Moving-Target Indication

(MTI), a method of discriminating radio signals reflected from a target against a background of passive interference (seeRADAR COUNTERMEASURES). The method, which is based on the use of a difference in the speeds of the target and the source of the interference, is employed in coherent-pulse radar sets (see alsoRADAR). When both a useful signal and interference signals are received by an MTI system, the difference in speeds makes it possible to cancel or suppress the interference and preserve the useful signal at the output.

MTI systems are based on the use of memory units. The memory may be an ultrasonic delay line, a visual storage tube, or a shift register. In a very simple system, a difference between signals received in consecutive pulse repetition periods is obtained at the output by means of, for example, an ultrasonic delay line and a subtractor. The subtraction may be carried out at either an intermediate frequency or a video frequency. If the signals are subtracted at a video frequency, phase detection of the signals is used before subtraction. In this case, the signal level at the detector output depends on the phase of a signal with respect to reference oscillations, the phase of which is fixed relative to the phase of the scanning pulses.

Target signals do not appear at the subtractor output, or are canceled, if the target is stationary—that is, is a source of passive interference—or if it moves at a blind speed—that is, moves a distance equal to an integral number of half-waves toward or away from the radar set in a pulse repetition period. However, signals from a target that moves at a speed other than a blind speed are not canceled and will be detected. If the source of the interference is moving, the interference signals may be canceled by introducing the required additional phase shifts into the MTI system.

Besides the blind-speed effect, the factors that reduce the effectiveness of video-frequency MTI include unwanted pulse-amplitude modulation at the system’s output. In this case, the modulation frequency is equal to the Doppler frequency if the Doppler frequency is less than half the repetition rate of the scanning pulses. Such modulation is eliminated by means of two quadrature networks with reference oscillations that differ in phase π/2. The blind-speed effect is controlled by using a variable scanning-pulse repetition period, that is, by wobbulation of the scanning-pulse repetition rate.

The effectiveness of MTI, which is evaluated from the improvement of the signal-to-interference ratio, depends on the speed of the target and the spectrum of the interference. For narrow interference spectra, the effectiveness may be raised by using several interference cancellation stages. An even higher effectiveness is provided by systems that combine multiple subtraction and weighted signal processing.