Emerging New Trends in Hybrid Vehicle Localization Systems - Global Navigation Satellite Systems: Signal,Theory and Applications - page 291

In order to put these outstanding issues in practical context the following performance criteria
are proposed.
7.1. Accuracy: Accuracy of a vehicle location estimate is defined as the degree of closeness of a vehicle's
location estimate to its actual (true) location.
7.2. Availability: Availability of a vehicle location estimate is defined as the ratio of the number of
estimates produced to the number of estimates expected per one unit of time.
7.3. Response Time: Response time is the time required by a localization technique to produce a
location estimate.
7.4. Integrity: Integrity is defined as the level of confidence that can be placed in the correctness of the
location estimate Bakhache & Nikiforov (2000); ESA (n.d.); Quddus (2006).

Based on the above performance criteria, a benchmark can be established in order to compare
the performance of different localization techniques based on reported best achievable
accuracy localization performance. Localization performance is compared with respect to
reliability as well. Table 1 provides a summary of the comparison in terms of modality
used, best case accuracy, environmental constraints, synchronization requirements, and
dependency on infrastructure. Table 2 reports emerging applications and their requirements

Modality(ies) Best Case Accuracy (m) Availability Synch. Infr.str.

GPS 10-20 Hoshen (1996); Leva (1996) Out Door-Open Sky Ye s No

DeadReckoning (DR) Worsen with time Kao (1991) Anywhere No No

DGPS with Visible Satellites 0.01-7.6 Meguro et al. (2009) suburban-Open Sky Ye s Ye s

DGPS+DR+Map Matching 0.5-5 Lahrech et al. (2005) Out Door-Open Sky N/A Yes

GPS+Vision+Map Matching 0.5-1 Chausse et al. (2005); Jabbour, Bonnifait & Cherfaoui (2006) Out Door-Open Sky No Ye s

Cellular Localization 90-250 Chen et al. (2006); 25-69 Porretta et al. (2008) Under Network Coverage Yes Yes

Location Services Submeter Zhang et al. (2008) In Door N/A Ye s

Relative Ad hoc Localization 2-7 (Simulation Drawil & Basir (2008); Parker & Valaee (2006)) Suburban Yes No

Table 1. Specifications of Localization Techniques.

with respect to localization accuracy. It is evident from Tables 1 and 2 that current localization
techniques do not live up to the required integrity and availability performance. In other
words, the delivered performance of the localization techniques listed in Table 1 is not
always above the target performance specified by the applications, and that is due to the
unavailability of their measurements or the decrease in their accuracy in some environments,
such as urban canyons, foggy weather, and dark areas.

Hence, performance needed by applications can constitute a challenging issue in the fusion
process of a multi-sensory system. Therefore task driven integrity issues relevant to vehicle
localization are highlighted next.

8. Task driven localization integrity
From the above discussion it is obvious that for localization systems to meet the expectations
of emerging applications it is imperative that they employ diverse location measurement
sources and effective strategies to fuse these sources so as to achieve the Quality of Service
expected of them. Of course this Quality of Service is multi-dimensional as it pertains to
expected accuracy, availability, response time and integrity. The Quality of Service as a
function of these performance criteria is application and task dependent. The more stringent
is the required Quality of Service with respect to a given performance criterion, the more
resources are needed and the higher is the computational cost. This presents a challenge for