Patent GB-1433356-A (GB)
Title: Marine radar transmission and reception system
IPC: H01S3/22
Filed: 1973-05-10
Abstract: 1433356 Radar TOKYO KEIKI KK 10 May 1973 [10 May 1972] 22333/73 Heading H4D In a marine pulse radar, echoes containing undesirably high proportion of sea clutter are rejected by transmitting the pulses in two different modes, e.g. vertical and horizontal polarization, high and low carrier frequency, narrow and broad pulse width and narrow and wide beam widths, such that if:-# 01 = radar X section of target at first mode, # 02 =radar X section of target at second mode, # c1 =radar X section of sea element at first mode, # c2 =radar X section of sea element at second mode, then:- In practice the signal strengths S 01 , S 02 , S c1 , S c2  of the target and sea echoes are used instead of the X sections (a constant of proportionality being involved) and the total echo signal strengths at the two modes, i.e. (S 01 +S c1 ) and (S 02  + S c2 ) compared in such a manner that when the ratio α 0c , (α 0 =ratio of signals from target at two modes, α c =ratio of signals from sea at two modes) becomes higher than a predetermined value α s , indicating a too-large content of sea clutter, the echo signals are supressed. In Fig. 2, equation (1) is shown with α 0 =1, α c =5, α s =4À64. In the arrangement of Fig. 1, the radar pulses from transmitter 1A0 are fed to vertical and horizontal polarized antennae 5A1, 5A2 via distributer 3 and circulators 401, 402. The echo signals EA1, EA2 after, convertion to IF as FA1, FA2, are summed at 7 and fed to gate 8. A divider 9 forms the ratio α 0c  which is compared with the threshold α s  at 10 to give an inhibiting signal for the gate 8 when α s  is reached. The high and lower carrier frequency modes embodiment, Fig. 3 (not shown), uses two transmitters and the same receiver/analysis circuitry as Fig. 1. The narrow and broad pulse width, Fig. 4 (not shown), uses two pulse modulators alternately applied to the transmitter, with the two interlaced trains of corresponding echoes, after synchronous separation using a switching control in synchronism with that of the modulation, and at half the standard repetition frequency, being converted into simultaneous trains of echoes at said standard frequency by means of 1 p.r.p. delay and direct path combining circuits. The two resulting signals are then treated/analysed in divider 9 &e. as in Fig. 1. The narrow and wide beam widths embodiment, Fig. 5 (not shown) uses the same receiver/processing circuitry as in Fig. 4. The pulses from the transmitter being however alternately applied to narrow and wide beam antennae. The foregoing analysis and embodiments have assumed zero phase difference between the echoes from the target and the sea. If in fact:-  (this reduces to equation (1) for #=0). Curves for α 0c  verses (S 02 /S c2 ) for #=0, 90, 120, 180 degrees, are shown in Fig. 6. Assuming that # will not go beyond 120 degrees, a second threshold detecting and gating circuit, i.e. 10, 11, 8 of Fig. 1 will be needed in parallel with the first and operating on a different threshold, e.g. α s '=5À5. Embodiments using the two threshold detecting and gating circuits in parallel and corresponding to those of Figs. 1, 3, 4 and 5, are described.