Measurement, analysis and processing of VHF ground clutter

Abstract

Radar echo measurements of stationary ground targets (clutter), illuminated at grazing incidence, have been conducted with LARISSA at its location in Wachtberg. The measurements were conducted with horizontal polarisation and the frequency used in the LARISSA system was upper VHF. The antenna is a 4-element array that can be directed to an arbitrary azimuth direction for data acquisition. It provides an antenna beamwidth of 20/spl deg/. A range resolution of 78 meters is achieved. The objective of the measurements was to provide a database for the development of clutter cancellation procedures for electronic scanning low frequency radars to detect low level air targets. Rather than measure precisely the reflectivity of different types of ground or vegetation, the intention was to acquire knowledge about the dynamic range and statistical properties of low frequency clutter at grazing incidence. The clutter measurements were analysed with respect to their spatial distribution and more important to their temporal fluctuation and coherence. The terrain structure varied widely from forested rolling hills to flat country fields and orchards. Coherency analysis demonstrated a high phase stability of consecutive pulses, containing clutter returns from one resolution cell. First, the spatial distribution of clutter areas is predicted and measured and the burst to burst fluctuation is analysed to obtain an overview of the temporal clutter stability. An in-depth analysis of the pulse to pulse clutter statistics and time coherence leads to a proposed coherent clutter map processor to remove clutter energy from the received signal.