Abstract: This Doppler radar for the detection and localization of helicopters through blade flashes works in a band located between 300 MHz and 20 GHz and identifies the blade flashes by the width of their frequency spectrum and their duration. It uses, at reception, a multilobe antenna associated with several parallel signal processing chains specialized in the detection of helicopters, enabling detection over a widened zone as well as precise localization. Each chain has a phase amplitude detector PAD (10, 10') followed by a Doppler filter MTI (20, 20') selecting the wide frequency spectrum, a module computing circuit (30, 30'), a contrast circuit (40, 40') eliminating excessively long echoes, a threshold circuit (50, 50') and a false alarm reduction circuit FAR (60, 60') eliminating excessively short echoes. The chains lead to a relative bearing computation circuit (100) performing angle measurements when this is possible.

Abstract: In a doppler radar, in order to compensate for the speed of moving clutter, one first establishes during a burst of order (i) the phase variation .phi.r.sub.i from one period to the next and which is due to clutter in a given range cell. Then, this phase variation .phi.r.sub.i is memorized and is used during the course of each of the periods of repetition of the next burst to modify the phase of the signal received from the clutter from the same range cell in a maner as to displace its Doppler frequency towards the zero frequencies band, and band which corresponds to the rejection zone of the fixed echoes eliminating filter.

Abstract: In a moving-target indicator in which the pulse-repetition period of an associated Doppler radar is subject to variation, incoming echo pulses appearing in the output of an i-f stage of the radar receiver are demodulated with the aid of two coherent reference signals in relative quadrature and the results are digitized for transmission over two parallel channels. In each channel, digitized signals corresponding to two consecutive echoes in a given range cell are subtracted from each other to provide a pair of difference signals which are passed through a weighting device for multiplication by factors which are trigonometric functions of a selected Doppler frequency and of the difference between the actual repetition period and a theoretical constant value of that period. The weighted signals are subjected to further filtering before being utilized for further processing or display.