GPS World, June 2014

consumer OEM Wireless LBS using them The details of the incident are as follows The GLONASS control system uploaded incorrect orbit data to several satellites When receivers used these satellites they had position errors of hundreds of meters or no positions at all At that time the BCM47531 was being tested alongside a GPS GLONASS receiver and we have the data to show what happened The receiver using only GPS GLONASS suffered position errors of ten thousand meters and long periods with no position at all at the same time the multi constellation receiver produced continual positions with normal accuracy Figure 13 shows the test data the left most image shows the route being driven the middle image shows the data from the GPS GLONASS receiver and the right image shows the data from the BCM47531 multi GNSS receiver Figure 14 shows the details of the multi GNSS receiver you can see that no GLONASS satellites are being used This incident may raise the question Why use GLONASS at all why not just GPS The answer is that in urban canyons such as where this test was done GPS alone does not have enough satellites to give the performance now expected in consumer products for the reasons explained in the beginning of this article Also GPS although it has been more reliable than GLONASS is not immune to failures or jamming itself The lesson of this incident is that reliability and accuracy comes from the combination of all the available constellations with a receiver that can use the signals interchangeably Conclusion We have shown the preferred architecture for a consumer GNSS receiver that includes all of the available constellations We have addressed the major requirements of such a receiver for the consumer market in particular for cell phones and tablets A receiver that meets these requirements is now available the Broadcom BCM47531 has been designed into a new generation figure 12 Interchangeability Assisted cold start first fixes Blue numbers show the satellites used in the position fix top two GPS and two BeiDou middle one GPS one GLONASS and two BeiDou and bottom four BeiDou only The receiver is in San Jose California where four BeiDou satellites can be seen some of the time some of the BeiDou GSOs can be seen and all the BeiDou MEOs can be seen for a few hours each day FIGURE 13 Side by side tests of GPS GLONASS receiver and multi constellation receiver during the GLONASS incident of April 2 2014 The GPS GLONASS receiver produced errors of ten thousand meters and long periods with no position at all while the multiconstellation BCM47531 operated seamlessly cell phones and tablets for 2014 Finally we have shown how with this receiver the ultimate GNSS goal of interchangeability can be achieved frank van diggelen is vice president of technology at Broadcom Corporation a consulting professor at Stanford University and inventor of coarse time GNSS navigation co inventor of Long Term Orbits for A GNSS and author of A GPS Assisted GPS GNSS and SBAS kathy tan a senior principal engineer at Broadcom Corporation She has worked on GNSS development and Assisted GNSS for Ashtech Magellan Global Locate and Broadcom She received her MS and BS in electrical engineering from Fudan University China FIGURE 14 Detail from the multiconstellation receiver when there is a problem with some satellites The errors are recognized automatically by algorithms comparing the measurements to redundant measurements from the extra constellations and the erroneous signals are not used GPS World June 2014 www gpsworld com 52