Differential GPS (DGPS) Method
VINS belongs to a class of real time kinematic positioning whose precision is relatively low. To increase its precision, one method is to introduce differential GPS (DGPS) technique. DGPS can reduce or cancel error sources. According to the different modes of operation, DGPS can be divided into three classes: position-based DGPS, pseudorange DGPS and carrier phase DGPS (Wang, et al., 1996; Hofmann-Wellenhof, et al., 1997). The principles are basically the same but corrected sophistication and precision levels of each technique are quite different. In VINS, position-based DGPS and pseudorange DGPS are usually used.
Figure 1: Comparison of accuracy of single point positioning and DGPS positioning
Fig. 1 shows that the precision of VINS can be improved to a meter level by using position or pseudorange DGPS (Wang, et al, 1996). Therefore, DGPS is a relevant method to increase the precision of VINS. With improved positions, the calibration of the sensors of DR system will have consequential improvements as well. The effect of using DGPS method is documented in Allison, et al., (1999).
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 precision. 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 exist two possible cases:
- If vehicle is static, select the street whose D is the shortest;
- If vehicle is moving, choose the street which has the smallest angle to the direction of vehicle movement.
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.
Fig. 2: Search Area for Map Matching
