More-Sophisticated Codes to Track Deep-Space Probes

This article is part of our exclusive IEEE Journal Watch series in partnership with IEEE Xplore.

As deep-space probes continue to explore throughout the solar system and beyond, scientists on the ground are racing to keep in touch with the many increasingly far-flung missions.

To that end, researchers in China have developed deep-space-communications processing algorithms that will allow scientists to pinpoint probes with remarkable, meter-level precision—as far as 180 million kilometers from Earth.

Xiaoyu Dang—a professor at Hangzhou Dianzi University’s Space Information Research Institute (SIRI), in China—says that communicating with a spacecraft millions or billions of kilometers away is incredibly difficult. “Signals get very weak, and existing ranging codes struggle to pinpoint a spacecraft’s exact distance precisely over these enormous gaps,” he says.

To maintain contact with space probes, the radio signals sent back and forth between Earth and deep space contain special ranging codes, which act as a kind of measuring tape or ruler. The code contains mathematical “notches” in it that enable mission planners to track individual signals as they travel to the probe and back. The travel time of the signal is then used to calculate the probe’s distance from Earth.

Dang and his colleagues have developed a set of eight new codes (called Legendre Sequence Ranging Codes, or LS codes) which offer a much longer “measuring tape.” The team’s LS codes offer extended pattern length in deep-space signals compared with existing deep-space-communications codes. And that means the new codes can track probes increasingly farther away.

Testing Deep-Space Signal Codes

In simulations, the new LS codes reliably measured distances that are between 12 and 2,375 times as far as the distances that existing measuring codes can measure. As a result, the codes could enable precision communication with probes as far as 180 million km away from Earth, or 1.2 times the Earth–sun distance.

On the other hand, the deepest deep-space probes travel billions of kilometers and often require specialized technologies and codes to maintain contact. For example, Voyager 1, which launched in 1977, has traveled nearly 25 billion km from Earth. Remarkably, NASA still communicates with the probe, as recently as earlier this year.

But communication with Voyager 1 is also very complex, Dang says, noting that scientists must combine Voyager’s original ranging code with additional low-frequency signals. This technique, he adds, is like using multiple rulers of different sizes and frequencies.

By contrast, the new LS codes will enable precise, long-distance communication without complicating the signals and signal processing. “This is invaluable for faster, simpler navigation during critical operations,” Dang says.

He also notes that the new LS codes will offer more flexibility than previous ranging codes, allowing for fine-tuning of the codes to suit specific mission needs. Scientists can also choose to modify the codes to achieve optimal long-distance precision, or other desirable goals. For instance, the code can be optimized for faster signal “locking” times when making contact with the probe, depending on the nature of the mission.

Dang says the researchers aim to test their codes using hardware in laboratories. Then, if the tests are successful, the team expects to propose the code to space agencies for use in future missions. He says the team also plans to further develop and harden the codes to ensure they are robust against the harsh realities of space.

The researchers’ deep-space signal-communications codes are described in a study published 11 August in IEEE Communications Letters.