An X-band radar with back-to-back horizontally and vertically polarized antennas of equal gain and beamwidths was built. The antennas were enclosed in a symmetrical aerodynamic radome permitting speeds of rotation from 20 rev/min to 600 rev/min. The radar power was switched every half revolution from one antenna to the other, so that a sea-clutter picture presented on the top 180° of the p.p.i. display was identically repeated on the lower 180°, but with the orthogonal polarization. Some of the different characteristics of sea-clutter on the two polarizations were strikingly illustrated by this virtually simultaneous presentation. The sea-clutter smoothing effects obtained by decorrelation and integration are demonstrated. The r.m.s. sea-clutter amplitude was brought down to that of the noise by a true i.f. (amplifying logarithmically at the input frequency) type of constant false alarm rate (c.f.a.r.) receiver developed during this work. Recent work on logarithmic receivers has shown that the output noise fluctuations do not always become constant and independent of input r.m.s. noise amplitude, even though a near perfect logarithmic input-output law may be obtained on pulse or continuous wave (c.w.) inputs. The performance of the true i.f. logarithmic amplifier has been investigated using a wide-band noise source (the output of which may be varied through 50 dB) to feed the amplifier. The conditions which must be observed if the output noise fluctuations are to remain constant while the input noise varies over this range have been established. When these conditions are met there is no significant impairment of detectability of weak signals against noise when using a logarithmic receiver, compared with a linear receiver.