Coverage and Performance Model for the Siemens Datatrak Vehicle Location System

摘要

The Siemens Datatrak automatic vehicle location system (AVLS) was originally designed as a high reliability tracking solution for cash-in-transit vehicles in the UK. As GPS has increasingly been employed for the less-demanding, low-cost, location and tracking applications, Datatrak's exceptionally high availability and low vulnerability have made it an attractive solution for applications that demand the highest standards. As a result, its coverage is being expanded into other countries, where it is employed both independently and in co-operation with GPS. The Datatrak system uses a network of LF transmitters positioned to optimise tracking performance. The mobile units are equipped with exceptionally narrow-band receivers that establish the vehicle's position by making phase comparisons between the signals from these stations. Location measurements and other data are then sent to the control centre via highly-redundant uhf links that employ precise timing controlled by the low-frequency network. The process of designing the new Datatrak networks now being planned is a complex one, especially as coverage must be optimised whilst minimising the number of transmitter sites. This paper presents a new coverage and performance prediction model developed for this purpose by the authors. The model employs data from International Telecommunications Union sources and from Datatrak's own extensive monitoring records. It predicts, point-by-point through arrays that span the region, the groundwave and skywave field strengths of each of the transmissions, and the levels of atmospheric and vehicle noise. It also considers the complex interference received from the many stations with which the system shares its frequency band, taking into account the high rejection of the narrow-band receiver. At each geographical point, the quality of the navigation signals is determined in this way. Then, by mimicking the operation of the receiver, the model determines the repeatable location accuracy obtained. The paper describes the ideas and techniques used in the model, and demonstrates its use in quantifying the performance of the UK Datatrak system.