GPS World, March 2011

Road TRANSPORTATION 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 roadside infrastructure As charging schemes to date have focused on spe infrastructure major motorways tunnels and bridges this type of technology provides an adequate solution To meet future policy goals howev charging schemes covering greater areas more road types more classes of vehicle and which will vary charges depending on location and time of day ing element to be onboard the vehicle GNSS based devices possibly augmented with other sensors have been 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 Operation fraud prevention is potentially com accommodate infrequent users GNSS performance is limited in terms of the positions with high availability and integrity in all operating conditions congested areas OBUs must work in all environments including urban areas Urban canyon problems with 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 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 and ideally much lower When positions can be determined 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 that they are being charged correctly for schemes to be effective and to gain public acceptance Whilst GNSS based solutions are 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 charging that can provide the required high integrity and high availability in a cost effective and scalable manner This robust high availability high 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 These features are intended to make a positive contribution in terms of the key RUC performance criteria as de tion for Road User Charging Accuracy right cost per trip Integrity probability and amount of overcharging Availability amount of charged usage Assistance Server An assistance service supplying suitably equipped OBUs is one means to maintain rapid TTFF and meet the requirement for high positioning availability The most can make to TTFF is to provide the ephemeris data which takes around 30 seconds to download from a satellite signal Assistance data can also reduce the frequency search space when a re pected Doppler frequency can be com and satellite positions The assistance server in SIGNATURE is based on EDAS currently available as a beta version EDAS allows a user to plug into EGNOS to receive the data collected by all the current EGNOS Ranging and Integrity Monitoring Stations RIMS This makes it possible to access EGNOS data when there is no clear sight to the EGNOS geostationary satellites which can often be the case in urban areas particularly at higher latitudes As well as supplying EGNOS messages EDAS also provides GPS obser www gpsworld com March 2011 GPS World 29