A New Attitude Control Mechanism for Leo Satellites

ORBCOMM is a LEO communications spacecraft, providing global point to point packet messaging. Two are in operation now, and the rest of the (eventually 36) constellation design spacecraft are currently in integration and test. The ORBCOMM ACS mission is to point a communications antenna to nadir, while maintaining continuous Sun tracking via spacecraft yaw maneuvers. This mission is achieved using almost entirely magnetic control~ providing a new standard of performance for this type of system. OSC's new design includes substantially improved attitude determination and control relative to the first two spacecraft. Using knowledge gained from the on orbit performance of Microlab 1, and ORBCOMM 1 & 2, both algorithms and hardware design have been augmented to provide accurate pointing without adding any mass. A unique new attitude determination and control architecture and supporting actuators were developed to meet these severe objectives. Accuracy is more than doubled, robustness to sensor and actuator failures (though there have been none so far) is added and autonomy is increased. Perhaps most important, tolerance to magnetic disturbances has been increased threefold, replacing manufacturing restrictions and complexity with onboard estimation. Since substantial gains have been made through improved algorithms, some of these benefits are retroactively available to the spacecraft already on orbit. Further, these improvements enable a new class of spacecraft bus that provides three axis pointing in a 60 lb. weight class "core vehicle." OSC hopes to use this new core vehicle to provide flexible control systems support to a family of spacecraft, hosting a variety of sensor payloads. This paper describes the hardware and algorithms used to achieve the ORBCOMM Constellation mission: it specifically provides a reference example for effective nadir pointing, yaw steering spacecraft design, using substantially magnetic controls. 1 The ORBCOMM Spacecraft and ACS Mission ORBCOMM is a lightweight, low Earth orbit satellite constellation providing global messaging service. Each spacecraft forms a packetized communications link between a remote subscriber terminal and a "gateway Earth station" which provides access to landline communications. In support of this mission, the attitude control system onboard ORBCOMM spacecraft is tasked to autonomously point a body fixed communications antenna to nadir. The spacecraft is shaped like a hockey puck, with two circular solar panels that deploy out 90 degrees from either face of the puck. With the antenna pointing to nadir and the disk oriented vertically, edge on to the Sun, the solar panels can be articulated about the spacecraft y axis to track the Sun. The figure at right gives a general spacecraft layout. showing the antenna and solar panel configuration. Final Configuration