This article presents a statistical study on the availability of location sources. In particular, it rates the availability of cellular network base stations and GPS satellites in urban scenarios. Individual and joint availability density functions have been obtained for both types of signal sources, as well as statistical information that complements these functions. This information is of great importance due to the growing interest in fusion (i.e., combination) of location techniques. The results indicate that the improvements in coverage from fusion techniques depend both on the fusion mechanism and the scenario, obtaining noticeable benefits in constrained situations.