Emerging New Trends in Hybrid Vehicle Localization Systems - Global Navigation Satellite Systems: Signal,Theory and Applications - page 289

Lack of adequate location anchors and/or prolonged multipath conditions remain unsolved
problems that continue to degrade localization accuracy.

5. Multi-level fusion approaches
As it has been reported above, a verity of multi-modality localization methods have evolved
in recent years. Typical modalities include satellite signals, VANET communication, vision
features, laser rays, etc. This variety of information has motivated the concept of multi-level
fusion.

For instance, in Boukerche et al. (2008), a data-fusion model is proposed in the form of a
three-level fusion localization system. In the first level, a variety of location information is
gathered as row data and processed separately using local filters that are suitable for each
type of location information. As with the system in Skog & Handel (2009), the second level
combines the output of the first level and produces a better location estimate. In Boukerche
et al. (2008), the results are then fused in the third level based on contextual information (e.g.,
digital maps and traffic information). In this scheme, the final location estimate is fed back to
the second level in order to improve future estimations.

Multi Level fusions aims to tackle data fusion as a hierarchical process so as to allow for
combining measurements at various levels of abstraction in a simple manner. Nevertheless,
if the estimates in the lowest-level filters are evaluated for reliability, the fusion of these
estimates in higher-level filters will then be more robust.

6. Integrity of localization systems
Due to the inherent errors in the positioning information, a level of uncertainty in location
estimates is inevitable. Therefore, it is essential to measure the reliability of the positioning
information in order to identify any hidden anomalies. To achieve this task, a level of trust,
integrity, in every estimate must be determined.

In the last two decades, a significant effort has been made in aviation to develop integrity
monitoring systems Hewitson (2003); Walter & Enge (1995). Integrity is defined as a measure

Information
sent to
VANET

X ˆ

Localization
Enhancement
Unit (LEU)

Information
extracted
from
VANET

b

Uncertainty
Measure

a

( - S )

~

INS
measureme
nts

b

b

~

Multipath
Detection
Unit (MDU)

GPS INS Fusion
Unit (GPS/INS-FU)

a a

( x,y ) IVCAL

Location
Estimate

GPS

GPS
reading

Discrepancy
value

Fig. 4. Block diagram of the inter-vehicle-communication-assisted localization technique.