Abstract. A simple, two-dimensional ray tracing scheme is developed and is applied to several propagation problems of importance to over-the-horizon radar. In particular, it is used as the basis of schemes for the simulation of oblique and backscatter ionograms. The efficiency of the scheme makes it ideal for applications where fast calculations are required, and an algorithm for inverting backscatter ionogram leading edges is developed as a particular example. IntroductionOver-the-horizon radar (OTHR) is made possible which, in the case of sky wave radar, consist of ionospheric refraction. Besides the obvious need to model propagation for the purpose of OTHR performance predictions, there is a need for such modeling to assist in the real-time operation of these radars. In particular, propagation modeling is required to fulfill the coordinate registration (CR) requirement. CR is the process whereby radar coordinates (propagation time and beam azimuth) are transformed into geographic coordinates (longitude and latitude). There is no simple method for translating between these coordinates, and the necessary relationships can only be found by characterizing propagation over the radar coverage. Such a characterization is normally achieved by building an ionospheric map from the available ionosonde (vertical, oblique, and backscatter) observations and then ray tracing through this map. In order to satisfy the real-time requirements of the radar, the ray tracing and ionogram inversion must be extremely fast. Section 2 of this paper describes an efficient ray tracing scheme that can help fulfill much of this requirement. Although OTHR functions such as frequency management can make direct use of the ionograms provided by ionospheric sounders, activities such as CR require their inversion. There exist well-developed techniques for the inversion of vertical ionograms [Davies, 1990], and these will not be considered any Copyright 1998 by the American Geophysical Union. Paper number 98RS01523.0048-6604/98/98 RS-01523511.00 further in this paper. In the case of oblique and backscatter ionograms, however, the process of inversion is far less well developed. Obviously, an ability to simulate these ionograms is an important first step toward their inversion, and this will rely on a ray tracing algorithm of the sort described in the present paper. Oblique ionosondes measure the time of flight (normally expressed in terms of group range) and signal strength of radio waves propagating between two fixed points. Such measurements are performed for a range of frequencies and result in an oblique ionogram (a plot of signal strength as a function of time and frequency). Section 3 of this paper develops an algorithm for the simulation of oblique ionograms and demonstrates its effectiveness through simulations for which there existed observed ionograms and an independent ionospheric characterization. Backscatter ionosondes provide a means of investigating the ionosphere at locations that are inaccessible to vertical and oblique sounding...
[1] While the Haselgrove ray tracing equations are well suited to situations where the ray launch direction is known, they are less effective for situations where only the end points of the ray are known. In such cases, many rays must be traced from the launch point in order to home in on the landing point. An alternative approach is to directly solve the variational principle from which the Haselgrove equations are derived. Such an approach is well suited to the point-to-point ray tracing, but poses several technical difficulties. In this paper we overcome these difficulties and show that a direct approach can indeed provide an effective means of point-to-point ray tracing.Citation: Coleman, C. J. (2011), Point-to-point ionospheric ray tracing by a direct variational method, Radio Sci., 46, RS5016,
Ionospheric scintillation originates from the scattering of electromagnetic waves through spatial gradients in the plasma density distribution, drifting across a given propagation direction. Ionospheric scintillation represents a disruptive manifestation of adverse space weather conditions through degradation of the reliability and continuity of satellite telecommunication and navigation systems and services (e.g., European Geostationary Navigation Overlay Service, EGNOS). The purpose of the experiment presented here was to determine the contribution of auroral ionization structures to GPS scintillation. European Incoherent Scatter (EISCAT) measurements were obtained along the same line of sight of a given GPS satellite observed from Tromso and followed by means of the EISCAT UHF radar to causally identify plasma structures that give rise to scintillation on the co‐aligned GPS radio link. Large‐scale structures associated with the poleward edge of the ionospheric trough, with auroral arcs in the nightside auroral oval and with particle precipitation at the onset of a substorm were indeed identified as responsible for enhanced phase scintillation at L band. For the first time it was observed that the observed large‐scale structures did not cascade into smaller‐scale structures, leading to enhanced phase scintillation without amplitude scintillation. More measurements and theory are necessary to understand the mechanism responsible for the inhibition of large‐scale to small‐scale energy cascade and to reproduce the observations. This aspect is fundamental to model the scattering of radio waves propagating through these ionization structures. New insights from this experiment allow a better characterization of the impact that space weather can have on satellite telecommunications and navigation services.
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