INITIAL BACKSCATTER OCCURRENCE STATISTICS FROM THE CUTLASS HF RADARS

A statistical study of the occurrence of ground and ionospheric backsc atter within the fields-of-view of the CUTLASS HF radars, at an operat ing frequency of 10 MHz, during the first 20 months of operation has b een undertaken. The diurnal variation of the occurrence of backscatter and the range at which such backscatter is observed is found to be hi ghly dependent on seasonal changes of the ionospheric electron density in both the E and F region, determined from ionosonde observations. I n general, ionospheric backscatter is observed at far ranges during th e local day in winter months and at near ranges during the local night in summer months. The Iceland radar observes more near-range E region backscatter than the Finland radar as a consequence of its more zonal look-direction. The dependence of the occurrence of backscatter on ge omagnetic activity and radar operating frequency are also investigated . The occurrence of ground and ionospheric backscatter is discussed in terms of HF propagation modes and ionospheric electron densities as w ell as geophysical processes. A brief assessment of the possible impac t of solar cycle variations on the observations is made and frequency management is discussed. Such a study, with its focus on the 'instrume ntal' aspect of backscatter occurrence, is essential for a full interp retation of HF coherent radar observations.