Low-Cost Navigation Systems : A Study of Four Problems

Sammanfattning: Today the area of high-cost and high-performance navigation for vehicles is a well-developed field. The challenge now is to develop high-performance navigation systems using low-cost sensortechnology. This development involves problems spanning from signal processing of the dirty measurements produced by low-costsensors via fusion and synchronization of information produced by a large set of diverse sensors, to reducing the size and energyconsumption of the systems. This thesis examines and proposessolutions to four of these problems. The first problem examined is the time synchronizing of the sensordata in a global positioning system aided inertial navigationsystem in which no hardware clock synchronization is possible. A poor time synchronization results in an increased mean squareerror of the navigation solution and expressions for calculating this mean square error are presented. A method to solve the timesynchronization issue in the data integration software is proposed. The potential of the method is illustrated with tests onreal-world data that are subjected to timing errors. The second problem examined is the achievable clocksynchronization accuracy in a sensor network employing a two-waymessage exchange model. The Cramer-Rao bound for the estimation of the clock parameters is derived and transformed in to a lower bound on the mean square error of the clock offset.Further, an approximate maximum likelihood estimator for the clockparameters is proposed. The estimator is shown to be of low complexity and to have a mean square error in the vicinity of the Cramer-Rao bound. The third problem examined is the detection of the time epochswhen zero-velocity updates can be applied in a foot-mountedpedestrian navigation system. Four general likelihood ratio testsfor detecting when the navigation system is stationary based onthe inertial measurement data are studied. The performance of thefour detectors is evaluated using levelled ground, forward-gaitdata. The results show that the signals from the gyroscopes holdthe most reliable information for the zero-velocity detection. The fourth problem examined is the calibration of a low-costinertial measurement unit. A calibration procedure that relaxesthe accuracy requirements of the orientation angles the inertialmeasurement unit must be placed in during the calibration isstudied. The proposed calibration method is compared with theCramer-Rao bound for the case when the inertial measurementunit is rotated into precisely controlled orientations. Simulationresults show that the mean square error of the estimated sensormodel parameters reaches the Cramer-Rao bound within fewdecibels. Thus, the proposed method may be acceptable for a widerange of low-cost applications.