This is an electronic version of a conference paper presented at the 43rd Congress of the International Astronautical Federation (paper no. IAF-92-0751), August 28 - September 5, 1992. Converted to HTML March 29, 1995. I have have spelled out "degrees" and "micrometers" as the symbols I used do not translate into HTML. Also, superscripts and subscripts do not translate well (e.g., the formula for water is shown as H2O).
Robert Rosen*, NASA Ames Research Center, Mountain View, California, U.S.A.
Gordon I. Johnston**, NASA Office of Aeronautics and Space Technology, Washington, D.C., U.S.A.
* Associate Director for Program Development, Associate Fellow, AIAA.
** Space Science Technology Thrust Manager, Member, AIAA.
This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States.
With the launch of the Upper Atmosphere Research Satellite (UARS), the United States National Aeronautics and Space Administration (NASA) initiated the Mission to Planet Earth. A key component of the Mission to Planet Earth is the Earth Observing System (EOS), composed of a series of low earth orbiting spacecraft, as well as the EOS Data and Information System (EOSDIS), an unprecedented system for the distribution and use of scientific information. The Mission to Planet Earth is NASA's contribution to the national and the international global change research programs.
Much attention is given at this and past International Astronautical Federation (IAF) Congress meetings to the scientific aspects of the Mission to Planet Earth. NASA has a significant responsibility to address scientific questions directly related to policy issues associated with the potential for global and regional climate change. This responsibility requires substantial, long-term investment to obtain an adequate understanding of the Earth as a total system.
Within NASA, the Office of Aeronautics and Space Technology (OAST) is conducting a major, on-going engineering research and technology program directed toward the support of future programs, with a major thrust targeted at technology for future space science missions. OAST is conducting a substantial effort to identify the technologies needed to support the evolution of Mission to Planet Earth. The effort consists of studies, workshops and technology research programs to explore:
This paper examines the technological needs of future earth science systems, surveys current and planned activities, and highlights significant accomplishments in the research and technology program. NASA OAST has identified the set of critical technologies required to support a long-term, comprehensive and evolving Mission to Planet Earth. Development of these technologies is required to enable and enhance the long-term observation, documentation, and scientific understanding of the Earth as a system. The specific tasks and criticality of the elements have been identified through systems analysis studies, and through NASA internal and external review.
No activity is more critical than understanding our home world, and the NASA OAST 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.
This paper is a compilation of the results of studies and research by technologists, advanced mission planners, and scientists at the NASA field centers, universities, industry, and NASA headquarters.
To further identify and refine our understanding of the technology needs for future missions, key "landmark" missions were selected, in consultation with the advanced mission planners for OSSA, for further systems analysis study. These studies were usually cooperative and co-funded by both OAST and OSSA. Earlier papers[2, 3] have discussed the results of these studies. The Earth Observing System (EOS) was one of these landmark missions. An Earth Observing System Technology Working Group was instituted in March of 1985 to identify enabling and enhancing technology for the EOS polar orbiting missions of the 1990's. This working group was composed of technologies of all disciplines from the Goddard Space Flight Center, the Jet Propulsion Laboratory, the Langley Research Center, the Marshall Space Flight Center, and the National Oceanic and Atmospheric Administration (NOAA). The working group had panels on Utilities and Servicing, Data Systems, Precision Pointing and Control, and Instrument Technology. The working group produced its final report in March of 1986[4].
That NASA, in concert with the Office of Management and Budget and the appropriate Congressional committees, establish an augmented and reasonably stable share of NASA's total budget that is allocated to advanced technology development. A two- to three-fold enhancement of the current modest budget seems not unreasonable. In addition, we recommend that an agency-wide technology plan be developed with inputs from the Associate Administrators responsible for the major development programs, and that NASA utilize an expert, outside review process, managed from headquarters, to assist in the allocation of technology funds.
To implement this recommendation, OAST adapted its on-going long-range planning and technology assessment activities to create the Integrated Technology Plan[13] (ITP). In preparing this ITP, OAST received inputs from the Associate Administrators responsible for the major development programs. For the expert, outside review process OAST utilized a review team with representatives from several senior advisory committees and numerous technical experts[14]. Some of the participating organizations included: the NASA Advisory Committees (the Space Systems and Technology Advisory Committee, the Space Science and Applications Advisory Committee, and the Aerospace Medicine Advisory Committee), the National Research Council (the Aeronautics and Space Engineering Board and the Space Studies Board), the Aerospace Industries Association, the United States government's Departments of Commerce, Defense, Energy, and Transportation, and U.S. Industries and Universities.
Figure 1. OSSA Technology Needs Matrix.
Table 1. Technology Requirements as identified by the OSSA Earth Science and Applications Division
Mission to Planet Earth includes polar orbiting satellites, low inclination orbiting satellites, earth probes, geostationary satellites, aircraft, buoys, ground stations, balloons, the data and information system, and the accompanying basic research. OAST continues to work with OSSA to understand the technology needs through cooperative systems analysis studies and workshops. The 1990 Report of the Advisory Committee On the Future of the U.S. Space Program also recommended (recommendation number 3):
That the multi-decade set of projects known as Mission to Planet Earth be conducted as a continually evolving program rather than as a mission whose design is frozen in time. A combination of different size spacecraft appears to be most appropriate to meet the needs of simultaneity, accuracy, continuity and robustness. NASA also should re-establish research and development in support of environmental satellites to meet the NOAA-stated requirements. ...
Continued investment in mission systems studies and analysis over the past decade has evolved a comprehensive understanding of the technology needs for the initial Earth Observing System and for its evolution and improvement over the next two decades. In addition, concepts have been studied for missions beyond EOS, such as the introduction of geosynchronous platforms that provide a synoptic, continuous view of our planet. The following text of this paper identifies technology needs and reports on-going activities, progress, and accomplishments[16, 17] within the NASA OAST Research and Technology program.
Active and passive remote sensing scientific observations of the earth utilize the full range of the electromagnetic spectrum (Figure 2). In their presentation to the NRC ASEB/SSB review, the OSSA Earth Science and Applications Division identified technology requirements in infrared direct detectors, cryogenic systems, submillimeter and terahertz microwave sensing systems, lasers sensing systems, onboard data storage systems, large antenna structures, software and data analysis, and power systems.
Figure 2. Measurement Regimes for Remote Sensing Observations of the Earth.
Current 12 to 20 micrometers infrared detector materials operate at temperatures below 65 degrees K and require active cooling. Many of these detector materials, especially those that operate at very low temperatures (i.e. below 10 degrees Kelvin), are only appropriate for astrophysics applications and cannot accommodate the long-term heat load from observations of the warm Earth. The detector materials for future Earth science sensors must be highly reliable and stable, with minimum cooling requirements (i.e. operate at >65 degrees 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 wavelengths shorter than about 15 micrometers. Manufacturing and fabrication problems with HgCdTe prevent the fabrication of two dimensional arrays, and allow only limited linear arrays, at high cost, especially for wavelengths greater than 12 micrometers. Concerns exist over the long term stability and spectral response of these detectors. Further, the best performance in HgCdTe come from photoconductive (PC) 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.
While the OSSA EOS program is working to develop 13-18 micrometers photovoltaic (PV) HgCdTe detectors for the AIRS instrument, OAST is pursing promising technologies to replace HgCdTe. Mercury-zinc-telluride (HgZnTe) promises the same theoretical performance as HgCdTe, and to be an easier material to work with, but requires further evaluation of its materials properties, and fabrication and test of breadboard arrays. Under the CSTI Science Sensor Technology program, Mercury-Zinc-Telluride (HgZnTe) linear array detectors were developed in 1990 through LaRC by the Hughes Santa Barbara Research Center. Two 1-by-270 element detector arrays have demonstrated sensitivity in the 12 to 20 micrometers wavelength range, at operating temperatures above 65 degrees Kelvin. These linear arrays were subjected to an accelerated life test along with HgCdTe arrays for comparison. After a vacuum bake for 26 days at 100 degrees C, the HgZnTe arrays have shown no degradation in responsivity, while sample HgCdTe arrays degraded significantly.
A broadly-based program is in place at the Jet Propulsion Laboratory (JPL) to develop advanced long-wavelength infrared detectors for earth science missions. Two infrared detector array approaches that are potentially easy and inexpensive to manufacture, but which require lower operating temperatures than HgCdTe (i.e. lower than 55 degrees K), are quantum well infrared photodetectors (QWIP's) and heterojunction internal photoemission (HIP) detectors. The JPL effort includes novel work on strained layer superlattices, which is not as mature a technology as HgZnTe, QWIP's, or HIP detectors, but which theoretically offer higher performance than HgCdTe at equal operating temperatures. Another approach is to take advantage of the development of high temperature superconductors (HTS). HTS bolometers are based on silicon micro-machining and yttrium-barium-copper-oxide thin films. They can respond to any wavelength, and are potentially easy to manufacture. Although far from technical maturity, the theoretical performance of HST bolometers is about equal to HgCdTe detectors at equal temperatures. All of these approaches require thorough evaluation and fabrication of test arrays, with the ultimate goal of developing two dimensional detector arrays that operate above 65 degrees K in the 12 to 20 micrometers range.
At JPL, a 128x128 element silicon germanium/silicon (SiGe/Si) heterojunction internal photoemission (HIP) sensor array has been developed, sensitive to wavelengths beyond 18 micrometers. This silicon-based technology should prove easy and inexpensive to manufacture, because of the industrial base in silicon microprocessors. However, these arrays have not yet demonstrated the quantum efficiency of HgCdTe detectors, and currently require lower operating temperatures (40 degrees K). Future efforts will focus on improving both the sensitivity and operating temperature.
The development of new detector technologies will offer lower cost, higher operating temperature, higher performance, and larger format arrays. The wider capabilities of technologies will allow smaller, less complex instrument designs, plus higher system reliability and greater science return.
Figure 3. Present Cryogenic Cooling Capabilities and the Earth Observing System Needs.
The EOS project is funding the development of the 55 degrees K cooler for the initial EOS-PM1 atmospheric infrared sounder (AIRS) instrument, based upon currently available technology. As a result of the high priority given to technology developments in this area by OSSA, OAST has initiated under the CSTI Science Sensors program a concentrated effort to understand the capabilities and limits of the Oxford cooler, and to improve the Stirling cycle cooler components and verify these improvements on an advanced Stirling cooler breadboard. Studies are continuing of alternate cooler concepts, with plans to develop the most promising alternate concept as a back-up for the Oxford heritage cooler. OAST is developing through the Goddard Space Flight Center (GSFC) a two-stage 30 degrees K cooler to meet the EOS SAFIRE requirements. The first stage of this cooler system has the potential of meeting the 55 degrees K cooling requirements of other EOS instruments. The military also has requirements for similar cryogenic coolers, and the efforts of the Department of Defense (DoD), OSSA, and OAST are well coordinated.
OAST efforts at the Jet Propulsion Laboratory (JPL) have demonstrated improved thermal efficiency and reduced vibration of the British Aerospace (BAe) Stirling cycle cryogenic cooler. JPL has developed a unique 6-degree-of-freedom dynamometer to accurately measure cooler vibration, and has used this vibration facility to developed detailed understanding of vibration harmonic structure of the Stirling cycle cooler. Previous work at JPL has resulted in a four-fold increase in thermal efficiency at 60 degrees K, improving the cooling power of the BAe cooler from 100 to 400 milliwatts. By replacing the standard BAe controller electronics with a new, low-distortion controller with multiple-harmonic, narrow-band vibration control techniques, JPL has been able to greatly reduce the vibration generated by a pair of back-to-back BAe coolers.
The MLS uses a heterodyne receiver, in which the signal from the atmosphere of the earth is mixed with a known, reference frequency (generated by a local oscillator), and then the difference (which is at a much lower frequency, in the range that can be handled by conventional electronics) is measured and analyzed. The Upper Atmosphere Research Satellite (UARS) MLS measures atmospheric thermal emission from chlorine monoxide (ClO), ozone (O3), water vapor (H2O), sulfur dioxide (SO2), and molecular oxygen (O2), using high spectral resolution heterodyne radiometers that observe the emissions of the upper atmosphere at frequencies of 63, 183 and 205 gigahertz (GHz). Measurements are performed continuously day and night giving global maps of the vertical distribution of these molecules. The vertical resolution is approximately 3 km. One percent accuracy in the measurement of ozone has been demonstrated.
OAST supported technologies in use by the UARS MLS include the local oscillator injector, the dual mode feed-horn, quasi-optical filter technology, and gallium arsenide (GaAs) Schottky diode development.
OAST continues to play a role in technology development for the follow-on MLS on the Earth Observing System (EOS), targeted for flight on the EOS-CHEM mission. Key enabling technologies, needed by 1997, include 2.5 terahertz (THz) planar Schottky barrier diodes/mixers, a local oscillator source at 2.522 THz with mass less than 30 kg and that can provide at least 5 milliwatts (mW) of output power while consuming less than 100 W of input power, a solid-state local oscillator operating at 1000 gigahertz (GHz) with 0.2 mW output power, planar "lift-off" Schottky barrier diodes that operate at 600 GHz, a space-qualified solid-state local oscillator source at 325 GHz with 10 mW output power, a stable low power digital auto correlator (DAC), and stable low power acousto-optical spectrometers that are high resolution, multi-channel, and space qualifiable.
Current efforts under the Space R&T Base Information and Controls program, the University Space Research program (Center for Space Terahertz Technology at the University of Michigan), and the CSTI Science Sensor Technology program support basic research in terahertz technology; university and JPL research in planar Schottky barrier diodes and submillimeter local oscillator sources; and development of the digital autocorrelator. Work under the OAST Exploration Technology Program at JPL has developed and demonstrated acousto-optical tuned filter (AOTF)-based imaging spectrometers for the identification of samples for remote robotic collection. These AOTF spectrometers operate in the visible and infrared, but the technology has possible spin-off application for submillimeter acousto-optical spectrometers. OAST is also funding, under the Space R&T Base University Space Research program the Space Engineering Research Center for VLSI (Very Large Scale Integration) Circuit Design at the University of Idaho, which is investigating low power designs for the digital auto correlator. There is no currently funded activity in laser local oscillators, another approach for providing the local oscillator reference.
Researchers at the University of Michigan Center for Space Terahertz Technology have developed the first square antenna array to operate near the one terahertz range. This 16x16 focal plane antenna array has been miniaturized to operate at 0.8 THz. Although currently planned earth science submillimeter missions, such as the EOS MLS, are not planning to use array technology, development of this capability will allow higher resolution instruments that no not need to scan to obtain spatial data. Eliminating the need to scan simplifies the instrument design by removing mechanical parts, which reduces the mass and size, and enhances the reliability of the instrument.
OAST has developed a 52-channel, 125 MHz bandwidth digital autocorrelator spectrometer for a JPL/University of California, Santa Barbara balloon experiment. Also under this effort, a 32-channel correlator chip was custom-designed and fabricated. This custom-designed chip requires less power and provides higher spectral resolution compared to state-of-the-art analog spectrometers. These developments directly support plans for future airborne microwave instruments, the EOS MLS, as well as future submillimeter astronomy missions.
The atmosphere is mostly opaque in the submillimeter region of the spectrum. For this reason, there has been very little interest and support for this technology by the military (the one exception is some support by the Strategic Defense Initiative Office, for detecting cold warheads, emitting in the submillimeter, against the even colder background of space). At longer wavelengths, there are a few "atmospheric windows," regions in the spectrum where observations of space can be made from the ground. This technology has been transferred to ground based astronomy community and is currently in use at ground-based observatories in the U.S., Europe, and South America
Solid state lasers require an optical pump, an input of light to excite the electrons in the laser crystal to the upper, lasing level. Any optical pump light energy that does not excite electrons to the proper level ends up heating the crystal. This creates several problems: it wastes energy and makes the overall system less efficient; and it heats the laser crystal, which makes the laser less efficient, and in the worst case can damage of destroy the laser. In some cases a mechanical cooler is needed to cool the laser, adding to the complexity, mass, and power requirements of the overall laser system. Early solid state laser system used flash lamps for optical pumping, which produce a broad spectral output, most of which ends up as waste heat in the laser. In addition, flash lamps are fragile, and not reliable enough for long duration space missions. Recently, flash lamps have been replaced by semiconductor diode pumps. Since most of the light from these arrays is at the right wavelength to be absorbed by the laser crystal, they are more efficient and produce less waste heat. They are also inherently more reliable, and since they are in large arrays, the failure of a few of the array elements means a graceful degradation rather than catastrophic loss of laser performance. A semiconductor array pumped solid state laser system has been space qualified for the Mars Orbiting Laser Altimeter (MOLA) instrument for the Mars Observer mission.
Laser measurements of wind velocities detect the Doppler shift in the return signal from the earth's atmosphere. This requires coherent detectors with high quantum and mixing efficiency (greater than 75%), wide electrical bandwidth (10 gigahertz (GHz)) and large area or two-dimensional array capability. Ideally, this detector should have full operational performance at room temperature. Current plans for the detector for the EOS Laser Atmospheric Wind Sounder (LAWS) requires use of Stirling cycle mechanical coolers.
The Laser Atmospheric Wind Sounder (LAWS) was selected for EOS because of the critical importance of accurate wind velocity measurements to global atmospheric modeling, as well as for weather prediction. Current weather forecasting models infer wind direction and speed from space and ground-based pressure measurements. Current plans for the LAWS instrument call for a pulsed, frequency-stable CO2 laser transmitter, a continuously rotating 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. OAST supported the technology development for LAWS by co-funding, along with OSSA, the development of two competing brass-board instruments that will be used to obtain lifetime and performance data. In previous years, researchers at LaRC developed the catalyst materials used to restore the CO2 gas.
Solid-state laser technology is rapidly emerging which could replace the more complex CO2 gas laser, plus operate at wavelengths of 2 micrometers versus 10 micrometers. Since there are more 2 micrometers diameter particles in the atmosphere than 10 micrometers diameter particles, atmospheric back-scatter at 2 micrometers is significantly higher than at 10 micrometers. This could significantly increase the signal-to-noise ratio, reducing the required size of the transmit/receive telescope for a lidar system. However, careful measurements and systems analysis studies are required to ensure that differences in the other components of the system, such as the speed and accuracy of the coherent return signal measurement, do not outweigh the advantage in return signal strength. Even if the net system performance is comparable, a solid state laser based system should prove to be inherently less susceptible to mechanical failures.
Key technology developments for 2 micrometers laser wind sounding are the development and optimization of the solid state laser crystal material, development of semiconductor array pumps, and development of the coherent detectors. Leading candidate laser crystal materials are holmium, thulium doped yttrium aluminum garnet (Ho, Tm: YLF) and holmium, thulium doped yttrium lithium fluoride (Ho, Tm: YLF).
Differential Absorption Lidar (DiAL) is a technique to measure concentrations and vertical profiles of trace gasses in the troposphere and stratosphere. The Lidar Atmospheric Sounder and Altimeter (LASA) instrument, a 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. The key technology development for DiAL measurements is a frequency agile, tunable laser that can be tuned on and then off of the absorption bands of key atmospheric constituents.
Satellite laser ranging has been used for almost two decades in the study of a variety of geophysical phenomena including global tectonic plate motion, regional crustal deformation near plate boundaries, studies of the earth's gravity field, and studies of the orientation of the earth's polar axis and variations in the earth's spin rate. In 1964, NASA first successfully demonstrated laser ranging to satellites. OAST has supported the continued improvement in this technology to the present, and has developed lasers, rapid detectors, and timing circuits.
In satellite laser ranging, ground-based laser stations transmit ultra-short, intense laser pulses to retroreflectors mounted on the satellite. Many satellites now carry these reflectors. The times of the pulse and the return of the reflected signal are accurately measured and compared, and corrected for atmospheric delay to obtain an accurate measurement of the geometric range from the ground station to the satellite. Using a global network of ground laser stations, NASA determines both the precise orbit of the satellite, and the precise location of the ground station. By monitoring the location of these stations over time, researchers can deduce the motion of the earth-based observing sites due to plate tectonics and other processes such as subsidence.
OAST developed technologies are a key part of this worldwide network managed by the Goddard Space Flight Center (GSFC). Advances in this technology now allow sub-centimeter precision in the range measurement, attracting the attention of a new community of scientists, notably those interested in high resolution ocean, ice and land topography. Over the next several years, the international SLR network will provide precise orbit determinations to two new oceanographic satellites, ERS-1 and TOPEX/Poseidon, which range to sea and ice surfaces using microwave altimeters.
To date, these SLR measurements have utilized ground-based lasers, and the only space hardware required has been the retroreflector. Advances in laser technology have allowed the development of space-based laser systems that will accurately measure the range to the ground. The Mars Observer spacecraft will include the Mars Orbiting Laser Altimeter (MOLA) instrument, a space-based laser ranging system based on OAST developed technologies. A related instrument, the Geoscience Laser Altimeter System (GLAS), is planned for earth science measurements. Current OAST supported technology efforts are focused on developing ultra-short pulse, two color lasers. Comparing the round-trip light time of two colors allows more accurate measurement and compensation for atmospheric delay.
Solid state lasers are important for a wide variety of applications, and technology efforts are underway both through NASA and the Department of Defense. These research efforts are well coordinated through mechanisms such as the joint NASA/Department of Defense (DoD) Space Technology Interdependency Group (STIG), the Advisory Group on Electron Devices (AGED), and the NASA Sensors Working Group. The premier research center for the identification and characterization of new solid-state laser crystals is in the Commonwealth of Independent States, and efforts are underway to identify and develop the appropriate cooperate efforts with this unique capability.
Basic research is being conducted under the Space R&T Base Information and Controls program, as well as work under the CSTI Science Sensor Technology program in laser materials research, laser transmitter design, and lifetime and efficiency improvements. A flight experiment, the Laser In-space Technology Experiment (LITE) under the Space R&T Base Space Flight program will demonstrate application of laser technology to Earth science needs. After the initial technology experiment flight on the Space Shuttle scheduled for 1994, this instrument will be turned over to OSSA for scientific flights.
Past work at LaRC has developed a frequency agile (i.e. easily tunable) titanium: sapphire solid state laser source in the 0.65 to 1.1 micrometers wavelength region for lidar (light detection and ranging) and DiAL (differential absorption lidar) applications. LaRC is continuing to study and improve the performance of known solid state laser materials, and to characterize candidate new solid state laser materials. The LaRC has demonstrated improved performance in two materials that operate near 2 micrometers. This research has shown improved efficiency and spectral purity in holmium, thulium doped yttrium aluminum garnet (Ho, Tm: YLF) laser crystals, and has verifies a 13% overall efficiency for holmium, thulium doped yttrium lithium fluoride (Ho, Tm: YLF) laser crystals operating at room temperature. Higher temperature operation is important to reduce the size, mass, complexity, cost, and power required for lidar instrument systems by eliminating or reducing the need for mechanical cryogenic coolers.
On-board data storage systems are needed that have the capability to acquire data at variable rates up to and possibly exceeding 300 megabits-per-second (Mbps), and to output the data at rates up to 150 Mbps. Smaller, light-weight data storage systems are need for Earth Probe applications. Technologies that offer these capabilities, such as solid state memories or magneto-optical disc recorders, have the additional advantage of having fewer moving parts, and promise to be inherently more reliable. These new systems can be designed to conform to the data packetization standard recommendations of the Consultative Committee for Space Data Systems (CCSDS), the organization that establishes international space data standards. For all of these data storage approaches, data compression techniques could be used to increase the data storage capacity.
Current research under the Space R&T Base Information and Controls program, and under the CSTI High Rate/Capacity Data Systems program, are developing space qualifiable component technologies such as electro-optical memories with no moving parts. The CSTI program is developing a flight optical disk system which could be accelerated for application on the EOS. Efforts at the LaRC on the Space-flight Optical Disc Recorder have demonstrated the parallel access capability, resulting in an eight-fold increase in the data transfer rate (133 megabits-per-second), faster than any other known disc technology, and a hundred-fold increase in storage capability (5 gigabytes on one disc surface, equivalent to 2 million pages of text). This effort has developed the components of a space optical disc storage system, including a nine-element laser diode array for reading and writing data and the magneto-optical storage media. A breadboard of the system controller has been developed and tested. Plans for this program include the development and in-space flight test of a full, read/write optical disc data storage system.
The longer wavelengths in the microwave region (as compared to the visible and near infrared) require correspondingly larger apertures in space in order to obtain the necessary resolution. In the nearer term, technology developments are needed to provide large antennas and structures for future space missions that are lightweight (less than 20 kg/m2), foldable and deployable, and with high surface metric accuracy that can be actively controlled and corrected. In the longer term, the desire to place microwave sensing systems in geostationary orbit (which requires considerably more energy to obtain) places greater demands for reducing the mass of the antenna system. Geostationary platform-based microwave antenna structures should be ultra-lightweight, with mass less than 5 kg/m2. These ultra-light antennas could be either deployed directly at geostationary orbit, or erected in low earth orbit and boosted to geostationary.
Currently, rainfall is measured by ground-based instruments, which results in extremely poor measurements over the ocean and unpopulated areas such as 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 missions such as the Tropical Rainfall Measurement Mission (TRMM) will use active microwave sensing (i.e. rain radar) to provide a statistical sampling, but only continuous geostationary measurements provide 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 gigahertz (GHz) with an Earth footprint of 10 kilometers require an antenna diameter of 40 meters (Figure 4). 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 are being studied as alternative approaches 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 the proposed ESTAR instrument, an unfilled aperture 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.
Figure 4. Earth Footprint vs. Antenna Diameter for Geostationary Observations.
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 multi-beam antenna feeds and monolithic microwave integrated circuits under the Space R&T Base Information and Controls 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. The mass and power required for the EOS SAR were the principle reasons for its removal from the EOS payload. The EOS SAR is now baselined for its own free flying spacecraft.
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. OSSA has developed the 12 by 4 meter aluminum structure antenna (at 75 kg/m2) for the Shuttle Imaging Radar-C (SIR-C) mission. The SAR instrument on the ERS-1 spacecraft uses a carbon fiber antenna array at 79 kg/m2. These, along with the deployable mesh reflector antennas on spacecraft such as the Tracking and Data Relay Satellite System (TDRSS) and Galileo, reflect the current state-of-the-art in space-based antenna structures. The military has recently declassified information on the development a 16 foot precision surface panel deployable reflector antenna system.
Efforts under the Space R&T Base Materials and Structures and the Information and Controls programs are developing the technology to support passive microwave remote sensing, developing technologies for large deployable or erectable antennas and radiometer phased array feed systems. The Space R&T Base Systems Analysis program is supporting study efforts to further refine the technology options for these measurements. The OAST In-Space Technology Experiments Program (In-STEP) includes planning for a flight experiment of an inflatable reflector antenna concept.
At LaRC, a Controls-Structures Interaction (CSI) testbed has been developed based upon preliminary concepts for an earth science geostationary platform. The CSI program is developing methods to design, analyze, and test lightweight, high-performance, controlled structures. This test-bed is being used to test and validate the results of this research. Initial ground-test experimental results indicate a 20 to 30-fold increase in vibration damping using CSI technology. Application of this technology to space missions could result in increased pointing precision, reduced jitter for large structures such as microwave antennas, and decreased interaction an interference among pointing and vibrating instruments on multi-instrument platforms.
The premier data system for earth science data will be the Earth Observing System Data and Information System (EOSDIS). The OAST Space R&T Program has no focussed activities directed toward EOSDIS; however, there are a number of relevant R&T Base programs that are applicable and some that, with modification, could be useful to EOSDIS[19] and the software and data analysis needs of the Mission to Planet Earth. These include technologies relevant to scientific visualization & data/information technologies, such as, software engineering, advanced computing, data storage, networks, and computer architecture 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.
Technology development in software and data analysis for the coming generation of earth science missions has been extremely well coordinated at the working level between scientists and technologists. The goal of many of the OAST supported programs such as the Center of Excellence in Space Data and Information Sciences (CESDIS), and of OSSA supported programs such as the Applied Information Systems Research Program (AISRP) is to encourage space scientists and computer scientists to work cooperatively together to solve critical problem in space science computing.
OAST manages the NASA component of the Federal High Performance Computing & Communications program[20], which will produce the integrated computational systems addressing NASA Earth & Space Science Grand Challenges. Hardware and software technologies under development will offer teraflops (one trillion computations per second) of computing power and support processing data sets of petabyte (1,000 trillion bytes) scale in reasonable time. These capabilities will provide the technology base for future enhancement to the EOSDIS. As part of the HPCC program, NASA is competitively selecting grants through a joint OSSA/OAST NASA Research Announcement (NRA) in earth and space science applications[21], which will specifically fund research proposals in massive data analysis, among other topics.
Information visualization requires the development of an integrated family of tools, procedures and visualization environments for interactively visualizing science data, and merging and comparing the data with science models. Converting the "fire-hose" of earth observation data into useful information that can be understood by the scientific community is 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.
OAST supports two activities, the Center of Excellence in Space Data and Information Sciences (CESDIS) at the Goddard Space Flight Center (GSFC), and the Virtual Environment Workstation (VIEWS) at the Ames Research Center (ARC). Related human factors research is under the Aeronautics R&T Program (for cockpit information displays).
CESDIS is a cooperative program between GSFC and the University of Maryland, and is funded jointly by OAST and OSSA. Special projects involve principal investigators from universities across the United States. Table 2 lists the five (out of a total of 10) research projects funded by CESDIS that have some relevance toward EOS/EOSDIS:
Table 2. CESDIS projects with relevance to EOS and EOSDIS:
VIEWS (Virtual Environment Workstation) is a powerful user interface device under development at the Ames Research Center that provides the user with a vived experience of three-dimensional space. VIEWS is synonymous with Virtual Reality, and enables users to interact in a computer-generated environment which appears real and responds dynamically to the actions of the user. An instrumented glove may be used to manipulate objects in computer generated or remote environments. The goal is to allow real-time visualization of remotely sensed data, scientific data base access and management, and mission planning visualization and simulation. Nearer term goals are to conduct moderate-fidelity simulations and naturalistic experiments on the perception and control of interactive 3D displays, and demonstrate visualization of real-time remote terrain data.
Research is needed in 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.
The work currently supported by the OAST Space Research and Technology Base program, and under the Civil Space Technology Initiative (CSTI) Artificial Intelligence program, can be categorized into the areas of: 1) data archiving, access, and retrieval; 2) techniques designed to decrease the cost and schedule risk of software development; and 3) the development of novel techniques such as neural networks.
The Heterogeneous Distributed Object Type Management task at the Goddard Space Flight Center (GSFC) provides research on the uniformity problem so that NASA space data users can access data objects of different types, e.g., databases, spreadsheets, manuscripts, software tools, images, and graphics, independent of physical distribution and specific organization. DAVID ( Distributed Access View Integrated Database) is focussed on distributed data archiving, access and retrieval and advanced user interfaces for data visualization and interpretation. Although currently being applied as a demonstration to astrophysics data, the same approach could be used to improve scientific access to diverse earth science data sets.
The Automatic Classification and Theory Formation task at the Ames Research Center (ARC) is developing and applying artificial intelligence (AI)-based tools for automatic classification of very large scientific and engineering databases. The most current tool developed is AutoClass IV. AutoClass does the tedious task of identifying patterns and correlations in the data, and allows the scientist to concentrate on explaining these patterns and correlations. An earlier version of AutoClass identified a previously unrecognized class of infrared objects in the data set from the Infrared Astronomy Satellite (IRAS). AutoClass IV was recently demonstrated on LANDSAT data. Future work will explore other algorithms that require less computation and that are even better at dealing with noisy data sets. This work is being conducted at ARC and at Vanderbilt University.
The objectives of the Gaussian Windows task at ARC are the development and demonstration of an interactive data-exploration technique which is capable of screening, analyzing and processing the unprecedented large, high-dimensional data sets that will be produced by NASA missions. The work is done under the auspices of the Research Institute for Advanced Computer Science (RIACS) and is an outgrowth of the Sparse Distributed Memory Project (described later in this paper).
The Intelligent Data Management (IDM) task at GSFC is developing techniques for characterizing, cataloging, and retrieving satellite data in near real time. Research has centered on using, extending and deriving artificial intelligence techniques to the types of databases systems required by NASA. In 1991 this project developed and demonstrated intelligent user interface for the International Ultraviolet Explorer database and EOSDIS.
The Automatic Image Data Encoding and Analysis task at GSFC performs fundamental research in automated approaches for encoding multi-spectral imagery data into image segments based on the spatial structure of the data. The data are encoded into image segments, or regions, and each segment is analyzed as a whole, and in relationship with neighboring segments.
The JPL Scientific Analysis Assistant (SAA) under the Artificial Intelligence program applies artificial intelligence machine learning techniques to the development of an automated tool for the reduction of a large scientific data set. The program is currently focussed on astrophysics but is applicable to many types of scientific data, both ground- and space-based observatories.
The Atmospheric Modeling Project (a sub-task in PI-in-a-Box) at ARC is co-funded with the OSSA Information Systems Program. The task consists of basic research and tool building that focuses on the modeling of observed phenomena to allow the discovery of new information from prior knowledge and observations.
The purpose of the Software Engineering Research Center (SERC) at the Johnson Space Center (JSC) is to develop a new generation of computer systems software that is required to support extremely large, complex, distributed Mission & Safety Critical (MASC) components that must be cost effective and must operate ultra-reliably, nonstop over 30 years. Its applicability is primarily toward Space Station Freedom, and the Lunar/Mars missions.
The objective of the Software Management Environment (SME) program at GSFC is to improve NASA's ability to develop large, reliable software systems. This activity will produce and verify an automated software management environment, a set of tools that use past experience to help software development managers understand what to expect. SME will use measures such as lines of code developed, cost, and manpower, along with empirical models, to predict future project behavior, estimate key project parameters such as cost and reliability, evaluate a project's quality, and support management decisions for correcting project problems.
Also at JPL is the Neural Networks for Space Science Data Analysis task, which is aimed at evaluating the suitability of leading lossless data compression algorithms for implementation on a large-scale CCD/CID neural chip, and develop a methodological framework for solving selected partial differential equations on synchronous neuroprocessors. The overall goal is to demonstrate the unique capabilities of massively parallel charge-domain neurocomputing to address complex information processing and modeling needs. This would result in orders of magnitude speed-up for modeling of important geophysical phenomena.
The Knowledge Base Technology task is investigating the Sparse Distributed Memory (SDM) concept, a massively parallel associative memory that uses very large patterns, hundreds to thousands of bits in length, as both addresses and data. SDM has very fast learning capabilities even for large problems, and can look directly at information in the memory to determine what the memory has learned and to make decisions using feedback. The technique is promising for applications to the analysis of massive data sets obtained from earth-observing satellite systems. The SDM is conducted at the Research Institute for Advanced Computer Science (RIACS) at Ames.
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. Technology developments are needed in the areas of: efficient, lightweight, high-capacity solar arrays (both silicon-based and in the longer-term InP/Ge based arrays) that are easy and inexpensive to manufacture; advanced and improved power storage and retrieval technology, including nickel hydrogen (NiH2) battery technology; and improved power management and distribution technologies, that can reduce the parts counts and complexity of spacecraft power systems.
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.
OAST built and flew the Solar Array Flight Experiment (SAFE) on the Space Shuttle in 1984. More recently, OAST researchers at the LeRC have developed a 12-element solar array sub-module for space flight later this year. This sub-module uses mini-dome Fresnel lens concentrators to focus the light on the active part of the array. The sub-module demonstrates a power output greater than 300 W/m2, three times that of the Space Station Freedom solar panels. Rigid solar panel designs based on this sub-module show a power to mass ratio of 95 W/kg, a twofold increase over state-of-the-art rigid panel values.
The advanced photovoltaic solar array (APSA), developed by through TRW, is a flexible, thin sheet solar array system. APSA has been vibration tested at a ground test facility. The test demonstrated that the APSA, an ultra-light-weight solar array (140 W/kg), can survive the vibration loads of being launched into space.
The LeRC is also working to advance the state-of-the-art in battery technology. The nickel hydrogen battery was developed by the Department of Defense (DoD). LeRC improved this battery design, resulting in a breakthrough in battery life as well as increased system energy to mass ratio. This ten-fold improvement in charge/discharge cycle life (from 4,000 to greater than 40,000 cycles) is of critical importance to low earth orbit missions which spend part of each orbit in the shadow of the earth. These improvements led to the selection of this battery technology for the Hubble Space Telescope.
More recently researchers supported through the LeRC have developed a high energy density rechargeable lithium battery. This cell has shown a specific energy of 100 Watt-hours per kilogram at cell level with a lifetime of 1000 cycles at 50% depth of discharge. Although this is 40 times fewer cycles than the nickel hydrogen battery, these lithium batteries have three times the energy-to-mass ratio of state-of-the-art nickel hydrogen batteries, for cases such as geostationary orbits when there are fewer period when solar energy is not available.
Battery technology developments have direct application to all space missions, and could be critical to missions requiring high power capabilities, such as future radar missions (SAR, TRMM, and TOPSAT), especially for night-time observations when solar power is not available.
Significantly lighter, more efficient power systems allow for more of the satellite mass to be dedicated to science and applications. Improvement in the technology equally benefits all space missions.
As mentioned above, numerous study efforts have identified candidate technologies for future earth science and Mission to Planet Earth spacecraft. Not all of these technologies have made the OSSA list of most critical technology needs. Some are of lesser importance, some have longer-term payoffs (beyond the planning horizon), some have broad application to multiple mission users (but are of less importance to any one user), and some need further study and development to demonstrate their usefulness to future missions. These technologies are categorized into Observation, Information, and Infrastructure technologies. These areas were not specifically addressed and identified by the OSSA Earth Science and Applications Division at the ASEB/SSB review, but may have been identified by other OSSA divisions in the OSSA technology needs matrix.
To allow quality, long-term, continuous observation of Earth processes on local to synoptic scale from geosynchronous orbit, optical systems technology research is required. Visible and infrared observations from low earth orbit will benefit as well. This technology need includes large field aperture optics, diffraction gratings, ultraviolet thin films, electro-optic crystals, hologram optical elements, and optical system performance modeling.
Beginning in 1974, OAST co-funded with OSSA the development of charge coupled device CCD) technology that led to flight sensors on the Hubble Space Telescope, Galileo, Yohkoh, and the shuttle still camera. This CCD technology will be used by EOS on the MISR instrument. Currently there is no CCD research within the OAST program. Development of large array (20,000 by 20,000 elements) CCD's is needed to support ultra-violet, visible, and infrared high resolution (spatial, spectral, and temporal) observation of the Earth, especially from geostationary orbits.
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 under the CSTI Artificial Intelligence program.
Voyager's near encounter with Neptune in August of 1989 gave NASA the opportunity to introduce automation and artificial intelligence to the process of monitoring spacecraft operations. The OAST developed Spacecraft Health Automated Reasoning Prototype (SHARP) provided telecommunications personnel with computer tools which allow them to have a more complete understanding of how the telecommunications link is functioning between the spacecraft and the Deep Space Network tracking stations. The benefits of this technology were underscored prior to Voyager's Neptune encounter. SHARP helped find the cause of a science data error which appeared in the telemetry from the spacecraft. After SHARP detected the problem, its graphic displays were used by telecommunications personnel to identify the problem and characterize its magnitude. In a matter of hours, SHARP was able to assist operators in solving an anomalous condition which could have easily escalated to a more serious problem during the encounter, and which could have taken human operators days or weeks to isolate without SHARP.
Mission to Planet Earth platforms will require on-board processors capable of handling 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 Information and Controls program, and under the University Space Research program (which supports the Space Engineering Research Center for VLSI System Design at the University of Idaho), and under the CSTI High Rate/Capacity Data Systems program, are developing space qualifiable component technologies such as satiable 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. Further work will develop tools and techniques to design, simulate, produce, and test application specific integrated circuits.
Key to the success of the Mission to Planet Earth will be the transmission of data between spacecraft, from the spacecraft to the ground, and between ground stations. 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. The OAST Space Communications program is developing technology in radio frequency communications, digital communications, optical communications, mobile communications, and advanced research in satellite communications.
The ancillary technologies to enable long term continuous observations of the earth need to be developed. The same arguments that support the development of improved power systems for earth science spacecraft 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 system technologies require development.
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, 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. Technologies to support future servicing capability developments have not been addressed in this paper.
Through a decade of mission and systems analysis studies, and more recently through the development of the Integrated Technology Plan, NASA has identified the set of critical technologies required to support a long-term, comprehensive and evolving Mission to Planet Earth, including the upgrade/replacement platforms for EOS as well as future geostationary platforms. 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 systems analysis studies, and through NASA internal and external review.
The 1987 NASA study Leadership and America's Future in Space 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." There are many examples of scientific objectives which cannot fully be achieved with existing technology. A strong advanced technology program can reduce the technical risk and cost and improve the quality and quantity of science data. Certainly before moving to an operational global change detection system, the results of the currently planned EOS program and evolving new technologies must converge toward an effective, next step.
No activity is more critical than understanding our home world, and the NASA OAST 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.