GPS satellites don't transmit strong enough to reach indoors, through the roofs and walls of buildings, like cell phones do. GPS signals that enter buildings through windows are unreliable since they often have bounced and thus give the wrong distance measures by even hundreds of meters. Since cell phones work fine indoors, and GPS uses the same frequency, I suppose there's no need to use another frequency.
Would it suffice for today's kind of GPS satellites to orbit in low Earth orbits, such as planned communication satellites constellations like Starlink? Would it be feasible to equip them with larger solar arrays, or would nuclear generators be required?
One can imagine many commercial applications for precision indoor navigation (like more efficient robotic vacuum cleaners ;-) Just reaching through the roofs of single storage factories and shopping centers should make for a great market. The alternative today is to install radio or ultrasound beacons (or perhaps a camera system) in every room where one wants to have a location service. A single ubiquitous system that's already standard would have economic advantages.