Radio signals from artificial earth satellites propagated through the earth’s ionosphere are subjected to phase delay, refraction, and attenuation. These effects cause uncertainties in the determination of the vacuum Doppler shift of CW signals. To understand the nature of these ionospheric contributions to the Doppler shift, a theoretical expression for the refracted Doppler shift that is an inverse power series in terms of the frequency of the signal has been derived. A computer program has been written that uses ray theory in conjunction with a model ionosphere to evaluate each term of this expression. The nature of each term is discussed, and its relative importance is shown. The ionospheric contributions that are proportional to (1/ƒ) and (1/ƒ³), which result from phase delay and refraction, respectively, are the predominate terms; therefore the effects of refraction on a radio signal can be connected by using three‐frequency Doppler data. The ionospheric contribution that is proportional to (1/ƒ²) is insignificant. Three‐frequency Doppler data were recorded to evaluate the refraction effects in the refracted Doppler data. Two sets of experimental data are included that show the refraction effects.
Recent developments in remote sensing have made it feasible to review high-risk, large-scale, earthwork sites on the highway network that have a history of instability. This paper describes two such sites in the UK: the Crackenthorpe landslide on the A66 in Cumbria; and the Leys Bend landslide on the A40 near Monmouth. Both sites have been the subject of long-term monitoring regimes, but because of dense vegetation and steep access it has proved very difficult to quantify ongoing slope creep movement accurately. This paper presents the case histories of the two slip sites, and demonstrates how the use of remote-sensing techniques has made it feasible to compare current topography accurately with historical records, to evaluate and plan future maintenance and stabilisation interventions.
The accuracy of satellite Doppler data for navigation, geodesy, and ionospheric electron content determination has been suspect because of failure to eliminate the effects of transmission path inequality caused by refraction. Previous examination of ionospheric refraction has revealed a frequency dependence allowing the required correction when three frequencies are received simultaneously. A ray‐tracing program on a digital computer is used to simulate satellite passes, and the resultant errors in two‐frequency and three‐frequency data are presented and compared. The three‐frequency Doppler technique is shown to effectively reduce residual ionospheric refraction errors to negligible amounts for the conditions considered.
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