GPS World, May 2016

46 GPS WORLD WWW GPSWORLD COM MAY 2016 WITH RICHARD B LANGLEY QUO VADEMUS By Martin Escher Mirko Stanisak and Ulf Bestmann I n the near future we will see the introduction of more and more next generation advanced driver assistance systems ADASs targeting the field of automated or autonomous driving These systems will have to be considered as safety critical In contrast to conventional localization systems they will have to guarantee a higher overall accuracy and integrity to their target applications Of course the overall performance of any localization system is mostly limited by its behavior during the worst conditions Such behavior is a very limiting factor especially for an ADAS that uses a GNSS such as GPS The accuracy and integrity of GNSS depend on the quality and availability of satellite signals The more signals that are available the greater are the accuracy and integrity However as GNSS signals can be blocked easily the worst time behavior is difficult to characterize especially in challenging urban scenarios important for an ADAS To face these challenges additional sensors such as inertial measurement units IMUs or odometers can be used for positioning as well These sensors can increase the availability and accuracy for situations where GNSSbased positioning is not available Additionally the characteristics of these sensors are complementary to satellite navigation The combination of these sensors with satellite navigation thus has the potential to achieve a positioning accuracy and integrity superior to that of single system performance As the number of GNSS measurements is crucial for a precise position fix the parallel use of different GNSS constellations can improve the overall performance significantly Four global satellite positioning systems are now available The American GPS and the Russian GLONASS have been in operation for years and are already used in a wide variety of applications Additionally newer systems like the European Galileo and the Chinese BeiDou systems are under construction Even though these systems do not have continuous worldwide availability at the moment their currently available satellites can already be included in multiconstellation GNSS positioning Once more satellites are in orbit the benefit of multi constellation GNSS will increase even further In this article we take a look at the current performance of multiconstellation GNSS positioning in an urban scenario contrasting it with GPS only positioning as well as GNSS positioning aided by additional sensors SATELLITES IN ORBIT To characterize multi constellation GNSS performance stationary GNSS data has been collected using different receivers in Braunschweig Germany GNSS data from GPS GLONASS Galileo and BeiDou was recorded over a 14 hour window on November 20 2015 Based on the broadcast ephemeris data and the receivers position the availability of GNSS measurements was calculated for the duration of the campaign TABLE 1 shows the number of all satellites of the different constellations as well as the minimum and maximum number of available satellites for each system during the recording period down to an elevation angle of 0 FIGURE 1 shows the satellite availability for the measurement campaign To obtain a position fix using a single GNSS constellation range measurements to at least four satellites of this constellation must be acquired Thus assuming optimal reception of GNSS signals single constellation positioning was possible for the full observing window using only GPS only GLONASS and only BeiDou satellites On the other hand Galileo only position fixes were not possible at any time due to the low number of simultaneously visible satellites However combining measurements from different GNSS constellations in parallel multi constellation GNSS provides the most benefit MULTI CONSTELLATION GNSS All major GNSS constellations operate independently and use different reference frames for position and time To combine measurements of different GNSS constellations the correct handling of the diverse reference frames needs to be ensured On the one hand the different coordinate systems have to be taken Future Automotive GNSS Positioning in Urban Scenarios GNSS constellation No available satellites Maximum no available Minimum no available GPS 31 14 8 GLONASS 23 10 6 Galileo 7 3 1 BeiDou 14 7 4 TABLE 1 Number of satellites in orbit and in view during a 14 hour window