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EP-3372511-B1 - SERVICE SATELLITE FOR PROVIDING IN-ORBIT SERVICES USING VARIABLE THRUSTER CONTROL

EP3372511B1EP 3372511 B1EP3372511 B1EP 3372511B1EP-3372511-B1

Inventors

  • REITMAN, MICHAEL
  • SPITZER, ARNON
  • HALSBAND, Arie
  • AZRIEL, OFIR

Dates

Publication Date
20260506
Application Date
20180305

Claims (16)

  1. A service satellite (10) for providing station keeping services to a host satellite (200), the service satellite comprising: a body (12); a gripping mechanism attached to the body and adapted to attach to an interface ring extending from an external surface of the host satellite, to thereby form an interconnection between the host satellite and the service satellite through the externally extending interface ring, and to thereby result in an interconnected unit having a combined center of mass (126), wherein the gripping mechanism includes at least two arms (90,92) configured to engage with the interface ring; at least two thrusters (36, 38, 40, 42), each thruster being mounted on a thruster deployment mechanisn (64) extending outward and away from the body of the service satellite; and at least one controller (120) configured to maintain the interconnected unit in a substantially stationary orbit by selectively moving each of the at least two thrusters, each thruster being mounted to a different thruster deployment mechanism, to angular orientations such that during thrusters firing, thrust vectors from each of the at least two thrusters avoid passing through the combined center of mass, are each offset from the combined center of mass, wherein at least one controller is further configured to fire each of the at least two thrusters such that their thrust vectors (122, 124, 132, 134) intersect within a body of the host satellite (200).
  2. The service satellite (10) of claim 1, wherein the at least one controller (120) is configured to selectively move each of the at least two thrusters (34,36,38,40) such that the combined center of mass (126) is disposed between intersection points of the thrust vectors (122,124,132,134) and a longitudinal plane of symmetry passing through the combined center of mass of the interconnected unit.
  3. The service satellite (10) of claim 1 or 2, wherein the at least one controller (120) is configured to: during a first time period position a first thruster of the at least two thrusters at a first angular orientation and fire the first thruster; during the first time period position a second thruster of the at least two thrusters at a second angular orientation different from the first angular orientation and fire the second thruster; during a second time period following the first time period, move the first thruster to assume the second angular orientation and fire the first thruster; and during the second time period, move the second thruster to assume the first angular orientation and fire the second thruster.
  4. The service satellite (10) of any one of claims 1-3, wherein the at least one controller (120) is further configured to adjust a thrust level of each of the at least two thrusters (34,36,38,40) such that torques generated by the thrust vectors (122, 124, 132, 134) about at least one coordinate axis of the interconnected unit are substantially balanced.
  5. The service satellite (10) of claim 3, wherein the controller (120) is configured to: fire the at least two thrusters (34,36,38,40) at a first thrust level when the first thruster is oriented at the first angular orientation; and fire the at least two thrusters at a second thrust level different from the first thrust level when the first thruster is oriented at the second angular orientation; wherein the at least two thrusters optionally include: a first north thruster and a second north thruster disposed on a first side of the service satellite, the first north thruster being spaced apart from the second north thruster; and a first south thruster and a second south thruster disposed on a second side of the service satellite opposite the first side, the first south thruster being spaced apart from the second south thruster.
  6. The service satellite of claim 5, wherein the controller (120) is configured to determine the second thrust level by: firing the at least two thrusters (34,36,38,40) at the first thrust level when the first thruster is oriented at the first angular orientation; determining an angular momentum about the at least one coordinate axis of the interconnected unit; and determining the second thrust level at which the at least two thrusters must be fired when the first thruster is oriented at the second angular orientation such that the angular momentum is substantially reduced to zero.
  7. The service satellite (10) of any one of claims 1-6, wherein the at least two thrusters (34,36,38,40) includes a first pair of thrusters and a second pair of thrusters, and the at least one controller is further configured to move one of the first pair of thrusters and the second pair of thrusters to a first angular orientation and move another of the first pair of thrusters and the second pair of thrusters to a second angular orientation different from the first angular orientation, such that the thrust vectors from the first pair of thrusters and the second pair of thrusters are offset from the combined center of mass (126).
  8. The service satellite (10) of claim 7, wherein the at least one controller (120) is further configured to fire thrusters (34,36,38,40) located at the first angular orientation at a first thrust level, and fire thrusters located at the second angular orientation at a second thrust level different from the first thrust level, such that torques generated by the thrust vectors about at least one axis of the interconnected unit are substantially balanced.
  9. The service satellite (10) of claim 7 or 8, wherein the controller (120) is further configured to simultaneously fire the first pair of thrusters for a first time duration, and simultaneously fire the second pair of thrusters for a second time duration.
  10. The service satellite (10) of claim 9, wherein the controller (120) is further configured to move the first pair of thrusters to the third angular orientation and the second pair of thrusters to the first angular orientation after a second time duration, and simultaneously fire the second pair of thrusters for the first time duration after moving the first pair of thrusters and the second pair of thrusters.
  11. The service satellite (10) of any one of claims 7-10, wherein the at least one controller (120) is further configured to fire thrusters located at the first angular orientation at a predetermined thrust level for a first time duration, and fire thrusters located at the second angular orientation at the predetermined thrust level for a second time duration different from the first time duration, such that torques generated by the thrust vectors about at least one axis of the interconnected unit are substantially balanced; optionally wherein the controller is configured to determine the second time duration by: firing thrusters positioned at the first angular orientation at the predetermined thrust level for the first time duration; determining an angular momentum about at least one coordinate axis of the interconnected satellite; and determining the second time duration for which thrusters positioned at the second angular orientation must be fired to decrease the angular momentum substantially to zero.
  12. The service satellite (10) of claim 10, wherein the controller (120) is further configured to sequentially fire the first pair of thrusters and the second pair of thrusters.
  13. The service satellite (10) of claim 11, wherein in a third time duration after the second time duration, the controller (120) is further configured to move the first pair of thrusters to the second angular orientation and the second pair of thrusters to the first angular orientation, and sequentially fire the first pair of thrusters and the second pair of thrusters after moving the first pair of thrusters and the second pair of thrusters.
  14. The service satellite (10) of claim 13, wherein the first time duration is an amount of time after which a reaction wheel of the service satellite is fully loaded; and the second time duration is the amount of time after which the reaction wheel of the service satellite is fully unloaded; optionally wherein: the first pair of thrusters includes the first north thruster and the second north thruster; and the second pair of thrusters includes the first south thruster and the second south thruster; or optionally wherein: the first pair of thrusters includes the first north thruster and the first south thruster; and the second pair of thrusters includes the second north thruster and the second south thruster.
  15. A method of providing station keeping services to a host satellite (200), the method comprising: launching a service satellite (10) according to any of the preceding claims into space and maneuvering it within docking distance of the host satellite; connecting the service satellite to the host satellite to form an interconnected unit by engaging the at least two arms (90,92) of the service satellite with an external interface ring of the host satellite, the interconnected unit having a combined center of mass (126); and selectively moving each of the at least two thrusters (36, 38, 40,42) of the service satellite for firing during station keeping maneuvers, each thruster being mounted on a thruster deployment mechanism (64) extending outward and away from the body of the service satellite, such that the thrust vectors (122, 124, 132, 134) from the at least two thrusters intersect within a body of the host satellite.
  16. The method of claim 15, wherein each of the at least two thrusters (34,36,38,40) is selectively moved such that the combined center of mass is disposed between intersection points of the thrust vectors (122, 124, 132, 134) and a longitudinal plane of symmetry passing through the combined center of mass (126) of the interconnected unit.

Description

BACKGROUND The commercial and governmental space sector faces high costs in placing and maintaining communication and other satellites in orbit around the earth. A satellite's life is typically governed by the amount of on-board propellant carried by the satellite. Even when onboard electronics are still functional, once the fuel runs out, a satellite typically loses its usefulness. As demand for satellite connectivity has increased over time, so has typical satellite size. A substantial initial investment is required to launch a large geostationary communication satellite. The initial investment for a satellite service provider includes not only the cost of the satellite itself, but also the cost of the launch vehicle required to inject the satellite from earth into an injection orbit. A satellite typically includes its own propulsion system, which allows the satellite to transition itself from an injection orbit to a final geosynchronous orbit and to maintain its orbital position for up to 15 years. Propulsion system technology has also matured throughout the years and allows the satellite to make use of not only chemical systems but also electrical propulsion systems that have a much higher efficiency. This increased efficiency saves fuel, which in turn allows placement of larger and/or heavier satellites in orbit. A satellite in a geostationary orbit around the earth experiences gravitational and solar forces, which tend to move the satellite away from its desired geostationary position. The satellite relies on its propulsion system to take corrective measures to compensate for the displacements caused by the gravitational or solar forces. The propulsion maneuvers required to maintain the geostationary position of the satellite are sometimes referred to as station keeping. When a satellite depletes is entire on-board fuel, it can no longer use its propulsion system to maintain its geostationary position and may have to be replaced. In some cases, the satellite launch vehicle may underperform, injecting the satellite into an incorrect orbit. The satellite may then have to expend its on- board fuel to transition itself from the incorrect orbit to its correct orbit. Thus, some of the fuel intended for station keeping may have to be used for initial orbit correction, which in turn may result in a shorter operational life of the satellite, providing less revenue to the satellite service provider. In other cases, a satellite may be required to change its orbital position and/or orientation for various commercial or operational reasons. These changes may also require the satellite to expend its on-board fuel, resulting in a shorter operational life. Because of the high cost associated with replacing a satellite in space, there is a need for technology that may help to extend the life of a satellite already in orbit. WIPO Publication WO 2016/030890 to Halsband et al describes a service satellite having a body, a controller and a docking unit. The docking unit includes at least two foldable, adjustable gripping arms pivotally mounted on the satellite body, each gripping arm being pivotable relative to the satellite body, and a gripping end at each free end of the gripping arms, wherein the gripping ends are adapted and configured to capture and grip a target portion of an orbiting satellite. Each gripping arm is controllable independently by the controller, which coordinates the motion of the arms. The service satellite also includes a propulsion unit including a first thruster mounted adjacent a Nadir end of the service satellite body, and a balance thruster, the balance thruster being distanced from the first thruster and facing a different direction than the first thruster, propellant for the thruster and the balance thruster; and means for aligning the thrusters so that a thrusting vector passes through a joint center of gravity of the service satellite and the serviced satellite. US Patent Application Publication 2005/0040282 to Wingo describes a satellite life extension spacecraft, comprising a mechanical implement adapted for connection to a parent spacecraft, a first and second boom, a first thruster pod and a second thruster pod attached to the first and second boom, the first pod positioned relative to the second pod such that the center of mass of a parent/child spacecraft can be determined relative to the first and second pod. US Patent 5,299,764 to Scott describes artificial life robotics utilized in a system for in-space service of spacecraft. An Earth Launch Vehicle (ELV) carries an Exoatmospheric Mothership Vehicle (MSV) into a space trajectory. The MSV, in turn, carries at least one Operational Service Vehicle (OSV). The MSV separates from the ELV and rendezvous within station keeping proximity of the spacecraft. The OSV then separates from the MSV for operative approach to the spacecraft. The OSV includes a propulsion/attitude control system and spacecraft servicing apparatus. The OSV carries a