Continuous-Wave Radar with High Range Resolution and Unambiguous Velocity Determination

The conventional pulse radar has two shortcomings. First, since the pulse width and pulse repetition frequency are constrained by resolution and maximum range requirements, the average transmitter power can be increased only by increasing the peak transmitter power. Second, the limitation imposed by the sampling theorem prevents unambiguous measurement of Doppler frequencies higher than one half the pulse repetition frequency. These disadvantages can be circumvented by operating with continuous transmission, suitably modulating the transmitter power and correlating returned echoes with a delayed replica of the modulating signal. An excellent waveform for modulating the transmitter is the pseudo-random code generated by a shift register with multiple feedback paths. The autocorrelation function of the code has a single narrow peak each code length and low sidelobes. The code is easy to generate. Long delays are possible, since it is necessary only to memorize the code generation logic and not the code itself. The binary nature of the code makes it easy to perform such operations as multiplication. Realization of the desirable properties of the pseudo-random code depends on the manner in which the radio frequency carrier is modulated and demodulated. It is shown that phase-reversal modulation results in little distortion of the code, while frequency modulation can give rise to false range indications.