GB862059A - Improvements in or relating to radar systems - Google Patents

Abstract

862,059. Radar. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Aug. 24, 1959 [May 27, 1958], No. 16925/58. Class 40(7) In a frequency-modulated C W Doppler radar the beat signal obtained by heterodyning the received signal with a portion of the transmitted signal comprises components of frequency nf 1 Œd, Fig. 1 (not shown), where f 1 is the modulation frequency, d is the Doppler shift frequency and n is any integer, the amplitude of the nth order component being given by J n (2# sin f 1 T) where J n is a Bessel function of the first kind of order n and argument 2# sin #f 1 T where #is the modulation index and T is the echo time. As illustrated in the Figure accompanying the Complete Specification (not shown), the coefficients J 0 , J 1 ... vary with T (and hence with range) in different ways and this property is utilised in the present invention to determine range. As described with reference to Fig. 3 and Fig. 2 (not shown), two beat signal components of order n and n+1 are selected and converted into derived Doppler signals each of frequency d, the amplitude ratio of the derived Doppler signals as determined in a comparator being indicative of range; alternatively some parameter of the system which affects the relative amplitudes of the derived Doppler signals, e.g. the modulation frequency f 1 or the modulation index #, is adjusted until these signals are of equal amplitude, the magnitude of said adjustment being indicative of range. The invention is described as applied to a radar mounted on a helicopter for determining the height and vertical velocity of the helicopter, the aerials being directed vertically downwards; alternatively the system may comprise part of a conventional F M Doppler airborne navigation system. As shown in Fig. 3 the transmitter oscillator 1 is frequency modulated by an oscillator 6 of frequency f 1 and the signals applied to and received from a single aerial 3 via a duplexer 2 are heterodyned in a mixer 7 whose output is applied to a wideband amplifier 8. Frequency components (f 1 Œd) and (2f 1 Œd) of amplitude J 1 and J 2 respectively are selected by filters 9 and 10 and heterodyned in mixers 11 and 12 with inputs f 1 and 2f 1 derived from the modulation oscillator 6 to give signals of frequency d which are rectified at 14 and 15, converted to 400 c/s A.C. in chopper modulators 16 and 17 and applied to the stators 21, 22 of a resolver 20 whose rotor output feeds a servo amplifier 28 and motor 26 which adjusts the angular position # of the rotor 23 such that tan #=<SP>J1</SP>/J 2 . The velocity is indicated on a meter 35, the magnitude of the displayed velocity being controlled by a frequency meter 34 coupled to the output of the mixer 12 and the sense of the displayed velocity being controlled by the output from a phase-sensitive rectifier 31 having as inputs a 400 c/s reference source 32 and the 400 c/s output from a tacho-generator 29 coupled to the servo motor 26; the latter output is also applied to the servo amplifier 28 to stabilise the servo loop. The inputs to mixers 11 and 12 are coherent and it is shown that because of this the relative phase # of the two derived Doppler signals must not be zero and should preferably be 90 degrees; this is achieved by adjusting the relative phase of the input reference signals. In the mode of operation in which a system parameter is adjusted to equalise the amplitudes of the derived Doppler signals, # may constitute said adjustable parameter. Fig. 2 (not shown) utilises amplitude comparison of the J 0 and J 1 components; in this case the output of the J 0 filter is of frequency d and a second heterodyne stage is not required but separate transmitting and receiving aerials must be used to reduce excessive coupling between the transmitter and receiver. The J 0 component is more susceptible to short range spurious signals and such effects may be reduced by using higher order components as in Fig. 3 but this reduces the maximum range that can be determined without ambiguity. The Specification includes a mathematical discussion and numerical values of optimum system parameters are given for different ranges of operation; in general the modulation frequency f 1 must be greater than twice the maximum Doppler frequency and must be less than <SP>1</SP>/T.