Opinion ID: 781499
Heading Depth: 3
Heading Rank: 1

Heading: introduction

Text: 9 Lockheed is the assignee of U.S. Patent No. 4,084,772 (the '772 patent), which discloses an apparatus and method for steering a satellite. In 1995, Lockheed's predecessor, Martin Marietta Corporation, brought this patent infringement action against Space Systems/Loral, Inc. (SSL), alleging that certain SSL satellites infringe the '772 patent. The '772 patent, entitled Roll/Yaw Body Steering for Momentum Biased Spacecraft, claims a structure and method for reducing the pointing errors of a satellite that has entered into an inclined orbit by varying the speed of the satellite's transverse momentum wheel in a sinusoidal manner. 10 A communications satellite typically orbits the earth in a geosynchronous equatorial orbit, circling the earth once every twenty-four hours in the equatorial plane. A geosynchronous orbit in the equatorial plane allows a satellite to maintain the same position relative to fixed points on the earth's surface, and is often referred to as geostationary. A satellite in geostationary orbit, when viewed from the ground, appears to remain stationary in the sky. Therefore, a geostationary orbit enables a communications satellite to maintain a constant relationship with transmitters on earth. In addition to preserving a geosynchronous equatorial orbit, a communications satellite must also maintain a proper attitude, or pointing direction, so that its antennae remain pointed at the desired target on the earth. 11 While in orbit, however, a satellite is subject to various destabilizing forces such as the gravitational effects of the sun and the moon. Such forces may cause a satellite to drift out of its equatorial orbit, into an inclined north-south orbit. A satellite in an inclined orbit may still orbit the earth in a geosynchronous manner (once every twenty-four hours), but may no longer appear stationary in the sky to an earthbound observer. The following diagram, Figure 2 of the '772 patent specification, depicts an inclined orbit relative to an equatorial orbit: 12 NOTE: OPINION CONTAINING TABLE OR OTHER DATA THAT IS NOT VIEWABLE Therefore, in an inclined orbit, the antennae of a communications satellite point north of the equator for half of the orbit and south of the equator for half of the orbit. Unless the satellite corrects for solar and lunar gravitational effects, the orbit of a geosynchronous satellite acquires an inclination at the rate of about 0.8 degrees annually. 13 The '772 patent discloses a system for allowing a communications satellite to continue to operate effectively after entering an inclined orbit. It does so by changing the attitude, or pointing direction, of the satellite when the satellite is north and south of the equator. The method described by the '772 patent rotates the satellite so that its antennae point north or south, and remain pointed at the same earth target over the course of twenty-four hours. 14 The attitude of satellites is described in terms of movement and rotation about three axes. The pitch axis lies in a north-south direction, the roll axis points in the direction of satellite orbital movement, and the yaw axis points to the center of the earth. In an inclined orbit, roll pointing error occurs when a satellite is north or south of the equator. There is no roll pointing error when a satellite is above the equator twice each orbit (depicted above in Figure 2 at points B and D). Conversely, yaw pointing error is greatest when a satellite crosses the equator. There is no yaw pointing error when a satellite is furthest north or south of the equator (depicted above in Figure 2 at points A and C). 15 The structure and method described by the '772 patent rotates a satellite around its roll axis such that its antennae point to the south when the satellite is north of the equator, and point to the north when the satellite is south of the equator. In this manner, even though a satellite is moving north and south during its orbit, it remains pointed at the same earth target, and behaves as if it were still in geostationary orbit.