GDOP(Geometric Dilution of Precision)
The idea of Geometric DOP is to state how errors in the measurement will affect the final state estimation. Geometric Dilution of Precision Estimate of satellite conditions for a given location & time, Given in distance units (meters or feet), When visible navigation satellites are close together in the sky, the geometry is said to be weak and the DOP value is high; when far apart, the geometry is strong and the DOP value is low. Consider two overlapping ring of different centres . If they overlap at right angles, the greatest extent of the overlap is much smaller than if they overlap in near parallel. Thus a low DOP value represents a better positional precision due to the wider angular separation between the satellites used to calculate a unit’s position.
Ideal GDOP: One Satellite directly overhead w/an abundance of additional satellites spaced evenly around the sky
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
User clock error:
Same as above these are eliminated by using an extra satellite equation.