GPS is also coupled with WAAS which relies on a network of ground stations at known coordinates to determine GPS error. These errors are caused by bulges in the ionosphere that throw off timing. Error is calculated, broadcast up to satellites and then rebroadcast down to GPS receivers that are WAAS enabled. This is known as a satellite based augmentation system. The other satellite nav systems that compete with GPS are building similar augmentation systems (like GAGAN, EGNOS, MSAS and SDCM). These systems are required for precision approaches like LPV for aircraft on instrument flight plans. Regular GPS precision due to ionosphere changes is too low. They've enabled the FAA to lower the minimums for GPS approaches down to levels that are close to ILS, removing the need for expensive ILS installations at many regional airports. So replacing GPS isn't just about creating a system to locate a point in space. You'd need to replace critical systems like WAAS that enable precision approaches during instrument conditions, for example.