Direction of arrival measurements of oblique chirp sounder signals at a high latitude site

The performance of high frequency direction finding systems is known to be related to the mode content of the incoming signal and to the frequency of operation. Furthermore, HF propagation via the high latitude ionosphere is often disturbed with, for example, the signal arriving at the receiver over paths well displaced from the great circle. Observations at Alert, located high in the Canadian Arctic, indicate that large off great circle bearings on fixed frequency signals are associated with well defined features on oblique ionograms taken over the paths of interest. In order to investigate further these phenomena, a system has been developed which can measure the bearings of signals radiated by the worldwide network of BR chirp sounder transmitters across the full range of propagating frequencies. Preliminary tests of the system were undertaken in Ottawa during August 1991 and at Cheltenham during January 1992 with further measurements at Alert during April 1992