Requirements of ITS Applications

The general PNT requirements for ITS applications are specified in terms of the four parameters of accuracy, integrity, continuity and availability as follows (GALA Project, 2000; GENESI Project, 2007).

Accuracy. Accuracy is used to define the degree of conformance between the

estimated position and the true position. Position accuracy performance is expressed as an error in metres with an associated statistical value (e.g. 95%). The constellation geometrical strength which can be predicted from the satellite ephemeris, the approximate position of the user and the quality of observations, influence the accuracy of the positioning.

users regarding when the GNSS system should not be used for navigation. There are two types of integrity error. Type one is a false alarm, which rejects a measurement from the true population; Type two is missed detection, which accepts a measurement from the outlier population (GENESI Project, 2007). In order to measure the integrity performance, some parameters are used as follows:

Alarm limit. The alarm limit is where the size of the position error exceeds the limit,

which the user must be informed of.

Time to Alarm (TTA). TTA is the maximum allowable time between the onset of the

error and a warning message reaching the user.

Integrity Risk (per hour). This is the probability that the GNSS terminal provides

position information with an error exceeding the alarm limit for a period of longer than the TTA.

Continuity. Continuity is defined as the probability that a system will provide the

specified level of navigation accuracy and integrity throughout an operation of a specified period, given that navigation accuracy and integrity are available at the start of the operation.

usable, i.e. during which the user terminal provides the required levels of accuracy, integrity and continuity. Unavailability arises from apparently random unpredictable factors.

Based on these parameters, safety related ITS applications require high levels of accuracy and integrity, as discussed below and illustrated in Table 2.

Automated Highway: The automated highway application assigns a safe separation

distance which should be maintained by subsequent vehicles in the same lane in a highway. This is a very safety critical application which requires high levels of accuracy and integrity. In addition, a large number of users are required for its effective operation and it would therefore have a significant potential market.

Intelligent Speed Assistant: The Intelligent Speed Assistance (ISA) application uses

Geographic Information Systems (GIS) to determine the speed limit associated with the current section of road. This information may then be relayed to the user in the form of warnings. The liability of such a system must be clear to avoid possible speeding charge counter claims.

Lane control: Positioning information may be made available to users within a

particular lane. Such information could be used in a lane keeping control system to improve safety, especially on motorways, highways and intercity routes (Pohl and Ekmark, 2003).

Restraint Deployment: In order to avoid collisions on the highway, high update

position and velocity estimations will be used. As a further application, as soon as an unavoidable collision is detected, restraints and air bags could be deployed before a crash takes place. This could help to limit injury to the passengers.

Table 2 presents the requirements for the safety related ITS applications introduced above, as determined by the GENESI project (GENESI Project, 2007). It can be seen that for the safety related applications, which are all based on lane level positioning, sub-metre positioning accuracy is required. In terms of integrity, the requirement varies for different applications. The continuity parameter is not quantified for the safety related applications. Furthermore, availability is not quantified for some applications.

Table 2 Safety Related ITS Applications (GENESI Project, 2007)

Application Name Accuracy Integrity Continuity Availability (% per 30 days) Alarm

Limit TTA Integrity Risk Automated Highway 0.1m-1m 1m 1s 2∙10-7 NI 99.8 Intelligent Speed Assistance 0.01m-1m 5m 2s 10-5 NI NI Lane Control/Geodetic control 0.01m-1m 1m 1s 2∙10-7 NI NI Restraint Deployment 1m NI 0.1s NI NI 99.9

The above requirements and applications are just examples of general information for the positioning system used for the ITS applications. From the table above, accuracy is an important requirement for the PNT system in many ITS applications. Currently, however, there are no specific requirements defined for lane level irregular driving

detection systems, despite the fact that this forms an important basis for many safety related ITS applications. Based on the general requirements for the PNT system, the accuracy is developed as the requirement for the precise positioning algorithm of the lane level irregular driving detection system. Integrity is also an important requirement for safety related ITS applications. Currently, however, there are no defined parameters to measure the integrity of the irregular driving detection algorithm. For the safety related ITS applications, it is important to detect the onset of irregular driving promptly and, therefore, the time to first detection has been developed as a parameter for measuring the integrity of irregular driving detection systems. In addition, correct detection rate, which is the percentage of the accurate detection of irregular driving, is also used as a parameter for the integrity, in order to evaluate the performance of irregular driving detection. In addition, availability, which is the percentage of a journey when it is possible to perform a detection, is developed as a requirement for the irregular driving detection. These parameters are quantified in this thesis through simulation and real field data validation.