Via Nature.com

Keeping in touch with far-flung space probes could become much cheaper and easier if space agencies used arrays of millions of tiny transmitters based on existing wireless technology.

That’s the proposal of Louis Scheffer, an electrical engineer from Cadence Design Systems, an electronics and software company in San Jose, California.

NASA currently talks to distant craft, such as the Cassini probe now in orbit around Saturn, using its Deep Space Network, which links radio antennae at three facilities, in Spain, Australia and California’s Mojave Desert.

These antennae have huge, steerable dishes up to 70 metres across that focus beams of microwaves carrying messages between space and Earth. The network also allows radar studies of planets and asteroids.

Scheffer points out that, although the system is effective, it has some serious disadvantages. “Big, steel, pointable structures are rather expensive to build”, he says, and repairs can take an antenna off-line for weeks at a time.

Big, steel, pointable structures are rather expensive to build.

Louis Scheffer
Cadence Design Systems, California.

The ability both to transmit and to receive microwaves also requires special filters to ensure that the dish doesn’t deafen itself with its outgoing signals, and these filters impair the receiver’s performance.

Chips with everything

In the latest issue of Radio Science1, Scheffer suggests that it would be easier and cheaper to use an array of miniature antennae, similar to the Bluetooth chips found in mobile telephones and laptop computers. About 100 antennae could be printed on a standard circuit board, about 25 centimetres square, and controlled by a computer chip.

Although each antenna would churn out a mere 10 milliwatts of power, Scheffer argues that 50 million of them could be united to achieve twice the power of the large transmitter dish at Arecibo in Puerto Rico.

The circuit boards could be laid like tiles across a wide expanse of flat ground and, with no moving parts, could be installed or replaced by relatively unskilled workers. Scheffer imagines a routine maintenance situation: “Just send a guy out to fix board number 42.” The rest of the array would continue to work as normal while the faulty board was replaced.

As the array would only transmit data, there would be no need for the electronic filtering required for the bigger antennae that both send and receive signals. And there are plenty of radiotelescopes around the world that can receive data from space probes.

Scheffer estimates that each tile should cost about US$20, and could be produced in the factories that already churn out high volumes of consumer electronics. “The costs are pretty speculative, but at first glance it looks about four times cheaper than building a radio telescope of equivalent power,” he says.

“This seems reasonable from a technical perspective,” says Chris Rose, a wireless networks expert at Rutgers University, Piscataway, New Jersey. “And it’s appealing not having to time-share an antenna between receiving and transmitting.”

Team message

The idea of uniting several smaller telescopes instead of building a single behemoth is not new. The Very Large Array in New Mexico, for example, has 27 radio antennae, each 25 metres in diameter, which work in unison.

But Scheffer says that with the rapid progress of mass-produced electronics, the time is ripe to take the idea to its logical extreme, in the form of his flat array. Even without a focusing dish, the direction of the microwaves from the array could be controlled by carefully tweaking the phase of each antenna, so that they add up to give a powerful beam in just one direction, he explains. As the array of tiny transmitters would cover an area similar to the Arecibo dish, Rose thinks it should have a comparable ability to direct its beam.

Larry D’Addario, a radioastronomer at the Jet Propulsion Laboratory in Pasadena, California, says that the idea is interesting, and adds that the Deep Space Network is in need of an upgrade.

“These systems, especially the 70-metre antennae, are nearing the ends of their useful lives and must soon undergo major refurbishment or be replaced,” he says. But D’Addario favours an intermediate approach, using 217 dishes, each 3.8 metres in diameter, a proposal that he presented at the International Union of Radio Science conference in New Delhi, India, this week.