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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.
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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
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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
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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.
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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
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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
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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
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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.
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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
Representative terms from entire chapter:
reaction wheels
