Which statement correctly describes GNSS atmospheric delays and common mitigation methods?

• A. Ionospheric delays affect earlier carrier phases and pseudorange; tropospheric delays affect propagation; mitigated via dual-frequency receivers, ionospheric models, and zenith delay corrections
• B. Ionospheric delays affect only amplitude; tropospheric delays are negligible; mitigated by ignoring atmosphere
• C. Atmospheric delays only affect the receiver's power; mitigated by higher gain antennas
• D. No atmospheric delays affect GNSS; they are constant

Answer: (A)

Two main atmospheric effects shape GNSS timing and positioning: ionospheric delays and tropospheric delays. The ionosphere is a dispersive medium, so its impact varies with signal frequency and changes the travel time and phase differently at each frequency. That means both the pseudorange (range) measurements and the carrier-phase measurements are affected. The troposphere, on the other hand, is non-dispersive and introduces a delay that depends on the path through the atmosphere rather than the signal frequency, influencing the propagation time but not in a frequency-dependent way.

Mitigation relies on approaches aligned with these behaviors. Using two frequencies lets you form ionosphere-free combinations or estimate the total electron content to remove the ionospheric delay. Ionospheres models provide corrections when you can’t rely on dual-frequency data by estimating the ionospheric delay from TEC estimates. For the troposphere, corrections come from zenith total delay estimates (separating into dry and wet components) and applying a mapping function to convert that zenith delay to the slant path corresponding to the satellite elevation you’re observing.

This description matches the statement because it correctly assigns the ionospheric delay to the carrier phase and pseudorange, places tropospheric delay in the propagation domain, and lists the common mitigation methods—dual-frequency data, ionospheric modeling, and zenith delay corrections. The other options imply incorrect facts, such as delays affecting only amplitude, being negligible, or no atmospheric delays at all.