Air Speed or Ground Speed?


#1

What speed are we seeing in our stats? Is it air speed or ground speed? I can see how either one would be valuable. Ground speed could be calculated from the coordinates and time, but air speed would need to take wind speed into account, too, so if it were wind speed, that would actually represent more data.


#2

Speed over ground. Certainly not IAS.


#3

It depends on the aircraft capabilities. Check out A.1.4.5 in the FAA spec:

http://adsb.tc.faa.gov/WG3_Meetings/Meeting30/1090-WP30-21-Appendix_A%20Mods.pdf

Subtypes 1 and 2 of the Airborne Velocity format will be used when the transmitting aircraft’s velocity over ground is known.

Subtypes 3 and 4 of the Airborne Velocity format will be used when the transmitting aircraft’s velocity over ground is not known. These Subtypes will substitute Airspeed and Heading for the velocity over ground.


#4

Thanks. Interesting that they mention velocity when, apparently, they mean speed (which is velocity’s absolute value).


#5

Assuming you’re referring to the message types: they really do mean velocity - the messages contain either (signed east/west, north/south velocity) or (airspeed, heading) depending on the message subtype, plus vertical rate. So it’s a vector quantity.


#6

You’re right. Looking back at the document, I was looking at the wrong place.


#7

For all practical purposes it’s GPS based ground speed.


#8

It must be ground speed. The flight I’m tracking shows 774 mph, which would be supersonic. Actual air speed I have no idea, as FlightAware doesn’t seem to show that information.