# What is meant by the pointing vector and pointing vector magnitude in ECI?

In Siouris's Missile Guidance and Control Systems, it is stated that the angle of attack can be approximated in a 3DoF (degrees of freedom) simulation by the following equation:

I know that ECI refers to the Earth Centered Inertial Frame. The only thing I don't understand is what this pointing vector, and its magnitude meant to be. It is not the thrust vector because the units do not match up (m for meters, otherwise it would be N for Newtons which is used commonly in the book). And also, if it is a pointing vector, why would it even have units?

• Could it be the direction in which the missile is pointing?
– Jim
Commented Dec 17, 2021 at 16:00

Indeed the pointing vector $$R_{NT}$$ is the direction that the missile is pointing in, as @Jim mentions in a comment. Which is not necessarily in the direction of the velocity vector, in which case the angle of attack would be zero.
And the magnitude of vector (x,y,z) is $$\sqrt{x^2 + y^2 + z^2}$$