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The demonstration by Bowles of the radar detection of incoherent radio-wave scattering from free electrons in the ionosphere has shown the possibility of measuring from the earth's surface the electron density the upper side of the F-layer. The distribution in volume and the relative motion of the scatterers gives rise to a range-distributed and Doppler-spread echo on a radar. The choice of the best radar system for the detection of such targets is complex. The paper analyses the response of f.m. and pulse radars to volume-distributed targets, both with stationary and with relatively moving scatterers. The equipment parameters necessary to ensure the best signal/noise ratio are found for the two radar systems, and their merits are compared. The relative performance for a given range-resolution and post-detector integration is shown to depend on the ratio of maximum peak to maximum mean power for the radar transmitter. As an example, the relative performance of particular 90Mc/s f.m. and 400Mc/s pulse systems are compared. The signal/noise performance is similar, but the pulse system is shown to have advantages which render it more attractive.