1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-13a71 Center Point of Contact: GSFCa71 Submitted by: Wil HarkinsSubject: Coordinate Systems for Attitude Determination and Control Practice: This guideline provides a procedure which specifies and documents consistent, use
2、ful, and well-defined coordinate system (or frame) definitions for spacecraft attitude control design and analysis. Several example coordinate frames and transformations are presented to show how these definitions are used to address various Attitude Control System (ACS) design issues. Past experien
3、ce has shown the most efficient convention varies from project to project as a function of mission type, constraints, and performance requirements. This procedure addresses the process and documentation to reliably define the most efficient reference frame convention for a given mission or spacecraf
4、t.Programs that Certify Usage: N/ACenter to Contact for Information: GSFCImplementation Method: This Lesson Learned is based on Reliability Guideline Number GD-ED-2211 from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:The primary benefit is increa
5、sed mission reliability due to a reduction in design errors occurring during spacecraft development caused by inconsistent coordinate frame definitions. A document will Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-be created early in the developme
6、nt of a spacecraft mission defining Attitude Control System (ACS) coordinate frames which will facilitate data transfer among subsystem engineers, speed documentation and communication during design and analysis reviews, expedite verification of instrument and sensor pointing, and assure that a reco
7、rd of the coordinate frames used will be available throughout mission planning, design, analysis, and flight.Implementation Method:Early in the development stages of a mission program, a document should be created, published, and distributed to all ACS and ACS related mission engineers. This documen
8、t will list coordinate frame definitions needed for ACS design and analysis. It should also be periodically updated as mission objectives evolve and hardware changes are made. The following discusses ACS coordinate frame definitions and the format for listing them in the ACS Coordinate Frames Defini
9、tion Document.1. Overview of Coordinate Frame Definitions for ACS Design and Analysis:ACS coordinate frames contain an origin location and three unit vectors emanating from that origin. “The most convenient set of these vectors is a dextral (i.e., right-handed), orthonormal (i.e., mutually perpendic
10、ular and of unit length) triad“ reference 4, p. 6. Vector quantities can be expressed as projections onto each of the three triad unit vectors of a coordinate frame. Triads or frames can be related to each other through the use of rotation matrices reference 4, pp. 8-10, thus permitting the expressi
11、on of vectors in any desired frame. With the use of coordinate frames and vectors, the orientation and changes in orientation of spacecraft, celestial bodies, instruments, mechanisms, and other ACS related hardware and objects can be described.An overall base coordinate frame must be defined relativ
12、e to which all other coordinate frames (discussed below) are defined. In many cases, this overall base frame will be an inertial frame which is used to determine overall mission success. For example, if the primary mission of the spacecraft is to point instruments at the sun, a good choice for the o
13、verall base frame might be the heliocentric reference frame reference 7, p. 29 since the suns motion can be easily established in this frame.Typically, within the ACS subsystem, several design issues must be addressed. These design issues can often be arranged into categories, such as overall spacec
14、raft pointing; environmental disturbances; spacecraft mass properties; sensor, actuator, and instrument motion; and flexible body dynamics. A category reference frame should be established to address each design issue. For example, when modeling environmental disturbances in Earth orbit, an Earth ce
15、ntered inertial frame is usually used as the category reference frame. For defining the spacecraft mass properties, sensor, actuator, and instrument motion, and flexible body dynamics the category reference is some sort of spacecraft body fixed coordinate frame. If information is to be transferred b
16、etween these ACS categories, transformations can be established through the overall base coordinate frame discussed above.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Additional coordinate frames may be needed to define the motion or effect to be
17、modeled within an ACS category. The effect to be analyzed may be defined in terms of an intermediate axis with this intermediate axis related back to the category reference frame. The coordinate frames needed for defining spacecraft motion within the orbital plane provide a good example of this proc
18、ess. A frame which is fixed to the spacecraft is defined first. This frame is used to define the motion of the spacecraft relative to the orbit plane. Then, a frame which is fixed to the orbital plane is used to define the motion of the orbit plane relative to an inertial frame. The result will dete
19、rmine the spacecraft motion relative to the inertial frame.Another example of the use of intermediate axes for addressing ACS design issues is the relationship among sensor and instrument reference frames. One axis of these frames is almost always defined along the boresight of the sensor or instrum
20、ent. The other two axes should match some other characteristics (e.g., parallel to the edges of a square field of view). The origin is at any convenient point. The relationships of the nominal and “tracking“ (a frame that moves with the boresight to track the sensor motion) boresight frames to the c
21、ategory reference can be achieved in many different ways depending on accuracy and knowledge requirements. Several intermediate frames might be needed to achieve these relationships. Often, both the nominal and tracking boresight frames must be related to a payload interface frame, and all requireme
22、nts of alignment are specified between this interface frame and another frame, the spacecraft optical frame. Typically, the interface frame axes are nominally parallel to the spacecraft optical axes, and the optical axes are defined with respect to an optical master reference cube. The nominal posit
23、ion of this cube relative to the spacecraft mechanical build axes (used for defining hardware locations within the spacecraft) must be defined next. Finally, this mechanical build frame may be used as the category reference or is then related to the category reference. The figure below shows the nom
24、inal orientations of these frames used in the SOHO spacecraft reference 1, p. 2.8.This example demonstrates the process of how coordinate frames are used to define the sensor and instrument pointing relative to its category reference frame.Provided by IHSNot for ResaleNo reproduction or networking p
25、ermitted without license from IHS-,-,-refer to D descriptionD A discussion of the frames needed to model how actuators are used for attitude control is presented as a final example of the use of intermediate frames. Momentum wheels, control moment gyros (CMGs), torque rods, and thrusters are commonl
26、y used control actuators. Frames are needed to represent the nominal orientation and location, misalignments produced when installing, and movement of the actuators. Also, rotation matrices which relate these frames to the category reference, usually the spacecraft ACS axes, must be determined. As a
27、 specific example, consider the frames needed in distributing control torques among a reaction wheel set containing 4 wheels. The wheels are usually aligned in a pyramid configuration as shown below. A frame is first defined for each wheel with one axis along the spin axis of each wheel. Then, rotat
28、ion matrices are created relating each wheel frame to the spacecraft ACS frame (called roll, pitch, and yaw for this case). This example demonstrates how intermediate and category frames are used to relate the orientation and motion of actuators (in this case, reaction wheels) to achieve desired tor
29、ques.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD 2. Document FormatA suggested format or outline for the coordinate frame definitions document is summarized below. However, this format is only a guide, and the user may nee
30、d to change or add to the format depending on the spacecraft mission. Since the choices of ACS coordinate frames to be defined are dependent on the overall spacecraft pointing objectives and the proposed ACS mission hardware required, these topics should be discussed first. To avoid any ambiguity, c
31、oordinate system symbols and nomenclature to be used should be listed next. Specific coordinate frame definitions should follow - an overall base frame, category reference frames, and frames needed within each category. Finally, a way of relating all the coordinate frame definitions should be includ
32、ed.ACS Coordinate Frames Definition OutlineDocument Title Table of Contents Mission Objectives, Requirements, and Criteria for SuccessState overall spacecraft pointing objectives and specificationsOverview of ACS HardwareState what instruments, control actuators, and other mechanisms are being used
33、for sensing, data collection, and control actuationNomenclature and SymbolsDiscuss the nomenclature and symbols to be used for the coordinate frame definitionsOverall Base Frame Definition Define a frame to which all other frames are referenced.Provided by IHSNot for ResaleNo reproduction or network
34、ing permitted without license from IHS-,-,-Category FramesGroup design issues into appropriate categories, e.g., spacecraft, instrument, and sensor pointing, actuator sizing, environmental disturbances, spacecraft mass properties, etc. Within each category, a category reference frame should be liste
35、d along with all other frames needed to address design and analysis issues. Figures showing the physical relationships among these frames would be helpful.Coordinate Frame TransformationsRelate each frame to the overall base frame.The first section of the document (after the table of contents) state
36、s the overall mission objectives and the criteria for a successful operation. The objectives include a list of celestial, Earth based, or other bodies to which the spacecraft and instruments must point. A discussion of the pointing accuracy and knowledge error definitions and specifications for perf
37、ormance needs to be given. Orbit parameters, spacecraft mass properties, and any issues that might affect the mission objectives or success criteria are provided in this section. This section will aid the reader in understanding the rationale behind the choice of coordinate frames.The second section
38、 of the document contains an overview of ACS hardware. Included in this discussion are locations, orientations, and functions of all ACS related hardware. The locations and orientations are best shown with a figure or a reference to an interface drawing. If the hardware moves or reorients itself (su
39、ch as solar array rotation to track the sun) relative to the spacecraft, this change is to be documented. The anticipated effects of flexibility should also be considered.Instrument and attitude sensor functions are given in relation to the overall ACS concept. For example, a magnetometer is used to
40、 determine the magnetic field of the Earth relative to the spacecraft. The location and orientation of the magnetometer relative to the spacecraft needs to be given, along with a statement of how the magnetometer may be used in conjunction with other ACS hardware and software. The magnetometer outpu
41、t may be used for attitude sensing or for determining when to pulse a torque rod to provide an attitude control moment. These different functions for the magnetometer may result in different coordinate frame choices.The third section of the document needs to discuss the nomenclature and symbols to b
42、e used for the coordinate frame definitions. The format may vary depending on the spacecraft mission. An example definition taken from reference 6 p. 4, and shown below, demonstrates a possible format which may be used for defining reference frames. A descriptive or commonly used name is given first
43、. A one or two letter symbol is listed next, which is also used for labeling the vectors comprising the axes of the frame. Then, a description of the frame is provided, and this description is to contain enough detail to unambiguously locate the frame.Provided by IHSNot for ResaleNo reproduction or
44、networking permitted without license from IHS-,-,-Equatorial Inertial Coordinate System, E (E1, E2, E3) This is the basic inertial coordinate system. All other coordinate systems are defined with respect to E. The origin is at the center of the Earth. The E3 axis is in the equatorial plane and it is
45、 positive toward the vernal equinox. The E2 axis is perpendicular to the equatorial plane, and it is positive toward the Earths North Pole. (The E1 axis completes the orthogonal triad.) The vernal equinox position is defined as its mean position at 1950.0.All the frames included in the document are
46、related to the overall base frame. Rotation matrices are commonly used to convert components of vectors from one frame to another, and the development of the mathematics is available in the literature reference 4, pp. 6-31, reference 6, pp. 10-20, and reference 7, pp. 410-420, 758-759. To avoid any
47、ambiguity in the definitions of coordinate frame rotations and their matrices, a discussion of this topic is to be included at the beginning of this section of the document. This discussion should include definitions of Euler angles, quaternions, direction cosine matrices, or other mathematics to be
48、 used to relate the frames. Then, a table or any convenient format is included at the end of the document which contains information relating each frame back to the overall base frame. Finally, figures illustrating the nominal relationships among all these frames and the possible reorientations of t
49、he frames during flight is essential and is included in the document.Technical Rationale:Due to the increased complexity of ACS work for spacecraft, a document is needed in the early stages of the project development which contains consistent and well-defined coordinate system definitions. Definitions are needed to accurately communicate within and between various design and analysis disci
