GPS World, June 2017

SENSOR FUSION The phase windup update is used to constrain azimuth error growth during low dynamic conditions that are typically not favorable to inertial navigation However it does require uninterrupted GNSS tracking and is therefore applicable only in GNSS benign environments This approach is expected to show the greatest benefit in low dynamic conditions and be directly attributable to azimuth accuracy but only in conditions where GNSS availability is relatively secure Equipment and Test Setup We paired OEM grade GNSS receiver cards with a selection of IMUs in different performance categories Since the OEM GNSS platform is capable of tracking all GNSS constellations and frequencies we configured each receiver to use triple frequency quad constellation RTK positioning The receivers were coupled with a wideband antenna capable of tracking GPS L1 L2 L5 GLONASS L1 L2 BeiDou B1 B2 and Galileo E1 E5b signals Three IMUs were tested an entry level MEMS IMU UUT1 a tactical grade MEMS IMU UUT2 and a highperformance fiber optic gyro based IMU UUT3 All GNSS receivers and IMUs were set up in a single test vehicle and collected simultaneously for all scenarios IMUs were mounted together on a rigid frame and all receivers ran the same firmware build that were connected to the same antenna The tests were conducted using a single GNSS antenna with no additional augmentation sources such as distance measurement instrument DMI or wheel sensor These are extremely helpful in aiding the solution but as previously mentioned this testing seeks to demonstrate the possible performance without the benefit of additional aiding sources Dependence on aiding sources is a very important distinction when comparing such systems The GNSS positioning mode used was RTK via an NTRIP 14 GPS WORLD WWW GPSWORLD COM JUNE 2017 feed from a single base station with baselines between 5 30 kilometers This was done to try to minimize GNSS positioning differences between the three systems L band correction signals were not tracked and PPP positioning modes were not enabled A basic setup diagram of each system under test can be seen in FIGURE 1 Test Scenarios Four test scenarios will be examined using all the equipment and algorithms described above They are urban canyon low dynamics parking garage and extended GNSS outage The urban canyon test is designed to show the performance of the system in restricted GNSS conditions The challenge to this scenario is to maintain a high accuracy solution when GNSS positioning becomes intermittent or even unavailable The low dynamics test is intended to illustrate the benefits of the land profile and specifically the phase windup azimuth updates in maintaining the azimuth accuracy The parking garage test will show the efficacy of the velocity constraint models over the different IMU classes as the extended outage provides no external information to the INS filter whatsoever Again no other aiding sources were used Urban Canyon Test The urban canyon environment has been and remains one of the strongest arguments in favor of using GNSS INS fusion in a navigation solution Because urban canyons are common densely populated and of course a demanding GNSS environment they represent both an important and challenging location to provide a reliable navigation solution Typically they contain major signal obstructions strong reflectors and complete blockages depending on the city For this reason they provide an excellent use case for INS bridging to maintain stability of the solution During most urban canyon environments it is typically rare to incur total GNSS outages of more than 30 seconds Therefore this scenario examines the stability of the solution in continuously degraded but not generally absent GNSS In this case the coupling technique of the inertial algorithms rather than quality of the IMU dominates achievable position accuracy The receiver platform is capable of tracking all GNSS constellations and frequencies This provides a significant benefit to test scenarios such as the urban canyon where the amount of visible sky is significantly restricted In this case the more satellites that are observable the more the tightly coupled architecture can exploit the partial GNSS information FIGURE 2 Urban canyon test trajectory