The Mobile Differential GPS Positioning System Based on Virtual Reference Station Technique and PDA Platform

Abstract: Since the fully operation of GPS system in 1990s, with the development ofelectronics, computer and communication technologies, GPS's application fieldsexpanded from professional ones such as military, marine navigation, survey andgeology to some daily applications such as LBS (Location Based Service), trafficcontrol, land resource survey and logistics etc. But due to the complex influencefactors of distribution of GPS satellites, signal obstruction, atmospheric delay andmultipath etc, the accuracy and integrity of GPS positioning is insufficient for manyrequirements. For the mobile user of GPS positioning, VRS (Virtual ReferenceStation) and SBAS (Satellite Base Augmentation System) are two useful techniquesto improve the positioning quality.VRS is a kind of real time kinematic positioning technique developed fromCORS (Continuously Operating Reference Stations).VRS system continuouslycomputing differential corrections based on the data collected from multi-referencestation network and send these corrections to rover users under certain protocol viaInternet. Since these corrections are provided according to the current positions ofrover users, i.e. the 'Virtual' reference station is installed just near the rover user,the degradation of accuracy caused by the less correlation between rover-referencestations are substantially avoided, and integrity and accuracy are improved as a result.SBAS is a kind of satellite base GPS augmentation technique. Mobile users canreceive differential corrections from extra GEO satellites, and these satellitesthem-selves also transmit ranging signals on the same frequency as GPS L1, thusboth the accuracy and integrity will be enhanced.The aim of this dissertation is to provide a reliable, stable and high precisionmobile positioning service for various mobile applications by the combination ofVRS and SBAS. Firstly, several kinds of GPS augmentation techniques are reviewed,and the characteristics of these techniques are discussed according to the special needof mobile positioning. Then the basic concepts of VRS and SBAS are introduced, as well as the operation model and systematic error source. Based on these theories, aSBAS GPS mobile positioning system is developed utilizing the GPS IntermediateDriver and a SiRFIII chipset on a Widows Mobile 5.0 platform PDA. Based onresearches of format of RTCM data, NTRIP protocol and Windows Mobile 5.0Internet communication, a high precision RTK mobile positioning system isdeveloped on PDA with Ashtech Z-Xtreme receiver. In the finality, the precision andintegrities of the two systems are analyzed by the data collected from a field testconducted in the campus of Tongji University…
Key words: GPS Mobile positioning; Virtual Reference Station; Satellite Augmentation System; NTRIP protocol; SiRF protocol; Windows Mobile 5.0; GPS Intermediate Driver

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