Aeronautics and astronautics – Spacecraft – Attitude control
Reexamination Certificate
2001-12-07
2004-02-24
Carone, Michael J. (Department: 3641)
Aeronautics and astronautics
Spacecraft
Attitude control
C244S158700, C455S427000, C342S354000, C342S357490
Reexamination Certificate
active
06695262
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to spacecraft and, more particularly, to spacecraft attitude determination and control.
2. Description of the Related Art
Points on the earth do not move relative to a geostationary spacecraft. Accordingly, the spacecraft can be maintained in an appropriate service attitude and the positions of earth points, relative to that attitude, will remain constant over each solar day. An exemplary geostationary spacecraft is a communication spacecraft that provides a payload beam which serves a communication service area on the earth and facilitates communication between points in the communication service area and the spacecraft.
In particular, the payload beam is configured to define a payload footprint on the earth that is preferably identical to the communication service area. Because earth points remain fixed relative to the service attitude of a geostationary spacecraft, the payload footprint remains substantially fixed over each solar day. This characteristic of geostationary spacecraft facilitates minimization of service error which is any difference between the payload footprint and the service area.
In contrast to a geostationary spacecraft,
FIG. 1
illustrates a communication spacecraft
20
that orbits the earth
22
in an orbit whose orbital plane
24
is inclined by an inclination angle
26
from the earth's equatorial plane
28
. The spacecraft carries an antenna system
30
and solar wings
32
and is shown in its service attitude at positions
34
A,
34
B and
34
C which correspond to times T
O
, T
O
+6 hours and T
O
+12 hours. The spacecraft may use signals from a beacon station on the earth
22
as an attitude reference.
Because the spacecraft
20
of
FIG. 1
is in an inclined orbit, earth points will move relative to the spacecraft's service attitude. In particular, they move along a figure-eight path such as the path
40
of
FIG. 2A
which indicates movement direction by a path arrowhead. Earth points initially drift southward and eastward during the first quarter of the orbit from its ascending node (spacecraft at position
34
B in FIG.
1
). For example, an inclination (
26
in
FIG. 1
) of 5.4 degrees will cause an earth point (e.g., a beacon station) at approximately 25° north latitude to trace a path having a north-south angular extent
42
on the order of 0.72 degrees and an east-west angular extent
43
on the order of 0.14 degrees.
More generally, the path
40
will define distorted figure-eight patterns that are tilted from a north-south axis
45
as shown in
FIGS. 2B and 2C
. The path
40
and the area within the path can be considered to define a beacon-station window
44
(i.e., a window, as observed from the spacecraft, that always contains the beacon station).
Various optimal steering laws have been utilized to realize different spacecraft pointing objectives (e.g., see U.S. Pat. Nos. 5,184,790, 5,738,309 and 6,135,389). A spacecraft's service attitude is determined by its respective steering law and the motion of an earth point, relative to the spacecraft, is a function of the steering law, orbital eccentricity, spacecraft mean longitude error, orbit inclination and longitude/latitude of the earth point relative to the spacecraft's location.
The service attitude of a communication spacecraft is typically maintained with an attitude-control system that receives attitude input signals from various attitude sensors. An exemplary set of attitude sensors comprises a sun sensor and a beacon-receiving antenna that receives a beacon signal from a beacon station on the earth. The beacon-receiving antenna is typically realized with several similar antenna beams that are arranged in a pattern such as the three-beam pattern
50
of FIG.
3
.
In this pattern, the beam widths of the three beams are represented by similar circles
51
which define beam points that have a common power level and are arranged to intersect at a beacon-receiving boresight
52
. Other exemplary beacon-receiving beam patterns in
FIG. 3
are the four-beam pattern
54
and the four-beam pattern
56
which also define beacon-receiving boresights
52
. Because each beam's power slope increases off-peak, the beam patterns of
FIG. 3
form sensitive beacon-receiving antennas.
The field-of-view of a beacon-receiving antenna is defined as the area over which it provides a useful attitude signal and is substantially determined by signal-to-noise considerations. The three-beam pattern
50
forms a substantially-triangular field-of-view
53
and the four-beam pattern
54
forms a substantially square field-of-view
55
. The four-beam pattern
56
includes a first pair of beams
57
that are alternated with a second pair of antenna beams
58
. Because each of the second pair has a beam width that is substantially broader than that of each of the first pair, the four-beam pattern
56
forms an elongate field-of-view
59
. Typically, the fields-of-view of beacon-receiving antennas (e.g.,
53
,
55
and
59
in
FIG. 3
) have been enlarged to a size that insures they will contain the beacon-station window (e.g.,
44
in
FIGS. 2B and 2C
) throughout a communication spacecraft's orbit.
FIG. 4
is a view of elements within the curved line
4
of FIG.
1
.
FIG. 4
illustrates that the antenna system
30
generates a payload beam
60
that has a payload vector such as the payload-beam boresight
62
fixed in the payload beam. The payload beam illuminates a payload footprint
64
on the earth
22
that is preferably identical to a communication service area. It has been found that the service error (between the footprint and the communication service area) is reduced if the payload vector is directed over each solar day at a subterranean target
66
as taught, for example, in U.S. Pat. No. 6,135,389.
The spacecraft
20
has two attitude sensors in the form of a sun sensor
68
and a beacon-receiving antenna that is realized with the antenna system
30
. The sun sensor
68
is preferably one having a wide field-of-view (e.g., 120°) that can provide an attitude signal for a significant portion (e.g., 4-6 hours) of each solar day. The spacecraft's attitude control system preferably includes a gyroscope system that estimates attitude about the yaw axis for the remaining portions of the day. With attitude input signals from the sun sensor and the gyroscope system, the spacecraft's attitude control system is able to control the spacecraft's attitude about its yaw axis which is generally coaxial with the payload-beam boresight
62
.
The beacon-receiving antenna (part of the system
30
) has a beacon-receiving boresight
70
which generally differs from a beacon line-of-sight
72
from the spacecraft
20
to a beacon station
74
which radiates a beacon signal. The beacon-receiving antenna provides a difference signal which corresponds to the difference angle
75
between the beacon-receiving boresight
70
and the beacon line-of-sight
72
. The difference signal is a useful attitude signal over the beacon-receiving antenna's field-of-view (e.g.,
59
in FIG.
3
). With the difference signal and yaw information from the sun sensor and gyroscope system, the spacecraft's attitude-control system is programmed to direct the payload-beam boresight
62
at the target
66
over each solar day.
The payload beam is typically formed with a plurality of spot beams (e.g., on the order of
200
). Reduction of service error has typically been realized by uploading beam coefficients and beam weights throughout the solar day that appropriately steer and reshape the spot beams. The specifications of many modern communication systems, however, are quite demanding and these systems have generally observed that the service error remains excessive and further reduction of service error would be useful.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to methods and structures that enhance service attitude accuracy of inclined-orbit spacecraft and, thereby, f
Fowell Richard
Wang Grant
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