Statistical Sensor Fusion of Ultra Wide Band Ranging and Real Time Kinematic Satellite Navigation
Khan, Muhammad Waqas Ahmad
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Position, velocity and time (PVT) can be calculated from Global Positioning System (GPS). Two types of GPS measurement models are present, code phase measurement model and carrier phase measurement model. Range measurement in GPS is affected by different types of errors including atmospheric, multipath, satellite and receiver clock and ephemeris errors. Atmospheric errors are the biggest source of error amongst these. Receivers within close proximity to each other face mostly same atmospheric errors from GPS signal. Several differential techniques have been developed during the last few years to mitigate these common errors. It means that the accuracy can be improved by using multiple receivers which mitigate the majority of errors. Real Time Kinematic (RTK) concept uses carrier phase measurements, which have high accuracy. RTK concept was originally developed for application such as surveying. The unknown ambiguity in the number of cycle between each satellite and receiver node is the main issue in RTK technique, moreover these ambiguities are integer numbers. Once the ambiguity is solved, it remains constant as long as the receiver maintains a phase lock on satellites signals. However, the loss of phase lock results in cycle slips and the ambiguity needs to solve again. In this work, RTKLIB, an open source software, is used for the RTK GPS positioning. Ultra-Wide Band (UWB) is known since early 1900s with synonymous terms such as impulse, time domain, nonsinusoidal, baseband, carrier free, orthogonal function and large relative bandwidth radio signals. The huge frequency bandwidth of UWB makes it suitable for positioning and navigation applications. Multipath resistance, high accuracy, low cost and low power implementation are other features of UWB. The huge bandwidth in frequency domain corresponds to short pulse in time domain, usually of nanosecond (ns) order. The Time of arrival (TOA), the time difference of arrival (TDOA) and the received signal strength (RSS) are known methods to calculate the range between the source and the target through UWB. TOA and TDOA are highly accurate but have clock synchronization problem. To overcome this problem, a modified method known as two-way time-of-flight can be used. BeSpoon phone equipped with UWB is used here for UWB ranging. To summarize the previous discussion, RTK GPS positioning has a high accuracy but has integer ambiguity resolution problem which causes cycle slips and requires good satellite visibility as well. Moreover RTK GPS positioning solution is for outdoor applications only and has high dynamic outdoor range. UWB, on the other hand, can give highly accurate positioning solution but has low dynamic range. UWB can be used for both indoor and outdoor applications. Moreover, high bandwidth of UWB makes it multipath resistant and as result can be used in shadow areas. Thus, the fusion of RTK GPS and UWB positioning may compensates the limitations of both and result in better performance system. In this thesis a Kalman filter is used for fusion of UWB and RTK GPS positioning solutions. UWB gives range from tags which are in meter and relative to BeSpoon phone while RTK GPS positioning solution is in geodetic coordinates form (latitude and longitude). Three steps are involved in fusion; first, convert UWB ranges to position in local coordinate by using trilateration, second, convert geodetic coordinates of RTK GPS to local coordinates through rotation matrix and third, use Kalman filter for fusion of both positioning data. The main goal of the thesis is the fusion of both RTK GPS and UWB positioning solutions with the help of Kalman filter in order to obtain better performance compare to stand-alone RTK GPS. Tampere University of Technology (TUT) parking area is used for testing. One corner of TUT parking area has the known coordinate point which is used for the base station of RTK GPS. Reference track and tags positions are drawn through Laser instrument Leica TPS1200 which has millimeter level of accuracy. Measurement results show that the fusion of UWB and RTK GPS positioning solutions have better performance compared to stand-alone RTK GPS solution. Whenever measurement from RTK GPS gives erroneous/missing result, the measurement from UWB sensor corrects it and the resulting solution from filter has better performance.