This will describe the follow-up research related to positioning technology capable of use in services up to AHS-a.

  First of all, a basic survey of two-dimensional moving body position detection technology was conducted in fiscal 1996, and the principle of operation was selected on that basis. Selection of joint test methods followed from fiscal 1997 to 1998. Magnetic markers were chosen for position detection and modulated radio markers were chosen for starting point detection, and these were submitted for joint tests.

  From fiscal 1999 to 2001, the results of joint tests and research conducted to improve other Promising methods were taken into consideration in a comprehensive comparison of lane markers. The two methods of magnetic markers and passive multiplier radio wave markers were selected. Positioning technologies were reevaluated in fiscal 2001, and the three methods of Pseudo-Satellite, Direction of Arrival (DOA), and application use of Dededicated Short Range Communication (DSRC) were selected as the more promising technologies for AHS-i services. Baseline operation tests were implemented for evaluation of positioning accuracy at high speeds and other basic aspects of performance.

  Based on this baseline data, specific examples of ITS services applying positioning technology were studied in fiscal 2002, and a comprehensive comparison of positioning technologies was carried out to obtain an organized view of the advantages and disadvantages of the individual technologies.

  The advantages and disadvantages, issues, and so on not only of lane markers but also of DSRC, high-accuracy GPS (including pseudo-satellite), DOA, and application use of DSRC, which are considered promising positioning technologies, are presented here in organized form.

  Lane markers provide high levels of accuracy and reliability, but costs of construction and maintenance, and measures to promote widespread use of on-board unit are issues. DSRC is advantageous in providing both information provision and positioning functions, but cost reduction is an issue. GPS systems can cover the entire country at low cost and have a wide range of applications, but there are issues of accuracy and reliability. Pseudo-satellite has the potential to resolve GPS issues and is promising as a supplement to GPS, but it is at the development stage and the technology must be evaluated. DOA is advantageous in that it can be shared with DSRC, but transmission of position becomes an issue in dense traffic flows. Application use of DSRC is advantageous less as a positioning technology than as a means of information provision. The phase of research for each technology is as shown in the table.

  Two types of lane marker (magnetic markers and passive multiplier radio wave markers) have been developed that are capable of positioning in a lane with an accuracy within several centimeters laterally and within one meter longitudinally in real time and with high reliability regardless of location, weather, or setting.

  Various test bed, test course, field, and other such tests were conducted from fiscal 1997 to 2001. These confirmed that the markers are capable of highly accurate positioning regardless of road surface conditions, weather, road construction, traffic environment, etc.

  Joint tests conducted on a test course in fiscal 2000, as shown in the figure, verified the possibility of positioning with an accuracy of 4 cm or better laterally in the lane for vehicles travelling at 20-120 km/h.

  It was also confirmed through testing that adopting magnetic tape sequential layout for magnetic markers with differing N-S polarity or radio wave reflection phase makes it possible to detect positions along the direction of the road. Examples of typical marker use are shown in the figure with the rate of marker diffusion and social benefit on the horizontal and vertical axes, respectively.

  As shown in the figure, the magnetic marker system has multiple magnetic sensors mounted on the vehicle side that detect the magnetic field distribution of magnets (markers) buried in the road surface and determine the position of the vehicle relative to the lane. The detection range is adjusted by the number of magnetic sensors mounted on the vehicle and other such means. It is also possible to create a starting point position that can provide service covering two-wheeled vehicles by arranging markers (starting markers) that have a strip magnetic field distribution oriented laterally across the road and placing them at different intervals than the position detection markers. As shown in the substance and flow of evaluation figure, these markers were subject to a variety of evaluations that included performance, environmental durability, on-board mountability, installability, safety, and so on. Based on the results, the recommended specifications indicate the use of two types (plates and nails) according to the pavement construction, with a magnetic field intensity of 170 µT or more at 30 cm above the road surface.

  The figure shows the actual vehicle test results for lateral position accuracy according to sensor mounting height on the vehicle and markers at different locations in the road surface structure. In all cases, the accuracy 2-was 40 mm or less. Magnetic markers are closest to the point of practical application in Europe and America, and their distinctive features are that they have a simple construction with high durability and high reliability.

  As shown in the figure, the radio wave marker system has a transmission antenna mounted on-board the vehicle that emits radio waves. The waves are reflected by markers installed under the paved road surface. The reflections are received by multiple receiving antennas on-board the vehicle and the position of the vehicle relative to the lane is determined accordingly. The detection range is adjusted by the number of receiving and transmission antennas mounted on the vehicle and other such means. It is also possible to create a starting point position that can provide service covering two-wheeled vehicles by arranging marker clusters with rows of multiple markers (starting markers) that are oriented laterally across the road and that have a reflection phase different from the position detection markers.

  As shown in the substance and flow of evaluation figure, these markers were subject to a variety of evaluations that included performance, environmental durability, on-board mountability, installability, and so on. Based on the results, the recommended specifications indicate the use of passive multiplier markers that double the reflection for the medium to low frequency, micro-power specification radio wave markers. Features of this radio wave marker system include accuracy equal to magnetic markers but somewhat greater flexibility in installation location, and where the magnetic markers have two values, the radio wave markers can be combined to yield at least four or more values.

  A pseudo-satellite emits positioning signals equivalent to those of GPS satellites. This technology expands the coverage of positioning using installations at locations where GPS positioning is not possible because a smaller number of GPS satellite signals is acquired, such as in the shadows of buildings. These installations supplement the blocked GPS signals. This can also be expected to improve the accuracy of positioning. However, since they use signals on the same frequency band as GPS satellites, there are concerns about deterioration of accuracy due to multipath and interference with GPS signals. The system pulses the pseudo-satellite signals in order to limit this effect.

  Verification testing was conducted at the National Institute for Land and Infrastructure Management test course, where the pseudo-satellite was evaluated by creating an environment equivalent to GPS blockage by using software to limit acquisition of GPS satellites. From a condition where positioning was not possible with two GPS satellites, movement into a pseudo-satellite area and acquisition of the pseudo-satellite made positioning possible. Accuracy at that time was confirmed to be 40 cm or better. At higher speeds, the vehicle would pass through the pseudo-satellite area before the positioning computation had converged. Consequently, inability to determine position became an issue. However, the possibility of application to AHS services was discerned as a result of expansion of the area of positioning inability and the realization of high-accuracy positioning.

  Research is underway on application of DOA for DSRC starting point function aimed at improving vehicle positioning accuracy and reducing equipment cost. The DOA sensor can determine a vehicle's position by detecting the direction in which radio waves are transmitted from on-board unit that is using road-to-vehicle communication. This system can therefore substitute for the starting beacon that was previously required for DSRC, can track a vehicle's cruising lane, and so on.

  The current testing was conducted at the National Institute for Land and Infrastructure Management test course with vehicles cruising at speeds up to 120 km/h. The testing confirmed accuracy, measurement time, the influence of nearby vehicles, and so on. The stand-alone accuracy of the DOA sensor is 1.5 m or better. After measuring vehicle position with that accuracy, the stand-alone DOA sensor mediated by the road-to-vehicle communications device had a lag time of 20 ms or less for completion of processing by on-board unit.

  Given these results, the total accuracy of the DOA system is 2.0 m or better, confirming the validity of DOA as the positioning system for use in AHS road-to-vehicle communications. This also confirmed the potential applicability of DOA in various DSRC systems. Verification on actual roads will be conducted in future, and study will be done of DOA use in various AHS and ITS applications.

  Application use of DSRC is a technology that limits DSRC functions to enable handier provision of information. The system utilizes ETC on-board communications module as its roadside unit, thus achieving compactness and lower cost. Testing of the communications performance of AHS on-board unit carried out in fiscal 2002 confirmed that the system could assure a bit error rate of about 10-5 within an area 15 m long in the direction of movement and 7 m wide. It also confirmed the possibility of information provision of about 3 KB (estimated simple figure at VICS level 2) by broadcast transmission. This technology envisions the use not only of AHS on-board unit but also ETC and all other such on-board unit that has DSRC communications capability.