Demands of the Road
March 1, 2011 By: Kevin Sheridan, Tomas Dyjas, Cyril Botteron, Jérôme Leclère, Fabrizio Dominici, Gianluca Marucco GPS WorldAn Assisted-GNSS Solution Uses the EGNOS Data Access Service
For use in billing drivers in road-user charging schemes, onboard units employing GNSS must meet stringent reliability and availability requirements, and at the same time, be based on low-cost equipment systems. The SIGNATURE unit includes an assistance service which provides ephemeris data and corrections from EDAS, optimized for user location.
Figure 1. (Click to enlarge.)
As roads become more congested, governments and regional authorities seek better ways to manage their existing networks. Road-user charging (RUC) is increasingly promoted to tackle the congestion challenge: charging drivers a fee, perhaps on a monthly billing basis, derived from the distance their vehicles have traveled, time of travel, and whether congested roads were used.
Recording trip information with a GNSS receiver in an onboard unit (OBU) provides a convenient and flexible means to support automated fee collection. For GNSS positioning to be used as the basis for billing drivers, however, it must meet stringent reliability and availability requirements, and at the same time be based on low-cost equipment.
We have developed a prototype to provide both the positioning availability and integrity required for this application. The Simple GNSS Assisted and Trusted Receiver (SIGNATURE) includes an assistance service that provides ephemeris data and corrections from the European Geostationary Navigation Overlay Service (EGNOS) Data Access Service (EDAS), optimized for the user location. Assistance messages are sent to OBUs that can either host an experimental receiver or a commercial-off-the-shelf (COTS) receiver. Data from the receiver is processed with application-specific navigation algorithms on the OBU that aim to improve the integrity of the position solution relative to standard solutions.
Field trials have assessed the contribution that assistance can make to positioning performance, and illustrate options for enhancing standard assistance solutions. Enhancements to assistance encompass modifications to the message content and alternative means of communications, showing the benefits and feasibility of a broadcast service. The impact of including EGNOS corrections through a broadcast assistance service in urban areas is also under investigation.
GNSS Road-User Charging
RUC has the potential to reduce congestion, lower vehicle emissions, and generate revenue streams for infrastructure improvement. It can ensure that revenues are based on actual road usage, creating a financial incentive for changing driving behavior. This might include lower overall use of private cars and, in particular, reducing peak-time travel levels in urban areas by effectively spreading out the morning and evening rush hour. RUC can also encourage commuters to use alternative forms of public transport.
To automate the process of collecting charges, electronic fee-collection (EFC) systems have been developed based largely on dedicated short-range communications (DSRC). In a DSRC solution, a simple tag on the vehicle receives a signal when it passes a roadside beacon and a charge is computed accordingly. Cameras with automatic number-plate recognition (ANPR) technology are also widely used, mainly as an enforcement tool.
Both technologies rely on fixed roadside infrastructure. As charging schemes to date have focused on specific areas (individual cities) or road infrastructure (major motorways, tunnels, and bridges) this type of technology provides an adequate solution.
To meet future policy goals, however, this is not feasible. More extensive charging schemes covering greater areas, more road types, more classes of vehicle, and which will vary charges depending on location and time of day require a far more flexible solution. Flexible schemes require the positioning element to be onboard the vehicle. GNSS-based devices, possibly augmented with other sensors, have been identified as the best option to achieve this.
Using GNSS, the OBU tracks the location of the vehicle, and this is matched against the road network to calculate a charge. A GNSS solution can support many different charging strategies including time distance and place (TDP) based charging for road sections, geographic areas, and cordon schemes. While GNSS offers great potential, several operational and performance limitations have prevented more widespread adoption. Operationally, OBUs are relatively expensive, fraud prevention is potentially complex, and charging schemes must also accommodate infrequent users. GNSS performance is limited in terms of the ability to provide sufficiently accurate positions with high availability and integrity in all operating conditions.
To be fully flexible and to target congested areas, OBUs must work in all environments including urban areas. Urban-canyon problems, with satellite signals blocked and reflected, are well documented. In some cases, not enough signals are available to determine a position, and when there are enough satellites, the ranges will be prone to errors and the geometry is likely to be poor. Signals are more likely to be available in the longitudinal direction of the street, but with few or no satellites on either side of the vehicle. Signal blockage is a particular problem when the GNSS receiver is started up, and it attempts to decode satellite ephemeris data. This requires around 30 seconds of uninterrupted tracking with a relatively strong signal for each satellite, and in an obstructed urban environment it may take many minutes to determine the first receiver position.
Charging schemes typically aim to charge for at least 99 percent of road usage. If a typical journey length is 30 minutes, this means that only 18 seconds with no usable position solution can be tolerated and hence time to first fix (TTFF) must be below 18 seconds, and ideally much lower.
When positions can be determined, they must be sufficiently accurate to identify the correct road segment that the vehicle was on, and they must be reliable. Reliability, or integrity, becomes critical if road users are to be charged on the basis of GNSS-derived positions. Users must have confidence that they are being charged correctly for schemes to be effective and to gain public acceptance.
Whilst GNSS-based solutions are entering the market, for example in Germany and Slovakia for heavy goods vehicles, barriers to wider adoption remain. Many countries are considering GNSS-based road pricing, and they all face similar challenges in ensuring the accuracy, integrity, and availability of a GNSS-based solution for nationwide deployment.
SIGNATURE Solution
The principal objective of the SIGNATURE project is to prototype a GNSS-based solution for flexible road-user charging that can provide the required high integrity and high availability in a cost-effective and scalable manner.
This robust, high-availability, high-integrity solution is delivered firstly through providing reliable assistance (A-GNSS) data from EDAS to optimize receiver acquisition and tracking capabilities and reduce TTFF, and secondly through implementation of embedded GNSS reliability algorithms into an OBU, providing assurance of positioning information (Figure 1, at top).
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