Which factor can exacerbate the effects of atmospheric errors in GPS positioning?

Each of the factors listed can contribute to an increase in atmospheric errors in GPS positioning.

Having fewer satellites in view reduces the amount of information available for calculating a precise position. GPS systems rely on signals from multiple satellites to triangulate a location accurately. With fewer satellites, the system may not adequately correct for atmospheric errors, leading to less reliable positioning.

Increased PDOP (Position Dilution of Precision) values indicate that the geometry of the satellites in view is poorer, which can result in greater uncertainty in the calculated position. High PDOP values suggest that the satellites are not well distributed in the sky, which can exacerbate the effects of atmospheric errors because the calculations become more sensitive to inaccuracies in the satellite signals.

Using single-frequency receivers also contributes to this issue. These receivers typically rely only on one frequency band for signal reception, making them more susceptible to ionospheric errors. Dual-frequency receivers can mitigate some atmospheric effects by using signals from two different frequency bands, allowing for better corrections for ionospheric delays.

All these factors combine to amplify the impact of atmospheric errors on GPS positioning, making the comprehensive answer 'all of the above' the most accurate representation of the situation.