OK, I sm somewhat of an expert in satellite data so here:

Each GPS manufacturer must launch or pay to have launched their own GPS satellite(s). The GPS unit in your car can only "speak" to it's own company's satellite. The satellites in turn "speak back" to the individual GPS units. That is why they were so expensive several years ago.

The bigger companies have more units communicating with the distinct possibility of overload or crash. This has happened many times recently with Garming and TomTom since they are the most popular. If too many people are using their GPS at the same time, the satellites overload and can actually crash into another planet, star, meteor or asteroid...or worsed case back down to Earth.

Also, the beams that are transmitted as the GPS units "speak" to the satellites can cause beam deflection errors which usually occur on bridges or the entrances to tunnels. Additionally, GPS radar microwave beams can be warped and/or "wrapped" from satellites that are about to transpond and overload because of too many people beaming to them at once.

Also, the satellites can lock their beams to the wrong GPS units and it is possible to reeive someone elses meant transmission which force people to get totally lost. For example, you live in San Francisco but you receive the satellite beam from the TomTom satellite unit of a person living in Prague. This is why I only buy offbrand GPS units like Nextar