Here we go - someone who speaks my language - a Federal judge.-g- A Federal District Court opinion in a patent case between Space Systems/Loral and Lockheed Martin, about - - what else? Devices used by satellites for station-keeping.
>>SSL is the assignee of the Chan and Rahn patents, both of which relate to control systems for communication
satellites. Communication satellites typically operate in a geosynchronous equatorial orbit, circling the earth once every twenty-four hours in the equatorial plane, thereby maintaining the same position relative to the earth’s surface. See Space I, 1998 WL 1045304, at *1. Such positioning enables the satellite to maintain a consistent relationship with the ground transmitters from which it receives radio signals. See id. For optimal performance, the satellite must also maintain a proper attitude, i.e., orientation of the satellite relative to its direction of motion, to ensure that its on-board communication devices (e.g., antennae) are pointed in the proper direction. See id.
While orbiting the earth, satellites are subject to various destabilizing forces, such as the gravitational forces of the sun or moon, that can cause the satellite to drift behind or move ahead in its orbit (east-west drift), or drift out of its
equatorial orbit and into an inclined orbit (north-south drift). See id. In order to maintain a satellite’s "pointing accuracy," i.e., its ability to receive radio signals from a particular ground transmitter and relay them to a specific target area, a satellite’s position and attitude must be periodically adjusted. See id.
"Stationkeeping" is the process by which a satellite maintains its designated position in orbit around the earth, and it is typically accomplished through the use of small thrusters located on the surface of the satellite. See id. at *2. An east-west station-keeping maneuver, for example, involves firing thrusters to either speed up or slow down a satellite’s orbit to return the satellite to its proper position. See id. Alternatively, a north-south stationkeeping maneuver involves firing thrusters to return a satellite that has drifted into an inclined orbit back to the equatorial plane. See id.
Satellites generally employ one or more momentum wheels to maintain proper attitude. See id. A spinning momentum wheel creates angular momentum, thereby providing resistance to the twisting forces that can affect a satellite’s attitude. See id. While the angular momentum created by a spinning momentum wheel does not provide any resistance to forces that would twist the satellite around the axis of the wheel, such forces may be offset by changing the speed of the wheel. See id. Eventually, however, the momentum wheel will become "saturated" with excess angular momentum. See id. A saturated momentum wheel, i.e., one that is spinning too fast or too slowly to control a satellite’s attitude, requires desaturation to return the wheel to its nominal spin rate. See id. Desaturation may be accomplished by using the wheel’s motor to either slow down or speed up the wheel, while firing thrusters to prevent the satellite from twisting around the axis of the wheel. See id.
The Chan patent is directed to an apparatus for autonomously performing stationkeeping maneuvers for three-axis
stabilized spacecraft such as geosynchronous satellites. See Chan patent, Abstract. The claimed apparatus includes means for performing desaturation of the momentum wheel "while simultaneously accomplishing a preselected compensation of the spacecraft’s east-west orbital position." Id. at col. 1, ll. 60-62. In other words, the claimed apparatus "automatically compensates for . . . east-west drift, while simultaneously allowing for desaturation of the wheels." Id. at col. 2, ll. 53-55. The written description states that one of the advantages of the claimed apparatus is that "[a]ll manual east-west stationkeeping maneuvers are eliminated." Id. at col. 1, ll. 64-65.
Claim 1, the only claim in the Chan patent at issue, reads as follows:
1. Apparatus for controlling a 3-axis stabilized spacecraft, comprising:
[a] at least one momentum/reaction wheel mounted on board the spacecraft’s attitude with respect to an
axis;
a set of thrusters mounted about the periphery of the spacecraft for desaturating the
momentum/reaction wheel and for accomplishing change in velocity maneuvers;
[c] coupled to the momentum/reaction wheel, means for determining when the momentum/reaction wheel
reaches saturation; and
[d] coupled to the determining means, means for performing any desired desaturation of the
momentum/reaction wheel while automatically and simultaneously accomplishing a preselected
compensation of the spacecraft’s east-west position.
Id. at col. 6, ll. 28-44 (emphasis and formatting added).
The Rahn patent is directed to a method and apparatus for controlling the attitude of a momentum bias spacecraft that has drifted into an inclined orbit. See Rahn patent, col. 5, ll. 50-53. Once a satellite’s thruster propellant is substantially exhausted, the satellite will no longer be able to perform stationkeeping maneuvers to maintain an equatorial orbit, and will begin drifting into an inclined orbit. See id. at col. 1., ll. 42-48. As the angle of inclination of the satellite increases, it becomes increasingly difficult to maintain the pointing accuracy of the satellite. See id. at col. 1, ll. 49-51. The Rahn patent discloses an attitude control apparatus that includes an on-board computer that uses orbit information to control the pointing trajectory of a satellite’s aim point according to a predetermined attitude tracking model. See id. at col. 5, ll.
54-58. According to the written description, by maintaining the pointing accuracy of a satellite after it has expended
most of its propellant and has drifted into an inclined orbit, the claimed apparatus extends the operational life of the
satellite without increasing the amount of propellant required, or the costs associated with such propellant. See id. at col. 6, l. 62 to col. 7, l. 8.
Claim 3, the only claim in the Rahn patent at issue, reads as follows:
3. In a satellite attitude control, the apparatus comprising:
[a] a first and a second sensor to passively determine alignment errors for a first and a second axis of the
satellite;
a momentum biasing attitude control apparatus for effecting attitude control in response to roll
commands, pitch commands, and yaw momentum commands;
[c] a model generating circuit for receiving a generalized attitude definition for the spacecraft;
[d] a modelling circuit, coupled to said model generating circuit, for receipt of said generated model and
determination of appropriate roll commands, pitch commands, and yaw momentum commands to
established [sic] a particular aim point for a selected target;
[e] a short term tracker circuit, coupled to said modelling circuit, said first and second sensors, and said
attitude control apparatus, for receipt of said commands and to provide attitude control information in
response to said commands appropriate to position the spacecraft at said commanded position to provide
tracking and damping of nutational dynamics; and
[f] a long term tracker to observe long term orbit rate dynamics.
Id. at col. 11, ll. 3-30 (emphasis and formatting added).
Lockheed manufactures and sells the Series 7000 and A2100 Series satellites, which are three-axis stabilized,
geosynchronous communication satellites that utilize thrusters and momentum wheels to maintain their position and attitude. While the Series 7000 satellites employ a single momentum wheel that is connected to the satellite body by a pivot mechanism, the A2100 Series satellites employ four momentum wheels that are arranged in a pyramidal configuration. See Space II, 1998 WL 1045302, at *9. Both satellites are capable of performing momentum wheel desaturation while simultaneously performing stationkeeping maneuvers that are initiated by a ground operator. See Space I, 1998 WL 1045304, at *7.<<
finnegan.com
Hmm. This site claims that Globalstar's satellites use hydrazine for station-keeping. Oh, well. Hydrazine, plasma, either way, they use it to keep the birds in orbit. But it is easier to do with LEOs than GEOs.
ee.surrey.ac.uk |
