A mobile device including a positioning device for determining position by timing analysis of received signals, the positioning device including a system for determining timing accuracy, the system including first signal generating means for continuously generating a first clock signal whilst the positioning device is inactive, second signal generating means for discontinuously generating a second clock signal during a plurality of spaced apart time periods, and accuracy determining means for determining the accuracy of the first clock signal using timing data from the second clock signal, the positioning device being arranged to determine position by use of the determined timing accuracy of the first clock.