CDS Lecture Series 1999

Friday, August 13, 1999, 2:00 p.m.

Li-Sheng Wang
Institute of Applied Mechanics
National Taiwan University

Long-Duration Carrier-Smoothed-Code Algorithm for GPS Positioning

GPS (Global Positioning System) has been extensively used in recent years for surveying, navigation, time transfer, etc. In order to get a centimeter level accuracy positioning, the carrier phase observables must be adopted, along with differential techniques. However, the use of carrier phase alone may require a long convergence period of intialization, and is thus subject to serious problems caused by cycle slip, if used in real-time. On the other hand, algorithms based on pseudo-ranges (or code) can provide less accurate position data once the signal is tracked. To overcome the cycle slip problem, and still be accurate most of the time, the Carrier-Smoothed-Code (CDC) algorithm was developed to synthesize the carrier phase observables and the pseudo-range observables. However, for long-duration positioning, in which problems caused by cycle slip, satellite changes, frequently arise, the CSC algorithm may exhibit positioning jumps. It is desirable to alleviate these jumps to yield a better positioning algorithm. In this talk, an improved scheme, called the Long Duration CSC (LCSC) algorithm, is introduced. A Kalman filter is used to predict the occurrence of cycle slips and to smooth the carrier-smoothed pseudo-range. A fuzzy scheme is designed to obtain the weightings between the carrier phase and the pseudo-range once the cycle slip happens. A least-square scheme based on the weightings of satellite signals is used to handle the problem of satellite changes. From both static and dynamic experiments, the LCSC algorithm indeed provides more stable positioning data in long term positioning.

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