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"Skyhawk 872ND, airport at your 12 o'clock in 15 miles."

"Baron 981K, traffic, 2 o'clock 5 miles, opposite direction, a Piper southbound, 1000 feet below you."

Does ATC use nautical miles or statute miles?

Sanchises
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slantalpha
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2 Answers2

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ATC uses nautical miles, this is because it's the ICAO standard documented in Annex 5 - Units of Measurement to be used in Air and Ground Operations, which states that Nautical Miles are to be used for distance and Knots for speed as an alternative non-SI measurement. Kilometers are actually the primary distance measurement because they are the SI standard, however nautical miles are so entrenched they've kept them and there's no termination date.

The ICAO annex is hard to find because everyone wants you to buy it, however the standards have been copied into this CAAS document word for word if you want some light bedtime reading.

GdD
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Broadcast weather METARs use statute miles. Navigational information, such as that from ATC, would use nautical miles.
Nautical miles are used for navigation, mainly because a nautical mile is one minute of a degree on the surface of Earth's geoid.(basically one minute of latitude.)
Kilometers and statute miles are each a fixed distance unit, nautical miles are not a fixed straight line distance but are a fixed fraction of the earth's circumference. Very useful for long range navigation and triangulation, 60 miles to each degree and exactly 21600 nautical miles per circumnavigation.

Max Power
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    I don't think the nautical mile is defined the way you're talking about anymore. It's exactly 1852 meters nowadays. – Erin Anne Jul 04 '18 at 20:28
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    METARs don't use statute miles everywhere, internationally it's usually kilometers. In any case if it's statute miles it says SM after. – GdD Jul 04 '18 at 20:29
  • You're right @ErinAnne, Annex 5 specifies exactly that. – GdD Jul 04 '18 at 20:30
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    The question was about statute miles vs nautical miles, clearly Km are Km, at every station that uses miles the METAR uses statute miles, the station is considered a 5 statute mile radius, the "vicinity" is 10 statute miles, and up to 30 statute miles for "distant".(DSNT is only found in remarks section) The one exception is for lightening which uses NM, though I don't know why. And wind speed is in knots, because aircraft speed is generally in knots. – Max Power Jul 04 '18 at 21:54
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    The meter originally was the same concept as the NM. It was originally defined as 100 km per gradian latitude, the gradian being the original metric unit of angle. In both cases, it was realized the Earth is not a sphere and the measurements kept being refined anyway, so they were decoupled. – user71659 Jul 04 '18 at 22:24
  • The NM was not defined as a strict linear distance, the meter was. The difference is the NM lends its self to positional navigation on a sphere, -finding accurate locations on the geoid regardless of changes in shape, size, or local distortion(mountains). Linear distance units have limited value in this due to curvature and distortions, they require substantially more initial computation, lack the accuracy of integer fractions, and all the data must be recomputed and adjusted for relatively minor changes in earths shape. The angular NM can be used equally on any planet using the same tables. – Max Power Jul 04 '18 at 22:48
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    @Max, even if it was true (I doubt, although it may be partly), it is not anymore. NM is considered a fixed unit of distance (and ultimately defined via SI, as everything nowadays). – Zeus Jul 05 '18 at 00:44
  • It is true, currently, if anybody uses the NM in that fashion and I know sailors who do use it in that fashion. A fiat declaration in a book some place does not magically change maths and metrology, no more or less than that town that attempted to declare Pi=3.2 exact. 1852meters is just 1852 meters, in this form it is a close enough approximation for estimating speed in knots but of little use otherwise. – Max Power Jul 05 '18 at 01:37
  • What, it is the definition! Units of measurements change their definitins uite often. There was a revisiin of the SI ystem this decade. They units did change then and ven the value of certain physical constants can change that way. Pi id not defined by its value, but many uinits are defined using otherr units. – Vladimir F Героям слава Jul 05 '18 at 06:17
  • The SI units themselves don't change they are only refined to a more accurate description. – Max Power Jul 07 '18 at 02:04
  • Dictionaries have allowance for multiple definitions under a single word for a reason. Just consider pumpkins and tomatoes, they are defined as vegetables by most statutes and treated as vegetables in cooking, but botanists define them as berries. One word two definitions. – Max Power Jul 07 '18 at 02:08