Highway tolling using a GPS on-board unit in each vehicle relies on the availability of accurate positions along the path of each vehicle. Conventional GPS receivers exhibit loss-of-lock and poor position availabilities whenever the satellite visibility is impaired by obstacles like buildings, trees, or topography.
High-sensitivity (HS) receivers use novel tracking loop techniques to lock onto GPS signals which may be 20dB (i.e. a factor 100) below the sensitivity threshold of conventional receivers. This increases the position availability significantly and allows using GPS even in harsh urban environment and indoors. However, weaker signals are usually associated with larger errors and so the enhanced availability may come at the cost of reduced position accuracy.
Based on our previous experiences in high precision GPS data modeling, quality control, and quality enhancement, we have investigated several aspects of HS GPS related to its reliable application in vehicle positioning and highway tolling.
The objectives of the research project were
(i) to define a standardized test for HS GPS sensors (receiver + antenna) which allows to objectively assess the performance of a sensor in terms of signal acquisition time, availability, accuracy, and power consumption;
(ii) to assess and compare 3 different state-of-the-art HS GPS sensors (SiRFStar2e/LP Xtrac, SiRFStar3, u-blox TIM-LH) using the above standardized test, including an investigation of the impact of the vehicle cabin and antenna location within the cabin;
(iii) to develop an optimum variance model for HS GPS pseudo-range observations providing enhanced positioning accuracy and reliability;
(iv) to investigate the feasibility of using an HS GPS sensor as basis of an accurate odometer.