TTC 2010 5th ESA International Workshop on Tracking, Telemetry and Command Systems for Space Applications ESA-ESTEC, 21 – 23 September 2010 TRACKING OF GLONASS SATELLITES BY VLBI RADIO TELESCOPES V. Tornatore 1 , R. Haas 2 , G. Maccaferri 3 , S. Casey 4 , S.V. Pogrebenko 5 , G. Molera 6 , D. Duev 7 1 Politecnico di Milano, DIIAR,Italy. E-mail: vincenza.tornatore@polimi.it 2 Chalmers University of Technology, Onsala Space Observatory, Sweden. E-mail: rudiger.haas@chalmers.se 3 INAF, IRA, Italy, E-mail:g.maccaferri@ira.inaf.it 4 Onsala Space Observatory, Sweden, E-mail: simon.casey@chalmers.se 5 Joint Institute for VLBI in Europe, The Netherlands. E-mail: pogrebenko@jive.nl. 6 Aalto University, Metsähovi Radio Observatory, Finland. E-mail: gofrito@kurp.hut.fi 7 Moscow State University, Russia E-mail: duev@jive.nl ABSTRACT Several tests to observe signals transmitted by GLONASS (GLObal NAvigation Satellite System) satellites have been performed using the VLBI (Very Long Baseline Interferometry) technique. The radio telescopes involved in these experiments are Medicina and Onsala. The satellites transmit artificial radio signals that were considered in a similar way as natural radio signal that are emitted by natural radio sources. The signal strength of satellite signals is much stronger than the signal strength of natural radio signals, so in order to avoid overload of the telescope frontends, corresponding signal attenuation was applied. Observations at the stations were performed using the standard Mark4 VLBI data acquisition rack and Mark5A disk-based recorders. The goals of the observations were to develop and test the scheduling, signal acquisition and processing routines to verify the full tracking pipeline, foreseeing the cross-correlation of the recorded data on the baseline Onsala- Medicina. The natural radio source 3C454.3 was used as a calibrator before the beginning of the satellite observation sessions. We present results of a detailed analysis of the signal recorded at the Onsala telescope together with a first attempt of cross-correlation of data recorded on the baseline Onsala–Medicina. Further plans to perform new GNSS observations are in preparation, and a study to develop the VLBI delay model for GNSS observations with astronomical VLBI telescopes is in progress. I. INTRODUCTION GNSS (Global Navigation Satellite Systems) are used worldwide for the determination of coordinates of points or of moving objects on the Earth and in Space with different purposes and different precision. The today existing and operationally working GNSS are the American GPS system and the Russian GLONASS system. Further GNSS-systems are currently under construction, for example the European Galileo system and the Chinese Compass system. GNSS coordinates are linked to the Conventional Terrestrial Reference Frame (CTRF). On the other hand, the Celestial Reference Frame (CRF) is defined by Very Long Baseline Interferometry (VLBI) observations. To achieve the best link between these two frames it is important to observe and process GNSS signals using the same optics, electronics and processing pipeline as for natural radio sources observed with the VLBI technique to insure the cross-calibration of two frames. Several tests, using the VLBI technique, have been carried out to observe GNSS satellites. In particular the use of a network of the three radio telescopes Medicina, Noto (both in Italy) and Onsala (Sweden,) was planned to simultaneously observe GLONASS satellites. These three stations are equipped with L-band receivers. We chose to observe GLONASS instead of GPS satellites, since the L-band systems of the Italian station Medicina cannot observe at the GPS frequency (1575 MHz). A geodetic VLBI observing mode was chosen: considering that the satellites emit artificial radio signals like natural radio sources emit natural signals, the time delay of the arrival of the same wave front at several widely spaced radio telescopes can be determined. The stations used the standard Mark4 VLBI data acquisition rack, and Mark5A recording mode. However, because of a serious technical failure at Noto, only Onsala and Medicina could finally participate in the observations. II. OBSERVATIONS AND ANALYSIS OF GLONASS SIGNALS Several attempts were performed to simultaneously observe GLONASS satellite signals with the Medicina telescope (32 m) and the 26.5-meter telescope at Onsala (conventionally catalogued as Onsala85). In this work we present preliminary studies performed on the data recorded during a test on June 28, 2010. Just before the starting of the session the radio source 3C454.3 was observed for 1 minute scan length as a calibrator. The same setup was used as for the first GLONASS satellite to be tracked. The GLONASS satellites