| With the development of mobile communication technology and mobilevalue-added services in full swing, the mobile positioning service based on theAssisted Global Navigation Satellite System (A-GNSS) has gotten more and moreapplications, which takes the Assisted Global Positioning System (A-GPS) as arepresentative. In present, the A-GNSS based mobile positioning function modulehas been included in most of the User Equipment (UE).The application of assisted information not only reduces the Time To FirstFix (TTFF), but also enhances sensitivity of the system, which enables thepositioning under weak signal. However, four sets of observation conditions arestill minimum requirements for the positioning. In fact, it is hard to keep UEalways in the environment with complete visibility, and the correspondingpositioning will fail without the basic positioning conditions. Therefore, othermethods are needed to provide additional positioning information when the basicpositioning conditions cannot be met, which is the key for the research of A-GNSSbased positioning technology. Around this problem, this thesis researches on theproblems about the service range of the reference receiver, function of the assistedinformation during satellite acquisition, enhancement of positioning in openenvironment and the realization of positioning in non-open environment.First, based on an in-depth study on GNSS and A-GNSS, an evaluationmethod for services scope of the referenced receivers in A-GNSS is proposed. Itmodels the spatial model of satellites and the application scenario of thereferenced receivers and UE. It provides the changes for observation time lengthof the referenced receiver and UE to the same visible satellite with differentdistance between them. Then the service range of the receiver can be obtainedaccording to the change of observation time, which is the foundation for theassisted positioning and provides reference method for the work about A-GNSS.Second, based on analyzing the principles of signal acquisition and thealgorithms for signal acquisition in the Global Navigation Satellite System(GNSS), this thesis offers a deep research about the influence of the assistedinformation on the signal acquisition. The range of Doppler shifts aroused by the relative motion between the receiver and satellites and the forecast for the Dopplershifts of the receiver by assisted information are also deduced. Then, the influenceof the assisted information on search range of Doppler shifts is analyzed and thecomparison between the forecasted Doppler shifts and the actual measurement isalso provided. Additionally, a quantitative analysis on the influences of theassisted information on coherent accumulation time and the acquisition time ofweak signals acquisition with the assisted information are also provided.Third, taking pseudo-range as the object of study, taking A-GNSS as thesystem architecture, this thesis offers an in-depth research on the A-GNSSpositioning enhancement in open environment. Based on the analyzing theinfluence of measuring error of pseudo-range, a method of selecting compositeweighted coefficients of Weighted Least-square (WLS) algorithm is proposed, andthe relative verification is also provided. Based on all these, according to thecomplementary advantages between WLS and the Extended Kalman Filter (EKF),a joint WLS-EKF positioning algorithm is proposed. The initial value is providedby the WLS algorithm, and then making a choice between the residual errors ofpseudo-range with EKF and WLS. After EKF filtering, the optimal position andclock bias can be obtained, and an enhanced positioning performance is achieved.Finally, the calculations for positioning in non-open environment with otherassisted information are studied. Based on the analysis of altitude-assistedpositioning errors, several methods are proposed for the situation when the numberof visible satellites misses the basic requirements of positioning. When the numberof visible satellites is three, a method assisted by the position of the based stationand the difference of elevation jointly, a method assisted by compensated range, amethod assisted by the measurement of Doppler, a method based ondouble-observation of three satellites and a double-observation based methodassisted by the base station are proposed. When the number of visible satellites istwo, a method assisted by the position of the based station and the measurement ofDoppler jointly, a method based on double-observation of two satellites and adouble-observation based method assisted by the base station are proposed.Through the application and simulated analysis of the above positioningcalculation algorithms, a better positioning performance compared with thecellular-based positioning can be observed. When the number of visible satellites does not meet the requirements of traditional positioning methods, the applicationof assisted information is able to help users to obtain their positions, whichextends the application of satellites positioning technology and also enhances theperformance of A-GNSS. |
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