Question for the ATC folks
Went playing today with fairly high winds aloft in a Cessna 172. While poking along, I decided how slow could I go in GS.
Attached is what I got from KJAN approach showing 2 knot ground speed. My GPS on a forthcoming video I will post under this thread showed 14 knots yet when I panned out side the window, plane was hovering and ever so slightly backing up. ASI was indicating 0 knots. Winds aloft forecast seemed spot on at 30 knots.
My question would be is how is the GS computed if GPS is not where the data is derived from?
As you will see from the forthcoming video, the ATC reported GS seem to true out to what I saw out the window and the GPS in the plane seemed incorrect?