4
Small Satellite Buses

CAPABILITIES OF SMALL SATELLITE BUSES

Satellites are frequently described in terms of a payload and a service module or "bus." The capability of a satellite bus relates to its ability to accommodate payloads and to meet mission requirements. Payload accommodation requirements are many and include mass; geometry (volume, mechanical interfaces, fields of view); thermal interfaces; power (wattage, voltages, duty cycles); data (rates, interfaces); contamination environment; electromagnetic interference limits; and spacecraft pointing knowledge and control (slewing and settling rates, stability, jitter).

The mission architecture places further requirements on the spacecraft bus such as on-board data processing; data memory and communication links; battery capacity; and the need for propulsion (orbit insertion, orbit maintenance, formation flying, end-of-mission de-orbit). Additional mission requirements include spacecraft life (expendables, radiation dose, solar array degradation); reliability; and degree of redundancy.

All space missions are constrained by launch vehicle performance (mass to orbit) and fairing—i.e., the aerodynamic cover that protects the spacecraft as it travels through the atmosphere—volume. These constraints can be severe for small expendable launch vehicles such as the Pegasus (see Chapter 5) and can lead to complex designs for "deployables" (such as the solar panels) in order to stow the satellite within the fairing, as in the Air Force Space Test Experiment Program Mission 1. Within these constraints, the satellite designer generally wants to maximize resources available to the payload and minimize those required for the spacecraft bus. Consequently, much small satellite technology development effort has been directed toward reducing bus volume, mass, and power consumption, while providing robust capability by increasing battery capacity, solar array efficiency, data memory, processing rates, and so on (NRC, 1994). This trend is likely to continue in avionics as well as in the still embryonic field of microminiature electromechanical systems.

Partly because of substantial investments by the National Aeronautics and Space Administration (NASA), Department of Defense, Department of Energy, and industry over the past decade, small satellite technology has already advanced to the point where a great deal of capability can be provided in a relatively small package. Table 4.1 shows typical performance parameters for current low- and high-end small spacecraft buses derived from information presented in NASA's Rapid Spacecraft Acquisition contract offerings. These are all off-the-shelf flight-qualified spacecraft buses NASA is making available to potential users through the Goddard Space Flight Center's Rapid Spacecraft Development Office (RSDO). Most are available in a basic or "core" version with options for enhancing performance.



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The Role of Small Satellites in NASA and NOAA Earth Observation Programs 4 Small Satellite Buses CAPABILITIES OF SMALL SATELLITE BUSES Satellites are frequently described in terms of a payload and a service module or "bus." The capability of a satellite bus relates to its ability to accommodate payloads and to meet mission requirements. Payload accommodation requirements are many and include mass; geometry (volume, mechanical interfaces, fields of view); thermal interfaces; power (wattage, voltages, duty cycles); data (rates, interfaces); contamination environment; electromagnetic interference limits; and spacecraft pointing knowledge and control (slewing and settling rates, stability, jitter). The mission architecture places further requirements on the spacecraft bus such as on-board data processing; data memory and communication links; battery capacity; and the need for propulsion (orbit insertion, orbit maintenance, formation flying, end-of-mission de-orbit). Additional mission requirements include spacecraft life (expendables, radiation dose, solar array degradation); reliability; and degree of redundancy. All space missions are constrained by launch vehicle performance (mass to orbit) and fairing—i.e., the aerodynamic cover that protects the spacecraft as it travels through the atmosphere—volume. These constraints can be severe for small expendable launch vehicles such as the Pegasus (see Chapter 5) and can lead to complex designs for "deployables" (such as the solar panels) in order to stow the satellite within the fairing, as in the Air Force Space Test Experiment Program Mission 1. Within these constraints, the satellite designer generally wants to maximize resources available to the payload and minimize those required for the spacecraft bus. Consequently, much small satellite technology development effort has been directed toward reducing bus volume, mass, and power consumption, while providing robust capability by increasing battery capacity, solar array efficiency, data memory, processing rates, and so on (NRC, 1994). This trend is likely to continue in avionics as well as in the still embryonic field of microminiature electromechanical systems. Partly because of substantial investments by the National Aeronautics and Space Administration (NASA), Department of Defense, Department of Energy, and industry over the past decade, small satellite technology has already advanced to the point where a great deal of capability can be provided in a relatively small package. Table 4.1 shows typical performance parameters for current low- and high-end small spacecraft buses derived from information presented in NASA's Rapid Spacecraft Acquisition contract offerings. These are all off-the-shelf flight-qualified spacecraft buses NASA is making available to potential users through the Goddard Space Flight Center's Rapid Spacecraft Development Office (RSDO). Most are available in a basic or "core" version with options for enhancing performance.

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The Role of Small Satellites in NASA and NOAA Earth Observation Programs Table 4.1 Characteristics of Small Satellites Parameter Low-End Buses (w/o options) High-End Buses (w/ options) Design life (years) 1–3 >>5 Reliability (at design life) .8–.9 .8–.9 Avionics redundancy Limited Extensive to full Bus mass (kg) 150–300 425–650 Payload mass (kg) 100–300 300–500 Payload power (orbital average, W) 60–125 100–500 Propulsion authority (kg Hydrazine) 0–25 33–75 Pointing accuracy (deg 3-sig) 0.02a–.25 0.01a–0.03a Pointing knowledge (deg 3-sig) 0.001a–0.2 0.003a–0.008a Data storage (Gbit) 2–64 12–200 Downlink (Mbps) 2–4 at S-band; 100 at X-band available on SA200S 2 at S-band, 320 at X-band NOTE: The low-end buses are the Spectrum Astro SA200S, Swales, and the three-axis TRW STEP; the high-end buses are the Ball RS2000, Lockheed Martin LM900, and TRW SSTI-500. a With star trackers. SOURCE: RSDO (1999). This level of performance, especially at the upper end, is adequate to support many—but not all—Earth observation missions. Some payloads are simply too large, too heavy, too demanding of power, or have moving parts that create too large a vibration source to be accommodated efficiently with a small satellite on a small launch vehicle (e.g., the Multifrequency Imaging Microwave Radiometer, Atmospheric Infrared Sounder, and Microwave Limb Sounder). Excessive payload size and weight can be addressed to some extent with more capable launch vehicles (e.g., NASA's FUSE [Far Ultraviolet Spectroscopic Explorer] mission), and needs for greater payload power with larger, more efficient solar arrays and higher capacity batteries. However, the limited inertia of a small satellite makes it difficult to control jitter without active isolation of large vibration sources. Table 4.1 shows that small satellites can provide robust capability with respect to data storage and downlink rates. However, a proliferation of small satellites in orbit will raise ground station capacity and frequency allocation issues. High-data-rate ground receiving stations are limited in number, and new ones are costly to install and support. Competition for frequency allocation is increasing around the world; this process is limited and controlled by the Federal Communications Commission for the United States and by the World Administrative Radio Conference internationally. With the number of satellites increasing, the competition for ground station contact time and uplink/downlink frequencies and the potential for interference are also increasing. These problems are an important aspect of the trade-offs entailed in system design and mission planning. SPACECRAFT BUS COSTS The cost of small spacecraft buses is a somewhat elusive parameter, depending as it does on the capability of the bus, the technological heritage, and the details of program management and bus production. Currently, recurring costs for spacecraft buses like those in Figure 4.1 range from approximately $10 million to $30 million. Nonrecurring costs can add another 150 percent if a complex, new, mission-unique bus must be developed, but substantially less if previously developed spacecraft can be adapted for use. A recent RAND study on small spacecraft offers some interesting perspectives on spacecraft costs (Sarsfield, 1997). Traditionally, cost modelers have used a cost estimating relationship based on mass to predict spacecraft development costs. However, as shown in Figure 4.1, variation in development costs for small spacecraft is much greater than for larger spacecraft. Very low costs—a key objective in the push toward small satellites—are

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The Role of Small Satellites in NASA and NOAA Earth Observation Programs Figure 4.1 The relative cost of small spacecraft (adapted from Sarsfield, 1997). Note that all acronyms are defined in Appendix E. experienced only with very simple spacecraft performing very limited missions. Small spacecraft can be relatively expensive when they retain the complexity required to meet demanding scientific objectives (pointing accuracy, power, processor speed, etc.). For demanding missions, development costs are a relatively weak function of spacecraft mass; thus, specific cost (cost per unit mass) increases as mass decreases. The RAND study also addresses the impact of development processes on spacecraft costs (Sarsfield, 1997). Streamlining the development process offers cost efficiencies for spacecraft of all sizes. However, it is easier to streamline processes with smaller, simpler satellites involving smaller development teams. Process-driven spacecraft cost reductions achieved with small satellites have ranged from 15 to 30 percent. Programmatic issues associated with small spacecraft missions are further discussed in Chapter 7. UTILITY OF "COMMERCIAL" SPACECRAFT Recently, there has been great interest in the possible use of commercial spacecraft buses to perform science missions as a way of avoiding or reducing bus development costs. As used here, "commercial" spacecraft buses are those for which there exists an operating production line serving a commercial market, as is the case for several manufacturers of communication satellites (e.g., Iridium). It is important to differentiate "commercial" from ''standard" buses. Several suppliers of small satellites offer a standard bus consisting of flight-qualified components configured for the particular mission at hand. These offerings generally involve a core bus plus a range of options to increase (or decrease) capability. As such, they really represent standard architectures employing standard spacecraft subsystems with defined interfaces. As opposed to a production line, such spacecraft are typically developed as individual projects by small teams co-located for efficiency. Several of the bus offerings available through NASA's RSDO (Table 4.1) fall into this category.

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The Role of Small Satellites in NASA and NOAA Earth Observation Programs The standard bus approach goes a long way toward lowering costs by reducing—but not eliminating—nonrecurring development. The commercial bus offers the potential for even greater cost reduction; if directly applicable to the mission, almost all nonrecurring development costs are avoided, and the recurring costs of manufacture benefit from the efficiencies of the existing production line. For defined payloads, most missions will not be able to use a bus directly off the line. Rather, in the great majority of cases, the bus will have to be tailored to the mission with some degree of modification. For example, Lockheed Martin modifies its LM700 Iridium bus for more demanding scientific missions (LM700B) and offers others (LM100 and LM900) for Earth observation missions in the NASA RSDO catalog.1 Recently, NASA sponsored a study of alternative architectures for performing the Earth Observing System Chemistry-1 mission. Eight suppliers with existing spacecraft buses performed studies to accommodate the HIRDLS and ODUS (High Resolution Dynamics Limb Sounder and Ozone Dynamics Ultraviolet Spectrometer), TES (Tropospheric Emission Spectrometer), and MLS (Microwave Limb Sounder) instruments on three spacecraft. No supplier had an existing commercial spacecraft bus that could perform these missions without significant modification and attendant costs. The question remaining then was the level of nonrecurring costs needed to modify a commercial bus versus those to configure a standard bus to meet mission requirements. The answer is mission specific and will be determined by the marketplace. In the case of the Chemistry-1 studies, cost estimates for the two cases were similar. SPACECRAFT CAPABILITY AS A PAYLOAD DESIGN PARAMETER An alternative paradigm that has been suggested is to transfer the burden of accommodation from the spacecraft to the payload; that is, to treat the spacecraft capabilities as payload design requirements much as the spacecraft designer currently treats launch vehicle capabilities as design requirements. This approach minimizes the costs of using either commercial or standard spacecraft buses. It also introduces several vexing issues: It places an increased burden on the payload developer who is often less experienced in space systems than are spacecraft manufacturers. Instrument development cycles (4 to 5 years) are typically much longer than those for small spacecraft (18 to 36 months). Thus, to define requirements, payload developers must select candidate spacecraft buses early in their design cycle and must somehow ensure availability when needed. More than one sensor supplier makes it very difficult to configure missions. System design integration for multisensor payloads is traditionally performed by the satellite manufacturer, which must ensure that all sensors are accommodated without interference. PRINCIPAL INVESTIGATOR-LED PROJECTS Recent NASA procurements (Discovery, Mid-size Explorer, Earth System Science Pathfinder, and Small Explorer) have embraced a "PI mode" wherein a principal investigator (PI) configures and leads a team to propose and compete for missions in response to a fairly broad Announcement of Opportunity (AO). In many cases, interested PIs have solicited industrial teammates with existing spacecraft buses to join their teams, thus defining the spacecraft capabilities and payload accommodation requirements early enough in the process to achieve efficient interface compatibility. This approach works when the PI has a sufficiently well-defined payload to select an appropriate bus (and hence teammate) at the time of the procurement. Most proposals submitted in response to recent PI mode AOs have involved payloads that have been under development for some time and are relatively mature. For example, the fourth Discovery mission—Stardust—carries aerogel capture cells proven on numerous Get Away Special Sample Return Experiments, and a camera that uses spare parts from the Voyager and Galileo missions. It remains 1    The catalog can be found online at <http://rsdo.gsfc.nasa.gov/rapidi/catalog.cfm>.

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The Role of Small Satellites in NASA and NOAA Earth Observation Programs to be seen how well the PI mode will work once the current inventory of sensor concepts, designs, and—in some cases—hardware have been exhausted. FUTURE TRENDS A key factor in the movement toward smaller spacecraft is the desire to reduce total mission costs.2 Smaller satellites can cost less, particularly if the mission payloads have less demanding accommodation requirements. Also, smaller satellites can be launched on smaller launch vehicles which, setting current issues of reliability aside, offer a lower cost to orbit. Cost reduction will continue to be a driver for small satellite missions and the spacecraft buses that support them. Continued technological development will further increase capability and lower costs. NASA's New Millennium program is an important vehicle through which new technologies will be validated through flight demonstrations. Avenues for additional cost reduction strategies (applicable to both large and small satellites) include the following: Streamlined procurement practices (PI mode, streamlined contractor practices, etc.); Low-overhead management techniques (concurrent engineering, integrated product development teams, customer insight through participation in lieu of oversight, reduction in formal reviews); Design and development improvements (computer-aided design, early definition of design requirements and interface control documents, hardware and software reuse, selected redundancy, spacecraft standards, and commonality); and Lowering the cost of operations (spacecraft autonomy, on-board processing). Several of these strategies are discussed further in Chapter 7. An excellent example of the trend toward streamlined procurement practices was NASA's Rapid Spacecraft Procurement Initiative (NASA, 1997). Through this solicitation, Goddard Space Flight Center has developed a "catalog" of industrial commercial and standard spacecraft bus offerings that can be quickly procured through indefinite delivery, indefinite quantity (IDIQ) contracts with seven suppliers. When applicable to the mission, these IDIQ contracts and catalog provide an efficient way for PIs to select industry partners when responding to AOs and for flight projects to acquire spacecraft buses. The goal is to shorten the procurement cycle from 9 to 12 months down to 30 to 120 days. This procurement approach was implemented for two missions—QuikSCAT (Quick Scatterometer) and ICESat (Ice, Cloud, and Land Elevation Satellite)—as of June 1998, with four more in competition and six (three outside NASA) under consideration (RSDO, 1999). On the other hand, an attempt to use this approach on SOLSTICE/SAVE (Solar-Stellar Irradiance Comparison Experiment/Solar Atmospheric Variability Explorer)—a program already well under way—was not successful. None of the standard bus offerings could accommodate the payload without employing a larger than desired launch vehicle with an unacceptable increase in cost.3 SUMMARY Small satellite technology has advanced to the point where very capable buses are currently available for performing many Earth observation missions. However, some Earth observation payloads are too large, too heavy, too demanding of power, or generate too much vibration to be efficiently accommodated with small satellite missions. Very low costs—a key objective in the push toward smaller satellites—are experienced only with simple spacecraft performing limited missions. Small spacecraft can be relatively expensive when they retain the complexity required to meet demanding science objectives (pointing accuracy, power, processor speed, etc.). 2    An October 1996 National Research Council workshop examined ways of reducing the cost of science research missions (NRC, 1997). 3    Daniel Baker, University of Colorado, Boulder, personal communication.

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The Role of Small Satellites in NASA and NOAA Earth Observation Programs Commercial production buses offer the potential for reducing costs. However, they generally have to be tailored—with attendant costs—to accommodate existing Earth observation payloads. Designing new payloads to match existing bus capabilities offers greater cost-effectiveness, but caution must be exercised not to compromise the scientific mission in doing so. NASA's Rapid Spacecraft Acquisition Initiative exemplifies an innovative approach to matching existing spacecraft buses to payload accommodation requirements. Efforts should continue to reduce bus volume, mass, and power, and to increase communications and data handling capacity, such that larger fractions of launch vehicle performance and fairings are made available to more demanding payloads. Lowest cost will be achieved when satellite size is matched to payload requirements and launch vehicles are matched to the satellite. REFERENCES National Aeronautics and Space Administration (NASA). 1997. Rapid Spacecraft Procurement Initiative, RFP 5-02816-001. Greenbelt, Md.: NASA Goddard Space Flight Center. National Research Council (NRC). 1994. Technology for Small Spacecraft. Washington, D.C.: National Academy Press. ———. 1997. Reducing the Costs of Space Science Research Missions. Washington, D.C.: National Academy Press. Rapid Spacecraft Development Office (RSDO). 1999. Available online at <http://rsdo.gsfc.nasa.gov>. Sarsfield, L. 1997. The Cosmos on a Shoe String: Small Spacecraft for Space and Earth Science. MR-864-OSTP. Santa Monica, Calif.: RAND, Critical Technologies Institute.