Aeronautics and astronautics – Spacecraft – Spacecraft formation – orbit – or interplanetary path
Reexamination Certificate
1999-12-21
2001-10-23
Jordan, Charles T. (Department: 3644)
Aeronautics and astronautics
Spacecraft
Spacecraft formation, orbit, or interplanetary path
C701S226000, C701S013000
Reexamination Certificate
active
06305646
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to systems and methods of stationkeeping satellite orbits, and in particular to a method and system for controlling eccentricity in inclined geosynchronous orbits.
2. Description of the Related Art
Over the past several decades, there has been a dramatic increase in the number of satellites in orbit. These satellites are maintained in different orbits, which are selected to allow the satellite to perform its intended mission.
Geostationary satellite orbits are highly valued, because they allow the satellite to remain in a fixed apparent location from a reference on the earth's surface. When satellites are placed in such orbits, the ground stations can direct the transceiving antenna at a fixed direction.
FIG. 1A
is a diagram of a satellite
104
disposed in an orbit
106
around the earth
102
. The satellite
104
is maintained in the desired orbit
106
by performing stationkeeping satellite maneuvers. These maneuvers can be performed autonomously by the satellite
104
itself using a satellite control system
110
having one or more thrusters
118
and one or more satellite processors
112
implementing one or more instructions for commanding the thrusters
118
to perform stationkeeping maneuvers. Alternatively or in addition to the autonomous stationkeeping technique, a command facility
114
having a command facility processor
116
communicatively coupled to a transmitter
120
can generate and transmit stationkeeping commands to the satellite
104
. Although illustrated as a ground-based command facility, the stationkeeping commands may be transmitted from another spacecraft as well.
FIG. 1B
is a diagram showing a first satellite
104
A in a geostationary orbit
106
A. Due to satellite-to-satellite communication interference issues, satellites in such geostationary orbits
106
A are assigned to geostationary “slots” that are 0.2 degrees wide (±0.1 degree about the nominal longitude). These satellites must remain within the assigned slot.
FIG. 1B
also depicts a second satellite
104
B in a geosynchronous orbit
106
B. Geosynchronous orbits, which are often used for communications to mobile customers (such as with “GEOMOBILE” satellites) are similar to those of geostationary orbits, except, they have a non-zero inclination typically in the range of three to seven degrees.
FIG. 2
is a diagram depicting the ground track
202
of a typical geosynchronous orbit
106
B. The center
204
of the “figure-8” depicted by the ground track
202
is at the equatorial plane of the earth
102
. The satellite
104
B passes through the ground track center
204
twice each day, once at the ascending node
208
and once at the descending node
210
. The motion is more complex in practice, due to orbit eccentricity, drift and perturbing forces. Despite the satellite
104
B motion, interference is still a problem, and the satellite
104
B is therefore still constrained to the ±0.1 degree slot near the equatorial plane. The geosynchronous satellite
104
B cannot stay inside the ±0.1 degree slot all of the time (the width of the ground track itself exceeds the ±0.1 degree window by itself when the orbital inclination exceeds about 4.8 degrees).
This problem is especially critical for current and future generation spacecraft. Such spacecraft often have large solar arrays and solar collectors, and therefore receive a strong solar force. This solar force produces a large steady state eccentricity when a single burn sun-synchronous perigee stationkeeping strategy is used. This eccentricity is difficult to control efficiently, even when a sun facing perigee stationkeeping strategy (which compresses eccentricity using drift control maneuvers) is used to conserve fuel. In some satellites, the east/west longitude excursion due to eccentricity can take up more than half the width of the slot. Other factors also consume slot
108
width, including drift over the maneuver cycle, maneuver execution error, bipropellant momentum dumping disturbances, orbit determination error, and orbit propagation error. A discussion of these contributors is presented in “The Operation and Service of Koreasat-1 in Inclined Orbit,” AIAA paper 98-1352, which reference is incorporated by reference herein. Hence, closer control of the excursion due to eccentricity is required.
There are many possible solutions to this problem. One is to introduce a maneuver scheduler, allowing drift maneuvers to be performed daily. This solution would require more complicated satellite processing. Further, this solution would be difficult to implement for satellites already on-orbit, and would only be helpful at “high-drift” longitudes.
Another possible solution is to use a “two-maneuver” satellite maneuver scheme which performs burns in both tangential directions to maintain tighter control over the eccentricity. However, this solution requires more propellant, additional thrusters, and raises issues of additional plume impingement.
Another potential solution is to implement an axial firing mode in which radial &Dgr;V is used to control eccentricity. This solution uses roughly twice the fuel of a tangential &Dgr;V scheme, and would require modified maneuver planning algorithms, and changes to the attitude control system (ACS).
Another potential solution would be to implement a shorter orbital stationkeeping maneuver interval. This would not provide any propellant savings, but would reduce longitudinal drift between maneuvers. However, this is a manual equivalent to the first proposed solution, and the shortened stationkeeping maneuver interval would increase support operations costs.
As is apparent from the foregoing, there is a need for a stationkeeping method for satellites in geosynchronous orbits that provides the necessary longitudinal control near the equatorial plane, without requiring the operational, hardware, or software costs outlined above. The present invention satisfies that need.
SUMMARY OF THE INVENTION
To address the requirements described above, the present invention discloses a method for controlling the eccentricity of an orbit.
One embodiment of the present invention is described by a method comprising the step of performing a program of eccentricity control maneuvers wherein the control maneuvers are executed at times and in directions selected to apply substantially all eccentricity control along a line of antinodes. In one embodiment, the control maneuvers are tangential maneuvers, and each control maneuver is applied near the line of antinodes. Provision is made for performing additional maneuvers, including node retargeting maneuvers during specified intervals. Another embodiment of the invention is described by a satellite in a substantially geosynchronous orbit having a satellite control system with a processor implementing instructions to perform control maneuvers as described above. Yet another embodiment of the present invention is described by a command facility comprising a processor for implementing instructions to perform the program control maneuvers described above, and a communicatively coupled transmitter for transmitting the commands to the satellite.
REFERENCES:
patent: 4827421 (1989-05-01), Dondl
patent: 4854527 (1989-08-01), Draim
patent: 5120007 (1992-06-01), Pocha et al.
patent: 5124925 (1992-06-01), Gamble et al.
patent: 5326054 (1994-07-01), Turner
patent: 5553816 (1996-09-01), Perrotta
patent: 5568904 (1996-10-01), Brock et al.
patent: 5669585 (1997-09-01), Castiel et al.
Fowell Richard A.
McAllister Jeoffrey R.
Best Christian M.
Gates & Cooper LLP
Hughes Electronics Corporation
Jordan Charles T.
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