L-Band TEC Measurements and Lower Frequency Scintillation

Signal amplitude measurements from the GPS satellites are currently limited to L-band frequencies above 1 GHz, which often remain unaffected by conditions causing even severe scintillation at more sensitive lower frequencies. Use of differential carrier phase data from dual frequency receivers to drive phase screen models and estimate scintillation at other frequencies is one potential means of monitoring scintillation over a wider range of frequencies. However, this process is complicated by the presence of a diffractive component in the L-band signal phase which can obscure the true structure in total electron content (TEC) needed as input to phase screen models. Signal amplitudes and phases at L1 and L2 frequencies (1.57 and 1.23 GHz, respectively) are calculated after propagation through one-dimensional power-law phase screens and then the resulting differential carrier phase compared with the initial phase values in the screen. Scintillation at a variety of frequencies is then computed from both the original screen and the simulated differential carrier phase, and the two results compared to examine the effects of the unobservable diffractive phase component contained in observational TEC data. Initial results show an increase of ~10% in S4 index computed at 250 MHz from simulated differential carrier phase compared to the direct phase screen computation. These results suggest that under many conditions L-band TEC observations can be used effectively to estimate VHF and UHF scintillation over a wide range of scintillation levels, and that the differences resulting from use of observed TEC instead of true ionospheric phase can be accounted for by a relatively simple correction factor.