GB1114531A - Improvements relating to radar - Google Patents

Abstract

1,114,531. Radar. ELECTRIC & MUSICAL INDUSTRIES Ltd. 11 May, 196 5 [12 Feb., 1964], No. 5992/64. Heading H4D. Radar for detecting a target (especially a breathing tube on a submarine) in the midst of clutter comprises a circuit for a received signal such as to produce an output signal only if the received signal exceeds a threshold value, the latter being so controlled that the circuit produces output signals in response to clutter at a predetermined average rate despite variation in the average amplitude of clutter in the received signal, and threshold values each relating to a respective group of range sectors are stored. In one of the digital systems described pulse radar echo signals are applied via a multiplier 4, Fig. 1, and an amplitude quantizer 5 affording binary digital output signals for application to a store 6 having 211 addresses corresponding to respective range sectors (e.g. each of 25 feet); at each address there are several storage elements corresponding to respective transmitted pulses. The addresses are read in sequence under the control of a unit 11 and the number of binary ones in an address is counted and converted to binary code by a unit 8 the output from which is added in unit 9 to the total of the particular range sector stored in the " total store " 10. Unit 11 also controls the reading from store 10 in synchronism with the range clock 7 and for each address the total is passed via a gate 12 to a unit 13 where a constant quantity K is subtracted therefrom. Unit 11 further selects addresses in a store 14 of threshold signal values for groups of range sectors, those values being up-dated by the addition of a portion of the stored value multiplied by the output of unit 13 in a multiplier 16. The output signal from store 14 is applied to a digital-to-analogue converter 18 to produce via a divider 19 an analogue threshold signal for the echoes received from the range sectors of the respective group. A reduced threshold signal is applied to multiplier 4. Separate threshold values may be stored for the different range sectors and an averaging circuit connected to converter 18. The full value of the analogue threshold signal is applied to a further multiplier 20, connected to a quantizer 21 and a store 22 similar to store 6; each store 25 is similar to store 10 but sums samples over a different period (e.g. 1 see., ¢ sec.,“ sec. . . .) for optimizing the integration period for a target which is occasionally obscured. Processing circuits 26 compare the accumulated values in stores 25 with different threshold values dependent on the summation periods and pass output signals to a display for indicating target presence. In summary, components 4 to 19 form a loop for determining a threshold value such as to produce a false alarm rate (F.A.R.) of e.g. 20% and, since such a F.A.R. would be impractical, in the " main " circuit coupled to the display a larger threshold value is employed so as to produce e.g. one false alarm per second. Tn an alternative embodiment, Fig. 2, the threshold value is established from the main circuit. Storage tubes or ferromagnetic cores may be employed; all the circuit may be of analogue form; and the data may be weighted in accordance with the azimuthal radiation pattern. The Specification includes a theoretical discussion of integration times and a scanning radar (all-round or sideways-sector) carried by an aircraft is considered. Mention is made of separating the breathing tube from " clump " clutter: the circuit output is fed to a groundstabilized direct-view storage tube and the aerial is made to scan all around; after a few minutes the ensuing picture will indicate slowly drifting clump clutter patches while the breathing tube (occasionally screened) will have moved further and generally in a different direction.