Estimating the refractive index of transionospheric HF radio waves

The radio receiver instrument (RRI) on the e-POP/Swarm-E satellite is affected by the motion of the satellite causing a Doppler shift in the received frequency. Traditionally, to determine the Doppler shift, only the motion of the receiver relative to the transmitter is necessary. However, when traveling through a dispersive medium such as the ionosphere, especially for radio waves in the HF band, the changing refractive index needs to be considered. Using a raytracing method, the refractive index is determined for an electron density profile along the path of the radio waves from the ground transmitter to the satellite receiver. The refractive index less than 1.0 causes the phase fronts to slow down. The resultant Doppler shift is lower than expected for the free space case. RRI data is used to validate the raytracing model for the Doppler Effect.