Proceedings with Extended Abstracts (single PDF file) - Radio ...

esulting in a widening of the bandwidth, this is still much narrower than the incoherent scattersignal bandwidth.Fig. 1 shows the ground-clutter profile of the EISCAT Svalbard Radar (ESR), which suffersparticularly due to its elevated location. This radar uses incoherent scatter for ionospheric studies.An incoherent scatter spectrum is shown in Fig. 2. For simplicity we chose here the Lorentziantypespectrum of scatter from the D-region. The power in the zero-frequency bin is stronglyenhanced in Fig. 2, which indicates a strong ground clutter component. The same feature alsoapplies for a typical double-humped incoherent scatter spectrum of a signal from the E- or F-region. The typical signal bandwidth of scatter from these altitudes is about 5-10 kHz, and themodulation does at most allow a frequency resolution better than a few hundred Hz (correspondingto a long pulse or coded pulse sequence of 1 ms duration). Typically, the total spectrumwidth can be up to ±25 kHz, corresponding to a code-baud length of 20 µs.The typical bandwidth of ground clutter is not exceeding a few Hz, even if the clutter from longranges is fading. The resolution bandwidth in the ISR applications is in the order of some 100 Hz(or less in the D-region applications), which means that in case of ground-clutter, the entire zerofrequencybin in the signal spectrum is spoiled. Since the signal covers usually covers 10-20frequency bins, this zero-frequency falsification has a substantial effect on the total power and onthe shape of the total spectrum, i.e. on the relevance of the analysed data (mostly the analysiswould not converge in such cases).One method to remove this ground clutter in the zero-frequency bin is to apply a spline interpolationusing the power in the adjacent non-zero frequency bins. However, this is a fairly crudemethod, which still leaves a significant error, particularly when the signal spectrum is even narrowerthan the assumed 10 kHz, i.e. in lower E-region or E-region observations. There are moreelegant and methods, which result in much smaller errors and which we will describe now.Weightingfunctionl = number of range gatesm = number of code sequencesn = number of interpulse periodsFig. 3 Block diagram of a hardware DC-clutter filter379