Japan has m ounted a successful and extensive series of technology demonstration programs that has served both to advance technology and to mature the specialized roles played by diverse industrial and government organizations. The series includes CS and CS-2, BS an d the Engineering Test Satellites, ETS-II, -III, -V and the current developments, ETS-VI and COMETS. ETS-VI represents a significant entry into large, three-axis-stabilized platforms suitable for a variety of communications services and other missions, w hile also integrating advanced payloads for experiments in Ka- and O-band millimeter wave communications, S-band mobile and intersatellite links to low orbiting spacecraft (relevant to TDRSS), laser links, and advanced fixed, broadcast and mobile services .
The objectives of ETS-VI are extensive and may be summarized into two primary goals: (1) the development and demonstration of a flexible satellite bus capable of supporting two- to three-ton spacecraft having long life, hi gh power, precise attitude control and stationkeeping and high payload capacity; and (2) the demonstration of advanced communications technologies embracing a spectrum of services, frequency bands, and future directed applications.
A diagram showing the principal features of ETS-VI, beam coverages, planned experiments, and comparisons with other large spacecraft of the world is shown in Figure 6.15.
O-Band (42/38 GH z). With the world's first use of this frequency plan, ETS-VI will explore communications with extremely small ground terminals.
Ka-Band (23/26 GHz). Ka-band ISLs will be implemented in experiments (as well as S- and laser links). These links may be emulated with fixed earth stations rather than orbiting spacecraft.
Laser ISLs (0.83/ 0.51 micron). A 7.5 cm telescope with 14 mW GaAlAs diode transmitter will be used.
S-Band (2.3/2.0 GHz). A phased array antenna is i ncluded for S-band ISLs. Experiments will include data relay from, and tracking of, LEO satellites. S-band is also to be used for telemetry tracking, command, and ranging. There is a 3.5 m diameter reflector multibeam antenna for both S- and Ka-band (2 0 GHz) to be used for fixed and mobile satellite service experiments in conjunction with on-board switching. The 3.5 m reflector is folded into three parts for launch and represents a significant achievement in precision reflectors.
Figure 6.15. The ETS-VI Featuring Its Planned Experiments, Beam Coverage And Relative Size.
Precision Attitude Control. Precise attitu de control is achieved by three-axis, zero momentum system; 0.05 degree for pitch and roll and 0.15 degree for the yaw axis are expected. This system continues the work started with the ETS-III, launched in 1982. Three- axis stabilization is provided in transfer orbit as well as on station. Control electronics use an IR earth sensor, a sun sensor, gyroscope and telecommand inputs. A fault tolerant, programmable microprocessor allows reconfiguration for different missions.
Electrical Power. Semirigid panels with 50 micrometer thin solar cells generate 4,100 W at summer solstice at the end of life. about 1,850 W is available to mission payloads through a 50 volt unregulated bus. four sets of 35 Ah nickel cadmium (NiCd) batteries provide b ackup, and nickel hydrogen (NiH(subscript 2)) batteries are flown as an experiment.
Structure. the body is 3.0 x 2.8 x 2.0 m; with the solar array deployed, the overall length is 30 m. the on-orbit weight is about 2,000 kg with 660 kg of payl oad (17.4 percent of launch weight); liftoff mass is 3,800 kg.
Launch. launch will use the h-ii launch vehicle on its second test flight, now expected in 1994.
Orbit. GEO at 154 degree E.