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The Moving Target Indicator (MTI) is part of the equipment set of radar systems; it is used for filtering out stationary objects like buildings or vegetation from radar displays so that they do not obscure, or distract from, moving targets (aircraft). Such things are also called 'permanent echoes', or more colloquially 'ground clutter'.

Generally, on an air traffic controller's radar screen the only things visible will be blips which correspond to aircraft, and perhaps some lines denoting airspace boundaries. Notwithstanding UFOs or Giant Flying Octopi (although UFOs and GFOs all look the same to air traffic controllers). Controllers cannot, for example, see if there is a huge cumulonimbus cloud directly in the path of an aircraft they're controlling. Thankfully, weather radar, not to mention the mk.1 eyeball (minimum one pair included), is a common fit in the instrument set of commercial airliners. Controllers rely on the pilots under their control, as well as weather reports from certified observers, to inform them about weather that may affect their aircraft, so that they may make the appropriate awesome adjustments.

Observe.

The MTI is unlikely to be a 'box' in modern radar systems, but rather part of the display software. It is certainly possible to alter the sensitivity of the MTI so that, for example, weather is visible. How easy this is probably varies from one radar system to another.

One particular intricacy of the MTI cropped up during my ATC training when I was being taught about military procedures for crossing controlled airspace. Neither I nor most of my colleagues had considered this aspect: as an interesting diversion from the usual death-by-powerpoint nonsense we were shown a recording of a military aircraft approaching some controlled airspace, which spanned a vertical section of airspace from FL45 (flight level 45 - about 4,500 feet) to FL460 (~46,000ft):


       |           |          4165
       |           |         / 050
       |           |        / 
       | FL45-460  |       *  ·  ·  ·  ·  ·
       |           |
       |           |
       |           |

Judging by the distance between the trail dots (the 'history' dots behind the blip that show where it was for the last few sweeps of the radar), the aircraft was flying at considerable speed. We all assumed it was about to enter controlled airspace.4165 is the aircraft's transponder code. 050 is the aircraft's 'Mode C' readout, indicating its flight level of five-zero, or about five thousand feet. Frame by frame, the playback continued (truncated):

       |           |         4165
       |           |        / 050
       |           |       / 
       |           |      *· ·  ·  ·  ·
       | FL45-460  |

...and the blip started to behave a bit oddly:

       |           |        4165
       |           |       / 055
       |           |      / 
       |           |     *·· ·  ·  ·
       | FL45-460  |

Ten seconds later:

       |           |       4165
       |           |      / 070
       |           |     / 
       |           |    *
       | FL45-460  |

Furrowed eyebrows, and a murmured chorus from about the class:

'the hell...

       |           |       4165
       |           |      / 105
       |           |     / 
       | FL45-460  |    *
       |           |
...
       |           |       4165
       |           |      / 170
       |           |     / 
       | FL45-460  |    *
       |           |
...
       |           |       4165
       |           |      / 290
       |           |     / 
       | FL45-460  |    *
       |           |

"ooOOoo..."

       |           |     4165
       |           |   / 455
       |           |  / 
       | FL45-460  | *··
       |           |

Grins and chuckles all round.

Rather than duck 500 feet underneath this bit of controlled airspace, the pilot had hit the burners and booted his aircraft into a 41,000ft vertical climb to crest it, one can only assume, just because he could. Absent any horizontal movement, the blip stopped moving altogether and we just watched its Mode C readout tick rapidly upwards.

       |           |  4165
       |           | / 465
       |           |/ 
       | FL45-460  *· · ·
       |           |

Thirty seconds later the blip had cleared controlled airspace, dived vertically back to its previous level and continued on as if nothing had happened.

Our instructor told us the MTI in this case took account of vertical movement as well as horizontal (in other words, it would still display the horizontally-stationary aircraft because of its high vertical speed), but some radar systems would remove the blip altogether. This could cause problems with hovering helicopters — they wouldn't have said vertical speed — so I suspect radar systems use the transponder signals of aircraft to verify that stationary radar contacts aren't simply clutter, and still get displayed.

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