Traveling Ionospheric Disturbance Mitigation for OTH Radar

Deflection of HF propagation paths by traveling ionospheric disturbances (TIDs) remains a troubling cause of coordinate registration errors for over-the-horizon (OTH) radar. Bandwidth and coverage limitations in ionospheric soundings preclude the ability to model TID structure in real time in most cases. It would be useful if TID-induced ray path deflections could be related to radar-measurable quantities like surface-clutter Doppler shift. In studying this possibility, we have considered the relationship between Doppler shift and range rate for point-to-point HF propagation paths in TID environments. The nature of range-rate/Doppler correlation is exposed in three ways: 1) simple theoretical modeling, 2) ray tracing in simulated TID environments, and 3) analysis of OTH radar measurements of a fixed beacon. It is shown that range rate and Doppler shift for fixed-endpoint propagation paths are usually proportional with a ratio that depends on whether ionospheric motion or density changes predominate in the TID environment. A mutual regression prediction of TID-induced range and azimuth ray path deviations based on ionospheric-induced Doppler shifts (as measured by surface backscatter returns) is shown to be an effective way of mitigating apparent wander of OTH targets due to TIDs.