Aeronautics and astronautics – Spacecraft – Attitude control
Reexamination Certificate
2001-01-25
2002-08-20
Jordan, Charles T. (Department: 3644)
Aeronautics and astronautics
Spacecraft
Attitude control
C244S172200
Reexamination Certificate
active
06435457
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to spacecraft and, more particularly, to orbital control of spacecraft.
2. Description of the Related Art
FIG. 1
illustrates a spacecraft
20
in a synchronous orbit
22
(e.g., a geostationary orbit) which defines an orbital plane
24
. In order to perform its designed service, the spacecraft
20
is typically required to maintain a controlled spatial relationship with the earth
26
.
This relationship is generally fixed with the aid of a station-keeping box
28
which has predetermined dimensions that define the maximum permitted excursions of the spacecraft
20
(particularly in longitude and latitude). The dimensions are determined by various operational and service considerations (e.g., beamwidth of spacecraft antennas, ground station tracking capabilities and specified interference limits between the spacecraft
20
and its neighbor spacecraft) and is on the order of 0.2° longitude and latitude (i.e., ~75 kilometers) for spacecraft which use regulated communication frequency bands.
Many spacecraft are restrained to a corresponding station-keeping box for their entire service life but service demands (e.g., military and commercial objectives) of others require that they also be capable of station changing, i.e., they must move from an initial box such as the station-keeping box
28
to one or more subsequent boxes such as the station-keeping box
30
. The initial and subsequent boxes are separated by a longitude
32
which may be of considerable magnitude.
In order to carry out its intended service, the spacecraft
20
must also maintain a predetermined service attitude such as one in which antennas of the spacecraft
20
are directed at respective service areas on the earth
26
(e.g., along an antenna boresight
33
). Disturbance of the service attitude causes interruption of service and avoiding the loss of expensive service time is a prime objective in the operation of spacecraft.
Although current thruster systems are generally effective for performing spacecraft stationkeeping and attitude control, they are generally less suited for spacecraft station changing. For the latter function, some systems employ high-thrust chemical engines (e.g., bipropellant thrusters) which can effect station changing in acceptable time spans. However, sufficient fuel must be carried by the spacecraft for these high-thrust engines and this reduces the spacecraft's payload.
Other systems employ low-thrust engines (e.g., ion thrusters) which can operate on electrical energy that is supplied by the spacecraft's solar cell arrays. In order to effect station changing in acceptable time spans, however, the spacecraft's attitude must typically be disturbed in order to more effectively align these low-thrust engines and this attitude disturbance causes loss of service time.
Current thruster systems, therefore, generally fail to effectively meet the combined demands of station changing, station keeping, momentum dumping and service-loss minimization.
SUMMARY OF THE INVENTION
The present invention is directed to thruster systems that can realize station changing, station keeping and. momentum dumping in spacecraft while minimizing any loss of spacecraft service time. These goals are realized with pairs of E and W thrusters, NE and SE thrusters and NW and SW thrusters whose thrust directions are each defined by respective polar angles &phgr; and slew angles &agr;.
The E and W thrusters are especially suited for rapid station changing while the remaining thrusters generate normal, tangential and radial thrust components required for station keeping. Because all thrusters are directed through the spacecraft's center of mass during station changing and station keeping firings, the spacecraft's attitude is not disturbed and loss of service time is avoided. The thrusters are preferably gimbaled so that they can track spatial changes of the spacecraft's center of mass or, for momentum dumping, be offset from that center of mass. The normal, tangential and radial thrust components of the thruster systems are arranged so that station changing, station keeping and momentum dumping are still realized after the failure of any thruster pair.
Different thruster embodiments are provided whose slew angles are rotated respectively from anti-nadir and nadir portions of the spacecraft's yaw axis. Although these systems can be realized with various thruster types, they are particularly suited for ion thrusters.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. dr
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a perspective view of a spacecraft, a spacecraft orbital plane and initial and subsequent station-keeping boxes that are longitudinally-spaced along the orbital plane;
FIG. 2
is a perspective view of a spacecraft and thruster system of the present invention;
FIGS. 3A and 3B
are views respectively of the anti-nadir and top sides of the spacecraft and thruster system of
FIG. 2
;
FIG. 4
is a perspective view which illustrates spherical coordinates of
FIGS. 2
,
3
A and
3
B; and
FIG. 5
is a plot of drift rate in an exemplary station-changing maneuver with the thruster system of
FIGS. 2
,
3
A and
3
B.
REFERENCES:
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patent: 4786019 (1988-11-01), Uken
patent: 5443231 (1995-08-01), Anzel
patent: 5458300 (1995-10-01), Flament et al.
patent: 5806804 (1998-09-01), Goodzeit et al.
patent: 5810295 (1998-09-01), Anzel
patent: 5813633 (1998-09-01), Anzel
patent: 6015116 (2000-01-01), Anzel et al.
patent: 6042058 (2000-03-01), Anzel
patent: 6213432 (2001-04-01), Koppel
Dinh T.
Gudmestad Terje
Jordan Charles T.
The Boeing Company
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