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.