Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
1999-07-07
2001-02-20
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C244S164000, C244S165000, C244S171000, C701S013000
Reexamination Certificate
active
06191728
ABSTRACT:
TECHNICAL FIELD
This invention relates to satellite attitude control, in particular, pointing an orbiting satellite at different ground based locations.
BACKGROUND OF THE INVENTION
Many satellites are reoriented during orbit to point (“target”) different earth sites at specific times, e.g., pan between targets
1
and
2
in the drawing. In the state of the art, the targets are scheduled by an “access time” on the ground and the target location along with the access time are transmitted to satellite through a satellite uplink. As the satellite moves along its orbit, attitude control devices, such as control momentum gyros (CMG), are operated to change the satellites attitude, preferably in a continuous maneuver without stopping at any attitude, to reduce the maneuver time, i.e., target as many locations in the least amount of time and access each target at the scheduled access time. The maneuver requires the use of known Quaternion Transforms (see for example, Bong Wie,
Space Vehicle Dynamics and Control
, AIAA), and satellite ephemeris to define a satellite's orbit location and attitude in a planning algorithm that allots a margin of time to each reorientation maneuver to guarantee, within an acceptable probability of success, that the maneuver will be completed in time to properly site the target, e.g., collect data form the site. But the accumulated margin of time over several targets can allow an additional target to targeted. In other words, the maneuver time margin reduces the number of targets per unit of time. Despite this, the target schedule may not be met for every target because in a typical agile satellite using a so called pseudo inverse to control CMG gimbal angles, the “robustness” or speed at which the satellite can be reoriented varies with the CMG gimbal angles, which determine the available angular momentum. If the gimbal angles bring the pseudo inverse close to a “singular” condition, where the array gain is reduced, the maneuver will take longer that expected.
SUMMARY OF THE INVENTION
Objects of the present invention include providing a technique to increase the number of targets that can be cited in a unit of time.
According to the invention, the margin of error per target is substantially reduced by allowing the control system on the satellite to “reschedule” the time at which it should be pointing at a target (access position) by a fraction of a second to several seconds from the original schedule if the original schedule cannot be met.
According to the invention, the time (access time) is determined separately based on attitude error and reorientation velocity. The time used by the Quaternion and ephemeris is the latest or maximum access time selected from three values: the scheduled access time, the access times for the velocity and the position error.
Other objects benefits and features of the invention will be apparent to one ordinary skill in the art from the drawing and following description.
REFERENCES:
patent: 4599697 (1986-07-01), Chan et al.
patent: 4858858 (1989-08-01), Bruederle
patent: 6047927 (2000-04-01), Heiberg et al.
Blum Theodore M.
Greenstien Robert E.
Honeywell International , Inc.
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