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Echoes that do not change in distance from the antenna (relative speed zero) of a ground radar with a Moving Target Indicator (MTI) are dangerous for the following reason
  • A
    The radar is not able to display two echoes moving in the same direction.
  • B
    The MTI does not recognise this as a moving object due to the blind speed.
  • C
    The frequency of the transmitted radar signals changes too much by the Doppler effect.
  • D
    The moving target indicator (MTI) eliminates such echoes.

Moving target indication (MTI) is a mode of operation of a radar to discriminate a target against the clutter. It describes techniques used to find moving objects, like an aircraft, and filter out unmoving ones, like hills or trees.

For a radar system, clutter refers to the received echoes from environmental scatters other than targets, such as land, sea or rain. Clutter echoes can be many orders of magnitude larger than target echoes. An MTI radar exploits the Doppler frequency shift of moving targets to allow those returns to show up on the display, whilst removing the returns which do not have any apparent frequency shift due to the doppler effect.

The doppler effect increases the frequency of a return when the object is travelling towards the radar, and decreases the frequency of a return from an object going away from the radar, the same as the doppler frequency in sound waves when a loud car or ambulance passes by, a high pitched sound coming towards you, and a lower pitch travelling away.

One of the problems with using this for MTI is that targets flying perpendicular to the radar beam will not have any apparent doppler effect, so may be seen as "clutter" by the radar and removed, even though they are moving.

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