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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