The characterization of ionosphere scintillations in terms of scintillation index is discussed. Scintillation index cannot be directly applied to the design of systems that use transionospheric propagation. The relationships of scintillation index to system design parameters, cumulative amplitude distributions, and fade levels, are shown. The determination of a spectral index is explained and its use for extrapolating the results of ionosphere scintillation measurements to a desired frequency is shown.
Using observations of the scintillations of beacons from synchronous satellites, a high latitude model of scintillation excursion is being developed. Phase 1 of the development has been completed and is described in this report. Several years of continuous recordings taken at Narssarssuaq, Greenland, Goose Bay, Labrador, and Sagamore Hill, Massachusetts, were reduced. The data base consists of values of 15-min scintillation excursions in dB at 137 MHz. Equations are developed which yield scintillations at this frequency as a function of local time, magnetic index, solar flux, and month of the year. The concept is to predict, at sub-auroral and auroral latitudes, scintillation at this frequency and at higher frequencies. The aim is to give to users an indication of expected scintillation excursion when one predicts magnetic activity and solar radio flux. The equations developed have been checked with one additional set of observations and are being checked with additional data. Phase 2 of this model will incorporate geometrical terms to take account of the propagation angle of the observer vis-a-vis the irregularities and will allow for frequency dependence to be ascertained. In addition, it is expected in Phase 2 to validate the model with additional sets of data, extending the model to auroral latitudes greater than 63 deg. and to polar latitudes. (Author).
Ionospheric scintillation effects encountered in the equatorial anomaly crest, polar cap and auroral regions have been contrasted to provide information for the design and evaluation of the performance of satellite communication links in ghese regions. The equatorial anomaly region is identified as the most disturbed irregularity environment where the amplitude and phase structures of VHF/L-band scintillations are primarily dictated by the strength of scattering rather than ionospheric motion. In the anomaly region, the spectra of intense amplitude scintillations at VHF and L-band are characterized by uniform power spectral density from the lowest frequency (10 MHz) to 4 Hz at VHF and to 1 Hz at L-band and steep rolloff at higher fluctuation frequencies with power law indices of -5 to 07. Such structures are compatible with intensity decorrelation times of 0.1 and 0.3 sec at VHF and L-band frequencies, respectively. The phase spectra are described by power law variation of psd with frequency with typical spectral indices of -2. 4. The strong scattering at VHF induces extreme phase rates of 200 deg. in 0.1 sec. The 90th percentile values of rms phase deviation at 250 MHz with 100-sec detrend are found to be 16 rads in the early evening hours whereas amplitude scintillation can cover the entire dynamic range of 30 dB not only at 250 MHz but at L-band as well.
In situ measurements of F-region irregularity amplitude and ambient electron density made by the retarding potential analyzer (RPA) on OGO-6 near perigee altitude of 400 km have been utilized to derive the variation of electron density deviation over the equatorial region. Based on these measured electron density deviations and other assumed model parameters, including a three-dimensional power-law form of irregularity spectrum of index 4, a model of equatorial scintillations is developed in the framework of diffraction theory. The percentage occurrence contours of estimated equatorial scintillations greater than or equal to 4.5 dB at 140 MHz during 1900 to 2300 LMT for the period November to December 1969 and 1970 have been derived. The model is found to depict a pronounced longitude variation with the scintillation belt width and percentage occurrence being maximum over the African sector. The latitude extent of the spatial scintillation belt narrows over the American sector without much decrease in the scintillation occurrence whereas over the Indian and Far Eastern sectors both the extent and the occurrence are found to decrease. The percentage occurrence of scintillations estimated from this model is found to be consistent with VHF scintillation measurements at Ghana, Huancayo, and Calcutta. In addition, the model was found to be in qualitative agreement with GHz observations at various longitudes made by the COMSAT group. The effect of varying model parameters on scintillation estimates at VHF, UHF and GHz are discussed. Implications of the observed longitudinal variation of scintillations on current theories of equatorial irregularity formation are indicated. (Author).
