SMALL COMMUNICATIONS SATELLITES

Introduction

Curiously, the last decade witnessed both the continued growth in the complexity and size of GEO communications satellites and the persistent entry of small low earth orbit (LEO) satellites into the consciousness of the engineering and scientific communities. Within the general subject of small satellites, small communications satellites and the capabilities they potentially enable represent topics of considerable controversy and interest. This section reviews European developments, programs and attitudes regarding small communications satellite systems. The information used in this section includes data gathered during site visits, literature review, and attendance at the first Small Satellites: Systems and Services Conference in Arcachon, France, July 1992.

Review of Factors Inviting Interest in Small Satellites

Satellite development and production costs are proportional to weight. Very small satellites, or microsatellites, typically in the 50 kg to 500 kg class, cost comparatively little, on the order of $1 million or $2 million, for development. There is also an overt trend towards producing spacecraft busses as well as incrementally improving previous designs with the objective of holding down cost growth and improving confidence in meeting budgets. Advances in microminiaturization of both digital and radio frequency electronics have made possible sophisticated capabilities in small satellites. Some key parameters are listed in Table 4.3 for maximum current capabilities of small satellites.

For communications, use of LEOs enables reduction of size and power of user communications terminals. This is the result of the reduction in the distance between the satellite and user, as compared to a GEO satellite (20 to 30 dB improvement). Hand-held, 1 W terminals are typically anticipated for either data or voice communications with small, LEO satellites, opening an era of truly mobile, global personal communications services (PCS). Lower altitudes both allow a very small transmit power from the satellite (leading to a small satellite) and result in a moving coverage, necessitating multiple satellites if near continuous visibility (or availability) is required.

Increasing availability with multiple satellites leads to multiple launch on large and expensive launch vehicles or multiple launches with smaller but still expensive launchers. To allow the small communications satellite systems to truly emerge, lower launch costs may prove necessary. Extraordinary launch costs also continue to burden the future of large GEO spacecraft, but truly low cost, small launch vehicles have not yet been realized (i.e., such that the ratio of launcher cost to satellite cost is closer to one) for economical launch of small satellites.

Table 4.3
State of the Art for Small Satellites (high end, 500 W class
)

Successful programs have demonstrated that development of spacecraft on the order of $1 million in cost are possible; that digital store-and-forward communications is very useful in a number of applications, even if there are significant time delays in message delivery; and that very small, low cost terminals are possible with such systems.

European Background

The University of Surrey (UK) initiated significant activity in small satellites in 1980 and launched its first small satellite in 1981. Since then, a number of small satellites carrying communications payloads have been built by the university or its spin-off, Surrey Satellite Technology, Ltd. Previous and current satellites in this series are outlined in Table 4.4. A drawing and photograph of the typical Surrey microsatellite are shown in Figure 4.3. The spacecraft design is now appropriately termed a bus, and has for the first time been exported to a large industrial company (Matra-Marconi) for payload integration and launch in the S80T program of CNES.

Current European Activities

ESA and major industrial organizations throughout Europe maintain an active but cautious interest in small communications satellites. Evidence for this interest is found in funded studies, proposed systems, bus designs, and experimental systems.

Table 4.4
State of the Art for Key Parameters


Figure 4.3. UoSAT Modular Microsatellite System: Exploded View and Deployed Configuration (Courtesy Matra-Marconi)

The IRIDIUM system proposed by Motorola has stimulated considerable discussion and analysis. For example, ESA sponsored a study of an IRIDIUM-like system in order to better understand all aspects of the proposal. There is considerable concern about the use of frequency spectrum by such a system. In particular, it may be that Europe, with its land mass at high latitude, may be better served by some form of inclined elliptical orbit (e.g., Molnyia type).

An important example of studies underway is Inmarsat's Project 21, for providing global PCS, and defining the evolution of its system to fourth generation satellites even as the third generation is under construction. Orbit configurations under study include some having a mixture of GEO and inclined, LEO small satellites.

The University of Surrey has established a collaborative three year program with South Korea in satellite engineering, comprising academic, on-the-job and project training of engineers from the Korean Advanced Institute of Science and Technology, establishment of satellite laboratories and a tracking station in Korea, and development and launch of two small experimental satellites, KITSAT-A and B. KITSAT-B was built in the new laboratories in Korea. The satellites provide digital store-and-forward communications, a digital signal processing experiment, earth imaging, and cosmic radiation experiments. Successfully launched on August 10, 1992, KITSAT-A sets a standard for rapid entry into space technology by developing nations.

Another first for small satellites is the S80T program in which the University of Surrey provides the satellite bus to industry -- Matra-Marconi Space, Toulouse (MMS) -- for integration, with final testing and launch carried out by a third entity, CNES. The program therefore represents an experiment in pan-European cooperation in a small satellite. The S80T provides an experiment in transponder (rather than store-and-forward) communications and position determination, both functions being accomplished by the same transponder. A global capability would be provided by about six satellites yielding an opportunity for either communications or position determination about once per hour. The tradeoff between waiting time to transmit (or message delivery time) and cost of service is a key issue in these small satellite systems. The S80T concept assumes that customers would embrace update intervals of an hour or more in exchange for lower cost.

A considerable number of funded studies are being reported, indicating active interest. These outline European focus on communications for Europe versus global communications, alternative concepts for digital message delivery, and combined position fixing and message delivery concepts. Industry in Italy has been particularly active in studies, with many studies performed by Italspazio. The interest is widespread, however, and not just confined to companies and institutions mentioned in this discussion.

It must be noted that there are no accepted definitions of small. Satellites of the order of one thousand pounds have been proposed for geostationary applications. ESA is sponsoring the Innovative Modular Satellite (IMS) bus development aimed at this intermediate size class, sometimes minisats. This approach is intended to lower development costs for a variety of satellites including communications satellites. The IMS would apply to inclined, elliptic orbits.

European entities are attempting to determine what, if any, viable markets exist for small satellites. While contemplating this question, they are also studying possible systems applications, conducting experiments (such as the S80T mission that will explore a particular form of position determination and message communications), and developing technology. These activities allow European agencies to be competitive should future markets or systems such as a future Inmarsat configuration require many small satellites.

Italian TEMISAT Program

Telespazio has developed and will deploy in mid 1993 a system primarily aimed at automatic data collection from distributed data collection platforms. Most of the applications are envisioned to be in environmental monitoring (snow cover, pollution, climatology, oceanography), agriculture, and geology. However, the system can provide data communications to mobile or transportable terminals and might be used entirely in this manner.

The program and the satellite are termed TEMISAT (for Telespazio Micro Satellite). Two satellites are being constructed, with the intention of launching the first in mid 1993 and the second in 1995. Both satellites are intended to have a five year life. The orbit is 950 km (110 minute period), circular and polar. One thousand "user terminals" are being manufactured to allow immediate operational use. The system is therefore said to be the first "professional" (i.e., commercial or operational) system. Data are collected from the distributed user stations (at 2,400 bits/sec) and held in digital memory until transfer to a central data collection center (at 9,600 bits/sec). The satellite is capable of receiving eight simultaneous single channel per carrier (SCPC) data streams. Access is regulated by polling from the satellite and multiple access is SCPC/TDMA (time division multiple access). The satellite weighs only 42 kg, and employs a novel semi-passive attitude control system that aligns the satellite to the local magnetic field. (Since the field lines are orthogonal to the gravity field except in the vicinity of the magnetic poles, this technique should align the satellite pitch and yaw axes very simply except in the polar regions where it may not be important to maintain alignment.)

Russian GONETS System

The Commonwealth of Independent States has launched a pair of demonstration satellites to be the first of an orbital constellation of 36 satellites. The system is termed GONETS and provides digital store-and-forward messaging. No crosslinks are employed. The system will compete with other systems (such as Italian TEMISAT, French S80T, U.S. ORBCOM X, and other proposed systems) for mobile and personal global data services. Information is not available on terminals or tariffs. GONETS is expected to be declared operational in 1995 and initially will provide banking data services. A second generation of GONETS may add crosslinks; and another system KOSKON, is expected to provide switched voice services similar to IRIDIUM using satellites of 800 kg weight and having crosslinks.

Small Satellites in Japan

Through the ISAS program, which involved satellites of 100 kg to 300 kg, Japan has gained experience with small satellites. The Amateur JAS 1a and JAS 1b (40-50 kg) satellites are additional examples. As in other centers of expertise, small satellites appear attractive for reduced cost and reduced launch cost. Japan is also studying concepts and trading designs against GEO approaches, attempting to identify appropriate applications for small satellites.

The paper presented at the IAF meeting (see bibliography) is the result of a study that considered a number of alternatives and spacecraft designs. For example, stabilization by means of micro-sized momentum wheels was found to be too heavy; thus gravity gradient stabilization with magnetic torquing was selected. The CRL program addresses several research topics posed by IRIDIUM, such as crosslinks.

A 1997 launch is planned for two satellites to be built in Japan. A diagram of the system and drawing of the satellite design is shown in Figure 4.4. The two 26-hedron bodies are attached by an expandable mast or truss. An S-band crosslink (ISL) is planned to operate at 1,200 bits/sec; VHF and UHF uplinks and downlinks operate at 9.6 kbits/sec. It is hoped that two more satellites of the same type can be launched in 1999. The J-I launch vehicle is to be used and the orbit altitude is 894 km (56 minutes). Each satellite weights 150 kg and the J-I maximum is 500 kg. The entire project is expected to cost in the range of $0.8 to $8 million.

Summary

The European satellite communications community has actually played a prominent role in introducing small satellites and developing the technology. The largest number of programs have been carried out by University of Surrey and Surrey Satellite Technology Ltd. These efforts have been complemented by participation of Matra-Marconi and CNES, by parallel developments in Italy and widespread studies. Small satellite communications requires system design and identification of markets; clever technology, particularly in microminiaturization of electronics; and mustering the necessary financial resources to proceed. In the case of experimental systems, the European efforts have been highly successful. The Russian systems will be successful technically, but may have financial problems in reaching a full capability.

Small communications satellites tend to be associated with LEOs and hence represent a revolution in all respects. The European community appears to have a slight bias towards evolution, suggesting the use of mid-altitude inclined orbits for serving European geographic areas, rather than moving directly to inherently global low altitude orbits. The intermediate step also favors mid-sized satellites that may be more matched to today's industry for development and manufacturing. Since developments in microminiaturization of electronics and other key technologies are enjoying steady progress in Europe and Japan, a technology base is available to support any future movement towards smaller satellites.


Figure 4.4. Experimental Store-&-Forward System (Japan)


Published: July 1993; WTEC Hyper-Librarian