Aeronautics and astronautics – Spacecraft – Attitude control
Reexamination Certificate
2001-01-29
2003-07-08
Barefoot, Galen L. (Department: 3644)
Aeronautics and astronautics
Spacecraft
Attitude control
C244S168000, C244S164000
Reexamination Certificate
active
06588708
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to spacecraft and, more particularly, to acquiring and determining spacecraft attitudes.
2. Description of the Related Art
FIG. 1
is a diagram
20
that illustrates exemplary orbits of a spacecraft about an Earth
22
. The spacecraft is shown both as a spacecraft
24
D with its solar wings
25
in a deployed configuration wherein they extend from its body
26
and as a spacecraft
24
S with its solar wings in a stowed configuration wherein they adjoin the body. The spacecraft is initially launched along a launch path
27
into a parking orbit
28
prior to its installation into a final orbit such as a geosynchronous orbit (GEO)
30
. Transfer between the parking orbit
28
and the GEO
30
is realized along transfer orbits such as exemplary transfer orbits
32
and
34
.
The elliptical transfer orbit
32
has a perigee
36
tangent to the parking orbit
28
and an apogee
37
tangent to the GEO
30
. Insertion of the spacecraft into and out of the transfer orbit
32
is typically accomplished with a motor that is generally referred to as an apogee motor which is of a motor type (e.g., solid propellant motor and bi-propellant liquid motor) that can realize a large thrust for a short time. The apogee motor is fired at the perigee
36
to attain a spacecraft velocity appropriate at that altitude for the transfer orbit
32
and is fired again at the apogee
37
to attain a spacecraft velocity appropriate at that altitude for the GEO
30
.
Because the apogee motor does not require large amounts of electrical power, the solar wings
25
are typically in a stowed configuration during the transfer orbit
32
and are then extended into a deployed configuration in the GEO
30
. The stowed configuration also protects the fragile solar wings from the acceleration of the apogee motor and from contamination by propellants of the apogee motor.
In contrast to the transfer orbit
32
, the transfer orbit
34
has an apogee
38
whose altitude exceeds that of the GEO
30
. This transfer orbit is typically converted to the GEO
30
by continuous firing of a low thrust engine such as an ion propulsion thruster. A thruster of this type must generate large electrostatic fields over long periods of time and, accordingly, the solar wings are in their deployed configuration during the transfer orbit
34
to provide the necessary electrical power. They are not in danger of being damaged during this transfer because the ion thrust is extremely low.
The attitude of the spacecraft
24
D is typically defined with reference to a body-fixed system of three orthogonal axes which are shown in the enlarged view of
FIG. 2A
to be a yaw axis
42
, a roll axis
43
and a pitch axis
44
. The axes are fixed relative to the spacecraft's body
26
and their arrowheads indicate the generally-accepted positive directions of the axes. The solar wings
25
extend from opposite sides of the body
26
, rotate about the pitch axis
43
and carry an array
40
of solar cells on one surface to facilitate power generation. Because the solar wings generally have considerable length, they are illustrated in a shortened form in FIG.
2
A.
When the spacecraft is in the GEO
30
of
FIG. 1
, its attitude must be carefully controlled to maintain it in a “service attitude” that permits it to carry out the service operations for which it was designed. An exemplary service attitude directs the yaw axis
42
at the Earth
22
with the roll axis
43
in the plane of the GEO and the pitch axis
44
orthogonal to the GEO plane.
Various anomalies in a spacecraft's attitude control system can cause it to depart from its service attitude and, further, to take on an unknown attitude in which its attitude sensors (e.g., star trackers) fail to provide attitude information. To prevent failure of the spacecraft and its operations, the spacecraft must promptly acquire an attitude in which its solar wings generate sufficient power to maintain a viable spacecraft. In addition, the spacecraft must determine its attitude so that it can be subsequently urged to its service attitude.
Spacecraft and their operation are generally expensive endeavors so that loss of the service attitude is of great concern. In a communication spacecraft, for example, revenues and customers are lost when the spacecraft's service is interrupted. The spacecraft's “return-to-service” time must be reduced to minimize these costs. When the service attitude is lost, it is therefore important to not only acquire and determine a power-safe attitude but to do it promptly.
Although the term “service” is typically applied to operations conducted in a spacecraft's permanent orbit, it is used herein to also indicate service operations during a preliminary orbit such as the transfer orbit
32
of FIG.
1
. When it is in this orbit, the enlarged view of
FIG. 2B
shows that the solar wings of the spacecraft
24
S are stowed to adjoin opposite sides of its body
25
and arranged so that a portion
46
, of each solar cell array is parallel to the yaw axis
42
. Because the spacecraft can be permanently lost if its attitude is not properly controlled throughout the transfer orbit
32
, it is important to promptly acquire a power-safe attitude when in the transfer orbit
32
.
Various methods have been proposed for acquiring and determining spacecraft attitudes. A method for determining the instantaneous attitude of a spinning spacecraft, for example, is disclosed in U.S. Pat. No. 5,020,744 (issued on Jun. 4, 1991 to Schwarzschild). The method requires inputs from a sun sensor, an earth sensor and a 3-axis gyroscope assembly. U.S. Pat. No. 5,255,879 (issued on Oct. 26, 1993 to Yocum, et al.) provides a method for directing the roll axis of a spacecraft along the sun line but it requires that a spacecraft carry three single-axis sun sensors.
A method for determining a spacecraft's attitude is provided by U.S. Pat. No. 5,412,574 (issued on May 2, 1995 to Bender, et al.). This method requires a terrestrial sensor (e.g., an earth sensor or a beacon sensor) and at least one star tracker or a cross-link sensor. U.S. Pat. No. 5,597,142 (issued Jan. 28, 1997 to Leung, et al.) teaches the use of a sun sensor, an earth sensor and a 3-axis gyroscope assembly to obtain a desired spacecraft attitude. Finally, U.S. Pat. No. 5,865,402 (issued Feb. 2, 1999 to Fischer et al.) discloses a method of acquiring a spacecraft attitude with a sun sensor, an earth sensor and a direction vector measurement device such as a star sensor or a magnetometer.
As evidenced in these examples, conventional methods for acquiring and determining spacecraft attitudes have typically:
a) required numerous attitude sensors which must often be added to those used in other spacecraft operations,
b) required numerous sequential maneuvers, and
c) reached a power, safe attitude that significantly departs from the spacecraft's service attitude and, therefore, lengthens the return-to-service time.
Because weight and space are limited assets in spacecraft and because increased return-to-service time causes loss of revenue, it is of significant importance to provide improved methods for acquiring and determining power-safe spacecraft attitudes.
SUMMARY OF THE INVENTION
The present invention is of particular use in spacecraft that have, for any reason, lost the spacecraft's service attitude that permits it to carry out the service operations for which it was designed. As part of this loss of service attitude, the spacecraft has typically also lost knowledge of its attitude, i.e., it cannot determine its attitude. In these cases, it is imperative that the spacecraft is quickly returned to its service attitude to minimize loss of revenue and, in extreme cases, loss of the spacecraft.
The present invention provides methods and structures for acquiring and determining a “power-safe attitude”—that being an attitude in which wing current is sufficient to support the spacecraft's housekeeping operations and from
Li Rongsheng
Needelman David D.
Wang Grant
Wu Yeong-Wei
Barefoot Galen L.
Gates & Cooper LLP
The Boeing Company
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