Space Telemetry: How Extraterrestrials Phone Home

The idea that measurements could be taken and commands given remotely using the same radio waves wasn't available midcentury

The trouble with radio waves is their invisibility. Rockets and spacesuits dominate the imagination. But human spaceflight -- let alone TV images of men walking on the moon -- would never have been possible without radio's invisible threads of information connecting the spacecraft to home.

In 1929, Richard E. Byrd made history -- not for reaching the South Pole, but for bringing on his Antarctic expedition 24 radio transmitters, 31 receivers, five radio engineers, three airplanes and an aerial camera. Unlike Ernest Shackleton's trans-Antarctic expedition, who 15 years earlier spent 17 months fighting for their lives after being trapped in the polar ice, Byrd's team was able to stay in constant communication with each other and with the outside world. It was the beginning of modern technology-aided exploration, and arguably the model for human spaceflight.

At midcentury, it wasn't clear that radio waves could pass beyond the earth's upper atmosphere. All we knew was that some of the shorter waves didn't get reflected back. In February 1945, Royal Air Force radar technician and aspiring science-fiction author Arthur C. Clarke, acting on behalf of the British Interplanetary Society, wrote a letter to the hobbyist magazine Wireless World noting that the German V2 rocket could be launched into the ionosphere and send measurements back to Earth. More powerful rockets still could launch what Clarke called "artificial satellites" into permanent geosynchronous orbit. "Three repeater stations, 120 degrees apart in the correct orbit," Clarke mused, "could give television and microwave coverage to the entire planet," a proposition he detailed at length in a full article that October.

In 1945, Clarke believed that communications satellites were "a possibility of the more remote future -- perhaps half a century ahead." He also believed that these satellites would have to be manned -- their crews "relieved and provisioned by a regular rocket service." The idea that scientific measurements could be taken and computational commands could be given remotely using the same radio waves -- what later became known as space telemetry -- wasn't available to him yet. That would come with the development of computer science, cybernetics and mathematical information theory, which would show how to send lots of information over long distances using very little power over increasingly noisy channels.

But automated information processing had its limitations, especially in the early years of space exploration. The first Sputnik satellite sent a beeping signal that anyone with a short-wave radio receiver could hear. To relay the atmospheric information it was gathering as efficiently as possible, Sputnik-1's radio used a simple binary code: the length of the pulses registered the satellite's internal pressure; the length between pulses, its temperature. Then its batteries died. In 1959, Luna-3's pictures of the far side of the moon were automatically developed, scanned and compressed; each image took twenty minutes to transmit, and the signal-to-noise ratio was still so poor that some Americans argued that the resulting images were a Soviet hoax.

Astronauts and cosmonauts weren't simply explorers or pilots; they were also scientist-engineers, charged with monitoring, repairing and providing a crucial set of eyes on the spacecraft and its data. When Vostok was launched into orbit on April 12, 1961, a break in data transmission shortly after take-off triggered a false signal indicating a failure in the booster rocket. It was only with Yuri Gagarin's vocal report that the capsule had reached orbit successfully, compressed and transmitted over short-wave radio to the command center at Star City, that the first men and women on Earth could know that the world had indeed changed forever.

Image: The Very Large Array (VLA).