Introduction to the spacecraft attitude concept
06-03-2009 06:28

Spacecraft attitude is briefly the angular orientation of a spacecraft body vector with respect to an external reference frame. A spacecraft attitude determination and control system typically uses several sensors and actuators and because attitude is described by three or more variables (apart from the angles, one can add the rotational speed etc), the difference between the desired and the measured state is complicated to be evaluated, most of the time being either undetermined or over-determined.

The attitude of a spacecraft is its orientation in space. The motion of a rigid spacecraft is specified by its position, velocity, attitude and attitude motion. The first two quantities describe the translation motion of the center of mass of the spacecraft and are the subject of what is variously called celestial mechanics, orbit determination, or space navigation, depending on the aspect of the problem that is emphasized. The latter two quantities describe the rotational motion of the body of the spacecraft about the center of mass. In general orbit and attitude are interdependent.
Attitude analysis may be divided into determination, prediction, and control. Attitude determination is the process of computing the orientation of the spacecraft relative to either an inertial reference or some object of interest such as the Earth. This typically involves several types of sensors on each spacecraft and sophisticated data processing. The accuracy limit is usually determined by a combination of processing procedures and spacecraft hardware.
Attitude prediction is the process of forecasting the future orientation of the spacecraft by using dynamical models to extrapolate the attitude history. Here the limiting features are the knowledge of the applied and environmental torques and the accuracy of the mathematical model of spacecraft dynamics and hardware.
Attitude control is the process of orienting the spacecraft in a specified predetermination direction. It consists of two areas – attitude stabilization which is the process of maintaining an existing orientation, and attitude maneuver control which is the process of controlling the reorientation of the spacecraft from one attitude to another. The two areas are not totally distinct, however. The limiting factor for attitude control is typically the performance of the maneuver hardware and the control electronics, although with autonomous control systems, it may be the accuracy of orbit or attitude information.
Some form of attitude determination is required for nearly all spacecrafts. Attitude control is required to avoid solar or atmospheric damage to sensitive components, to control heat dissipation, to point directional antennas and solar panels (for power generation), and to orient rockets used for orbit maneuvers. Typically, the attitude control accuracy necessary for engineering functions is on the order of 1 deg. Attitude requirements for the spacecraft payload are more varied and often more stringent than the engineering requirements . Attitude constraints are more severe when they are the limiting factor in experimental accuracy or when it is desired to reduce the attitude uncertainly to a level such that it is not a factor in payload operation. These requirements may demand accuracy down to a fraction of an arc-second (1/3600 degrees).
A convenient method for categorizing spacecraft is the procedure by which they are stabilized. The simplest procedure is to spin the spacecraft. The angular momentum of a spin-stabilized spacecraft will remain approximately fixed in inertial space for extended periods, because external torques which affect it are extremely small in most cases. However, the rotational orientation of the spacecraft about the spin axis is not controlled in such a system. If the orientation of three mutually perpendicular spacecraft axes must be controlled, then the spacecraft is three axes stabilized. In this case some form of active control is usually required orientation to drift slowly. Three axis stabilized spacecraft may be either non-spinning (fixed in inertial space) of fixed relative to a possibly rotating reference frame, as occurs for an Earth satellite which maintains one face towards the Earth and therefore is spinning at one rotation per orbit. Many missions consist of some phases in which the spacecraft is spin stabilized and some phases in which it is three-axis stabilized. Some spacecraft have multiple components, some of which are spinning and some of which are three –axis stabilized.
From the mission profile point of view, we can identify several profiles.
However, most missions have in common three more or less distinct phases:
1. Launch (and early orbit phase) –consisting of the activities from lift-off until the end of powered flight in a preliminary Earth orbit;
2. Acquisition consisting of orbit and attitude maneuvers and hardware checkout;
3. Mission operations consisting of carrying out the normal activities for which the flight is intended.
Launch is the most distinct and well defined phase and is normally carried out and controlled primarily by personnel concerned with the rocket launch vehicle and who will not be involved in subsequent mission operations. A limited number of launch vehicles are in use. Launch sites are similarly limited, depending either is an equatorial orbit or a polar orbit.
Once powered flight has ended and the spacecraft has separated from most of the launch vehicle, the acquisition phase of maneuvers and testing begins. The acquisition phase can last from a few minutes to several months and may be defined differently depending on the particular aspect of the mission that is involved.
Finally, once the proper orbit and attitude have been obtained and the hardware has been tested, the mission operations phase, in which the spacecraft carries out its basic purpose, is initiated. At this stage, attitude determination and control becomes a routine process. On complex missions, such as lunar or planetary explorations, the acquisition phase may be repeated at various intervals as new hardware or new conditions are introduced.

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