Velocity over the ground minus velocity in the air = wind velocity
i.e. it's the speed difference but, since directions matter, you have to use vectors (velocities).
Quoting UAL747 (Thread starter): Does it require the use of GPS, or can it be manually calculated somehow?
It does not require GPS but it does require some way to get velocity over the ground. GPS is one way to get that but you can also get it from taking periodic position fixes using any other navigation sources or using an inertial reference system.
Velocity in the air comes straight from the airspeed system and the heading indicator.
KAUSpilot From United States of America, joined Jan 2002, 1958 posts, RR: 33
Reply 2, posted (2 years 6 months 2 weeks 5 hours ago) and read 2700 times:
It's calculated by Inertial Reference Units/Attitude Heading Reference Systems using trigonometry. The systems compare the aircraft's heading to its ground track and its true airspeed to its groundspeed to solve for wind direction and velocity, which is then displayed on the flight deck. The ground track and groundspeed can be determined by IRU's alone, GPS, or navigation radios receiving ground based navaids. The heading and airspeed are determined using traditional aircraft systems.
GPS can be used but something has to tell the GPS unit what the aircraft's heading and true airspeed are, so without that interface it won't be possible to display wind direction and velocity, to my knowledge. There are a few papers out there about using GPS signal reflections from the surface of the earth to calculate independent wind velocities, but I'm not aware if these methods are actually being applied.