In-car GPS based satellite navigation systems are now a common part of driving, providing turn-by-turn navigation instructions on smartphones, portable units or in-car dashboard navigation systems. This paper uses interactional analysis of video data from fifteen naturalistically recorded journeys with GPS to understand the navigational practices deployed by drivers and passengers. The paper documents five types of 'trouble' where GPS systems cause issues and confusion for drivers around: destinations, routes, maps & sensors, timing and relevance and legality. The paper argues that to design GPS systems better we need to move beyond the notion of a docile driver who follows GPS command blindly, to a better understanding of how drivers, passengers and GPS systems work together. We develop this in discussing how technology might better support 'instructed action'.