[chroma]

I think your estimate is overly alarmist. Without GPS, we'd use ground-based stations. It wouldn't be as accurate or as available, but it would still be good enough for almost everything GPS does. In fact, we already have ground stations to augment GPS. And even without ground-based stations, modern accelerometers allow inertial navigation to be quite accurate.

Also, GPS satellites are in medium Earth orbit, which is rather sparsely occupied. In a Kessler syndrome scenario, GPS would likely survive.