A unique antenna developed by Georgia Tech Research Institute (GTRI) for the NASA-led International Space Station may soon keep astronauts safe and in touch as they prepare for challenging space walks.
Called an Orlan antenna, this two-foot-long "loop" design is being developed for the crew lock, a cramped cylindrical air lock that can hold two space-suited astronauts. Because of its positioning, the sensitive antenna — sometimes known as a "towel bar" model — must survive bashing by space packs, huge temperature swings, and serve as a hand-and foot-hold for astronauts clambering into space.
The project's divergent requirements have made it a challenging one, said Victor Tripp, a principal research engineer with the Sensors and Electromagnetic Applications Laboratory of the Georgia Tech Research Institute (GTRI). Lab researchers have been working with The Boeing Company on the project since last year. "This is a very special development and design for an antenna," Tripp said.
The greatest technical hurdle has involved the antenna's location within the crew lock. "Ordinarily antennas go on an exterior surface; this one needs to go inside a metal container and provide a radio-frequency field for communications there," Tripp explains. "Ordinary techniques don't apply, because a radiation pattern is meaningless inside a resonant cavity."
In such a resonant cavity, he adds, most energy comes back because it doesn't have any place else to go. Researchers have found that the special loop design couples a sufficient amount of this energy to the astronaut, rather than reflecting it back.
The antenna's job is to provide communications for International Space Station astronauts using Russian-designed spacesuits. These spacesuits employ the same communications frequency used for many years aboard the Russian spacecraft Mir. This frequency is four times lower than the frequency used by U.S. spacesuits (which have their own antenna within the crew lock). The longer Russian wavelength is barely short enough to resonate in the crew lock's 65-inch diameter, necessitating use of an antenna so large it must act as handrail, as well as give strong communications performance.
Dependable operation of the crew-lock antennas is essential. While astronauts wait within the crew lock sealed into spacesuits, these antennas carry verbal communication between astronauts and crew, and they also transmit vital signs and other data necessary to monitor each astronaut's physical condition. For instance, each spacesuit's umbilical cord provides coolant to keep astronauts from overheating inside the highly insulated spacesuit. Temperature data transmitted from the crew lock lets the space station's communication center make sure the cooling system is operating properly.
To develop an optimal antenna design, Tripp and other GTRI researchers first performed calculations using High Frequency Structure Simulator (HFSS) modeling and simulation software by Ansoft. They augmented those findings with their own 1:6 scale model of the crew lock. The researchers even developed an astronaut scale model with a conductive spacesuit that simulated the Russian spacesuit design. In it, a metallic layer lets the whole suit function as an antenna.
"The HFSS software approach was a big help because we got a good feeling for the difficulty and how things worked . . . but it was very sensitive to astronaut position," Tripp explained. "That's why we made a rather detailed scale model that we could also make measurements on."
The researchers even scaled the frequencies — the "full size" frequencies of around 120 MHz were adjusted upward to about 720 MHz in the scale model.
Such detailed effort seems to be paying off. At NASA's Johnson Space Center in Houston, a recent test of the loop antenna in The Boeing Company's full-scale crew-lock model produced data that closely resembled data from the scale model, Tripp said.
As in many projects, the Orlan designers had to be ready to compromise to satisfy the overlapping requirements for the antenna, he notes. Originally, the GTRI team wanted to use an antenna with a standoff of six inches from the crew-lock wall — making it taller than the other handrails. But after consulting with Boeing personnel and also with U.S. astronaut Leroy Chao, a member of the Space Station crew, researchers settled on a height of about three inches.
"And of course, as we predicted it didn't work as well (with the lower stand-off)," Tripp said. "But it still worked very well — much better than the original antenna." The test loop antenna was made of copper; the final, space version will be high- strength aluminum, which will perform almost identically to copper.
While at Johnson Space Center, the GTRI team also tested a second crew-lock antenna design — a "patch" model made of nickel-ferrite, a magnetic material that allowed reduced antenna size. The patch model outperformed the Mir antenna, but came in second to the loop design.
The International Space Station, called by NASA the largest international scientific and technological endeavor ever undertaken, is under construction in factories and laboratories of 15 nations around the world. It will become a permanent space laboratory and a test bed for future technologies. NASA's current schedule calls for the first unmanned element of the space station to be launched from Russia this summer. Shortly thereafter, a Space Shuttle flight will dock with that element. Construction plans call for 45 flights over five years to complete the space station.