Global Positioning System Reference
In-Depth Information
the estimated distances between the sidewalk edge and the vehicle are then used in fixing
the vehicle location. Although the memory constraints are overcome by using the GIS, the
accuracy of the estimate of the distances is not consistent due to occluding objects between
the laser scanner and the edge of the sidewalk. The training phase required for any traversed
region is also not insignificant.
3.2 Vision, digital maps, and GPS/DR
Visual data is also utilized in localization techniques since digital images can provide a wide
range of information about the surrounding environment. Due to the time required for image
processing Jabbour, Cherfaoui & Bonnifait (2006), only key images are maintained and linked
to the GIS database Jabbour, Bonnifait & Cherfaoui (2006). Again, both GPS/DR are used and
the proximity of the vehicle location estimate to the roads in the GIS database is examined. The
road segment closest to the location estimate is then selected, and key images of that road are
extracted in order to compare their features with the features of the images taken during the
navigation stage. The weakness of this strategy appears when the curvature of the vehicle's
path is significant, especially when the vehicle turns in orthogonal intersections.
Visual features can, however, be blended with other location measurements, such as GPS and
DR data in the EKF formulation Rae & Basir (2007). The main advantage of this strategy
is that the uncertainty of all the information sources is kept local to the EKF, namely, in the
error covariance matrix, which guarantees a minimum mean square error estimates. In Rae
& Basir (2007), the EKF structure is derived and validated where the curvature of the roads is
employed as a visual feature. It is shown that when the roads are curvy, the vehicle location
estimate is dramatically improved. On the other hand, if the road traversed is not curved,
then the accuracy of the location estimate remains the same as that produced by the GPS/DR
fusion localization technique.
3.3 Satellite visibility and DGPS
In urban areas, GPS multipath signals cause unpredictable localization errors due to the NLOS
satellite signals. Another approach is the localization system which is driven by tracking
visible GPS satellites using an infrared camera. An omni-directional infrared camera mounted
on the top of a vehicle is used to recognize obstacles and their height and to detect visible
satellites by observing their positions with a satellite orbit simulator Meguro et al. (2009).
This method allows the system to exclude any radio waves emitted by invisible satellites to
improve the localization accuracy.
The vehicle localization system used in this approach has high degree of accuracy since it
employs a DGPS receiver. However, in high rise building areas, the availability of location
estimates is low due to the lack of enough visible satellites, and even with enough visible
satellites, the geometric configuration of the constellation may result in a high Dilution of
Precision (DOP).
4. Cooperative localization
Cooperative Localization is a recent location estimation approach that has been implemented
in vehicular positioning and wireless communication systems. This localization scheme is
suitable for scenarios which involve the coexistence of several entities that independently
provide location information. The goal is to localize a mobile node or to enhance its location
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