Signed phase angle

Phase angle

The phase angle is the angle, at the satellite, between the direction of the Sun and the direction of the observer. At 0° the observer has the Sun behind them and sees the fully lit side; at 90° half of a sphere would look lit; near 180° the object is backlit.

Why a sign

Two lines of sight with the same phase angle can lie on either side of the anti-solar point. LightCurve signs the phase angle:

• negative when the satellite is west of the anti-solar point (evening side);
• positive when it is east of it (morning side).

For a geostationary satellite, the phase goes from negative to positive during the night. The sign matters for attitude-dependent objects: a nadir-pointing satellite does not show the same faces at −40° and +40°.

Phase plane and tilt

In phase mode, the line of sight turns by the signed phase angle away from the anti-Sun direction, inside a plane:

• tilt 0°: the plane contains the celestial east–west direction, like the GEO belt;
• tilt ±90°: the plane goes through the celestial poles.

The button Put it highest in the sky picks the tilt that maximises the elevation of the satellite.

The phase curve in the dock sweeps the signed phase from −180° to +180° in the current plane. Click a point to move the simulation there.