GPS World, May 2016

MAY 2016 WWW GPSWORLD COM GPS WORLD 51 recorded data to compare the different positioning performances As a baseline for comparison a GPSonly position solution was calculated This result represents the current state of the art navigation systems for most production cars All valid GPSonly position fixes are shown in FIGURE 10 For large portions of the test drive no GPS only position solution was possible because of insufficient GPS measurements To quantify the benefit of multiconstellation GNSS compared to GPSonly a combined position solution was calculated using the same data as before There was a significant improvement in the availability compared to the GPSonly position solution However even when using multiple GNSS constellations some areas with no valid GNSS fixes still exist The GNSS availability can be improved further by using differential corrections from a GNSS reference receiver The correction data is available in the research car using 4G mobile telecommunication links to different service providers Each provider uses a network of GNSS receivers to calculate differential corrections However all commercially available services are currently limited to GPS and GLONASS Thus another stationary multi constellation GNSS reference receiver at the Institute of Flight Guidance generated correction data for the test drives As the baselines are short in this scenario not longer than 10 kilometers no significant spatial decorrelation is expected As the majority of possible inter system offsets are already eliminated using the differential corrections of identical receiver types a multi constellation solution can be calculated here even with as few as four GNSS satellites in view This is shown in FIGURE 11 In this way the achieved availability increased again Finally using all the information available in the car a hybrid position solution based on differentially corrected GNSS inertial navigation and the test vehicles odometer has been calculated In sections without any GNSS positioning aiding marked red in FIGURE 12 the inertial navigation system was used in dead reckoning mode As these outages lasted only for short periods of time the accuracy of the combined position remained usable for the duration of the test In this way an accurate position solution could be calculated for the whole test drive using this tightly coupled positioning algorithm With increasing positioning complexity the computational burden increased as well For a tightly coupled system integrating the measurements of the different sensors significantly more calculations must be performed in real time than for current GPS only standalone positioning However even today these computations can be easily made using embedded devices CONCLUSIONS AND OUTLOOK For this article the achievable positioning performance of multiconstellation GNSS has be analyzed with a special emphasis on urban automotive applications Simulations of constrained environments have been compared with real data and show good agreement Multiconstellation GNSS outperforms GPS only positioning especially in situations where large portions of the sky are blocked by obstacles because significantly more satellites remain usable Multi constellation GNSS has thus the potential to be an important part of future safety of life positioning and navigation applications FIGURE 10 GPS only standalone positioning fixes for test drive in Braunschweig FIGURE 11 Differentially corrected multiconstellation positioning fixes for test drive in Braunschweig FIGURE 12 Tightly coupled positioning trajectory for test drive in Braunschweig