I have the orbital position and velocity and the orbital parameter of 2 spacecraft at LEO, how to define the angle between Sun - Probe 2 - Probe 1 in ECI frame (If you imagine at a point sun then behind it probe 2 and behind of that probe 2 is Probe 1, so at some point 3 objects could be one line so angle zero but most of the time it will be different as both probes are rotating around earth). like this picture,
From this source and from this souce (4.3 algo) I got the unit vector of sun position in ECI Frame. But how can I calculate the angle? Long story in short, Probe 1 is communicating with Probe 2 with laser beam but behind the Probe 2 if sun will appear, communication could be an issue, so I need to understand the angle variations.