Inospheric and Tropospheric delays of signals.
The ionosphere is that part of the upper atmosphere where free electrons occur in sufficient density to have an appreciable influence on the propagation of radio frequency electromagnetic waves.
Less than 1% of constituents ionized Lower boundary at ~ 60 km Upper boundary at ~ 1000 km,
The ionosphere introduces a variable time delay in the propagation of signals from the satellite to the receiver, which affects positioning.
Satellite clock error:
All satellites contain atomic clocks that control all onboard timing operations, including broadcast signal generation. Although, these clocks are highly stable still they lack of perfect synchronization between the timing of the satellite broadcast signals and GPS system time. Satellite clock correction terms and time of clock , account for this lack of synchronization. These will lead to inaccuracy in time measurement
