Attitude and Articulation Control

(From Issue 5, December 1982)

The dual-spin Galileo is a challenging departure from the traditional three-axis stabilized and all-spin spacecraft which have plied the solar system in the past. The configuration combines attributes of both types of vehicle: an inertially stable platform for precisely aiming optical instruments, and the sky-sweeping continuous rotation desirable for fields and particles experiments.

Though gyroscopic stability is a favorable aspect of this system, the dual-spinner poses a unique set of control problems by introducing the complexities of rotational dynamics into the critical pointing control considerations which occur in the three-axis design. Combine these factors with the need for advanced flight software capable of providing autonomous operation and onboard attitude determination, and it is apparent that the Galileo Attitude and Articulation Control Subsystem (AACS) represents the most sophisticated state-of-the-art application of control system technology.

The AACS (see figure) consists of many individual components, all governed by the Attitude Control Electronics (ACE), the heart of which is an ATAC-16MS 16-bit/word microprocessor. The ACE communicates with the Command and Data Subsystem (CDS) via the spacecraft CDS bus, receiving commands and transmitting telemetry as required. The ACE also controls the Retro Propulsion Module (RPM) via the Propulsion Drive Electronics (PDE), which contain the logic and drive electronics for the thruster valves and latching isolation valves.

The AACS utilizes a Star Scanner (SS) mounted to the spinning part of the spacecraft as its primary source of attitude control data. A photomultiplier tube senses star crossings through a V-slit aperture and lens system, providing information which is processed by the AACS software and compared against an onboard star catalog. This is the basis for Galileo attitude determination relative to an inertial coordinate system. Galileo has adopted the Earth Mean Equator of 1950 (EME-50) as its standard reference frame. Command and control of Galileo is facilitated entirely by the AACS through this reference system.

A set of gyroscopes is attached to the scan platform to provide three-axis inertial reference for the platform. This allows the scan platform to compensate for spacecraft motion. Gyro data is also used for spacecraft orientation during periods when the star scanner is not usable, as for example during maneuvers.

Acquisition Sun sensors on the spinning section sweep the sky and generate backup spin rate and Sun direction information. This provides a simple method by which a Sun-pointed orientation can be achieved from any random attitude. This is important for attitude recovery in the event of a failure.

Two accelerometers measure spacecraft velocity changes along the spin axis. This is used to control the magnitude of trajectory correction maneuvers as well as for compensation for non-gravitational forces which affect navigational accuracy. Galileo can also perform lateral velocity changes by timed thruster burns.

The Spin Bearing Assembly (SBA) couples the spinning and nonspinning parts of the Orbiter, referred to as the rotor and the stator, respectively. A brushless DC motor provides the torque necessary to spin or de-spin the stator. By counter-rotating the stator at precisely the rotor's rate of spin, the stator remains fixed in inertial space. The SBA also provides the means to articulate the scan platform, which is mounted on the stator, about the spin axis or "clock" direction. Electrical interfaces between the stator and rotor are contained within the SBA. These consist of slip rings for power transmission and rotary transformers for data transfer.

The Scan Actuator Assembly (SAS) is a bearing assembly with motor similar to the SBA. It is used to articulate the scan platform about an axis perpendicular to the spin axis, known as the "cone" direction. Thus, the scan platform can be pointed as desired by appropriate combinations of SAS and SBA motion commanded by the attitude control electronics. Both the SAS and SBA contain optical encoders which provide very precise position information. The ACE communicates with the SAS, SBA, gyros, and accelerometers through the Despun Control Electronics (DEUCE) unit.

Wobble control of the Orbiter is achieved by changing the angle at which the RTG booms are canted to the spin axis. This is controlled by Linear Boom Actuators (LBA), which utilize stepper motors driven by the ACE.

The AACS must satisfy very stringent requirements imposed by the Galileo mission. In order to provide communication with Earth, the High-Gain Antenna (HGA), which is aligned with the spin axis, must be pointed accurately. Control of the spin axis orientation is also important for the science instruments. Those on the scan platform must be pointed in a manner which corrects for spacecraft motion (wobble, nutation, etc.) as well as the relative angular motion between the spacecraft and a scan platform target (target motion compensation). Additionally, the fields and particles instruments mounted on the rotor require that the spin rate be carefully controlled. Probe release is initialized at a specific attitude and rate, also under the control of the AACS.

The AACS provides measurements of the Orbiter and scan platform attitudes and rates. Particularly remarkable is the fact that the AACS corrects this data for known errors that affect Orbiter and scan platform orientation and converts the information to the EME-50 coordinate system before it is telemetered (sent to Earth). This allows simplification of ground support systems, both in command preparation and pointing reconstruction, thus reducing operational costs.

The design of the AACS incorporates the ability to detect failures and switch to redundant components in order to continue as normal an operation as is possible in an anomalous situation. As was the case with other advanced planetary spacecraft such as Voyager, this is necessary because of the enormous distances and the attendant communications delays which could result in the loss of the spacecraft if there were no provision for self-diagnosis and correction.