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Navigational tools that assist travelling people when they visit new areas are becoming increasingly common. Some of these tools are designed to be used with small portable devices (palmtops and also cellular phones) to give users more flexibility in their utilization (e.g. while walking in a town). The limited storage capacity of such devices makes it unrealistic to think that all the useful information can be stored in such devices, apart from the most basic one, like road maps and location of some points of interests. Other more detailed information should be loaded on the fly when needed, e.g., by using a wireless connection to some remote information service. However, this could cause unacceptably high latency to get the information. Another important factor that must be taken into account is the energy consumption caused by the access to the remote information service through a wireless link. Hence, the design of an information service that integrates the capabilities of palmtop-based navigational tools must be carefully designed around the goals of reducing both the perceived latency and the consumed energy. To this purpose, we investigate the use of caching and prefetching techniques. We present analytical and numerical results obtained by defining a probabilistic model of the typical utilization scenario of the considered service and corresponding cache management policies.