Satellite-to-Indoor Broadband Channel Measurements at 1.51 GHz

Kurzfassung

The Global Positioning System (GPS) made outdoor localization broadly available and is commonly employed for automotive navigation for a large number of users. Low cost devices are available to track satellites at received power levels as low as −159dBm. With these high sensitivity receivers (standalone or in sensor combination) indoor satellite tracking becomes possible. But errors by signal blocking, diffraction and refraction still impede navigation by a Global Navigation Satellite System (GNSS) indoors, so new algorithms have to be developed to mitigate such errors. The first step towards those algorithms is to evaluate the propagation characteristics for the satellite-to-indoor channel. Therefore channel sounder measurements in terms of positioning like in [1], [2] and [3] are substantially needed. To gain insight into the propagation of electromagnetic waves emitted by a high flying object like a satellite into a building, DLR performed measurements in June 2008. The measurements were accomplished using the Medav RUSKDLR channel sounder at operating centre frequencies 1.51 GHz. While the receiving antenna was located in an indoor environment, the transmitter was mounted on a mobile crane similar to measurements described in [3] and [4]. In such a way the crane simulated a satellite by transmitting a 5W chirp signal with a rectangular spectral shape of 100MHz bandwidth. To obtain elevation angles up to 65° the transmitter was lifted up to a maximum altitude of 43m above ground. Overall twelve different transmitter positions were obtained from in three different azimuth positions and four different heights for each azimuth position. The heights were 12m, 16.5m, 25m and 43m above ground. A directional antenna with 3dB-beamwidth of 60° was used at transmitter side. The signal transmitted was right hand circular polarized with a time period of 12.8μs giving a maximum resolvable impulse response length of 12.8μs. The building itself can be characterized as a standard three story office building of concrete with standard non-metallic window glass. The rooms located on the second floor (approximately 9m above ground) are usually used for presentations and contain less furniture than a normal office room. Rooms included two presentation rooms, a lobby and a corridor without windows. As our primary goal was to simulate a moving receiver instead of point measurements, the receiving antenna was mounted on a model train running on different tracks through all rooms The position of the transmitter was precisely determined using the Leica TPS1200 tachymeter system giving a nominal accuracy in the sub cm domain. To get a similar accuracy for the receiver antenna on the model train while it was running, the model train was equipped with a rotary encoder giving 500 impulses per motor turn. To prevent wheel slipping the model train runs by a cogwheel. Together with each radio frequency measurement done in a cycle of 1ms the number of impulses given by the rotary encoder was saved which results in a precise travelled distance measure for each captured impulse response snapshot. To map the travelled distance to coordinates, the track as well as the starting point of the model train were measured in the same coordinate system as the transmitter by the Leica TPS1200 tachymeter system. As receiving antenna we used a hemispheric right hand circular polarized antenna. As the time of arrival is the most important value for navigation receivers and therefore the most important part in this measurement campaign, receiver and transmitter were always perfectly synchronized by using one common rubidium atomic clock serving as frequency normal. This setup prevents time drifts which usually occur in channel sounder measurements. In the paper we will give valuable insides into the propagation channel from satellite to indoors as well as statistical characterisations like power delay profile, rms delay spread, mean delay spread and coherence time. Further we will look into navigation relevant values like the difference of time between obstructed line of sight and the maximum of the channel impulse response. [1] F. Perez-Fontan, B. Sanmartin, A. Lehner, A. Steingass, J. Selva, E. Kubista, and B. Arbesser-Rastburg, “Measurements and Modeling of the Satellite-to-Indoor Channel for Galileo,” in ENC GNSS, 2004. [2] F. Perez-Fontan, V. Hovinen, E. Kubista, R. Wack, M. Schönhuber, and R. Prieto-Cerdeira, “Characterization of the Satellite-to-Indoor Channel at S-Band,” in EUCAP, 2007. [3] G. Hein, M. Paonni, V. Kropp, and A. Teuber, “GNSS Indoors, Fighting the Fading, Part 1,” InsideGNSS, vol. March/April, pp. 43–52, 2008. [4] J. Goldhirsch and W. Vogel, Handbook of Propagation Effects for Vehicular and Personal Mobile Satellite Systems. NASA, 1998.