GPS World, September 2013

Galileo and BeiDou GNSS DESIGN FIGURE 9 Measurement results of Galileo IOV FM3 FM4 E1 E5 and E6 IQ Constellation characterize their performance and imperfections as well as to predict possible consequences for user receivers Since the signals are well below the noise floor we use a specifically developed GNSS monitoring facility to characterize the signals The core element of this monitoring facility is a 30 meter high gain antenna at the German Aerospace Center DLR in Weilheim that raises GNSS signals well above the noise floor permitting detailed analysis In the course of this analysis we found differences in the signal quality in the various generations of the Chinese navigation satellite system BeiDou differences which influence the navigation performance This article gives an overview of new navigation satellites in orbit For selected satellites a first signal analysis reveals important characteristics of these signals The data acquisition of these space vehicles was performed shortly after the start of their signal transmission to get a first hint about the quality and behavior of the satellites For more detailed analysis these measurements should be repeated after the satellites become operational Then the acquired high gain antenna raw data in combination with a precise calibration could be used for a wider range of analyses signal power spectra constellation diagrams sample analysis correlation functions and codes to detect anomalies and assess the signal quality and consequently the impact at the user performance Measurement Facility In the early 1970s DLR built a 30 meter dish FIGURE 1 for the HELIOS A B satellite mission at the DLR site Weilheim These satellite PLVVLRQV ZHUH WKH UVW 8 6 HUPDQ interplanetary project The two German built space probes HELIOS 1 December 1974 March 1986 and HELIOS 2 January 1976 January FIGURE 2 The shaped Cassegrain system 1 parabolic reflector of 30 m diameter 2 hyperbolic sub reflector with a diameter of 4 meter 3 sub reflector 4 Cabin with feeder and measurement equipment FIGURE 1 30 meter high gain antenna 1981 approached the Sun closer than the planet Mercury and closer than any space probe ever Later the antenna supported space missions Giotto AMPTE Equator S and other scientific experiments In 2005 the Institute of Communications and Navigation of the DLR established an independent monitoring station for analysis of GNSS signals The 30 meter antenna was adapted with a newly developed broadband circular polarized feed During preparation for the GIOVE B in orbit validation campaign in 2008 a new receiving chain including a new calibration system was installed at the antenna Based on successful campaigns and new satellite of modernizing GPS and GLONASS and GNSSs under construction Galileo and COMPASS the facility was renewed and updated again in 2011 2012 This renewal included not only an upgrade of the measurement system itself but also refurbishment of parts of the high gain antenna were refurbished The antenna is a shaped Cassegrain system with an elevation over azimuth mount The antenna has a parabolic reflector of 30 meters in diameter and a hyperbolic sub reflector with a diameter of 4 meters A significant benefit of this antenna is the direct access to the feed which is located within an adjacent cabin FIGURE 2 The L band gain of this high gain antenna is around 50 dB the beam width is less than 05 The position accuracy in azimuth and elevation direction is 0001 The maximum rotational speed of the whole antenna is 15 second in azimuth and 10 second in elevation direction Measurement Set up 7KH DQWHQQD RIIHUV DQRWKHU VLJQL FDQW advantage in the possibility to have very short electrical and highfrequency connection between the L band feeder and the measurement equipment As mentioned earlier the challenge for future GNSS applications is the high accuracy of the navigation solution Therefore it is necessary to measure and then analyze the signals very accurately and precisely To achieve an uncertainty of less than 1 dB for the measurement results required a complete redesign of the setup which consists of two main parts paths for signal receiving and acquiring the measurement data calibration elements for different calibration issues The path for receiving the signal and acquiring the measurement data consists of two signal chains each equipped with two low noise amplifiers LNAs with a total gain of around 70 dB a set of filters for the individual GNSS navigation frequency bands and isolators to suppress reflections in the measurement system With this setup it is possible to measure right hand circular polarized RHCP and left hand circular polarized LHCP signals in parallel This provides the capability to perform www gpsworld com September 2013 GPS World 35