More people using GPS won't require more satellites, just like more radio listeners doesn't mean you need more radio transmitters, or more bums on seats in an auditorium doesn't mean you need more members in an orchestra.  However, there will soon be more positioning systems, when the Galileo constellation launches.  Galileo is an EU/ESA project to enhance and back up GPS.

I've not heard about GPS isn't the best technology for this sort of application, as it doesn't penetrate too well inside buildings.

However, there could well be domestic applications of GPS that we're just not aware of!  Please let us know if you know of any...

We put this to Professor Todd Humphreys:  

Todd -   Divers can't use GPS.  It’s because the high radio frequencies used for GPS just bounce off the surface of water.  And so, divers will have to resort to other means - sonar or other techniques - for navigating beneath the water surface.  

Ben -   Helen, you're a regular diver.  How do you find out where you are?

Helen -   Well, we have used GPS when I've been doing things like underwater surveys. What you can do is tie the GPS to a buoy on the surface and drag it along behind you.  Then you can download basically where you've been.  You can track your path and match that up with what you were looking at when you were underwater.

We put this to Professor Todd Humphreys:

Todd -   Well in fact, the early systems were ground based.  There was the LORAN (LOng RAnge Navigation) system developed around the second World War that worked faithfully for many years and is still in use in some parts of the world.  The trouble is that GPS enjoys great advantages over these ground based systems.  It’s got global coverage and its signals aren’t distorted by the local topography.  So it’s just been a question of success.  GPS has line of sight global signals.

We put this to Professor Todd Humphreys:

Todd -   Well, it’s important to recognize that the US has a strong incentive to ensure that GPS is reliable, and that’s because a good deal of the world economy depends critically on GPS, especially for timing.  So, any military initiated or US initiated denial of civilian GPS would most likely be a surgical pinpoint denial in some military area of operations.  It’s possible but it’s very unlikely because the US has such a strong incentive to make sure everybody understands GPS can be counted on.

Ben -   As a blanket thing doesn’t civilian GPS only work up to a certain altitude?

Todd -   Well yes.  Civilian GPS receivers are required to carry inside themselves certain checks against going too fast and going too high so that in the case that they were strapped to some intercontinental ballistic missile, they would fail to work.  So that’s one way that the US is trying to prevent unauthorised use of GPS in a various purposes.

Ben -   But obviously, assuming we’re not strapped to an intercontinental ballistic missile, we can still use it to find out where we are to go treasure hunting and to make sure we know where we are on a map.

Todd -   That’s right.  All of those good activities!