Large HF bearing errors for propagation paths tangential to auroral oval

Large electron density gradients in the ionosphere associated with the walls of the midlatitude trough produce tilted reflecting surfaces which lead to HF signals propagating over paths well displaced from the great circle. Measurements are presented of signals propagating over two paths along the midlatitude trough for which bearings deviations of up to 100° from the great circle path, i.e. the expected direction of arrival, were frequently recorded. The times of occurrence and magnitude of these large deviations are consistent with the known features of the midlatitude ionospheric trough. Such large deviations of the measured bearing from the great circle path, which far exceed the instrumental errors, have serious implications for the operation of position location systems operating within the HF band