GPS and GNSS relies on a network of satellites orbiting the Earth to determine your location. When you're indoors, the GPS and GNSS signals have to penetrate through walls, ceilings, and other structures, which can weaken or block the signals altogether.
The signals from GNSS satellites are relatively weak by the time they reach the Earth's surface, the average signal level at ground level is about -130dBm. Let's find a reference object, the standby RF power of a cell phone is -60dBm. The structures can further attenuate them, and the signals can reflect off surfaces indoors, causing multipath interference, where the signal takes different paths and arrives at the receiver at slightly different times, further introducing more interference to the positioning.
So there no GPS signal inside a BUS park underground.
Therefore, in the vast majority of indoor environments, GNSS signals are unavailable and GNSS receivers are unable to achieve positioning and timing.