An Analysis of the Vertistat Gravity Gradient Satellite Orientation

The Vertistat is a device that passively orients a satellite in a near circular orbit into a specific alignment with the local earth vertical. Basically, it consists of three long, orthogonally placed rods. The vertical rod, which is longest, causes a restoring torque due to the slight change in gravitational force along it. The three rods are interconnected by a spring-damper arrangement to provide damping of the satellite librations. This paper covers two important Vertistat performance areas, small deviation stability, and orientation error due to orbit eccentricity. Stability results include an analytical method for selection of Vertistat parameter values yielding optimum results for all altitudes. Analog computer time history data are given for these values; the data indicate that the device can remove an initial small deviation error within an orbit. It is shown that orbit eccentricity induces an error of 1.5°/0.01 of orbit eccentricity at any altitude.