Interplanetary GPS: Here is an interesting article about a proposed GPS system using x-ray pulsars to help navigate the solar system.  However, I'm more surprised that we we don't have a plan for conventional GPS or GNSS satellite systems for places we plan to visit in the near future - like the Moon, or Mars.

That said, there may be reasons why this isn't practical at the moment.  One issue is the lack of a significant magnetic field around either Mars or the Moon to help deflect solar or cosmic radiation.  This could significantly reduce the life and health of conventional GPS satellites.

Another issue, for at least the Moon is that the satellite constellation could make exploration of the Moon, and navigation to and from the moon more difficult in the near term.  Transportation coordination for Mars would also need to be worked out - we wouldn't likely launch the satellites one at a time like we do for the Earth orbits.  We'd need to send them in adequate groups to conserve on energy (but small enough to promote redundancy in case of a significant transport issue); and management of the satellites on Mars may require that a base be setup first.

Finally, it's also a question of timing; having the right resources in place at the right time.  Still it would be nice to see a plan for GPS tracking on either the Moon or Mars (or both) in the near future.