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Strangely enough, I'm having trouble finding the unit reported by ADS-B on the Internet. Many sites simply refer to it as "ground speed". Is it measured in KPH?

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ADS-B* encodes the ground speed in NM/s (nautical miles per second) in $2^{15}$ increments of ≅ 0.22 knots each such that:

0 ≤ Ground Speed < 2 NM/s

As of June 2015*, it is optional to report the ground speed (users can use two position reports to get the speed). Decoding to Knots (NM/h) is more common though for aviation for most of the world.**


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* Eurocontrol Standard Document For Surveillance Data Exchange
** ICAO (Annex 5) recommends km/h but permits the 'temporary use' (as they put it) of non-SI alternative units for vertical/horizontal distances and speed.

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