ETS-7 - Orbital Rendezvous and Robotic Mission
TOKYO - April 2, 1997 - The National Space development Agency of Japan (NASDA) has unveiled the flight model of its latest technology development satellite, Engineering Test Satellite-VII (ETS-7) at Tsukuba space center, in Ibaraki Prefecture, north east of Tokyo.

Comprised of a box-shaped 2480 kg main "chaser" bus and a much smaller square panel-shaped 410 kg "target" satellite, the ETS-7's two units will separate and recombine in orbit in a series of experiments next spring which will enable NASDA to develop automatic docking systems needed for re-supplying the International Space Station via Japan's future unmanned space shuttle Hope.

"The objective is to prove that these kinds of technologies are feasible and to show the taxpayers that they are useful. This means that we can then ask them for more money. NASA tried to develop something similar with the OMV and failed because it was too expensive," said NASDA's ETS-7 program coordinator Mitsushige Oda, who conceived the satellite's mission parameters seven years ago. At 300 million dollars, Oda said the twin satellite represented a comparatively cheap technology step when compared to NASDA's average 500 million dollar price tag per R&D satellite.

NASDA have renamed ETS-7's two units "Orihime" and "Hikoboshi," the Japanese names for Vegar and Altair. According to legend Orihime and Hikoboshi, were lovers. The couple, madly in love with each other, neglected their other responsibilities and were parted by the angry god of heaven, who permitted then to meet only once a year.

The modern Satellite couple will spend two months at Tsukuba Space Center undergoing pre-flight testing before assembly in June and shipping to NASDA's Tanegashima launch center, in Kyushu, southern Japan, on July 7. ETS-7 will be launched along with NASA's Tropical Rainfall Measuring Mission satellite (TRMM), in early November.

Once launched, TRMM will be injected into its 350 km orbit while ETS-7 will enter a transfer orbit which will be raised to 550 km over the following days. Upon completion of three months of in-orbit tests, NASDA will conduct a series of six rendezvous docking experiments, held monthly, with options on a seventh experiment. If problems occur or more data is needed, the fuel budget will allow up to 14 rendezvous approach runs during the satellites projected 18-month life span, said Mitsubishi Electric Corp.'s Hiroshi Koyama. Koyama has been responsible for designing the Mitsubishi-built "soft-docking" mechanism, which consists of three pincers and handles.

"While usual approach speeds are at 50 cm per second, ETS-7 will recombine at two centimeters per second," he said. "The unique feature of this system is its fully automatic nature, which makes these maneuvers much more difficult (than conventional docking,)" he said.

Because the satellite will be visible for only a few minutes on each orbit sweep, the experiments will be conducted via a GPS satellite and NASDA's two-ton Communications and Broadcasting Engineering Test Satellite (COMETS), which is scheduled for launch in August by via the firth H-2 mission and will be placed in a 36,000 121 degrees East geostationary orbit. All systems will be coordinated from Tsukuba's tracking and control center.

Despite the seven years of planning, rather surprisingly NASDA has failed to come up with an emergency plan if the complicated communication link should fail, said Katsuo Yonezawa, deputy director for public affairs at Tsukuba space center. Comets provides a vital link for the satellite, increasing communications time from 10 minutes to 40 minutes per 100 minute orbit. "We don't have any contingency plan yet. If there are problems we may have to ask NASDA about using the Telemetry and Data Relay Satellite. If the link fails it will be impossible to conduct the docking experiments in the 10 minute window, " he said.

"It's a good question," said Fumio Yamagata, a NASDA project manager for ETS-7. "The matter is under consideration and I can't tell you anything other than we are thinking about this situation," he said.

The twin system will employ its own internal navigation system comprising three sensors. A GPS receiver will be used to compute the relative positions and speeds of the separated satellite units from nine km out to the 600 meter final approach stage. A rendezvous laser radar (LR) will compute relative positions from 660 to two meters, whereupon a proximity sensor will make final attitude and positional adjustments.

The first three maneuvers will separate the satellites by two meters, 500 meters and nine km respectively to check each of the three systems. Flight experiments four and five, will check the system's ability to cope with a disabled abort, when the chaser deliberately misses and is repositioned, and collision avoidance, where the chaser will retire to a safe position after missing the target. Finally, a "r-bar" approach, where the chaser maneuvers into position from below and behind the target, simulating the approach path for future docking runs with the International Space Station, will be attempted.

Oda regards the two "problem" experiments as the most crucial. Assuming the three types of sensors cope well, the real task is to see how Orihime and Hikoboshi cope with problems. Two of these are one of the units getting hopelessly lost- the COMETS tracking area only allows for a twenty kilometer zone in any given position, while on-board systems have an even more limited 9 km vision - and the relatively limited processing abilities of Hitachi's RISC-based H32 32-bit microprocessors processors used in ETS-7. If the satellites get badly out of position, NASDA has a manually operated back-up system that can take over the automated functions at any stage.

ETS-7 will also conduct three types of telerobotic experiments which will boost Japan's future unmanned in-orbit construction and repair capabilities, with the focus on large antenna assembly technology. These objectives will be brought a step nearer through a Fujitsu-built advanced robotics arm, a Shimizu-built antenna assembly mechanism and an NEC-built truss structure telemanipulation experiment, sponsored by Japan's Ministry of International Trade and Industries, the Communications Research Laboratory, and the National Aerospace Laboratory respectively. All systems are mounted on the main bus and testing will begin in February or March next year.

"We must show that robots are useful, otherwise they're just toys," said Oda.

Despite the complexity of the satellite, integration posed no special problems for Toshiba Aerospace, said Toshiba's integration manager Toshihiko Shirotani, which followed on naturally from Toshiba's work on ETS-6. "Toshiba is now considering applying the ETS-7's bus technology for lunar orbiting or deep space satellites," he said. It will also enable Toshiba to develop its own in-orbit robot systems over the next decade, he said.