Everybody concludes there is motion no? The plane is shown flying straight while under autopilot heading and barometric altitude hold (that's what HDG and BALT mean on the bottom of the video – I like flight sims and you can find an explanation of radar modes on an F-18 here if you do too
Source: https://www.youtube.com/watch?v=xUukVLGOBLc). The thing is being tracked from 4°L to 8°R so it's moving to the left compared to the aircraft as it crosses its nose.
So it IS moving left. No doubt about that right?
They then use this movement to calculate velocity and then acceleration in the final frames of the video if I understand it correctly but I am still working my way through it.
p.s. range is key here. IF (that's a big if) my trig is correct and google gave me correct conversions (damn miles and knots): with 12 degrees of movement observed from start to end (67 seconds) and assuming distance is constant from the F18:
- at 40 nautical miles (max range of ATFLIR) we have the object crossing 8.6nm of space in 67 seconds. That's 6.71 nm/min so 402 knots. Not too shabby
- at 20 nautical miles (statements by Chad Underwood) half distance = half speed so 200 knots.
The F-18 in the video is going at 250 for reference. That's mach 0.55 at that altitude according to the indicator in the video.
IF (a big if) my calculations are correct (I doubt it) then this would be too fast for a balloon or your average weather event/cloud.