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Methods for Improving the Accuracy and Reliability of Vehicle-borne GPS Intelligence Navigation

Map Matching Method
The positions of a vehicle determined by GPS/DR or DGPS/DR could be displayed in electronic map. Because errors exist in both positions acquired by GPS/DR and also in digital maps, it is not possible to ensure that the positions of the vehicle register properly on a digital map. The result of this is that a vehicle may be seen to be moving over a building or into the sea. To avoid this phenomenon, map- matching method can be used to improve the displayed precision of vehicle over an electronic map. The principle of map matching method is to ensure that a position is snapped or matched to the nearest street. However, a street network can be quite complicated especially when there are several crossroads.

Determination of the correct street is not entirely straightforward. One such algorithm is proposed by Yi et al., (1998).



Figure 2 presents a section of a network and one point (P(X,Y)) representing a vehicle’s position. The position does not register correctly to a street. To find which street it belongs to, a circle with search radius R is drawn. With experience, a suitable value for R will be used. In this area, the objective is to find all streets that satisfy the following condition:

Distance (D) between street and P point is shorter than R.

There exists two possible cases:

1. if vehicle is static, select the street whose D is the shortest;
2. if vehicle is moving, choose the street which has the smallest angle to the direction of vehicle movement.

After determining the street, the next stage is to display the projected point in the electronic map. R could be equal to the width of the widest street. If no street satisfies the search condition, it can be concluded that the vehicle is traveling outside the street and it is not necessary to match it. Furthermore, R may be variable to adapt to different environments.



According to above principle, a study was done with VINS for Wuhan City in China. Figure 3 shows the effect without using map-matching method. Some vehicle trajectories are outside the streets. Figure 4 demonstrates the visual improvement of the map matching method.



Conclusions and Recommendations
Three methods for improving the accuracy and reliability of VINS have been described. These methods are not independent each other. Unfortunately the additional augmentation will increase the cost. Moreover, in the course of developing VINS, several issues pertaining to the accuracy and reliability of VINS were also noted:

1. the hardware like GPS receiver or OEM board or DR sensors should be stable and reliable;
2. multi-path is a serious error contributor in high-rise areas which should be reduced to minimum;
3. the 2D position of vehicle should be transformed from WGS-84 to the local coordinate system before it is displayed in the electronic map. The transformation steps could be:
   
   
   
   (X, Y)_M is the 2D position in electronic map.

References

• Allison N. Kealy, Maria Tsakiri & Mike Stewart (1999). Land vehicle Navigation in the Urban Canyon-A Kalman Filter Solution using Integrated GPS, GLONASS and Dead Reckoning. ION GPS’99, 14-17 September 1999,Nashville,TN. Pp509-518
• Haddad M.H.EL & E. T. Powner (1999). Charging for Load use by Using Navigation Systems. ION GPS’99, 14-17 September 1999, Nashville, TN.pp565-568
• He J.Zh, Li W.G (1998). Vehicle-borne GPS Navigation and Positioning System. Bulletin of Surveying and Mapping, 1998, No.12: 19-21
• Hofmann-Wellenhof B. et al. (1997). Global Positioning System: Theory and Practice. New York: 1997
• Hong D.Y (1997). GPS and its Applications. Xiamen: Xiamen University Press,1997
• Hu Guorong & Cui Weihong (2000). Discussion on GPS+GLONASS for Vehicle Navigation and Monitoring System, Engineering of Surveying and Mapping, Vol, 9, No.4, 2000, pp19-21.
• Wang G.Y, Guo B.Y., Li H.T (1996). Differential GPS and Applications. Beijing: Electronic Industry Press, 1996
• Yi Y.D., Ji S.B.,Bai Z.D.,Wang J.X (1998). Electronic Map for Vehicle-borne GPS Navigation and Positioning System. Engineering Reconnaissance, 1998, No.5: 46-50
• Yuan X., Yu J.X., Chen Z (1993). Navigation System. Beijing: Aerospace Press, 1993

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