Lane Determination With GPS Precise Point Positioning
Modern intelligent transport solutions can achieve an improvement of traffic flow on motorways. With lane-specific measurements and lane-specific control, more measures are possible. Single frequency precise point positioning (PPP) is a newly developed and affordable technique to achieve an improved position accuracy compared with global positioning system (GPS) standalone positioning. GPS-PPP allows for sub-meter accurate positioning, in real time, of vehicles on a motorway. This paper tests this technique in real life; moreover, it presents a methodology to map the lanes on a motorway using data collected by this technique. The methodology exploits the high accuracy and the fact that the most driving is within a lane. In a field test, a GPS-PPP equipped vehicle drives a specific motorway stretch 100 times, for which the GPS-PPP trajectory data are collected. Using these data, the positions and the widths of different lanes are successfully estimated. Comparison with the ground truth shows a dm accuracy. With the parametrized lanes, vehicles can be tracked down to a lane with the GPS-PPP device.
An overview of the GPS techniques is given in . For mapping the vehicle position to the road, sophisticated algorithms have been developed (e.g.,  or  with incomplete data). The usual problem in GPS positioning is that the accuracy is not within a lane-width, hence lane-mapping or mapping the position to a lane is impossible. In (urban) canyons, the accuracy decreases since there is no line of sight to some satalites, requiring further approximations (e.g., ). Anyway, solutions have to be found to improve the accuracy to get accuracy to a lane lavel. For instance D-GPS is being used, e.g. by  and . Reference  discuss the option of creating lane-specific maps, in a dedicated mapping campaign, by using high-end dual-frequency GPS receivers in a differential (carrier-wave) processing setup. They discuss problems with driver errors and difficulties finding the lane boundary demarcations with this approach. They propose an alternative method that uses geo-corrected aerial photographs and manual lane finding. Our solution, on the other hand, does not require any manual labour, special equipment or dedicated mapping campaigns, and thanks to the large number of GPS traces, driver errors can be expected to even out. Further fusion with other data from video is proposed by . Also further combination with more epochs of GPS measurements is possible, using Kalman filtering to combine inputs . In these cases, the accuracy of the location improves.
Since this new technique can be implemented on a relatively cheap GPS receiver for the user (as only single frequency measurements are needed) and it provides a highly accurate position, it opens a wide range of applications in different fields including automotive and hand held GPS devices. A specific application is the use for lane identification, for which it was already demonstrated to meet the accuracy requirement; a successful automotive experiment was presented at the ION GNSS 2011 conference . Precise Point Positioning is favored for many applications, thanks to its very high accuracy and independence of a local infrastructure, examples are crustal deformation monitoring, precision farming, to support seafloor mapping, and land surveying . However, due to the long (convergence) time to reach a high accuracy, conventional PPP is best suited to static applications, or other applications where uninterrupted satellite tracking is possible. Our suggested approach on the other hand, is to use single-frequency PPP, which shares the independence of local infrastructure, but does not reach the same level of accuracy, which makes it less interesting for most conventional PPP applications. However, thanks to the low cost hardware, and short convergence time, it is perfect for dynamic applications with quickly changing sky-view, and were many receivers may be needed with competitive accuracy and price, or simply where dual-frequency PPP is too expensive. Examples are hand-held devices, buoys, low-cost precision farming, and automotive applications
In this paper, we introduced a GPS single frequency Precise Point Positioning technique. The positions are sufficiently accurate to track a vehicle down to the lane level. To this end, a roadmap with individual lanes is required, which can be collected using the very same GPS single frequency PPP technique; the algorithms to do this have been outlined and demonstrated (in practice) successfully in this paper. The accuracy of lane mapping is very good, once traffic is spread over all lanes. This technique can be used to get lane-specific traffic information, or to perform lane-specific traffic management. In order to use the GPS-PPP map, a low bandwidth connection to the vehicles is necessary, in order to provide the system with the position of the satellites, the clock errors and the atmospheric conditions. Moreover, for lane-specific management, it is required that the vehicles communicate their own measured position in real-time (order: tens of seconds) with a central server. Finally, the current tests showed that the system and the methodology work for flat roads and open surroundings. Some GPS errors are related to receiving the GPS signal without obstruction or reflection. Figure 5 gave a first indication of this, by showing the degraded performance with only 4 GPS satellites available, and to a lesser extent with 5 satellites available. The sensitivity of the GPS-PPP measurements to the different physical environments (hills, urban environments) is topic for further research.
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