Ground Clutter Cancellation in Incoherent Radars: Solutions for Eiscat Svalbard Radar

Incoherent scatter radars measure ionosphere parameters using modi®ed Thomson scatter from free electrons in the target (see e.g. Hagfors, 1997). The integrated cross section of the ionospheric scatterers is extremely small and the measurements can easily be disturbed by signals returned by unwanted targets. Ground clutter signals, entering via the antenna side lobes, can render measurements at the nearest target ranges totally impossible. The EISCAT Svalbard Radar (ESR), which started measurements in 1996, su€ers from severe ground clutter and the ionosphere cannot be measured in any simple manner at ranges less than about 120±150 km, depending on the modulation employed. If the target and clutter signals have di€erent, and clearly identi®able, properties then, in principle, there are always ways to eliminate the clutter. In incoherent scatter measurements, di€erences in the coherence times of the wanted and unwanted signals can be used for clutter cancellation. The clutter cancellation must be applied to all modulations, usually alternating codes in modern experiments, used for shorter ranges. Excellent results have been obtained at the ESR using a simple pulse-to-pulse clutter subtraction method, but there are also other possibilities.