Computing pedestrian’s position by using vehicles as anchors, when GPS fails to work in urban canyon

What is it about?

On the roads dominated by high speed vehicles, pedestrians are susceptible to injury or even death in the collisions with vehicles. Pedestrian-to-vehicle communication, in which a pedestrian device estimates its position and sends to nearby vehicles, helps a driver learn the presence of a pedestrian even when the pedestrian is behind a building and is not directly visible. Position is usually computed based on GPS signals, which relies on measuring the direct distance between satellites and a GPS receiver (a pedestrian device). But in urban canyons, the direct path between satellites and a GPS receiver might be obstructed by roadside buildings, and a GPS receiver may not detect enough satellites, or a GPS receiver may receive reflected signals instead which have large errors in the distance estimation. Although different satellite systems (GPS, Glonass, Beidou, Galileo, etc.) can be combined together to increase the number of satellites, the directly visible satellites usually are over the head, with a poor distribution which may lead to large positioning errors. This paper aims at computing the position of pedestrians accurately, even in the urban canyons. This is achieved by using vehicles as anchors as well, besides GPS satellites. In this work, it is assumed that vehicles periodically exchange their position information to avoid collision accidents and a pedestrian device overhears signals from vehicles, based on which estimates its distances to vehicles and computes its own position.

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Why is it important?

Using vehicles as anchors in computing pedestrian position helps to solve the problem of satellite shortage and improve the distribution of anchors for positioning. Vehicle position can be computed more accurately than pedestrian position. Vehicles moving on the roads usually see more satellites than pedestrians moving on the sidewalk near roadside buildings. In addition, vehicles have other auxiliary means to improve their position precision with different sensors. In the moving (longitudinal) direction, vehicle's speed can be accurately measured by a speedometer and used for dead-reckoning. In the vertical (lateral) direction, vehicle positions can be constrained to roads by map matching, and further constrained to lanes by using cameras or LiDAR for lane detection Different from satellites, vehicles move on the roads and scatter in a more balanced way around pedestrians. But the direct path between vehicles and pedestrians may also be obstructed. Instead of computing distance from the overall signal strength, each pedestrian device collects the channel state information from a vehicle, and detects whether a direct path exists to the vehicle. Then, a pedestrian device uses the strength of the direct component to estimate the distance, and uses only vehicles with direct path for positioning. Vehicles frequently transmit signals. A pedestrian device overhearing these signals can frequently measure pedestrian-to-vehicle distance. These frequent measurements are further combined together by a filter to get more accurate pedestrian position.

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