How do ionospheric delays affect GNSS pseudorange measurements, and how are they mitigated?

• A. They cause random jitter; mitigated by averaging
• B. They are frequency-dependent biases; mitigated by dual-frequency ionosphere-free combinations or ionospheric models
• C. They do not affect pseudorange
• D. They only affect carrier phase, not pseudorange

Answer: (B)

Ionospheric delays act as a frequency-dependent bias in GNSS pseudorange measurements. The ionosphere is a dispersive medium filled with free electrons, so the signal’s travel time through it depends on frequency and the total electron content along the path. Because the delay scales roughly with TEC divided by the square of the frequency, the delay differs between frequencies (for example, L1 and L2). This creates a code-range bias in the pseudorange that must be mitigated. The standard approach uses a dual-frequency ionosphere-free combination, which cancels the first-order ionospheric term when combining the two pseudorange measurements with coefficients based on the squared frequencies. If only a single frequency is available, ionospheric models (such as Klobuchar) can be used to estimate and remove the delay, though higher-order effects and model inaccuracies leave residual error. So the ionosphere introduces a frequency-dependent bias and is mitigated by dual-frequency ionosphere-free processing or by ionospheric models.