Radio Frequency Interference Suppression Techniques in FMCW Modulated HF Radars

High-frequency (HF) radars are operated in the 3-30 MHz frequency range and need to share the frequency bands with other radio services. Due to their over-the-horizon (OTH) capabilities, HF radars play an important role in remote sensing and surveillance. The propagation conditions of the electromagnetic wave depend on the earth´s ionosphere and mailnly follow a daily cycle. Communication paths between the HF radar and other radio services, some thousands of kilometres off, open and close with a high variability. Special care must be taken to dynamically adapt the HF radar´s characteristics to the varying electromagnetic environment. The impact of a frequency modulated continuous wave (FMCW) HF radar on other radio services is not very strong, because of its low transmit power and utilisation of the radio spectrum. However, strong signals from other radio services can significantly reduce the performance of the oceanographic measurements. Several radar control and signal processing steps are discussed in this paper. All together form an effective procedure to reduce the impact of Radio Frequency Interference (RFI) on the oceanographic measurements.