Startup Idea : Underground Car Parking Locator

The problem with multi-level car parks is that there is no GPS signal and this is what most mobile based apps want in order to pinpoint your car location. In big malls, I've spent a lot of time trying to locate my car or these days I memorize exactly where the car is parked (why?) Maybe use a cheap custom chip that will emit a certain frequency that can be traced by your phone with an app? I know this is a big issue and can save people lots of times but how?