The importance of elevation angle measurements in HF radar investigations of the ionosphere

We evaluate the performance of three methods for determining the ground range and refractive index of ionospheric scattering volumes. Each method uses the same equivalent path analysis with the latter parameter derived through the additional use of Snell's law. Two of these methods make their predictions using the group range and a virtual height model for each scattering volume; the third method uses the group range and the elevation angle of the ray. The effectiveness of each of these methods is evaluated using ray tracing through a simulated reference ionosphere. Ray tracings provide determinations of the initial elevation angle and group range of the transmitted signal and the ground range and refractive index of the scattering volume. The first pair of parameters is used as inputs to the geolocation methods, and the second pair becomes part of a data set against which the predictions of the geolocation methods are evaluated. We find that the geolocation methods using virtual height models change the initial elevation angle to a value that is consistent with their virtual height model but inconsistent with ray tracing. Consequently, predictions of ground range and refractive index from methods using virtual height models are rarely consistent with predictions from ray tracing. In contrast, the third method uses the initial elevation angle from ray tracing and yields accurate predictions in the bottomside ionosphere. Two modifications were made to this method to extend the agreement between it and ray tracing to all backscattered signals.