SpaceDev

Abstract: This paper describes a simple satellite attitude controller designed to run on a fairly low capability processor. The controller is intended to be used in a contingency mode when the spacecraft flight computer and full sensor and actuator suite are not available, making use of only analog-output sensors (three-axis magnetometer and sun sensors) and analog-input actuators (magnetic torque rods or coils). The controller is designed to keep an axis near the principal axis of maximum inertia pointed at the sun. Because control torques may only be applied in a plane perpendicular to the local geomagnetic field the control scheme uses a very low control bandwidth in conjunction with gyroscopic spin stabilization. While originally designed for a satellite in a high-inclination orbit, the spacecraft was adapted for a new mission in a nearly equatorial orbit, where purely magnetic attitude control is not typically used because the direction of the local geomagnetic field is nearly constant during an orbit. This paper reviews the design of the original controller as well as the modifications which were made to improve performance in the nearly equatorial orbit. Statistical performance results based on Monte Carlo simulations are included.

Abstract: The Cosmic Hot Interstellar Plasma Spectrometer satellite (CHIPSat) was launched on January 12, 2003 and is successfully accomplishing its mission. CHIPS is NASA’s first-ever University-Class Explorer (UNEX) project, and is performed through a grant to the University of California at Berkeley (UCB) Space Sciences Laboratory (SSL). As a small start-up aerospace company, SpaceDev was awarded responsibility for a low-cost spacecraft and mission design, build, integration and test, and mission operations. The company leveraged past small satellite mission experiences to help design a robust small spacecraft system architecture. In addition, they utilized common industry hardware and software standards to facilitate design implementation, integration, and test of the bus, including the use of TCP/IP protocols and the Internet for end-to-end satellite communications. The approach called for a single-string design except in critical areas, the use of COTS parts to incorporate the latest proven technologies in commercial electronics, and the establishment of a working system as quickly as possible in order to maximize test hours prior to launch. Furthermore, automated ground systems were combined with table-configured onboard software to allow for "hands-off" mission operations. During nominal operations, the CHIPSat spacecraft uses a 3-axis stabilized zero-momentum bias "Nominal" mode. The secondary mode is a "Safehold" mode where fixed "keep-alive" arrays maintain enough power to operate the essential spacecraft bus in any attitude and spin condition, and no a-priori attitude knowledge is required to recover. Due to the omnidirectional antenna design, communications are robust in “Safehold” mode, including the transmission of basic housekeeping data at a duty cycle that is adjusted based on available solar power. This design enables the entire mission to be spent in "Observation Mode" with timed pointing files mapping the sky as desired unless an anomalous event upsets the health of the bus such that the spacecraft system toggles back to "Safehold". In all conditions, spacecraft operations do not require any time-critical operator involvement. This paper will examine the results of the first six months of CHIPSat on-orbit operations and measure them against the expectations of the aforementioned design architecture. The end result will be a "lessons learned" account of a 3-axis sun-pointing small spacecraft design architecture that will be useful for future science missions.

Abstract: Systems and methods for are adapted for automatic implementation of exclusion zone avoidance for target-tracking vehicles, such as spacecraft. The systems and methods are configured to monitor pointing commands (commanded attitude and angular rates) generated for target tracking, and modify these commands as necessary to avoid pointing a boresight into an exclusion zone.

Abstract: This paper presents a robust fault protection strategy for a low-cost single-string spacecraft that makes extensive use of COTS components. These components include commercial processors and microcontrollers that would traditionally be considered inappropriate for use in space. By crafting an avionics architecture that employs multiple distributed processors, and coupling this with an appropriate fault protection strategy, even a single-string COTS-based spacecraft can be made reasonably robust. The fault protection strategy is designed to trap faults at the highest possible level while preserving the maximum amount of spacecraft functionality, and can autonomously isolate and correct minor faults without ground intervention. For more serious faults, the vehicle is always placed in a safe configuration until the ground can diagnose the anomaly and recover the spacecraft. This paper will show how a multi-tiered fault protection strategy can be used to mitigate the risk of flying COTS components that were never intended for use in the space environment.