A GPS device comprising: a first circuit arranged to receive at least one first signal and arranged to output first timing information dependent on the first signal; a second circuit arranged to receive at least one second signal and arranged to output second timing information dependent of the second signal; and a third circuit arranged to determine timing information of the device. The third circuit is arranged to receive at least one of the first and second timing information, and further arranged to produce third timing information dependent on at least one of the first and second signals. The third circuit further is arranged to produce a location estimate dependent on the first and third timing information. The third timing information is initially synchronized to the first timing information and maintained substantially synchronized to the at least one first signal using the second timing information.