Gordon I. Johnston* and Wayne R. Hudson+
NASA Headquarters, Office of Aeronautics and Space Technology
Washington, D.C., U.S.A.
__________________________
*Program Manager, Global Change Technology Program and Space Technology University Programs
Member, AIAA
+Assistant Director for Space, Spacecraft Systems
Member, AIAA
This paper is declared a work of the U.S. Government and
is not subject to copyright protection in the United States.
Abstract
In her report to the NASA Administrator on Leadership and America's Future in Space, Dr. Sally K. Ride states that Mission to Planet Earth "requires advances in technology to enhance observations, to handle and deliver the enormous quantities of data, and to ensure a long operating life." These three themes (1) space-based observation, (2) data/information, and (3) spacecraft/operations, form the basis for NASA's efforts to identify the technologies needed to support the Mission to Planet Earth. In the observation area, developments in spacecraft and space-based instrument technologies are required to enable the accurate measurement of key parameters crucial to the understanding of global change. In the data/information area, technology developments are required to enable the long-term documentation of these parameters and the timely understanding of the data. And in the spacecraft/operations area, developments in spacecraft, platform, and operations technologies are required to enable consistent long-term collection of data through increased system reliability and operations effectiveness. This paper summarizes current activities related to these technology requirements within the NASA Office of Aeronautics and Space Technology (OAST) Research and Technology program and identifies areas where further efforts are needed. Two additional areas, technologies for transportation (including launch and orbit transfer) and technologies for servicing of Earth science spacecraft (including human, telerobotic, and robotic servicing), are not addressed in detail in this paper.
I. Introduction
Earlier papers and reports have summarized the criticality of scientific research into global change, the relationships between natural and anthropomorphic change, the connection between the global change theme of the International Space Year (1992, the 500[th] anniversary of the discovery of the Americas by Columbus) and the themes of the National and International Decade for Natural Hazard Reduction, the need for international cooperation in order to conduct scientific research into global change, and the coordinating role of the International Geosphere-Biosphere Programme under the International Council of Scientific Unions.[1,2,3,4,5] Other reports have summarized the scientific requirements for global change and Earth system science observations, and the plans for the Mission to Planet Earth.[6,7,8]
The Federal Coordinating Council for Science, Engineering, and Technology (FCCSET) Committee on Earth Sciences has responsibility for defining the national strategy as well as the roles and responsibilities of the multiple government agencies involved in Global Change Research, and has developed the national research plan.[9,10] The goal of the U.S. Global Change Research Program is to provide a sound scientific basis for national and international decision making on global change issues. The scientific objectives of the Program are to monitor, understand, and ultimately predict global change. The program's goals, objectives and research priorities, and strategy are consistent with current national and international global change planning and research efforts. Their FY 1990 Research Plan includes a priority framework for the U.S. Global Change Research Program.
NASA has a significant national responsibility in Global Change research, which will require a major agency investment over the next few decades to obtain the science data required to understand the Earth as a total system. Technology research and development is a natural complement to this national scientific program. A parallel technology program should be initiated to assure that technology is ready and tested before projects commit to hardware development, to provide a sound foundation to aid in solving problems that develop during the project, and to improve the scientific capability for the future. It is especially important in high cost and long term agency programs to conduct the predevelopment technology that will enable the ultimate programmatic objectives. Not making this technology investment invites cost growth and failure to meet the basic scientific potential.
The space component of the international effort to understand the Earth is a concept called "Mission to Planet Earth." This effort would consist of a number of sun synchronous polar platforms; a series of low earth orbit equatorial missions such as Space Shuttle payloads, Space Station Freedom attached payloads, and Explorer class Earth Probe missions; and five geostationary platforms. The core of NASA's initial partition is the Earth Observation System (EOS). International partners such as the European Space Agency (ESA) and Japan could provide two each of the polar and geostationary platforms. A complementary in situ observing system would use automated and manned ground stations, balloons, aircraft, ships, ocean floats, and sounding rockets.
In her report to the NASA Administrator, Leadership and America's Future in Space, Dr. Sally K. Ride states that Mission to Planet Earth "requires advances in technology to enhance observations, to handle and deliver the enormous quantities of data, and to ensure a long operating life."[11] These three themes (1) space-based observation technologies, (2) data/information technologies, and (3) spacecraft/operations technologies form the basis for NASA's efforts to identify the technologies needed to support the Mission to Planet Earth.
This paper summarizes current activities related to these technology requirements within the NASA Office of Aeronautics and Space Technology (OAST) Research and Technology program and identifies areas where further efforts are needed. The intent of this study is to include all current NASA OAST activities that even loosely apply to the technological needs of the Mission to Planet Earth, even if they only contribute to the base of understanding, or are focused on other mission applications. The areas identified where further efforts are needed represent a synthesis of inputs from three NASA inter-center workshops and the advice of the OAST Space Systems Technology Advisory Committee Ad Hoc Review Team on Mission to Planet Earth Technologies.[12]
II. Current Program and Future Needs
The Fiscal Year 1990 NASA OAST Research and Technology (R&T) Program is divided into Aeronautics and Space R&T Programs. The majority of technologies discussed in this paper are within the Space R&T Program, under the Space R&T Base Program, the Civil Space Technology Initiative (CSTI), the Pathfinder Program, and the In-Space Technology Experiments (In-STEP) Program. The exception is the High Performance Computing Initiative, which is included in the Aeronautics R&T Program. In general, the nature of the Space R&T Base Program is basic research, addressing new concepts, high risk/high payoff approaches, and proof-of-concept research. The CSTI program is focused on low Earth orbit (LEO) applications including EOS and astrophysics missions such as the Large Deployable Reflector (LDR). The Pathfinder program is focused on the needs of solar system exploration missions.
Through a series of inter-center workshops and on-going systems analysis studies, NASA OAST has identified the set of critical technologies required to support the comprehensive set of Mission to Planet Earth spacecraft including the upgrade/replacement platforms for EOS A and EOS B as well as future geostationary platforms. The technology set is divided into four thrusts covering Systems Analysis, Observation technologies, Information technologies, and Spacecraft technologies. The specific tasks and criticality of the elements has been identified through NASA internal and external review. The following summarizes the on-going activities within the NASA OAST R&T Program, and identifies areas where new efforts and/or additional emphasis is required.
Systems Analysis Activities
The Spacecraft Systems Analysis program under the Space R&T Base Program is the principle tool that the NASA OAST Space Directorate has for studying key future NASA and national space missions and identifying the critical technologies required to support these future mission options. Recently the focus for the Spacecraft element of the Space R&T Base Systems Analysis program has been the technology needs of future Earth science missions to address the concerns of global change. This effort has developed and refined the definition of the Global Change Technology Initiative described in this paper.
Future systems analysis efforts are needed to continually refine the definition and scope of the technology effort and to ensure continued relevance to the evolving requirements of the Mission to Planet Earth instruments, data systems, and spacecraft. These studies will identify technology needs and opportunities for Earth science and operational systems to meet future integrated scientific and operational objectives, including studies of alternative approaches to meet these objectives. This will foster effective technology transfer through cooperative studies with the potential users of the technology.
Development of EOS spacecraft and subsystem models will enable real time analysis of technology problems and potential solutions, as well as understanding the impacts of technology developments. This will permit accurate assessment of the impact of system modifications to meet evolving mission requirements, as well as to take advantage of technology opportunities, and would support program management decisions.
Observation Technologies
Included in the observation technology category are cryogenic coolers, infrared arrays, active microwave sensing, sub-millimeter sensing, laser sensing for light detection and ranging (Lidar) and differential absorption lidar (DIAL), passive microwave sensing, optics, large array charge coupled devices (CCDs) for visible and near infrared sensing, acousto-optical tunable filters, and the study of approaches to enhance instrument stability and calibration with decreased degradation from contamination and space environmental effects.
Thermal Infrared Measurement Needs. The thermal infrared range (5 to 20 microns) corresponds to the peak of thermal emission from the surface of the Earth, and is the region of the spectrum in which the "green-house" gases play a crucial role.[13] Observations in this range support a large number of scientific applications, including land, ocean, and ice surface temperature, cloud cover, vegetation identification and condition evaluation, and water vapor measurements.
Current infrared detector materials need to operate at temperatures below 65deg.K, requiring active cooling. Many of these detector materials, especially those that operate at very low temperatures (i.e. 10deg. Kelvin), are only appropriate for astrophysics applications and cannot accommodate the heat load of observing the warm Earth. To meet the needs of future Earth science missions, improvements in both detector materials (to operate at higher temperatures) and cryo-coolers (to provide the cooling power necessary to observe the warm Earth) are required.
There is only one refrigerator, the Oxford Stirling cooler, that is near to being space qualified. Plans for the EOS polar platforms call for the use of multiple coolers derived from the Oxford Stirling heritage, with perhaps as many as sixteen on a single platform. Concerns exist over the dependence of EOS on these coolers, which may have manufacturability, performance, vibration, and lifetime problems.
The detector materials for future Earth science sensors must be highly reliable and stable, with minimum cooling requirements (i.e. operate at >65deg. Kelvin), as well as minimum impact from contamination, radiation, and the space environment, in order to provide the long term and sensitive measurements necessary to detect long term changes in global climactic parameters that are buried beneath the day-to-day variations in weather. Currently, mercury-cadmium-telluride (HgCdTe) detectors are used for the 12 to 20 micron range. Manufacturing and fabrication problems with HgCdTe prevent the fabrication of two dimensional arrays, and allow only limited linear arrays, at high cost. Concerns exist over the long term stability and spectral response of these detectors. Further, the best performance in HgCdTe come from photo-conductive detectors which use an electric current to detect infrared radiation. This current adds to the heat load on the detector and thus to the overall cooling requirements for these detectors.
65deg.K Cryogenic Coolers. Current efforts under the CSTI Science Sensor Technology Program are addressing some of the concerns over the Oxford Stirling cooler as well as pursuing other approaches such as the pulse tube cooler. The pulse tube cooler appears to be less efficient, but to cause less vibration and to be more reliable as it has no moving parts in the cold end of the refrigerator. Further support is required for this critical technology for the EOS mission. A concentrated effort is required to understand the capabilities and limits of the Oxford cooler, and a focused program is needed to improve the Stirling cycle cooler components and to verify these improvements on an advanced Stirling cooler breadboard. Studies are required of alternate cooler concepts, followed by development of the most promising alternate concept as a back-up for the Oxford heritage cooler.
Infrared Detectors. Mercury-zinc-telluride (HgZnTe) promises to be an easier material to work with than HgCdTe, but requires further evaluation of its materials properties, and fabrication and test of a breadboard array. Under the Space R&T Base Information Sciences program as well as the CSTI Science Sensor Technology program, HgZnTe linear detector arrays, as well as a few other approaches such as quantum well devices are being investigated. A number of other approaches show promise, in a number of materials systems (both III-V and II-VI materials), but are not being pursued. These materials require thorough evaluation followed by the fabrication of the most promising candidates into test arrays, with the ultimate goal of developing two dimensional detector arrays that operate above 65deg.K in the 12 to 20 um range.
Laser Sensing. To measure winds, and to identify and measure concentrations and vertical profiles of trace gasses in the troposphere and stratosphere, laser remote sensing is the optimal method. No other technique can provide the required resolution on a global scale. Recent measurements of ozone in the arctic and antarctic, and of carbon monoxide in the tropics, used laser sensor systems mounted on aircraft. Space qualified laser systems need to be developed that are long lived, reliable, and efficient. The Laser Atmospheric Wind Sounder (LAWS) was selected for EOS because of the critical importance of accurate wind measurements to global science, but an underlying technology program would serve to reduce the technical risk for this critical instrument. The EOS Laser Atmospheric Wind Sounder (LAWS) instrument, baselined for flight on the Japanese polar platform, will use a pulsed, frequency-stable CO2 laser transmitter, a continuously scanning transmit/receive telescope (1.5 meter diameter), a heterodyne detector, and a signal processing subsystem. The signal processing subsystem automatically reduces the number of laser firings when return signals are not detected, in order to prolong laser life. The current effort under the CSTI Science Sensor Technology program is supporting the transition of this technology from research to advanced development through the co-funding with the Office of Space Science and Applications of a LAWS breadboard, and investigations of the lifetime of the catalyst materials that restore the CO2 gas.
All solid-state laser technology is rapidly emerging which could replace the more complex CO2 gas laser, plus operate at 2 microns versus 10 microns. Atmospheric back-scatter at 2 microns is significantly higher than at 10 microns, which significantly increases signal-to-noise (reducing the required size of the transmit/receive telescope), because there are a larger number of 2 micron diameter particles in the atmosphere than 10 micron diameter particles. However, the Lidar Atmospheric Sounder and Altimeter (LASA) instrument, solid state laser system, was not selected for the EOS because of technical problems including the inability to scan the instrument, and concerns over laser power and mass requirements. A small amount of basic work is being conducted under the Space R&T Base Information Sciences program, as well as work under the CSTI Science Sensor Technology program in laser materials research, laser transmitter design, and lifetime and efficiency improvements. Additional emphasis on this technology program could accelerate it and make it available for the future platforms. Two flight experiments, the Laser In-space Technology Experiment (LITE) under the Space R&T Base Space Flight program, and the Stanford University Laser In-space Technology Experiment (SUNLITE) under In-STEP, will demonstrate application of laser technology to Earth science needs, and basic capabilities of ultra-stable lasers which could be important for future applications, respectively.
Sub-Millimeter Sensing. The sub-millimeter range will require developments in sensors as well as supporting systems such as mixers and oscillators. These will enable highly accurate Earth viewing and limb sounding sub-millimeter wave heterodyne systems for measurement of stratospheric water vapor, aerosols, and trace gasses crucial to the understanding of ozone depletion, and will require cooling to 4deg. Kelvin or below. Currently, this requires use of stored cryogenic fluids, which have limited life. Long-life, space qualified multi-stage cryogenic cooling systems to provide cooling to these temperatures simply do not exist at this time. Current efforts under the Space R&T Base Information Sciences program, the University Space Research program (Center for Space Teraherz Technology), and the CSTI Science Sensor Technology program are working on some detector materials and multistage cryogenic coolers (using the 65deg.K cooler described above as an upper stage). A concentrated effort is needed to bring forward and evaluate other promising detector approaches, and to accelerate the development of both the detector and lower stage coolers.
Microwave Antennas. Currently, rainfall is measured by ground-based instruments, which results in extremely poor measurements over the ocean and unpopulated areas of the tropics. Resolution of water vapor and rainfall is needed on a global basis because the storage and release of energy by the evaporation and condensation of water is the driving force behind global circulation. Currently planned instruments such as the Tropical Rainfall Mapping Mission (TRMM) will use active microwave sensing (i.e. rain radar) to provide a statistical sampling, but only continuous geostationary measurements would provide for complete global monitoring. Since it is estimated that about half of the Earth's rainfall occurs in short lived, small scale storms, resolution corresponding to the size of these storms (10 kilometers) is needed to provide complete rainfall monitoring data. For example, geostationary observations at 36 GHz with an Earth footprint of 10 kilometers require an antenna diameter of 40 meters. These large antennas will require precision shape correction and steering to allow coverage of the globe, either mechanically or through receiver array adjustments. Measurements above about 36 GHz require solid surface reflectors while lower frequency measurements can use large mesh reflectors. Unfilled aperture or interferometric techniques could provide an alternative approach for the large (greater than 40 meters) antennas required for frequencies less than 36 GHz. Even in low Earth orbit, the size and antenna structure accuracy requirements for a passive radiometer system to measure soil moisture (using frequencies around 1.2 GHz) prevented it from being accepted for EOS. Special microwave transparent structural materials may be required to achieve the instrument performance requirements.
Current efforts in this area are included in a number of NASA OAST programs. These large antennas may either be deployed or assembled in space, and are large, flexible structures that require precision pointing and control in order to focus the beam and acquire the necessary resolution. Efforts under the Space R&T Base Materials and Structures, Space Data and Communications (for large antennas), and Controls and Guidance programs all apply to some extent, as well as the CSTI Control of Flexible Structures program (which is developing a model of a geostationary Earth science platform as a ground test article), the CSTI Precision Segmented Reflectors program (for both support structure and solid reflector surfaces), and the Pathfinder In-Space Assembly and Construction program (although this is focused on assembly exploration vehicles such as large aerobrake structures). A focused technology effort could build upon the base that these research activities have created for this application.
Active and Passive Microwave Sensing. For the reasons given above, active and passive sensing in the millimeter and microwave regions of the spectrum are important for precipitation monitoring and soil moisture measurements. In addition, active sensing using synthetic aperture radar (SAR) can provide all weather imaging of a wide variety of phenomena including surface topology (through vegetation and even dry sand cover), sea ice, leaf moisture, etc. Current efforts in multibeam antenna feeds and monolithic microwave integrated circuits under the Space R&T Base Space Data and Communications program could apply to active and passive measurements, but these are directed towards communications rather than remote sensing needs. OAST has supported SAR technology development in the past but is not currently supporting any effort directly tied to this application (efforts that support information processing for the SAR are described below in the Information section). The mass and power required for the EOS SAR were the principle reasons for its removal from the EOS B payload. The SAR is now baselined for its own free flying spacecraft. OAST technology developments could support improving the performance of this critical Earth observing instrument.
Optics, Large Array CCD, and AOTF. To allow quality, long-term, continuous observation of Earth processes on local to synoptic scale from geosynchronous orbit, optical systems technology research is required, as well as development of large element array charge coupled devices (CCDs) for visible and near infrared sensing, acousto-optical tunable filters (AOTF), diffraction gratings, ultraviolet thin films, electro-optic crystals, hologram optical elements, and optical system performance modeling. Currently there is no optics or CCD research within the OAST program. The only AOTF effort is under the Pathfinder Sample Acquisition, Analysis, and Preservation program. Development of large array (20,000 by 20,000 elements) CCDs and an optics technology base is needed to support ultra-violet, visible, and infrared high resolution (spatial, spectral, and temporal) observation of the Earth.
Calibration. Improvements must be made in the ability to calibrate and compare data from diverse instruments and spacecraft, for both relative and absolute measurement values, in order to allow the synthesis of information obtained over time and across the spectrum. Techniques to improve instrument performance and reduce instrument noise, such as chip-level integration of data preprocessing (i.e. including the pre-amp and analog-to-digital conversion electronics on the sensor chip) could maximize the performance of both existing and new sensor technologies. OAST has no current program in calibration technology or chip level integration.
In Situ
Information Technologies
Included in the information technology category are human factors research in scientific visualization, and research in both on-board and ground data/information technologies such as software engineering, advanced computing, data storage, and data/information networks, to support the technology needs for the Mission to Planet Earth. This will include the computer architecture and software techniques needed to support the development of integrated models for Earth system science, and to support an information system capable of enabling the analysis and understanding of integrated data sets from both space-based and in situ instruments. Optical communications developments will enable the extreme high rate collection and relay of data and information between spacecraft in low inclination, polar, and geosynchronous orbits.
By the mid-1990s, space-based remote sensing observations of the Earth will be acquired at the rate of about 10 trillion bits-per-day, and this trend will continue with the addition of the Earth Science Geostationary Platforms. The SEASAT spacecraft operated for three months in 1978, and it has taken eight years to analyze three months worth of data. An end-to-end information system capability from initial acquisition to handling and storage on the ground, with improved access and display would greatly aid the evaluation and ultimate understanding by the scientific user of the massive amounts of data that will be generated by Mission to Planet Earth.
Information Visualization. Converting this "fire-hose" of data into information that can be understood and used by the scientific community will be a high pay-off area for technology development. Future human factors research to enhance scientific visualization could seek ways to improve the man/machine interface for the interchange of scientific information, and to better use the unique pattern recognition and cognitive capabilities of human beings to review and assimilate the massive amounts of data that will be received. Current efforts in this area are under the Aeronautics R&T Program (for cockpit information displays) and under the Space R&T Base Human Factors program (for in-space operational rather than scientific display), CSTI Telerobotics program (virtual workstation for telepresence and telerobotic control), and Pathfinder Space Human Factors program (for virtual exploration and planetary data analysis).
On-Board Processing and Data Storage. Mission to Planet Earth platforms will require on-board processors and data storage systems capable of handling and storing the high data rates and large data volumes generated by the multiple scientific and operational instruments. Data system approaches with an "open" architecture, employing local area network management to support additional instrument and system upgrades, will allow use of a common platform data system design for multiple missions, as well as enable future servicing options. Current research under the Space R&T Base Space Data and Information, Information Sciences, and University Space Research (Space Engineering Research Center for VLSI System Design) programs and the CSTI Data: High Rate/Capacity program are developing space qualifiable component technologies such as electro-optical memories with no moving parts, switch-able fiber optic elements, neural networks and more conventional general and special purpose (SAR and imaging spectrometer) processors. The CSTI program is developing a test-bed based upon the EOS system for testing and evaluation of the advanced technologies developed under the program, as well as a flight optical disk system which could be accelerated for application on the EOS. The NASA component of the Federal High Performance Computing Initiative will conduct research into high performance parallel computing architectures appropriate for use in on-board data processing.[14,15] Further work will develop tools and techniques to design, simulate, produce, and test application specific integrated circuits.
Software Engineering. Software engineering research will develop the tools and techniques to enhance software reliability. These are becoming more critical as the software systems for mission operations and information processing become more complex, and as the durations of these mission increase. Failure of a major software system could drastically effect mission cost and even threaten mission success. Current efforts in this area are under the Space R&T Base Space Data and Communications program.
Design Knowledge Capture. As the complexity and duration of space missions increase, the importance of design knowledge capture increases as well. It is not reasonable to expect that the experts involved in the initial design and development of a fifteen year mission like EOS will be involved in the operations at the end of mission. Current efforts in this area, under the Space R&T Base Information Sciences program and the CSTI Artificial Intelligence program, are either basic research or targeted for demonstration on other missions such as the Hubble Space Telescope (HST).
World Modeling Computer Systems. The NASA component of the Federal High Performance Computing Initiative will facilitate the use of new parallel architectures for parallel processor computers, the processing architecture needed for global ocean-atmosphere-biosphere system simulations.[14,15] Advances in software and computing technologies will facilitate the development of a system for Global Computing.
Optical Communications. Geostationary science platforms could be used as relay stations for the collection of data from other geostationary platforms not in view from the United States, from low Earth orbiting platforms including the EOS, and from aircraft, ocean floats, and other in situ sensors. These platforms could be used as well for the dissemination of scientific information to the national and international scientific community. For these applications, technology development and a flight demonstration of optical communications are needed. Optical communications will increase the capacity and capability of information system data communications, as well as decrease antenna size, which will result in improved platform stability and pointing. Technology research needed includes the development of high performance semiconductor lasers. Current efforts in optical communications are under the Space R&T Base Space Data and Communications program.
Information Archives, Access, and Retrieval. The massive quantities of data generated by the Mission to Planet Earth will require development of new approaches to archive massive quantities of information, and to easily access and retrieve information from these massive data bases. Research is needed to develop advanced technology for archiving and managing large and complex science data sets collected from global scale space observations. This will enable development of data management systems that integrate high performance media, communications strategies, and distributed data bases for efficient storage and maintenance of global change data for distribution by discipline and organization. Research is also needed to develop the capability to organize, store, access, and retrieve information from large volume, distributed data bases of global scale space observations and model simulations. This will enable development of methodologies for building, classifying, and sorting large archives of data, and for extracting information by area of scientific interest, to allow efficient data organization and retrieval to support interactive analysis and visualization of global trends and to support global system computer modeling. Current OAST activities in this area are under the Space R&T Base Space Data and Communications and the CSTI Artificial Intelligence programs, including the development of advanced data handling concepts and neural network approaches for pattern recognition in complex data sets.
Operations Automation. The long operating life of Mission to Planet Earth spacecraft and platforms, the scientific requirements for synergistic and complementary observations (both from the same platform and between platforms, possibly in different orbits), and for rapid response to transient events such as forest fires, volcanic eruptions, and unusual or unpredictable weather events, make it desirable and cost effective to automate as much of the mission planning and operations of the platforms as possible to reduce the workload and facilitate rapid replanning and adaptation of sequences. Current efforts in this area are not targeted to Mission to Planet Earth needs. They are under the CSTI Artificial Intelligence program (targeted to applications for the space shuttle, the Cosmic Background Explorer (COBE), the Hubble Space Telescope (HST), and the Space Station Freedom power system) and under the Pathfinder Planetary Rover program (focused on Mars exploration requirements).
Spacecraft Technologies
Measurement of global change requires the detection of subtle, long-term trends in global parameters that are often hidden by the day-to-day variations of weather. Recent reports of an average rise in global temperature are based upon the use of temperature data from world-wide networks of weather stations acquired over the last hundred years, coupled with model corrections and adjustments. The EOS mission is conceived to have a fifteen year operating life, while the instruments are designed for five year lifetimes. Five year replacement of the platforms will enable the fifteen year collection of data. Potential use of geostationary platforms presents a greater challenge both because of the greater expenditure of energy required to place the initial and replacement platforms in the geosynchronous orbit, and the added complexity of servicing in GEO if this option is selected instead of replacement.
The ancillary technologies to enable long term continuous observations of the earth need to be developed. High performance, lightweight power systems would expand the potential payoff from future low Earth orbit and geosynchronous orbit platforms. A very similar case can be made for highly reliable propulsion systems. The long duration, high reliability required, especially for geostationary platforms, would be greatly assisted by a strong fundamental program in space effects on materials, structures, and mechanisms.
In the spacecraft technology area, the long-term, sustained nature of the measurements required for understanding the Earth as a system will be enhanced through basic technology research and development to increase spacecraft reliability and lifetime. This includes technologies in areas such as reliability and quality assurance, non-destructive inspection and evaluation (including "smart structure" techniques), long life materials and structures, thermal control systems, contamination and space radiation, and platform charging. In addition, multiple instrument pointing, platform structural concepts for both deployable and space assembled structures, and long-life/low-contamination propulsion and power system technologies require development.
Reliability Design and Validation. Research in reliability and quality assurance will include investigations into testing techniques and tradeoffs such as thermal cycling vs. thermal dwell for testing and flight certifying electronic assemblies, development of improved transient dynamic load test techniques for spacecraft structures and spacecraft equipment, and improved vibration test techniques. OAST does not currently have a program in this area.
Multi-Instrument Pointing. Earth science platforms in both LEO and GEO require the ability to acquire simultaneous and continuous observation of the Earth with multiple instruments with minimal interference, vibrational or otherwise. The current OAST Space R&T Base Materials and Structures program includes research into tribology and mechanisms (which can affect platform vibrations), the development of analytic tools and experimental techniques for use in the design, development, and analysis of the structures and prediction of the structural dynamics of complex spacecraft. The current Space R&T Base Controls and Guidance program also applies, developing control technology, shape sensors, actuators, and modeling design/evaluation tools. The Space R&T Base University Space Research program supports the Controlled Structures Technology Center. One of the activities at this center is evaluation of control technologies for multiple instrument platforms based upon the needs of Mission to Planet Earth spacecraft. The current CSTI Control of Flexible Structures program also relates. Focused efforts are needed in these areas to develop technologies for precision alignment and/or compensation for deformations, momentum compensation for scanning instruments, and inter-instrument isolation. Development of long life/low vibration bearings and space devices (reaction wheels, magnetic torque units, etc.) will be required to minimize interference with the observational systems. For example, the pointing requirements of the precision instruments on the geosynchronous platforms require that the large radiometric antennas not disturb the dynamics of the entire platform. The need to decouple the motions of these structures may require novel solutions such as tethered platforms or a network of free flying spacecraft.
NDI/NDE. The long life and servicing or replacement requirements of future Earth science platforms could be significantly enhanced by the development of Non-destructive inspection and evaluation (NDI/NDE) techniques. These could be used to develop "smart structures" with integrated fiber optics sensors within critical structural components to provide real-time information about strain, temperature, configuration, impact damage, and radiation degradation. Other approaches include using thermal and ultrasonic health monitoring and laser based optical monitoring systems, to provide reliable information about the state of the spacecraft during operations that could enable on-board fault detection and correction. Current efforts in this area are under the Space R&T Base Materials and Structures program. Focused technology research could develop these techniques for the specific needs of future Earth science platforms.
Propulsion. Propulsion research and technology will develop long life, high specific impulse, low contamination propulsion systems for orbit transfer, orbit maneuvering, and station keeping, with minimum contamination and resupply (if servicing is available) or platform replacement requirement. The long life and mass saving of these propulsion systems is of even greater importance for geostationary platforms due to the increased cost-per-pound in GEO. The Iridium/rhenium engine, which can operate at a higher chamber temperature providing more complete combustion of bi-propellant fuels, can allow in the near term the high thrust performance of these fuels for orbit transfer and safe de-orbit applications, while reducing the concerns over contamination of the science instruments by the propellant byproducts. Past OAST efforts have supported development of this technology, but it is not included in the current OAST program. Arcjets and (in the farther term) ion thrusters can support platform station keeping with reduced mass requirements (provided sufficient power is available). Current Space R&T Base Propulsion program activities support development of both arcjets and ion thrusters, and the Flight Experiments programs are pursing in-space characterization of the arcjet. A focused effort could develop and evaluate these technologies for both performance and contamination effects, specifically for the needs of Earth science platforms in both LEO and GEO. Under the Space R&T Base University Space Research program, the Pennsylvania State University Center for Space Propulsion Engineering and the University of Cincinnati Health Monitoring Technology Center for Space Propulsion Systems conduct basic research which could have application to future Earth science platforms.
Power. Developments in advanced power technology will enable improved science (especially active sensing using lasers and radar), improved communications, and the possible use of electric propulsion (arcjets and ion thrusters) to minimize propellant requirements for geostationary platform, low to geostationary orbit transfer, and on-orbit station keeping. Current efforts in advanced photovoltaic solar arrays, advanced batteries, and advanced power management and distribution (including switching and control "smart power" technology and power integrated circuits), are under the Space R&T Base Space Energy Conversion program. Further power research and technology will develop high performance, autonomous, light weight, and reliable power generation, storage and distribution components for high density power systems to support platform operation, active sensing, and propulsion with minimum maintenance and ground interaction.
Space Environmental Effects. Spacecraft charging and plasma interactions are of concern for Earth science spacecraft, especially for geosynchronous platforms, and the phenomena is increased as platform size and voltage levels increase. Also of concern are contamination, radiation damage, atomic oxygen damage (for explorer class Earth probes in LEO), and debris damage. Contamination can degrade the performance of science instruments and complicate the calibration process. Current research and technology development in space environmental effects is included in the Space R&T Base Space Energy Conversion program (for power system interactions) and Materials and Structures program (for effects on space durable materials, debris impacts, contamination modeling and understanding, and atomic oxygen effects). Analysis of the Long Duration Exposure Facility (LDEF) will provide a wealth of data on space environmental effects. The In-STEP program is intended to include experiments in debris effects, contamination (both around the shuttle and around an Explorer platform), and solar array/plasma interactions. Platform charging modeling capabilities will need to be developed to assess vulnerability to spacecraft charging of the Mission to Planet Earth platforms, as well as technical approaches for mitigation to allow long-term operation without interruptions or damage from geomagnetic substorms. Advanced materials will need to be characterized for their long-term stability in the low and geostationary Earth orbit environments. Degradation mechanisms of composite materials in these environments are not fully understood, and the database for new materials is very small.
Platform Thermal Systems. Current research in thermal management, such as fluid thermal control and heat pipe systems, are included in the Space R&T Base Space Energy Conversion program, as well as definition of a heat pipe experiment under In-STEP. Precision geostationary platforms present a greater challenge for thermal design and control because, unlike LEO platforms, they rotate slowly (once per day) and thus have greater thermal gradients across the platform. The capability for integrated thermal control systems and designs needs to be developed for advanced Earth science platforms.
Transportation and Servicing Technologies
Two additional areas, technologies for transportation (including launch and orbit transfer) and technologies for servicing of Earth science spacecraft (including human, telerobotic, and robotic servicing), are not addressed in detail in this paper. All space missions, regardless of purpose, require transportation from Earth to orbit and in many cases for orbit transfer. This paper has not addressed technology developments for improved launch vehicles, nor has it addressed technologies such as aerobraking that could be used for orbit transfer and the return of servicing vehicles from geostationary orbit. Propulsion technologies for platform attitude control and station keeping have been addressed. By virtue of their proximity to Earth, Earth science platforms and spacecraft are candidates for on-orbit servicing, including Space Shuttle and Space Station Freedom based extra-vehicular activity (EVA) servicing as well as remote telerobotic and robotic servicing. Current plans for the EOS do not include servicing, but these platforms will be designed to be serviceable, should a future polar orbit servicing capability be developed. Technologies to support future servicing capability developments have not been addressed in this paper.
III. Summary
NASA OAST has identified the set of critical technologies required to support the comprehensive set of Mission to Planet Earth spacecraft including the upgrade/replacement platforms for EOS A and EOS B as well as future geostationary platforms. The technology set is divided into four thrusts covering Observation technologies, Information technologies, Spacecraft technologies, and Systems Analysis. Development of these technologies is required to enable and enhance the long-term observation, documentation, and scientific understanding of the Earth as a system. This will require both basic and focused technology research and development. The specific tasks and criticality of the elements has been identified through NASA internal and external review. NASA OAST is proposing the Global Change Technology Initiative to address the critical and key technologies for the comprehensive set of Mission to Planet Earth spacecraft. No activity is more critical than understanding our home world, and this technology program will complement the on-going and planned scientific investigations by developing the technology needed for the coming generations of missions to observe the Earth.
IV. References
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2. Leonard A. Harris, Gordon I. Johnston, Wayne R. Hudson, Lana M. Couch, "Earth Orbiting Technologies for Understanding Global Change," AIAA-89-0254, 27th Aerospace Sciences Meeting, January 1989.
3. "Confronting National Disasters, an International Decade for Natural Hazard Reduction," Advisory Committee on the International Decade for Natural Hazard Reduction, Commission on Engineering and Technical Systems, National Research Council, U.S. National Academy of Sciences, U.S. National Academy of Engineering, National Academy Press, Washington, D.C., 1987.
4. "Reducing Disasters' Toll, the United States Decade for Natural Disaster Reduction," Advisory Committee on the International Decade for Natural Hazard Reduction, Commission on Engineering and Technical Systems, National Research Council, National Academy of Sciences, National Academy of Engineering, Institute of Medicine, National Academy Press, Washington, D.C., 1989.
5. Natural Hazards Observer, Volume XII, Number 6, July 1989.
6. "Space Science in the Twenty-First Century: Imperatives for the Decades 1995 to 2015, Mission to Planet Earth," Task Group on Earth Sciences, Space Science Board, Commission on Physical Sciences, Mathematics, and Resources, National Research Council, National Academy Press, Washington, D.C. 1988.
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8. "Potential of Remote Sensing for the Study of Global Change, COSPAR Report to the International Council of Scientific Unions (ICSU)," edited by S. I. Rasool, published for the Committee on Space Research (COSPAR) by Pergamon Press, Advances in Space Research, volume 7, number 1, 1987.
9. "Our Changing Planet: A U. S. Strategy for Global Change Research," Report by the Committee on Earth Sciences of the Federal Coordinating Council for Science, Engineering, and Technology, to accompany the U. S. President's Fiscal Year 1990 Budget.
10. "Our Changing Planet: The FY 1990 Research Plan, the U.S. Global Change Research Program (Executive Summary)," Report by the Committee on Earth Sciences of the Federal Coordinating Council for Science, Engineering, and Technology, July 1989.
11. S. K. Ride, "Leadership and Americas's Future in Space," A report to the (NASA) Administrator, August 1987.
12. "Technology for the Mission to Planet Earth," Report of the Ad Hoc Review Team on Planet Earth Technologies of the Space Systems and Technology Advisory Committee for the National Aeronautics and Space Administration, July, 1989.
13. Stephen H. Schneider, Randi Londer, "The Coevolution of Climate and Life," Sierra Club Books, San Francisco, pp. 195-198.
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15. "The Federal High Performance Computing Program," Executive Office of the President, Office of Science and Technology Policy, September 8, 1989.
Conference paper presented at the AIAA 28th Aerospace Sciences Meeting (paper no. AIAA 90-0767), January 8-11, 1990. Converted to HTML November 25, 1994, but not yet checked for translation errors, etc.