Discovery of persistent ionospheric frequency shifts of a few Herz and impact on time and frequency transfer

Abstract

Unexpected frequency shifts and occasional splitting of short-wave Standard Frequency Signals into components with persistent frequency shifts of up to a Herz or so have been `discovered' using SDR receivers with milliHerz resolution. Such spurious signals have also been observed on AM carrier frequencies and DRM signals between ~1MHz and 25MHz. The frequency shifts are in general downwards like `Hubble' shifts but at times can be upwards, depending on the time of day and direction of the path around the earth. Shifted components in general correlate with the existence of an ionospheric propagation path. At some times an unwanted frequency shifted component can be stronger than the original carrier signal and then the Standard Frequency Signal cannot be used as an accurate reference frequency. There are also problems with inaccurate pirate standard frequency signal transmissions on some frequencies. Small short term, diurnal and seasonal frequency variations are being investigated as a possible method of detecting gravity waves.