Why doesn't GPS work in multi-level parking lots?

GPS needs line of sight to the GPS satellites in order to triangulate your position.

At a very high level here are some common scenarios.

Open sky:
Above ground your phone will be able to receive the signal directly from the satellite, this is what it needs to triangulate your position. Your phone will also receive some reflections from nearby building however since it has the direct signal, it can time correlate the reflections and strengthen the overall received signal.




Below ground parking lot:
For the most part almost all of the signal is blocked and you won't be able to get a fix.




Above ground multi-level:
The direct line of sight gets block and even though some of the reflection can get in, without the direct signal the reflections are difficult to decode resulting in large inaccuracies.




Above ground multi-level:
If you're willing to sacrifice some convenience and want Find My Car Smart to be able to operate in a multi-level parking lot you can park on the uncovered top floor.