Nighttime equatorial ionosphere: GPS scintillations and differential carrier phase fluctuations

The presence of scintillation-producing irregularities in the nighttime equ atorial ionosphere, in the path of Global Positioning System (GPS) signals received at an equatorial station, causes dual-frequency measurements of th e differential carrier phase of GPS L1 and L2 signals to have a contributio n from phase scintillations on the two signals. Dual-frequency data for flu ctuations in the total electron content (TEC) along the path of GPS signals to the equatorial station Ancon (1.5 degrees dip), sampled at a rate of 1 Hz, are used to separate this contribution from the slower TEC variations. Rapid fluctuations in the differential carrier phase, usually on timescales < 100 s, which result from diffraction, are seen to follow the pattern of intensity scintillations on the L1 signal. Intensity scintillations are als o related to the variations in TEC which arise from density fluctuations as sociated with ionospheric irregularities. An approximate version of the tra nsport-of-intensity equation, based on a phase screen description of the ir regularities, suggests that a quantitative measure of intensity scintillati ons may be provided by the derivative of rate of change of TEC index (DROTI ), obtained from the second derivative of TEC. This equation also yields th e dependence of the scaling factor between DROTI and S-4 On the Fresnel fre quency. Comparison of DROTI computed from relative TEC data to correspondin g S-4 indices indicates that there may be lesser uncertainity in a quantita tive relation between the two than between the index ROTI, introduced in re cent years, and S-4 Power spectral analysis of TEC fluctuations and simulta neous intensity scintillations on L1 signal, recorded at Ancon, does not in dicate any simple dependence of the scaling factor between DROTI and S-4 on the spectral characteristics.