NASA-GSFC nano-satellite technology for Earth science missions

AcraAstronautica Vol. 46, Nos. 2-6,~~.287-296,200O 0 2000 Published by Elsevier Science Ltd. All rights reserved

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NASA-GSFC NANO-SATELLITE TECHNOLOGY FOR EARTH SCIENCE MISSIONS Jaime Esper*, Peter V. Panetta, Dr. Michael Ryschkewitsch, NASA-GSFC,

Dr. Warren Wiscombe, Steven Neeck

Greenbelt, Maryland 2077 1, USA

ABSTRACT The NASA-GSFC Nano-satellite Technology Program is currently formulating solutions for 21”’ century Earth Science requirements. We anticipate that nano-satellite (- 10 kg) and micro-satellite (10 to 100 kg) constellations will have important applications in both Earth and Space science. Such constellations, acting in unison and with a large degree of autonomy, could form “virtual platforms” of detailed remotely sensed measurements providing orders of magnitude more information than today’s thinly-populated networks of LEO and GE0 satellites. If the constellations include a variety of basic, versatile instruments, for example UV, VIS and IR hyperspectral spectrometers, then virtual platforms for different applications can be formed in space, on the fly, and “disassembled” later for other uses or to test other scientific hypotheses. Example applications include weather prediction, radiative/reflected energy measurements for global change studies, hazard warning and monitoring systems (fires, volcanoes, hurricanes, etc.), and in-situ measurements of Earths magnetic field. For a wide range of applications, nano- and micro-satellite technology is likely to further the way NASA explores not only the Earth, but the solar system and beyond. Identifying the strategies and technologies that provide strong benefit to both the Earth and Space science programs will provide the best return on NASA’s technology investment. This paper will highlight some possible Earth Science applications for nano- and micro-satellite constellations as well as the current status of planned 0 2000 Published by Elsevier Science Ltd. All rights reserved NASA-GSFC nano/micro-satellite technology development. Keywords: nano-satellite,

micro-satellite,

constellation,

Earth Science, remote sensing, in-situ measurements.

platforms bristling with instruments. Technologies to break future missions up into nano- and micro-satellites are already being developed at Goddard and elsewhere. Fleets of such satellites have all the benefits of large platforms, and few if any of the drawbacks. Protocols replace physical connections. GPS allows precise formation flying and precise pointing of fleet members at the same Earth scene. Subsets of the fleet can formationfly as a “super-instrument” for a while, then move on to other observing tasks rather than being mated for life. Other subsets can “race to the scene” of interesting or important events on Earth. Fleets of micro/nano-satellites inexpensive rapid cost-effective, allow and are replacement of aging technology, and are quicker to get into orbit (not only shorter development times, but many more opportunities to piggyback on launches of larger satellites).

1.0 ADVANTAGES OF SMALL SATELLITE CONSTELLATIONS FOR EARTH SCIENCE In 1988, the well-known physicist and popular author Freeman Dyson gave a series of lectures “On Being the Right Size: Reflections on the Ecology of Scientific Projects”. In particular, Dyson analyzed space missions with a view to the appropriate balance between large and small. He estimated an improvement in cost-effectiveness of at least lo4 between the IDE telescope (1978) and Wemher von Braun’s proposed Mars Project (1952), and noted that this huge jump came about from a radical change in style. He goes on to say: “When we look ahead to 2018, we should expect big steps forward in science to come once again from changes in style rather than from marginal improvements in technology... To improve costeffectiveness by a factor of 104, we need spacecraft that are radically cheaper, smaller, and quicker than anything we can build with 1988 technology. Spacecraft should weigh pounds rather than tons, they should cost $104 rather than $lO’, and they should fly missions at the rate of several per day rather than several per year.”

Earth observation from space is an attempt to populate an 8-dimensional space with data. The 8 dimensions are time, space (3), angle (2), wavelength, and polarization. The sum total of all Earth-viewing missions has explored only tiny disjoint fragments of this space. Given the specialized nature of current Earth Science missions, and the lead times involved in carrying out a particular

We believe that Dyson is pointing in the right direction. For Earth Science we foresee little further need for large * Swales Aerospace, 5050 Powder Mill Rd, Beltsville, MD 20705, USA

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implementation, we are normally forced to fix all or nearly all of the measurement characteristics years in advance of the time that observations can commence. In contrast, nano- and micro-satellite constellations offer an observational system that could tailor all measurement characteristics to the target of interest, to the intended data use, and to evolving science understanding. This maximizes the useful information content of the data gathered as well as the use of the asset. Such constellations have the flexibility both to adapt to evolving science needs and to implement the newest technologies during the course of an on-going flight mission system. In many cases, the “missions” of the past would be replaced by “investigations” involving manipulations of the existing constellation for new purposes, where we will be able to think of an udupzble observational infrastructure that enables science investigations. Hence, rather than referring to “a spacecraft mission”, the shift would be toward a “spacecraft constellation mission”, with multiple components and ensuing llexibility to modification of individual components (spacecraft/application). Like the World Wide Web, it will allow rapid adaptation and evolution to changing needs. Nano- and micro-satellite constellations will also give us the flexibility to cluster observations where they are most needed for prediction. There is a rising tide of research showing that weather prediction could improve greatly if observations were made in crucial limited space-time regions rather than uniformly in space and time. Inevitably, this can prove true for climate prediction as well. Unfortunately. surface observations, whether on land, ships, or in the air, cannot be moved around fast enough to implement such a challenging new paradigm; but constellations of very small satellites could “race to the scene” for intense measurements in regions of most quantitative value to the predictive models. Another advantage of very small satellite fleets would be the simplification and improvement of international cooperation, which at present is difficult to control and time-consuming. With a constellation, international cooperation would merely involve a country agreeing to meet the protocols of the constellation, and launching their own satellite to join the fleet. If they were late, or ran out of money, or their instrument failed catastrophically, the whole mission would not be dragged down with them. This in turn would provide relatively frequent and inexpensive opportunities for countries just entering the space game, which might otherwise be excluded. A proper system study to identify all of the needs of future Earth Science systems is beyond the scope of this paper. However, useful insights into the numbers of platforms needed can be gained by looking at a small

number of illustrative examples. For instance, a phenomenon that has much to gain from observation with a nano/micro-satellite constellation is the El Nino Southern Oscillation (ENSO), a short-term climate variation that drives significant changes in the temperature and rainfall over a large portion of the globe. Although the economic impact of these variations is large, good predictions would substantially ameliorate the resulting adverse effects. If one examines the shortfalls in our present predictive capability, and the capabilities needed to make dramatically improved seasonal to interannual predictions of the effects of the ENSO, nano/micro-satellite technology would make a large impact. In particular, a large number (-10’) of in-situ in-theplatforms (on-the-ground, in-the-ocean or atmosphere), and tens to hundreds of space-based remote sensing platforms will be necessary. These large numbers of platforms are needed to provide closely spaced subsurface measurements of temperature and salinity, tine spatial resolution, and high revisit rates for a variety of remotely measurable parameters such as surface winds, salinity, temperature, soil moisture, vegetative cover, etc. It is clear that new strategies will be necessary to enable such measurements at reasonable costs. Achieving the coupled nano-satellite program objectives of producing compact, lightweight spacecraft to lower launch costs and developing production techniques for both the spacecraft and instruments will enable dramatically new capabilities for Earth Science applications. 2.0 EXAMPLES OF SPECIFIC EARTH SCIENCE MISSION APPLICATIONS Although there are a wide range of Earth Science applications for nano- and micro-satellite constellations, we will touch on only a few representative examples. 2.1 Leonardo The first mission concept uses clusters of micro-satellites, each carrying but a single instrument. It is called “Leonardo” in honor of the famed Italian artist whose wide-ranging interests call to mind the flexibility and breadth possible in a small-satellite constellation. While Leonardo is re-configurable to support an enormous variety of scientific investigations, an early goal is to explore the variability in “radiative forcing” (net radiation flowing across a shell around the Earth above the atmosphere) and associate it with natural and man-made alterations of the Earth’s surface and atmospheric composition. If radiation leaving the Earth was a smoothly varying field, widely spaced measurements in time and space might suffice. In reality, radiation leaving the Earth has large variations in space, time, and angle, not least due to clouds. This enormous variability makes a constellation like Leonardo quite suitable to effect the required observations.

Small Satellites for Earth Observation

Leonardo would be composed of different micro-satellite clusters in different sun-synchronous LEO orbits. Initially, each cluster would have three micro-satellites, and each micro-satellite would carry hyperspectral spectrometers specialized to one region of the reflected or emitted spectrum: UV, VIS, and JR, respectively. Later, other wavelength regions and active instruments could be added. Twelve clusters would allow hourly measurements of every pixel on Earth. The failure of any one microsatellite or instrument would be only a small perturbation, since it could be quickly replaced from a spacecraft reserve possibly stored within the International Space Station (ISS).

2.2 Global Precipitation Mission Another mission example is the Global Precipitation Mission (CPM). Its objective is to measure global rainfall distribution at much higher spatial and temporal frequencies than current systems (e.g. the Tropical Rainfall Measuring Mission). This is the foremost measurement required to progress toward a quantitative knowledge of the water cycle, and arguably represents the most accessible hydrologic quantity for satellite remote sensing. The mission concept employs many microsatellites in medium inclination orbital planes (at 5001000 km altitude), each carrying real/synthetic aperture microwave radiometers that are miniaturized and have substantially reduced mass and power consumption. They also employ a high level of autonomy in operations and constellation maintenance, and facilitate a data collection architecture that can be sustained and augmented in capability at reduced cost.

2.3 In-Situ Measurements Examples where nano-satellites are used for in-situ measurements are provided by atmospheric, magnetospheric, and near Earth radiation research missions. A near Earth magnetospheric mission may be based on a design currently planned for NASA’s Solar Terrestrial Probe Program (STP), the Magnetospheric Constellation mission (MagCon). Here, a large number of nano-satellites (-100) are to be deployed at highly elliptical orbits in order to understand the filamentation of aurora1 currents, observe transitions between quiescent oscillations and geomagnetic storms and sub-storms, and other phenomena affecting the Earth magnetic environment. For the near-Earth environment, a multitude of orbits between polar or equatorial inclinations may be selected for multiple, 3-dimensional point measurements across the sphere of interest.

2.4 Other Examples Other potential examples of Earth Science applications of nano and micro-satellite constellations are under study. These include a proliferation of low-cost compact

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imaging radiometers, deployed on 10’s of micro-satellites in various orbits. This would complement existing meteorological and environmental research satellite capabilities in the measurement of global vegetation index and other terrestrial features; to enable land use/land change, seasonal to inter-annual climate change, and natural hazards research. Advances in micro-laser altimeters with two orders of magnitude reduction in mass and cost permit the possibility of micro-satellite constellations for altimetry over land, oceans, ice and clouds with much greater spatial and temporal sampling. Nano- and micro-satellite constellations enable exciting opportunities in microwave scatterometry through sensing of reflected L-band signals from GPSlGLONASS satellites or transmitter -receiver nano- or micro-satellite pairs. Measurements of sea surface wind speed, ocean salinity, and the extent of wetlands and flooded areas will be produced. Finally, nano--satellites will enable the measurement of Ozone and other atmospheric constituents at all levels in the troposphere, including the boundary layer, at high vertical resolution (
NANO/MICRO-SATELLITE INSTRUMENT TECHNOLOGIES FOR EARTH SCIENCE 4.0

Development of technologies that accommodate the required scientific measurements for remote sensing or insitu nano/micro-satellite platforms is an essential element in the process of developing a fully operational and highly capable nano-satellite. Hence, development of miniaturized instrument technologies go hand-in-hand with the total system architecture of a true nano-satellite. As spacecraft mass, volume, and power requirements are reduced, it becomes imperative to re-invent the method by which spacecraft are integrated. Instruments must be developed that conform to the available size and power requirements of a nano-satellite. Conversely, spacecraft may be developed which build around a particular instrument, in a similar fashion that a “science-craft” or “subsystem-less” spacecraft is created. NASA Earth Science technology programs recognize these needs and are consequently focusing on reducing instrument mass, power, and volume and enabling new sensor-to-science knowledge information architectures. The instrument incubator program (IIP) is currently enabling a number of the compact instruments that are part of the mission concepts described in Section 2.0 through fostering the development of innovative remote-sensing concepts and the assessment of these concepts in ground, aircraft, or

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engineering model demonstrations. Figure 4. I illustrates the technology vision for implementation of Earth Science Enterprise (ESE) missions of the future.

Figure 4.1: Earth Science Enterprise Technology

Vision

Making useful Earth science measurements with nanomicro-satellite and systems will present special challenges. Many of the measurements are aperture size driven and will require the development of a range of technologies for deployable optical and radio frequency filled and/or unfilled apertures. Instrument measurement capabilities will need to be improved through the development of micro-LIDARs and novel compact mechanism-less spectrometers. Lightweight techniques that also preserve high performance will have to be developed for the instrument systems. Reduced instrument size, power, and complexity will need to be achieved through highly integrated optical and microwave front ends, detectors, signal processing, and data product generation. In addition, production, characterization and calibration methods that allow for minimal touch labor while preserving scientific observational capability will have to be developed. Figure 4.2 describes the State of the Art (SOA) for a number of critical instrument technologies and future goals to achieve the ESE vision. A review of the representative mission concepts discussed in Section 2.0 provides insight into specific instrument technology development needs. The keystone of Leonardo is a range of UV, VIS, and IR imaging spectrometers. Each will have to be compact and have wide fields of view to eliminate life-limiting scanning mechanisms and to reduce aperture size. A novel all-reflective optical design will maximize instrument throughput and sensitivity and may be fabricated as an integrated opto-mechanical structure from SIC to reduce mass and ensure thermo-optical stability. Large format megapixel high quantum efficiency UV, visible, and infrared detector arrays will be required. Active Pixel Sensors (APS) may be used in the Siresponse region of the electromagnetic spectrum to reduce

for Earth Observation cost and to enable new instrument functional capabilities. Utilizing conventional CMOS fabrication techniques, APS sensors have lower power requirements, higher yields, and are inherently more compact than conventional CCDs. With equivalent or lower noise characteristics, APS technology permits on-chip analog signal processing and analog to digital conversion on a pixel by pixel basis and random addressability. In the future it will facilitate on-chip digital signal processing. Warm (i.e. near ambient temperature) PV HgCdTe or InGaAs detector arrays may be used in the SWIR, eliminating any need for special mechanical or radiative cryogenic coolers. Quantum Well Infrared Photodetectors (QWIPs) may be used in the MWIR and LWIR. These promise much lower fabrication costs and higher uniformity compared to conventional detectors with a broad range of wavelength tunability during the fabrication process. Microbolometer arrays may be an alternative if device sensitivities can be improved. They are uniquely attractive because they can operate at ambient temperatures like the aforementioned SWIR detectors. Slnlc-oi-A‘1

Figure 4.2: Instrument Technologies

2020 GOal

SOA and Goals

The Global Precipitation Mission relies on miniaturized, lightweight, high sensitivity microwave radiometers. These are enabled through a variety of technologies including a low-mass composite material synthetic aperture thinned array antenna, compact low power MIMICS-based receiver modules, and FPGA/ASIC digital correlators. Alternative technologies include a low-mass microstrip array antenna or an inflatable tensioned membrane waveguide antenna array. Other key technologies for the representative missions include “spectrometer on a chip” integrated MEMS variable etalon with megapixel detector arrays for the measurement of global vegetation index; high efficiency low power diode-pumped Q-switched Nd:YAG microlaser oscillator/amplifier with start diode and lightweight telescope for KHz altimetry over land, oceans, ice, and clouds; and a miniature low power

Small Satellites for Earth Observatiotl

GPWGLONASS receiver with a large number of correlators capable of fmely selecting Doppler shifts to isolate specific sections of the range-Doppler space for scatterometry measurement of sea surface winds, ocean salinity, and wetland areas. Instruments for in-situ particles and fields measurements consist of miniaturized low and hi-energy particle detectors (electrons and ions) and magnetometers. The low energy particle instrument contains a semiconductor telescope using ion implanted silicon detectors, and a magnet used to separate electrons and ions. It is scientifically desirable to include a sensitive electric field instrument to complement the magnetoplasma measumments, although innovative techniques need to be developed to make the required electric field measurements without the need for a long boom.

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other capabilities, while still able to carry out its intended compromise. The scientific objectives without development of these systems is bound to revolutionize the Earth and Space sciences by dramatically reducing the cost of building and flying scientific (and even commercial) missions. Figures 4.1 and 4.2 illustrate current conceptual designs for spin and three-axis stabilized nuns-satellites, respectively. Cmnm Antenna

Magnetometer

Patch Antenna \

\

3.0 NANO/MICRO-SATELLITE TECHNOLOGY AT NASA-GSFC Nano-satellites require technologies that radically reduce the mass and power of components without compromising performance. In addition to miniaturizing components, the technology efforts are looking at methods to integrate similar functions across subsystems. For example, all subsystem electronics, including instruments, could be integrated within the central data processing module. Multifunctional solutions also offer significant savings over traditional approaches. Technology investments are required to develop or adapt components to accommodate the expected radiation environment. Simple, effective methods of thermal control are essential to keep the operational during extreme nano/micro-satellite temperature variations. Autonomy is a critical technology that impacts every subsystem. Constellations with tens to hundreds of spacecraft must be highly autonomous to be practical. The nano/micro-satellite ground system must be kept inexpensive, simple, and made interoperable with other missions. The choice of technologies will depend on the particular application for which the nano- or micro- satellite is designed. Spin stabilized nano-satellites for instance are better suited for in-situ measurements, whereas three-axis stabilized micro-satellites are well suited for remote sensing applications. In addition, until advanced instrumentation technologies are developed, particularly in the area of optical and electro-optical components, such payloads will not be suitable for the smaller, 10 kg-class nano-satellites. It is for this reason that the distinction is made here between nano-satellites for in-situ measurements, and micro-satellites for remote sensing. The long-term goal is to produce filly capable remotesensing nano-satellites within the next 10 years. This 10 kg remote sensing platform would be capable of performing its own orbit insertion, station keeping, attitude control, power generation, full autonomy, and

Figure 4.1: Nano-Spinner Star Trackka

Sun Sensor

\

Telescope

..

I Oti Antenna

Figure 4.2: Nano-Pointer Constellations of 10s to 100s of spacecraft also pose an operations problem. Communication schemes need to be developed that minimize the burden on ground stations. For instance, one approach is to assign only a subset of spacecraft to communicate to the ground during scheduled passes. In addition, communication protocols for intersatellite communication will be needed to guarantee the acquisition of data for those spacecraft that do not communicate directly to the ground. Either a worldwide network of antennas or a few selected antenna locations may be used, depending on particular mission requirements. Whether one or ten antennas, they will receive spacecraft data and transfer them to a centralized Operations Control Center (OCC), which in itself may be shared with other missions in order to take advantage of reusability. Furthermore, the operations staff is minimized through a high level of automation, not only on the ground segment, but also through highly autonomous spacecraft. Although the command and control of scientific satellite constellations may benefit from the experience gained by current commercial communication networks, aspects including differences in orbit, data

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volume and processing, and orbit determination requirements promise to differentiate NASA constellation operations from their commercial counterparts. Finally, delivery and deployment systems represent ancillary elements that support the existence of satellite constellations. Minimizing spacecraft mass and volume allow for a large number of satellites to be deployed into space by a single launch vehicle. Deployment structures that can carry the nano-satellites to their desired orbit may be either passive or active, meaning that they may be designed for special maneuvers such as orbit insertion, or simply as a satellite carrier. An example of a “active” deployment structure is given in Figure 4.3, shown onboard a Delta II 7925A payload fairing.

Figure 4.3: Nano-Satellite

Carrier

The following paragraphs provide an overview of some selected technologies that can support the application of nano/micro-satellites for Earth science missions. 4.1 Propulsion The nano/micro-satellite propulsion system must be capable of carrying out all the functions expected of a large spacecraft, yet it needs to fit within a reduced framework of mass, power, and volume. Present chemical propulsion technologies, which rarely dominate the power system resources on large spacecraft, cannot fit within the power constraints of a spacecraft this small. Also, as propulsion systems decrease in size, the increasing mass/volume ratio of the propellant tanks and the fixed mass of the other components combine to decrease the propellant mass fraction. Furthermore, the standard cost of today’s thrusters and other components is prohibitive when multiplied across tens or hundreds of spacecraft. Our research to date has led us to focus on chemical propulsion technologies. While certain electric propulsion

(EP) technologies (e.g. pulsed plasma and field emission EP) can be made to operate at I W input power, they would provide only extremely small impulse bits (on the order of 10.’ to 10e6 N-s). This makes EP far less versatile, as they would not be applicable to spinstabilized nano-satellites, nor to three-axis-stabilized nano-satellites with significant reorientation requirements. Nonetheless, developments in ultra low power EP will be followed for possible applicability to a limited number of three-axis-stabilized missions. Propulsion system performance requirements (measured in terms of total AV) depend on what they are expected to accomplish. In this sense it is possible to differentiate propulsion systems functionally in terms of Orbit Insertion Maneuverability (OIM), Attitude Control (ACS), and Station-Keeping (SK). Which functional system demands the highest performance is specific to the mission goals. For instance, a Low Earth Orbit (LEO) mission may require a higher performing SK propulsion system, whereas a Medium Earth Orbit (MEO) may require a higher performing OIM. A number of propulsion systems have been considered, including solid and liquid propellants, and hybrid propulsion systems. The applicability of either system for a particular mission depends on the functionality requirements as outlined above. The following products are most desirable for our applications, and are actively being pursued for development: miniaturized solid propellant AV motors with a low cost/mass ignition system; miniaturized liquid propellant thrusters (hydrazine or advanced monopropellant); ultra low power cold gas micro-thrusters; low cost tanks and other feed system components; low power gas generators for liquid-storage cold gas feed systems; and micro-machined solid propellant motors for attitude control firings. Many challenges remain in the development of acceptable solid propellant motors for nano/micro-satellites. The motor must be able to accommodate a wide range of AV requirements without incurring costly changes to the nano/micro-satellite’s mechanical interface. The safe/arm system, typically a mechanical device on larger motors, must be downsized radically, and use of a non-mechanical switch would require the concurrence of range safety personnel. Low cost fabrication techniques must be employed to ensure that the purchase of one motor for every nano-satellite is not prohibitively expensive. Finally, an acceptable thermal design must be devised to limit heat input to the nano-satellite from the burned-out motor. Miniaturized liquid propellant thrusters are another promising technology. Given the lower specific impulse (1s~) of hydrazine systems compared to their solid propellant counterparts, they could be used for missions

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with lower AV requirements, or for attitude control on spin-stabilized or three-axis-stabilized nanolmicrosatellites. The development of miniature hydrazine thrusters also entails many challenges. The power required to operate valves must be reduced by an order of magnitude. For three-axis-stabilized applications, the thrust level must be reduced by two or three orders of magnitude. Additionally, smaller thrusters will require novel thermal design approaches to prevent flow choking or premature combustion. Advanced mono-propellants, such as those based on hydroxylammonium nitrate (HAN) and other chemicals, offer all the advantages of hydrazine with several additional benefits, including higher specific impulse, higher density, non-toxicity, and lower freezing point. Although the stability and performance of advanced mono-propellants have only begun to be characterized, it is expected that a system may be available for flight qualification in less than four years. One potentially near-term technology is the ultra low power cold gas thruster. Because of the low specific impulse of cold gas thrusters, they cannot be used for any substantial AV on a nano-satellite. However, their simplicity and multiple-pulse capability makes them a good choice for attitude control. Finally, small, solid propellant gas generators could be used as ACS thrusters. Such gas generators are currently under development at NASA’s Lewis Research Center. micro-electromechanical systems BY incorporating (MEMS) techniques, the devices can be produced relatively inexpensively. Propellant selection, low-power ignition, and thruster array packaging are some of the challenges ahead for this technology. 4.2 Guidance, Navigation and Control The Guidance Navigation and Control (GN&C) subsystem key technologies and concepts have been identified to enable the successful performance of spinstabilized and three-axis-stabilized nano-satellites for future missions. Technologies for spin-stabilized spacecraft include miniaturization of a sun sensor and horizon-crossing indicator. The miniature precision ‘fan’ sun sensor will pinpoint the sun virtually anywhere in the entire celestial sphere with every satellite rotation. The sun sensor will be required to weigh less than 0.25 kg, draw less than 0.1 watts, operate on no greater than a 3.3 volt bus, and meet a 0.1 o resolution requirement. The miniature horizon-crossing indicator has a small boresight FOV that is mounted at an angle off the spin axis. As the spacecraft rotates, a cone of coverage is formed. Total horizon crossing indicator weight and power will be less than 0.2 kg and 0.1 watt, respectively.

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Technologies for three-axis-stabilized spacecraft include a miniaturized gyroscope (micro-gyro), a wide-angle earth sensor, and a miniature star tracker. Future efforts will also include a miniature reaction wheel, a miniature threeaxis magnetometer, electromagnets, a two-axis sun sensor, and a two-axis earth sensor. The three-axisstabilized LEO missions are further classified into two sets of pointing requirements, coarse and tine. The coarse pointing missions will require the development of miniature wide angle and miniature two-axis earth sensors. Development goals include a factor-of-four weight reduction while maintaining 0. lo resolution. Fine pointing missions will require a miniature star tracker and a miniature two-axis sun sensor. The initial design iteration will focus on minimizing weight by sacrificing resolution to the order of an arc-minute. The second design iteration will bring the resolution to the order of an arc-second. Of particular interest to LEO missions is the incorporation of GPS onboard the nano-satellites, to eliminate groundbased ephemeris generation. Studies are underway to define the limits and required software to be developed in support of this application. In particular, algorithms that enable formation flying and autonomous operation of a space systemmay be expanded from the work already being done by NASA-GSFC for the Earth Orbiter 1 (EO1) mission. The Enhanced Formation Flying (EFF) software technology uses a mix of fuzzy logic and natural language capable of planning, executing, and calibrating routine spacecraft maneuvers. For the GPS hardware to fit within the constraints of a nano-satellite, the receiver electronics need to be miniaturized into a layer within the avionics module.

4.3 Avionics Module NASA-GSFC is developing an integrated avionics package that features reduced power consumption (0.8 W), low mass (0.25 kg), radiation hard, low power processing platforms, high capacity, low power memory systems (2 Gbits), radiation hard, re-configurable, field programmable gate arrays (RHrFPGA), and 3-D cube Multi Chip Module (MCM) design. Advanced, modular, and scalable-packaging microelectronics is being developed to both reduce cost and meet the requirements of various missions. The current avionics MCM concept is composed of six functional elements called “slices”, each dedicated to performing a particular task: processor, memory, instrument interface, ACS interface, housekeeping and communications interface [2]. The interface, development of these slices rely heavily on the use of RHrFPGA, in turn developed based on CMOS Ultra-Low Power Radiation Tolerant (CULPRK) technology. The

goal of CULPRiT is to enable a 32: I reduction in power over the current S-volt technology, foundry independence of die production, and radiation tolerance (100 Krad Si total dose. latch-up immune). The development of the nano/micro-satellite MCM for NASA-GSFC will utilize the most cost effective whether infusing commercially driven approach, semiconductor devices into spacecraft applications or partnering with industry to design and develop low cost, low power. low mass. and high capacity data processing devices. The packaging method chosen must have a small volume and small footprint (6 cm x 6 cm x variable height). The packaging technique must provide data on programmable substrates to accelerate the process of The packaging “prototype to flight” with less cost. data on compliant must also provide technique interconnects for space use. Figure 4.3.1 shows an example of a MCM made by Pica Systems Inc.

longer-term solution, and are discussed in section 4.7 of this paper. Circuits that have short, high current demands, such as thruster solenoids and fuses, need to be augmented with components that have a lower power density than batteries, but also have lower internal resistance. Ultra capacitors are a candidate for this application. Miniaturization of the power system electronics (PSE) to meet the weight and size requirements of nano-satellites is a considerable challenge. A fundamental principle is to match solar cell operating voltage with the operating voltage of the selected battery technology. The ideal approach is to eliminate the PSE completely, by having a fixed electrical load and batteries provide the needed bus regulation. This yields a simplified system consisting of the solar cells, batteries, and minimal circuitry. A more immediate approach to miniaturization is to produce hybrid modules for each PSE component, namely the solar array regulator, battery regulator, and low voltage power converter. Beyond this. the combination of these three components into one module will reduce the size and weight another order of magnitude.

4.5 Thermal System

Figure 4.3.1. Multi-Chip-Module 4.4 Power Systems The spacecraft physical size limits the area available for solar power conversion. A number of schemes may be developed to ameliorate this difficulty. appropriately applied to either spin or three-axis stabilized nano/microsatellites. Among the solutions, lightweight, efficient solar array panels with maximum packing factor are a possibility. Triple or quad-junction Gallium-Arsenide (GaAs) solar cells that yield 20 to 30% conversion efficiency at end-of-life (EOL) are being developed under a number of technology programs. Lithium-ion battery cells with high energy densities (- 200 W-hr/kg) and high cell voltage (up to 3.4 V) also represent energy-storage technologies under development. Spacecraft batteries used to cover eclipse periods may represent a significant mass impact. For missions where this impact may be unacceptable at times, a power management scheme may be used where (for instance) instrument data collection and communication to Earth may be traded off. Structurally integrated battery systems are a

Spacecraft thermal design is dependent on variables such as eclipse duration, instrument requirements, electronics power dissipation, spacecraft distance to the Sun, etc. Given the power constraints associated with a nano/micro-satellite, it is highly desirable to develop a thermal system that relies either on passive control, or which draws a relatively insignificant amount of power to operate. A promising technology particularly suited for a miniature spacecraft incorporates variable emittance surfaces. Either passively or actively enabled, variable emittance “coatings” can store heat during eclipse periods. and reject excess energy in sunlight. Possible implementation and electrochromic, MEMS, methods include electrophoretic technology. MEMS thermal louvers arc functionally equivalent to their mechanical counterparts, but as solid state devices incorporate orders of magnitude reduction in system mass. They can function through the active use of MEMS or passively through bi-metallic actuator motors, actuators. Thousands of MEMS louvers can be applied selectively to different areas and adjusted to yield the desired emissivity. Electrochromic technology provides variable IR reflectance through a chemically reversible Electrophoretic technology relies on the process. movement of particles in a medium under an electric field. Currently, NASA-GSFC is actively funding the

Small Satellitesfor Earth Observation

development of MEMS thermal louvers, and is partnering with industry for the advancement of variable emittance surfaces.

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stable oscillators that exhibit low power and low voltage are being developed for the nano-satellite RF system. 4.7 Structures and Mechanisms

A moderate amount of technology development will be necessary to enable a two-phase heat transport system for use specifically in a nano-satellite, such as a capillary pumped loop (CPL) or loop heat pipe (LHP), which “shuts off’ during earth shadows to minimize thermal losses. First, the small size and low heat transport requirements of the nano-satellite will necessitate significant downsizing of today’s flight qualified twophase systems (e.g. EOS AM). This reduction will be accomplished by leveraging recent successful tests of a small, cryogenic two-phase CPL. Another area of development will pursue the best method by which to “shut down” the system during nano-satellite eclipses. Currently, a certain amount of heater power is used to this effect, but in the nano-satellite application, the goal is to reduce or eliminate this need.

4.6 Communications The nano-satellite onboard RF communications must be capable of transmitting science data to the ground and receiving ground commands at a sufficient data rate to keep communication time short. Performance requirements are complicated by the small size, low mass, and low power available. A three-axis-stabilized nano-satellite may have a high gain antenna (HGA) that is pointed to the ground, but a spinning nano-satellite will generally rely on a low gain omni antenna. Three-axis-stabilized nano-satellites may also use low gain omni antennas if the pointing requirements are such that the use of a HGA is prohibitive. Electrically pointed HGAs that are being developed for compact, low power applications, require onboard knowledge of the orbit and ground station position. Chnni antennas are also applicable when inter-satellite communication is required. For spin-stabilized nano-satellites employing omni antennas, inter-satellite communications are limited by the sensitivity of the receivers/bit synchronizers, and advanced decoders. Current estimates limit this cross-link communication to data rates of about 2 kbps, and relative separations in the order of 2000 km. Orbits of more than 1000 km altitude (above the GPS satellite Ll beamwidth) are unable to use conventional GPS schemes because they can not simultaneously receive the required 4 GPS signals. New technology will eventually allow a sequential solution, but in the near term the tracking system should be coupled with the communications electronics to maximize efficiency in mass and power. These nano-satellites will either use new technology transponders or they will use a small stable oscillator and one-way doppler tracking. Highly

For traditional spacecraft, the reduction in structure mass although desirable, does not in itself account for more than about 10 to 15% reduction in the total spacecraft mass. Further, the mass associated with brackets, cables, or connectors may represent a significant percentage of the total spacecraft mass (up to 50%). Clearly, technology that reduces structural mass and miniaturizes avionics components would also benefit from the introduction of “Multi-functional Structures” (MFS) [3]. Their implementation promises to eliminate the “parasitic” mass associated with traditional spacecraft avionics by combining structural and functional components into one. MFS makes it possible to consider structural batteries, to embed electronic components within composite structural elements, and to couple live electronic components directly to mechanical surfaces. Ultimately, the goal of MFS is to create a cable-less spacecraft, where all functional and structural components are combined into one single unit. The ideal nano/micro-satellite mechanical design would consist of a one-piece structure to which all other components are mounted. Whenever possible, there would be no moving parts, and the structure would serve as a thermal conductor and radiator, and as the substrate for electronics boards and solar cells. Examples of MFS include diamond facesheet honeycomb panels, and structures with an embedded energy storage system (structural battery system). The diamond facesheet honeycomb panels can serve as a structure, thermal conductor and radiator, and printed circuit board substrate. The diamond facesheet provides 10 times greater thermal conductivity than aluminum and can dissipate heat from high power density electronics modules, while having a low mass comparable to carbon fiber composites. The structural battery system consists of a honeycomb panel where the cells of a nickelhydrogen battery (or other flight qualified cell technology) are formed into a honeycomb core. This approach provides a structural panel with an embedded energy storage system for significantly less mass and volume than if the panel and batteries were integrated separately. Structural materials that can inherently provide some radiation shielding are also desirable. Mass production techniques not traditionally used for science space flight hardware will be used, such as investment casting and injection molding. Options being considered for the nano-satellite structure material are: cast aluminum; cast aluminum-beryllium alloy; injection molded plastic; fiber reinforced plastic; and flat stock composite construction.

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Mechanisms which are small, reliable, and space qualified will be required for the deployment of nano-satellites. We are presently evaluating numerous types including: miniature pyrotechnic devices; paraffin actuators; nonexplosive initiators; memory metal devices; and thermal knife release units. The selected actuators will be based on reliability, timing, cost, availability and ease of use.

4.8 Nano/micro-Spacecraft

Integration

The development of a fully capable nano/micro-spacecraft with ambitious mass, power, and volume constraints presents the fundamental challenge of defining new ways for integrating the spacecraft subsystems and components together. It is through innovative, revolutionary approaches to systems integration that significant savings in mass and volume can be realized. To realize a truly integrated spacecraft of this mass class, problems will be encountered that have not been previously experienced in spacecraft development. No prior attempts have been made to seamlessly bring together the spacecraft functions to this level of cohesion. For this reason, spacecraft integration in itself deserves full attention as a “technology” which must be resolved. In order to achieve a total mass in the l-50 kg range while not sacrificing capability or performance, this class of miniaturized spacecraft must essentially possess a subsystem-less architecture, where the traditional physical boundaries between subsystems are removed. Although the above sections list technologies in terms of their functional definitions, it is clear that the goal is to no longer have a C&DH box, an instrument box, a power supply electronics box, etc., all physically separate and connected by harnesses and cables. Instead, the idea is to have a seamless integration of spacecraft/instrument electronics (avionics) in the core of the structure with sensors distributed throughout the spacecraft body. Traditional harnesses and connectors are no longer used to make interconnections. Instead, connections are now made by solder-less contact points and the mating of multi-functional materials. One to two orders of magnitude reduction in mass and volume that is expected for the production of nano-spacecraft will require the development of new spacecraft systems integration approaches, and NASA-GSFC is currently engaged in fostering its definition and resolution.

5.0 CONCLUSIONS At present, the NASA Earth Science Enterprise is in the process of formulating its implementation plans for 2004 and beyond. It is expected that these plans will evolve rapidly over the next few months, particularly as the broader scientific, national and international communities become involved. Current investments in the spacecraft, instrument, autonomy and operations technologies are

being made through the NASA-GSFC Nano-Satellite Technology Development Program supporting the Space Science Enterprise Solar Terrestrial Probes Missions, and through the NASA Cross Enterprise Technology Development Program. The Earth Science Enterprise is making instrument investments through its Instrument Incubator Program. At present the fundamental technologies are being identified for development with the expectation that an aggressive increase in innovation across-the-board will help to enable nano-satellite systems for a broad range of Earth Science applications. Nano/micro-satellite technology promises to further the advancement of Earth Science missions through a concerted, methodical, and continuous monitoring of a multitude of parameters. The availability of 10s to 100s of satellites, whether studying the near or extended Earth environment, promises to generate much needed measurements which can then be used as input to weather prediction software, or to better quantify the effect of human activity on the environment. In addition, the flexibility and robustness of these constellations will make them impervious to single (or even muhiple) spacecraft failures, and also provide ‘for:., tHe implementation of new technological advancements, or the re-orientation of scientific priorities, while the system remains fully operational. Applications of advancements in miniaturized components for Space & Earth science applications will undoubtedly permeate across the realms of human society, positively affecting sectors such as the commercial aerospace, health, the automotive industry, and even general household products.

6.0 REFERENCES 1. Panetta, P.V., H. Culver, J. Gagosian, M. Johnson, J. Kellogg, D. Mangus, T. Michalek, V. Sank, S. Tompkins, NASA-GSFC Nano-satellite Technology Development, 12” AIAA/USU Conference on Small Satellites, August 1998 2. Culver, H. L., P. J. Luers, J. Plante, Nuno-Satellite Avionics, The Second International Conference on Integrated Micro/Nanotechnology for Space Applications, April 1999 3. Barnett, D. M., S. P. Rawal, Multifunctional Structures Technology Experiment on Deep Space I Mission, IEEE AES Systems Magazine, January 1999