GPS World, June 2017

SENSOR FUSION on the center bottom of the image Unlike urban canyon environments that contain partial GNSS information this exhibits an extended period of complete GNSS outage During this type of scenario the IMU specifications provided in Table 1 become much more significant IMU errors directly translate to the duration the solution can propagate before the accumulated low frequency errors of the IMU grow to unacceptable levels System performance during the outage degrades according to the system errors at the time of the outage and the system noise The velocity errors increase linearly as a function of attitude and accelerometer bias errors The attitude errors will increase linearly as a function of the unmodeled gyro bias error The position error is a quadratic function of accelerometer bias and attitude errors Position results from each IMU are shown for UUT 1 in FIGURE 9 This plot shows the error with the land profile on and off Without the land profile the second order position degradation in an unconstrained system is clearly visible By enabling the land profile the filter constrains IMU errors by utilizing a velocity model for wheeled vehicles With the constraints the position errors are startlingly reduced for UUT1 and then progressively less impactful as the IMU quality increases in UUT2 and UUT3 respectively This makes sense as the IMU error growth is progressively smaller in those IMUs so the effect of mitigating them is also reduced Extended GNSS Outage Test An extension of the parking garage test is to evaluate the performance in a much longer outage Instead of 10 minutes an outage of one hour was tested Also due to the extremely long GNSS outage bridging the effects of adding a DMI sensor odometer will also be explored as they are able to be used as a major additional aiding source The most common measure of dead reckoning performance is error over distance traveled EDT Due to the very long duration outages in this test the errors will be reported in error over distance traveled to conform to the typical reporting method This test was conducted in a mixture of highways and suburban streets with an average speed of 65 Km h incorporating a moderate amount of dynamics This effect can be seen over the duration of the entire outage as well in Figure 9 In this case the points are the RMS error over several tests and the light background shroud represents the one sigma confidence as time progresses The confidence increases over time as the overall distance traveled also increases Results and Conclusions In testing a range of IMUs in some challenging scenarios this paper has sought to illustrate what kind of performance is achievable using each kind of system An added complexity is looking at what effect certain inertial constraint algorithms have IMU EDT Land Profile EDT DMI Profile UUT1 031 012 UUT2 009 006 UUT3 004 004 20 GPS WORLD WWW GPSWORLD COM JUNE 2017 on this solution Although low cost MEMs IMUs are continuing to greatly improve in quality and stability the end application is still highly correlated to the overall performance of a selected INS system For a great many applications the MEMS devices in combination with a robust inertial filter can meet requirements and provide excellent value However some applications continue to require higher end sensors and possibly post processing to meet their needs The ability of SPAN to utilize partial GNSS measurements such as pseudorange delta phase and vehicle constraints means even lowcost MEMs are capable of providing a robust solution in challenging GNSS conditions However this tightly coupled integration is limited in cases where GNSS is completely denied or when in low dynamic conditions INS prof iles using velocity constraints phase windup and robust alignment routines have been shown to provide substantial aid to the INS solution in tough conditions such as GNSS denied or low dynamics These improvements were shown to exhibit greater impact as the IMU sensor precision decreases These abilities in conjunction with the existing tightly coupled architecture of SPAN and the ever increasing accuracy of MEMS IMUs indicate that robust GNSS INS solutions will continue to proliferate at lower cost targets However very precise applications such as mapping FIGURE 8 Parking garage test trajectory TABLE 5 Percent error distance traveled over 1 hour GNSS outage FIGURE 9 UUT1 position error std vs land