The effect of fairly large-scale ionospheric irregularities on satellite Doppler shift

Abstract

At frequencies in the VHF band and above the refraction caused by the ionosphere may be regarded as a first order perturbation of the free space ray. A consequence is that the Doppler shift of a satellite signal passing through an ionosphere containing irregularities can be represented as the sum of contributions due to free space, the smooth background ionosphere and the irregularities. Thus it is possible to evaluate separately the contribution due to the irregularities, i.e. to determine the ‘signatures’ which irregularities produce on Doppler records. Mathematically the approach involves the determination of the second variation of the phase path. The theoretical analysis shows that fluctuations in satellite Doppler records may be due to (a) non-stationarity of the ionosphere, (b) irregularities lying below the satellite height and near the straight line path between the satellite and observer, (c) irregularities at the satellite or some combination of (a), (b) and (c). The fluctuation in Doppler shift is calculated for representative irregularities. A major contribution to the fluctuation arises from gradients orthogonal to the free space ray. Thus the Doppler shift is very sensitive to the orientation of the irregularity with respect to this ray. This means that while, for example, the orientation of the phase fronts of a gravity wave can be determined from the Doppler fluctuation, the absence of fluctuations does not mean irregularities are absent.