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7 Guidance and Control Technology BACKGROUND AND STATUS The function of the guidance and control system is to determine and control a spacecraft's position, attitude, and directional and angular velocity. A guidance and control system consists of sensors to measure required parameters, signal transducers and transmission circuitry to connect elements of the system, processors, storage devices, and electronics and actuators to effect control. As a result of significant investments by ARPA, BMDO, and corporate independent research and development, near-term NASA space missions are unlikely to be precluded or seriously inhibited by shortcomings in guidance and control devices, components, or subsystems. Fortunately, by leveraging the past research and development by DoD and industry, NASA has brought many key devices, components, and subsystems appropriate for small spacecraft to a level where they could be ready for use in a short time and at reasonable cost. Some equipment will be flown, essentially in commercial form, within the next two years. Pertinent designs of key devices, components, and subsystems should be completed, documented, and appropriately proof- tested. In this regard, the pane} considers the proposed TIMED program to be critical; it should be augmented with sufficient funds to ensure adequate "validation" and full documentation of hardware and software. However, for NASA to take full advantage of these developments, additional funding for space qualification; radiation hardening; adaptation; and, in some cases, further refinement, is required. in the present austere funding environment, NASA cannot depend on DoD technology as it has in the past. If existing components and subsystems are not qualified for space use, in the future, payload size and performance will be limited on small spacecraft. In addition to a short-term program to capitalize on existing guidance and control designs and developments, the pane! considers it important to maintain some level of effort on longer-term, high-potential developments to ensure that breakthrough opportunities are not overlooked. Also, and most important, attention needs be paid to ongoing development in other arenas (defense, commercial, Federal Aviation Administration) that may be of value to NASA if properly qualified. 57
58 Technology for Small Spacecraft Component requirements and system considerations vary with mission; however, many key guidance and control elements are common to many missions and are discussed below. GUII)ANCE AND CONTROL COMPONENTS Gyroscopes Gyroscopes are used to determine a spacecraft's attitude. Conventional mechanical (rotating mass) gyroscopes have been employed in most spacecraft flown to date. However, as the size of these gyroscopes is reduced, performance is limited. Gyroscopes based on optical techniques have been advanced in Air Force and corporate-sponsored programs to a level where they have displaced mechanical gyroscope-based systems in many applications, for example, commercial aircraft navigation. Two types of optical gyroscopes are gaining acceptance for space missions: ring laser gyroscopes and fiber-optic gyroscopes. Each is based on measuring the difference in time taken for two beams of light to complete a circular path when the beams are moving in opposite directions, and the medium in which they are moving is rotating. Ring laser gyroscopes were developed first. They are offered commercially by various companies, such as Litton, Kearfott, and Honeywell. A ring laser gyroscope is flying on the Clementine spacecraft and is scheduled for the proposed TIMED mission (see Appendix D). The NASA effort on ring laser gyroscopes has been limited largely to procurement and testing. The primary shortcomings of these gyroscopes are the difficulty and cost of achieving and maintaining the necessary mechanical alignment. Interferometr~c fiber-optic gyroscopes (also called fiber-optic-rotation-sensor gyroscopes), while not as fully developed as ring laser units, are considered to have greater promise than ring laser gyroscopes. Interferometric fiber-optic gyroscopes do not have the severe mechanical tolerances of ring laser gyroscopes. Design and fabrication are relatively simple and readily adaptable for different levels of performance. Interfaces for interferometr~c fiber-optic gyroscopes also can benefit from the ongoing development in optical communication. Interferometric fiber-optic gyroscopes employ optical fibers and electro-optical transducers similar to those used in optical communication links and, hence, will continue to benefit from ongoing developments in commercial communications. Developers believe that necessary performance and radiation resistance in interferometric fiber-optic gyroscopes are readily achievable with further effort. Company-sponsored development programs are underway at several locations, such as the Charles Stark Draper Laboratory, Litton, and Honeywell. JPL, with U.S. Air Force sponsorship, is conducting a developmental program in-house and at Lawrence lLivermore Laboratory. GSFC has an experimental program and has scheduled an interferometric fiber-optic gyroscope to fly on the proposed NASA TIMED spacecraft. Tnterferometric fiber-optic gyroscopes are flying on the BMDO Clementine mission and on the
Guidance and Control Technology ARPA/U.S. Air Force Technology for Autonomous Operational Survivability (TAOS) spacecraft (see Appendix D). A quartz hemispherical vibrating gyroscope has been developed by Hughes that appears simple, rugged, and inexpensive. Evaluation for space applications would be worthwhile. Micromechanical (vibrating) gyroscopes, fabricated with semiconductor manufacturing techniques, offer longer-term potential. A modest development and qualification effort on such devices could result in a major breakthrough in size, weight, and cost. Small programs are currently underway at the Charles Stark Draper Laboratory and JPL on in-house funds. Trackers Trackers, like gyroscopes, are used to determine spacecraft attitude. Sun and horizon trackers are being used extensively in space missions with modest attitude accuracy requirements. Star trackers employing focal plane arrays have proven successful in simultaneously tracking a number of stars and establishing attitude to a high degree of accuracy. Focal plane array star trackers have a wide field of view and can track a target body as well as reference stars, thereby eliminating transmission errors between the attitude reference and the target sensor. Since detector arrays are important for commercial applications, continued development and improvement is ensured. As a result of U.S. Air Force support and in-house-funded research and development at companies such as Ball Aerospace and Hughes, trackers of a size suitable for small spacecraft are now available. While current performance falls short of that which is desired for many applications, the commercial effort on detector arrays is almost certain to improve the discrimination and accuracy achievable in the near future. Even though flight tests of these devices are currently scheduled on the proposed NASA TIMED and the ongoing BMDO Clementine missions, the pane! considers it desirable to thoroughly test and document the designs to assure future availability (NRL/NCST, 1993; Ryschkewitsch and Plotkin, 19931. Accelerometers Accelerometers for small spacecraft do not appear to be a limiting item in the foreseeable future. Developments for other markets should satisfy space requirements. Reaction Wheels and Cones o! Moment; Gyroscopes Reaction wheels and control moment gyroscopes provide torque to correct and maintain spacecraft attitude. Since reaction wheels and control moment gyroscopes are heavy and have a short life, redundant wheels are frequently used to improve reliability, so
60 Technology for Small Spacecraft thereby intensifying the weight problem. Several small programs are underway to introduce magnetic bearings to increase the lifetime, but this adds complexity, cost, and weight. A number of smaller and lighter-weight reaction wheels are becoming available (e.g., from Ball Aerospace and Bendix) that have potential for use on small NASA spacecraft. The pane! believes that conventional bearings, when properly designed, are adequate and superior for most applications. Advantage should be taken of the conventional-bearing design skills in industry and in the Charles Stark Draper Laboratory, and the development of magnetic-bearing reaction wheels should be limited to those programs requiring the special properties of such bearings, namely, very long life and lower level of vibration. Magnetic bearings could become important, but the complexity of associated electronics, the added power requirements, and the increased cost and weight are disadvantages, particularly if properly designed conventional bearings can satisfy the requirements. Thrusters Thrusters are employed to correct and maintain the position and attitude of a spacecraft. They are discussed in Chapter 3 of this report. Control Electronics System architecture and spacecraft and data collection control electronics are largely mission/spacecraft specific, although some software, some electronics standards, and the general system approach carries over from spacecraft to spacecraft. As a result, system design and control electronics (levelonment for NASA scientific spacecraft are largely done at NASA centers like JAIL and USED. Designs reflect advances In the commercial world; they employ current microelectronics, packaging techniques, and automated design aids but recognize the special environmental, weight, and power requirements of space. With the exception of radiation hardening, NASA should be able to depend on industry for advancing the state of the art in control electronics. Hardened Solid-State Processor and Recorder Processors and recorders are user! for controlling the spacecraft and for storing and processing data. Computers and recorders of large capacity are advantageous, if not necessary, for control, storage, anal processing of data; spacecraft health monitoring; and autonomous operation. Although impressive advances in processing capability and storage capacity have been made for terrestrial uses, for space-based application these crevices must be hardened against radiation. Several reasonably modern, solid-state recorders and 32-bit computers have been hardened; the level of hardening and the missions for which this equipment is suitable should be established and designs documented.
Guidance aru' Control Technology Commercial requirements will ensure continued progress in capacity and speed. With the exception of radiation hardening, NASA can depend on industry for advancing the state of the art. However, since the processor/memory field is changing so rapidly and computer hardening has proven to be a major effort in the past, it is critical that an ongoing effort in processor/memory technology be ensured to develop means for simplifying hardening of new designs. Global Positioning System (GPS) Although developed for military use, current orbital-based guidance and navigation systems such as GPS and its Russian counterpart (GLONASS) provide extremely precise positional information for spacecraft within the operating range of the GPS constellation and now are available for civilian use. When viewed as position sensors, GPS receivers working in differential mode offer about i-meter accuracy for Tow-Earth-orbit spacecraft. Current JPL results suggest accuracies better than 10 centimeters under ideal circumstances. Since the GPS constellation is in a 20,000- kilometer orbit, precision is degraded for spacecraft with orbits significantly higher than 2,000 kilometers, although precision of a few tens of meters can be obtained even at geosynchronous Earth orbit. Relative positional precision can be further extender! to the centimeter level by a variety of differential techniques. These include such schemes as combining GPS receivers and ground-based (or other spacecraft-based) GPS transmitters with known locations, the use of antenna arrays, and relative measurements using GPS carrier waves. With this technique, the system is referred to as Differential Global Positioning System (DGPS). The information obtained using the various differential techniques can then be used to develop velocity, attitude, and even attitude rate signals, as well as extremely precise relative position data. Several specific schemes yielding position, velocity, and attitude information have been demonstrated experimentally with aircraft and ground vehicles. In fact, such uses are now being seriously considered for future low-Earth-orbit spacecraft. GPS could be applicable to several aspects of a mission. Combining GPS and an inertial measurement unit (with gyroscopes, accelerometers, or trackers) offers major advantages by bounding errors of the inertial set, providing exceptionally good long-term references and thereby ensuring precise, on-board navigation and, with appropriate complimentary techniques, providing a higher level of redundancy and/or accuracy for position, velocity, and attitude. GPS systems may enable a combination of several small spacecraft to serve as a surrogate for one very large spacecraft by providing time and position connections between sets of data gathered by the different small spacecraft. GPS could be used to simplify range safety during launch by eliminating the ground-based radar systems, or it could be used to assist in the maintenance of orbit position by determining orbit. The search for GPS applications has become an enormously fertile and expansive area. Receivers are already sufficiently compact and inexpensive to be applied to all manner of visionary systems, and alley promise to become even more available as 61
62 Technology for Small Spacecraft production levels soar. Because of the enormous market potential, transmitter and receiver technologies for GPS are advancing rapidly on many fronts. Unquestionably, commercial and additional military developments will serve to further advance the component technology. Consequently, no NASA assistance is needed at the component level. On the other hand, the application of this technology to small spacecraft will require novel systems engineering developments. Accordingly, it is recommended that funding emphasize system considerations, many of which will be unique to NASA, rather than emphasizing component technologies. The potential payoffs in weight, volume, and power savings from utilizing GPS can be substantial. GPS can, in fact, conceivably eliminate the need for components such as star trackers, Earth sensors, sun sensors, accelerometers, and rate gyroscopes in some spacecraft system arrangements. AUTONOMOUS SYSTEMS Advances in computing and data storage make possible on-board processing of data and instructions, which reduces the communication load and increases the opportunity for risk reduction through bit checking, redundancy, and backup systems. Technologies for autonomous operations are discussed in Chapter 2 of this report. RADIATION HARDENING The requirement for radiation hard and tolerant systems presents the major complication in adapting commercial products for spacecraft use and, hence, limits the opportunity for designers to use currently available hardware and software. Further, the uncertainties associated with radiation effects have, in the past, resulted in space flight being a necessary cart of a test Program. it is understood that some space flight testing _ ~ ~ ~ ~ - - -I--- - 0 will always be required, but hopefully not the extended space testing that was necessary in the past. Considerable progress has been made in predicting radiation effects and in ground testing, enabling radiation hard and tolerant design and reducing the need for flight tests. Funding for expanding and documenting these techniques and making design tools readily available could result in earlier technology insertion and substantially reduced program costs. The cooperative effort of GSFC and JPL in this field should be encouraged and expanded. ELECTRONICS PACKAGING Two trends are having a profound influence on the size and capacity of electronic circuitry: the combining of functions on individual semiconductor chips and the dense packaging of chips on stacked boards. A number of packaging designs are being pursued by both government and industry and should be continued. One design, employed by the
Guidance and Control Technology Charles Stark Draper Laboratory and others, appears to offer major advantages and development should be encouraged. This technique employs uncased chips; provides embedded electrical and electro-optical interconnecting circuitry; and results in a rugged, dense, easily tested package. Development is being funded largely on corporate independent research and development, with a small program underway at GSFC. INTERFACES Considerable emphasis has been placed on spacecraft bus standards by industry and government. The proliferation of bus standards suggests that standardization at this level may involve unacceptable compromise, except by class of application (e.g., satellite communications). Alternately, standardization of components and system architecture offers greater opportunity for time and cost savings and shouic} be pursued. However, even at this level, differences such as level of radiation exposure will necessitate deviation. Standardization at the interface level, with the resultant reduction in interface negotiation and documentation; integration; checkout effort and time; and cabling will produce the majority of cost savings (Krueger, 19931. The broad acceptance of Military Standards 1553 and 1773, indicates what can be accomplished. The many advantages of imbiber optics for a data bus dictate that emphasis should be on this approach. An existing standard electro-optical bus Military Standard 1773, has clemonstrated weight ant! power savings and reduced radio frequency/electromagnetic interference. Simple redundancy and fault tolerance have been achieved. Further, electro-optical transceivers and i~iber- optic cables stand to benefit from new developments in commercial communications, a rapidly evolving field. FINDINGS AND PRIORITIZED RECOMMENDATIONS In the recent past, DoD, through ARPA and BMDO, has funded major efforts on spacecraft and their guidance and control systems ant! components. This not only provided direct support to activities of interest to NASA but also promised a significant market in which industry was willing to invest. NASA has accordingly been in the enviable position of being able to procure equipment that could, with modest effort, be tested and qualified for its applications. This has had two effects. On the positive side, NASA's requirement for development funds was reduced. The negative side is that the development efforts have generally been funded as part of specific programs, where the natural tendency of a project manager to avoid risk and limit the cost and schedule impact of new technology often results in use of obsolete technology. Also, new technology, when accepted, tends to be mission specific. As DoD and corporate independent research and development activities are reduced, NASA's technology development requirements also are changing. If the goal of smaller, less expensive, and 63
64 Technology for Small Spacecraft more frequent spacecraft is to be achieved, NASA must now assume a larger role in research and development. To circumvent the problems inherent in the introduction of new technology or the use of older mission-specific, but adaptable, hardware, some of the NASA funding necessitated by the changed environment should be applied to the development and qualification of components and subsystems suitable for multimission applications and indepenclent of specific programs. To maintain the focus on timeliness and requirements some funding might be provided to other than NASA research centers. The full implications or the potential impact of GPS have not yet been completely recognized. The availability of GPS and the rapidly developing capability of associated products will have a profound influence on the applications and effectiveness of small spacecraft and, hence, deserve special attention. GPS, used in various combinations with other guidance components, will afford drastic reduction in size and weight and improvement of performance over current systems. GPS is likely to revolutionize guidance and control equipment and capability, at least for low Earth orbit. Further, GPS in differential modes can possibly enable the use of several small spacecraft instead of a large spacecraft for some missions. Products based on current technology are frequently excluded from critical roles in missions because radiation effects on the technology are uncleaned, which leads to fear that failure will occur in the flight environment. Even items that have been flown may be excluded because of longer mission duration or a different radiation environment. Potential small spacecraft applications are compromised by the size and weight of space- qualif~ed hardware. Smaller, lighter models exist and in some cases have been flown, but final design and proofing have not been completed. Investment in high-risk, high-payoff technology is limited. With reduced spending for DoD and corporate indepenclent research and development, additional funding for NASA will be required. In order for NASA to enhance small spacecraft systems for guidance and control, the Pane! on Small Spacecraft Technology makes the following prioritized recommendations. I. A high-priority program to realize the potential of GPS on small spacecraft should be established. The unique combination of capability and small size made possible by integrating GPS receivers/processors with other existing and emerging guidance components should be assessed. 2. The design, documentation, and appropriate qualification of the following components and subsystems should be completed: . fiber-optic interferometric gyroscope; miniature focal plane array star tracker; space-hardened GPS receiver/processor with attitude capability; advanced, miniaturized small reaction wheel; hardened 32-bit processor; and hardened solid-state recorder.
Guidance and Control Technology Advantage should be taken of work in programs such as the proposed NASA TIMED and BMDO Clementine missions. A senate amount of funding could significantly advance the capability of small spacecraft. ~ Design and grouncI-testing techniques should be developed that ensure acceptable performance in the space radiation environment. Additional support should be provided for the work in this field. The payoff in reduced flight-test time and funding will more than compensate for the investment in this effort. Further, the added assurance will encourage project managers to use more current technology. These techniques could be applicable to a broad range of electronic components and systems. 4. The advantages and disadvantages of applying standardization to specific interfaces for electronic and electro-optical components ant! subsystems (e.g., Military Standards 1553 and 1773) to simplify integration activities should be evaluated, and standardization should be implemented as indicated by the evaluation. 65