What is the unit for ground speed reported by ADS-B?
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 &lt; 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.**







* 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.