#### enigma

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hopefully someone can help me out here:

I'm looking at the Gibbs' method for orbit determination, and I'm losing the conceptual picture in the vector calculations.

I have no problems seeing how

**D**and

**N**point in the

**W**direction (

**PQW**coordinate system), but I'll be damned if I can get a grip outside of simply swimming through the math why

**S**: [(r2-r3)

**r1**+ (r3-r1)

**r2**+ (r1-r2)

**r3**] points in the

**Q**direction.

Anyone have any insights?

I'm reading Vallado

*Fundamentals of Astrodynamics and Applications*section 7.5.1 if it helps.