GPS World, June 2014

consumer OEM Wireless LBS figure 9 A GNSS provides broadcast satellite data over some other wireless network as well as time offsets between the different pairs of systems realizations of UTC is within several nanoseconds of the others To combine measurements from these different systems and avoid any time induced intersystem biases we need to resolve the time offsets Each system transmits the delta time between its system time and the systems that preceded it as listed in Figure 8 To combine the systems we either need to decode these data messages or obtain the delta time values from Assisted GNSS Note however that in the BeiDou broadcast Nav message the intersystem time offset data values are all set to zero even though the true offsets are not zero Assisted GNSS Including BeiDou Assisted GNSS or A GNSS increases sensitivity and decreases the timeto first fix of a receiver by providing assistance data in the form of the figure 8 Intersystem time differences and broadcast delta time values from each system receivers approximate position time and frequency as well as all data that the receiver might decode from the broadcast signals The assistance data may also include data beyond what is broadcast in particular lets focus on BeiDou time offsets The BeiDou time offset to the other systems is included in the BeiDou broadcast Nav message as shown in Figure 8 however at present these data values are all set to zero even though the true offsets are not zero Thus in order to get these offsets and integrate BeiDou properly into a combined GNSS system one must compute the offsets at a reference station and provide them as part of the assistance data as shown in Figure 9 Commercial Implementation The preferred architecture described in this article has been implemented in a commercial GNSS receiver that is now available for commercial hostbased products such as cell phones and tablets The chip Broadcoms BCM47531 is the first consumer GNSS chip with a tri band front end capable of acquiring and tracking satellites from GPS SBAS QZSS GLONASS and BeiDou constellations simultaneously and operating in host based mode for navigation and in host offload mode for Always On location Broadcom has collaborated with leading smartphone manufacturers to launch the first wave of BeiDou figure 10 GPS GLONASS phone and GPS GLONASS BeiDou phone being tested in Warsaw Poland Note the six BeiDou satellites red that are seen and tracked by the BeiDou phone GPS World June 2014 www gpsworld com 50