Feb. 16, 2021: Around the world, ham radio operators are doing something once reserved for national Deep Space Networks. „We’re monitoring spacecraft around Mars,“ says Scott Tilley, of Roberts Creek, British Columbia, who listened to China’s Tianwen-1 probe go into orbit on Feb. 10th. The signal, which Tilley picked up in his own back yard, was „loud and audible.“ Click to listen:
The signal from Tianwen-1 is dominated by a strong X-band carrier wave with weaker side bands containing the spacecraft’s state vector (position and velocity). Finding this narrow spike of information among all the possible frequencies of deep space communication was no easy task.
„It was a bit like a treasure hunt,“ Tilley says. „Normally a mission like this would have its frequency published by the ITU (International Telecommunications Union). China did make a posting, but it was too vague for precise tuning. After Tianwen-1 was launched, observers scanned through 50MHz of spectrum and found the signal. Amateurs have tracked the mission ever since with great accuracy thanks to the decoded state vector from the probe itself.“
So far, Tilley has picked up signals from China’s Tianwen-1 spacecraft, NASA’s Mars Reconnaissance Orbiter, and the United Arab Emirates‘ Hope probe–all orbiting Mars approximately 200 million kilometers away. How is such extreme DX’ing possible?
„It helps to have a big antenna,“ says Tilley, who uses a 60 cm dish, pictured above. „But the real key,“ he says, „is the advent of Software Defined Radios (SDRs) , which have become the norm for hams in the past decade or so.“
In a Software Defined Radio, computers digitally perform the signal mixing and amplification functions of circuits that used to be analog. SDRs are cheap, sensitive, and they give hams the kind of exquisite control over frequency required to tune into distant spacecraft.
„Amateurs really began listening to deep space probes in the late 1990s and early 2000s,“ says Tilley. „This sparked an awareness that it was possible. The combination of improving technology and growing awareness has resulted in more and more interplanetary detections.“
Next up: NASA’s Mars 2020 spacecraft carrying the Perseverance rover, due to reach Mars Feb. 18th:
Tilley plans to listen but he doesn’t expect a strong signal. „Perseverance does not have a very large antenna,“ says Tilley. „It doesn’t need one because it can use other NASA spacecraft in Mars orbit as relays. The signal will therefore be weak and I doubt many amateurs will record the landing in Jezero crater.“
Tianwen-1, on the other hand, has a relatively large antenna with a booming signal. „China probably plans to use it as a relay for future Chinese space missions,“ Tilley speculates. „This makes it a good target for hams hoping to bag their first Martian spacecraft.“
Stay tuned for more radio signals from Mars.