LARGE GEO SATELLITES

Size Trends

During the past five years, there has been a renewed emphasis on providing satellite-based services to consumers. The acceptance of these services is determined to a great degree by cost to the consumer, including the cost of the equipment as well as monthly service charges. Consumer electronics benefits from competition as well as cost decreases associated with volume manufacturing and distribution and this is vividly demonstrated by the rapid decrease in the cost of DBS home equipment. The power of the signal from the satellite is a critically important factor in the determination of the cost of the ground equipment or terminals. The more the power from the satellite, the less the cost of the terminal. The size of the antennas and the cost of the amplifiers decrease as the power from the satellite increases. Business customers benefit from this increased power for the same reasons. As these costs are driven down, new applications for satellite services emerge. An interesting example of this is the presence of 30 cm satellite antennas at gas station pumps, which are used for credit card transactions. Of course, the multitude of recently proposed mobile and high bandwidth data services are also dependent on the existence of low cost terminal equipment.

The need for more power and bandwidth from commercial satellites is obvious to all the satellite manufacturers. Typically, you would expect that increasing the power and bandwidth from the satellite would require a larger, and thus heavier, satellite. However, increasing the weight of the satellite adds to the cost of the launch. Indeed, the maximum weight of the satellites is often capped by the lift capability of the launch system. Thus the challenge of the satellite manufacturers is to design and deliver a satellite with increased power, without increasing its cost and weight.

Thus today the increased demand for power is the dominant factor in driving the development and utilization of new GEO satellite technology, especially to meet these weight and cost constraints. Bandwidth per satellite has been increasing as combined C and Ku-band satellites become more common. The need for more bandwidth is especially evident for the new data applications, which are expected to be met with Ka-band and possibly V-band satellites. Here again, more total power is needed to meet power per channel (or Hertz) requirements.

Other factors driving the increased size and weight of the satellite are the needs for larger antennas, onboard processing electronics, and intersatellite links. Considerable technology development is directed towards the reduction of this weight and the size of the satellite. The rocket fairing is typically 4 m in diameter, and the satellite has to fit into that cross section. We are seeing the insertion of new lightweight composite materials into the structural composition of the satellite, the use of more efficient propulsion systems and fuels to insert the satellite into its final orbit and for station keeping, the use of arc jets and ion engines to increase the efficiency of the fuel that is used for station keeping, the reduction in the number of feed horns and their associated wave guides by using shaped antennas, the use of higher efficiency power amplifiers (TWTA & SSPA) the use of optical fiber to replace copper wires for the busing of signals onboard the satellite, the use of higher efficiency solar cells such as "black" Silicon, GaAs (on Ge) and multiple junction, multiple material cascade cells to replace the workhorse Si cells of the past, the welding of solar cells onto the array to decrease costs and to eliminate heavy solder, the use of light structures for solar panels, the use of unfurlable solar arrays, the use of more efficient heat exchangers, and the use of more efficient high pressure Ni-H2 batteries.

Figure 3.2 illustrates how the weight of the typical GEO satellite has increased over the past 30 years.


Fig. 3.2. Spacecraft mass (kg) vs. time (year).

Power Subsystem

As mentioned previously, the demand for increased microwave power from the satellite is probably the most important factor in driving the insertion of new technology into modern GEO satellites. Higher power at the customers' antenna translates into lower cost equipment and the availability of new services and thus the need for the manufacture of more satellites and their associated launches. The demand for more power from satellites is driving the development of considerable new technology, with the requirement that this new technology does not add to the cost of the satellite or its weight, which translates into increased launch costs.

Figure 3.3 illustrates the trend of the increasing power capability of GEO satellites over the past 35 years.


Fig. 3.3. Spacecraft power/time.

The power subsystem is composed of the solar array (solar cells on the supporting structure including pointing devices), batteries and the power conditioning electronics. Considerable progress has been made in the last five years.

While this panel did not visit any of the companies or organizations that manufacture or develop solar cells, this is a subject that should not be passed over lightly since this component is such an important part of the power system and improvements in efficiency are key to the ability of satellites to deliver higher power. The efficiency of solar cell technology over the years is summarized in Table 3.1.

Table 3.1
Solar Cell Efficiency vs. Time

Year

Organization

Efficiency (%)

Material

Comments

1962

ATT Bell Labs

8 - 10

Si

Basic design, trendsetter

1970

COMSAT Labs

13.5

Si

Violet cell

1973

COMSAT Labs

15.5

Si

Non reflecting cell (black cell)

1976

COMSAT Labs

16.1

Si

Black cell, sawtooth cover slide

1993

Sharp

17 - 18

Si

Black cell, improved materials

1997

Spectrolab and Techstar

25.5

GaAs/Ge

Dual junction

2000

Spectrolab and Techstar

35

III-V comp'ds

Cascade cells



Remarkable progress has been made in the increase in satellite solar cell efficiency over the years, and R&D is being performed to make cells available with significantly higher efficiency in the near future.

In the mid 1990s, Sharp started delivering a high efficiency silicon cell, commonly referred to as the "black silicon cell" because of its appearance. This cell has rapidly become widely used for communications satellites with its efficiency of 17-18%. This advance was followed quickly by the availability of GaAs cells, with an efficiency of 18-19%. While it has long been known that GaAs has an intrinsically higher efficiency than silicon, the difficulty in fabricating GaAs cells that are competitive in cost to silicon has prevented large scale application in satellites. This changed with the development of techniques to grow and dope layers of GaAs that have been epitaxially grown on germanium substrates. Spectrolab (a division of Hughes) and the Applied Solar Energy Division of Techstar are two U.S.-based companies that are the primary suppliers of these cells. The shortage of germanium, since it is widely used in the fabrication of fiber for the communications industry, has led to shortages of these cells. As these cells become available they have been used on the solar arrays of many recently delivered satellites. Arrays have been constructed that contain both Si cell panels and GaAs/Ge cell panels due to this shortage, heritage designs and cost tradeoffs. These GaAs/Ge cells cost approximately 4-5 times more than Si cells. In addition to efficiency, resistance to radiation is another parameter involved in the design of the solar arrays. Since the GaAs/Ge cells are more resistant to the damage caused by high energy particles from the sun than is Si, it is not necessary to include as many additional cells to meet end-of-life (EOL) power requirements. The radiation damage is cumulative and causes the power output of Si cells on GEO satellites to decrease 10 - 15 % over their lifetime, requiring additional cells to achieve EOL power requirements. GaAs/Ge requires considerably fewer cells to compensate for this loss of power. The success of the epitaxial GaAs on Ge process has lead to the extension of this process to the design and fabrication of multi junction, or cascade, cells, which are also made by Spectrolab and Techstar. These cells are composed of several layers of III-V compound materials, such as GaAs, GaInP, GaInAsP and GaSb grown epitaxially on Ge. These cells are also quite resistant to radiation, and cells with an efficiency of ~ 26% have been delivered to customers for evaluation. With additional R&D, it is anticipated that cells having an efficiency of 35% will be developed in the near future. Since these cells are made by a process that is quite similar to that used to manufacture the GaAs/Ge cells, it is expected that these exotic cells will not be that much more expensive. If progress continues at the present pace, these high efficiency, cascade cells could be the dominant source of power for satellites in the near future.

Another promising solar array technology is the use of concentrators to focus the light down onto the GaAs cells. AEC-Able Engineering Co, Inc. of Goleta, CA is working on parabolic reflectors that gather 7-8 times the light that would normally fall on a cell. These reflectors would also shield the cells against the high energy particles that degrade the cells. Such a technology would offer the promise of reducing the number of cells and the weight and thus the cost of the solar array. With a 7-8 times light gathering power, it should not be necessary to have precise pointing of the array towards the sun.

The design of the solar array itself is also evolving to improve the total power handling capability of the satellites, as well as reducing the weight and volume of the array. WTEC panelists saw large area array designs at Mitsubishi and Loral that involve the addition of panels that fold out from the main array. At Lockheed Martin WTEC panelists saw lightweight "pleated shade"-like structures that fold out like an accordion on a boom. These structures are light in weight, take up little space and offer considerable promise as an array structure. TRW is building flexible solar arrays on blanket-like structures that also offer the promise of reducing the weight and volume of high powered arrays.

The only discussion about batteries during the WTEC site visits was at Hughes. At the present time, high pressure Ni-H2 cells are widely used for GEO satellites. However, as the need for more power onboard satellites increases, then so does the requirement for increased power storage capability, which is met by batteries. Since the weight of the batteries typically scales with the power storage capability, we are going to see an increased percentage of the total weight composed of batteries, unless we have more efficient batteries. This is a critical technology that needs R&D attention. Lithium-ion is a system that offers possible solutions to this critical battery technology problem. The ability to support the numerous charge/deep-discharge cycles during the lifetime of a satellite has to be demonstrated for this system. Hopefully the experience gained from the expected broad consumer use of Li-ion batteries (for such applications as laptop computers and cellular phones) will help solve some of the problems facing the system. The experimental STENTOR satellite of CNES is designed to use Li-ion batteries.

Flywheel storage of energy is a possible substitute for batteries as improved bearings and stronger, lightweight materials are developed.


Published: December 1998; WTEC Hyper-Librarian