential for Radio-Occultation and Reflectometry from the International Space Station

The ESA mission Atomic Clock Ensemble in Space (ACES) will operate a new generation of ultra-stable and accurate atomic clocks on board the International Space Station in 2013-2015. The ACES payload will be attached externally to the European Columbus module. A commercial-of-the-shelf GNSS receiver will be connected to the ACES clock signal. Primarily, the receiver will ensure orbit determination of the ACES clocks in order to apply relativistic corrections to the space-to-ground comparison between the ACES clock signal and the ground clock. Secondarily, the receiver offers the potential for remote sensing from space in the field of radio-occultation and reflectometry exploring the use of the new GNSS signals. The paper presents the current status of the ACES GNSS instrument

The ACES GNSS Remote Sensing Concept and Status of the GNSS Subsystem

Atomic Clock Ensemble in Space (ACES) is an ESA mission in fundamental physics based on a new generation of ultra-stable and accurate clocks operated in the microgravity environment of the International Space Station. A dedicated GNSS receiver on-board the ACES payload will ensure orbit determination of the ACES clocks in order to apply relativistic corrections to the space-to-ground comparison between the ACES clock signal and the ground clock. Furthermore, the ACES mission is exploiting remote sensing applications including high-rate radio-occultation and reflectometry data recordings arising from GPS L1, L2, L2C, L5 and GALILEO/GIOVE E1, E5a signals. In parallel to ACES continuous precise orbit determination measurements several daily radio-occultation and reflectometry measurements can be scheduled. The ACES GNSS remote sensing concept and status of GNSS subsystem will be presented in this paper

Orbit Determination and Prediction of the International Space Station

The International Space Station is equipped with Global Positioning System receivers that provide real-time position information at the 10m accuracy level. In preparation of the Atomic Clock Ensemble in Space experiment, measurements from Russian and American receivers have been used to assess the navigation accuracy that can be achieved through postprocessing of navigation solutions and raw data in a precise orbit determination process. In addition, the capability to accurately forecast the space station orbit for operation of microwave and laser terminals has been studied. It is shown that the orbit can be reconstructed with a 1 m position accuracy and a 1 mm/s velocity accuracy even from single-frequency Global Positioning System measurements. For the test period in mid 2006, short arc orbit predictions with a median error of 20 and 70 m could be obtained over forecast intervals of 6 and 12 h, respectively. The navigation accuracy obtained is compatible with the mission requirements for the relativistic correction of the atomic clocks and the quick look clock performance monitoring