The ASSISTANT project contributes to maintaining the mobility of older people in Europe, in order to safeguard their social and economic participation in an increasingly ageing society. It does this by helping them to travel safely and independently by public transport. This three-year project develops an application for the home PC and smartphone that designed to help older travelers to plan their public transport journeys and then receive guidance during their journey. This guidance will help them to find the vehicle they need, warn them when to get off, when and where to change to another route, and will provide assistance if something goes wrong. There are several stages in the guidance where uncertainties play a major role and have an effect on the quality of the trip. The major uncertainty is with the location services when GPS reception in poor or impossible due to urban canyons or the user being under ground or in a tunnel. In addition, when waiting at a stop where for instance several buses might arrive at the same time, it could be difficult to identify the correct bus to board. This paper explains the overall design of the ASSISTANT project and addresses some of the issues related to positional uncertainties.