Moving target indication (MTI) is a mode of operation of a radar to discriminate a target against the clutter. It describes a variety of techniques used for finding moving objects, like an aircraft, and filter out unmoving ones, like hills or trees. It contrasts with the modern stationary target indication (STI) technique, which uses details of the signal to directly determine the mechanical properties of the reflecting objects and thereby find targets whether they are moving or not. Early MTI systems generally used an acoustic delay line to store a single pulse of the received signal for exactly the time between broadcasts (the pulse repetition frequency). This stored pulse will be sent to the display along with the next received pulse. The result was that the signal from any objects that did not move mixed with the stored signal and became muted out. Only signals that changed, because they moved, remained on the display. These were subject to a wide variety of noise effects that made them useful only for strong signals, generally for aircraft or ship detection. The introduction of phase-coherent klystron transmitters, as opposed to the incoherent cavity magnetron used on earlier radars, led to the introduction of a new MTI technique. In these systems, the signal was not fed directly to the display, but first fed into a phase detector. Stationary objects did not change the phase from pulse to pulse, but moving objects did. By storing the phase signal, instead of the original analog signal, or video, and comparing the stored and current signal for changes in phase, the moving targets are revealed. This technique is far more resistant to noise, and can easily be tuned to select different velocity thresholds to filter out different types of motion. Phase coherent signals also allowed for the direct measurement of velocity via the Doppler shift of a single received signal. This can be fed into a bandpass filter to filter out any part of the return signal that does not show a frequency shift, thereby directly extracting the moving targets. This became common in the 1970s and especially the 1980s. Modern radars generally perform all of these MTI techniques as part of a wider suite of signal processing being carried out by digital signal processors. MTI may be specialized in terms of the type of clutter and environment: airborne MTI (AMTI), ground MTI (GMTI), etc., or may be combined mode: stationary and moving target indication (SMTI).
Operation
The MTI radar uses low pulse repetition frequency (PRF) to avoid range ambiguities. Moving target indicator (MTI) begins with sampling two successive pulses. Sampling begins immediately after the radar transmit pulse ends. The sampling continues until the next transmit pulse begins. Sampling is repeated in the same location for the next transmit pulse, and the sample taken (at the same distance) with the first pulse is rotated 180 degrees and added to the second sample. This is called destructive interference. If an object is moving in the location corresponding to both samples, then the signal reflected from the object will survive this process because of constructive interference. If all objects are stationary, the two samples will cancel out and very little signal will remain. High-power microwave devices, like crossed-field amplifier, are not phase-stable. The phase of each transmit pulse is different from the previous and future transmit pulses. This phenomenon is called phase jitter. In order for MTI to work, the initial phase of both transmit pulses must be sampled and the 180 degree phase rotation must be adjusted to achieve signal cancellation on stationary objects. A secondary influence is that phase rotation is induced by Doppler, and that creates blind velocities. For example, an object moving at 75 m/s (170 mile/hour) will produce 180 degree phase shift each 1 millisecond at L band.
Doppler = 180 , 000 ∘ / s = 720 ( 75 × 10 9 3 × 10 8 ) = 720 ( Velocity × Transmit Frequency C ) {\displaystyle {\text{Doppler}}=180,000^{\circ }/{\text{s}}=720\left({\frac {75\times 10^{9}}{3\times 10^{8}}}\right)=720\left({\frac {{\text{Velocity}}\times {\text{Transmit Frequency}}}{C}}\right)}
If the pulse repetition interval is 0.002 s between transmit pulses, then the MTI process will produce 360 ∘ {\displaystyle 360^{\circ }} phase rotation. That is the same as a stationary object, which renders the system blind to objects traveling at this radial velocity. MTI requires 3 or 4 pulses to reduce the effect of blind velocities. Multi-pulse strategies use staggered pulses with irregular pulse repetition intervals to prevent signal cancellation on moving objects. The summation process is slightly different so as to accommodate the additional samples. Phase jitter, Doppler effects, and environmental influences limit MTI sub-clutter visibility Measure of Performance to about 25 dB improvement. This allows moving objects about 300 times smaller to be detected in close proximity to larger stationary objects. Pulse-Doppler signal processing is required to achieve greater sub-clutter visibility.
… excerpt ends here. Continue reading the full article.
