A GNSS signal slows down as it crosses the ionosphere, an electron-rich layer roughly 200–800 km up. The extra path delay is proportional to how many free electrons the signal crosses — the Total Electron Content (TEC), measured in TECU (1 TECU = 10¹⁶ electrons/m²) — and inversely proportional to the square of the carrier frequency:
Δ_iono = 40.3 · TEC / f² (meters, TEC in el/m², f in Hz)
Slant TEC = Vertical TEC × F(el)
F(el) = 1 / √(1 − (Rₑ·cos(el) / (Rₑ+hᵢₒₙ))²) (obliquity/mapping factor)
A geomagnetic storm (driven by a CME or fast solar wind stream compressing the magnetosphere) dumps extra energy into the upper atmosphere, ionizing it further and can locally more than double the quiet-time TEC — this simulator scales baseline TEC by a storm multiplier tied to the Kp index, plus a day/night factor since sunlight is the normal ionizing source. Because delay grows at low elevation (a grazing signal crosses far more ionosphere — the obliquity factor F(el) rises sharply below ~20°), a satellite near the horizon during a storm can see many meters of extra range error, degrading GPS position fixes.
Dual-frequency correction:
ρ_corrected = (f1²·ρ1 − f2²·ρ2) / (f1² − f2²)
removes ≈99.9% of the first-order ionospheric term
- Kp slider — storm intensity; higher Kp inflates TEC and adds fast electron-density turbulence (scintillation) visible as flicker in the shell.
- Local time slider — the dayside ionosphere is far denser than the nightside because solar UV is what ionizes it.
- Elevation slider — moves the ground receiver and satellite; a low-elevation path crosses more ionosphere (larger obliquity factor F).
- Dual-frequency correction — real receivers combine two carrier frequencies (e.g. GPS L1 1575.42 MHz / L2 1227.60 MHz) to cancel almost all of this delay; toggling it shows the residual single-frequency error a cheap receiver is stuck with.
Real-world relevance: this is why single-frequency GPS receivers lose meters of accuracy during geomagnetic storms, why aviation and precision-agriculture receivers use dual- or triple-frequency corrections, and why the March 1989 and October 2003 "Halloween" storms caused documented GPS outages.