Summary: The angular momentum per unit mass of satellite is rotating in ECI x-y plane and I can't understand why. I'm very new to orbital mechanics.

What I'm doing: I'm taking the cross product of satellite position by satellite velocity in ECI frame; Turn this vector into a unit vector and plotting components of this unit vector over one year.

$P = \frac{Pos \times Vel}{|Pos|*|Vel|}$

What I'm expecting: Since the frame is ECI, I was expecting the orbit plane to be constant over time and so the unit vector orthogonal to this plane (P) to be constant as well.

What I'm observing: The orthogonal unit vector P is rotating in x-y plane suggesting orbit is rotating around the earth. Z-> green, Y->blue, X->red

Question: How is this constant rotation explained? What is the rate of change?

Summary: The angular momentum per unit mass of satellite is rotating in ECI x-y plane and I can't understand why. I'm very new to orbital mechanics.

What I'm doing: I'm taking the cross product of satellite position by satellite velocity in ECI frame; Turn this vector into a unit vector and plotting components of this unit vector over one year.

$P = \frac{Pos \times Vel}{|Pos|*|Vel|}$

Satellite orbit data is generated using AGI STK's orbit wizard with the following parameters: Circular orbit, Inclination 75 deg, Altitude 700 km, RAAN: 30 deg

What I'm expecting: Since the frame is ECI, I was expecting the orbit plane to be constant over time and so the unit vector orthogonal to this plane (P) to be constant as well.

What I'm observing: The orthogonal unit vector P is rotating in x-y plane suggesting orbit is rotating around the earth.

The graph shows the components of unit vector P ($\hat P_z$ : green, $\hat P_y$ : blue, $\hat P_x$ : red) over 364 days ( horizontal axis is in UNIX time format i.e. seconds).

Question: How is this constant rotation explained? What is the rate of change?