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Humanity has been a spacefaring species for barely sixty years now. In that brief time, we’ve fairly mastered the business of putting objects into orbit around the Earth, and done so with such gusto that a cloud of both useful and useless objects now surrounds us. Communicating with satellites in Earth orbit is almost trivial; your phone is probably listening to at least half a dozen geosynchronous GPS birds right now, and any ham radio operator can chat with the astronauts aboard the ISS with nothing more that a $30 handy-talkie and a homemade antenna.
But once our spacecraft get much beyond geosynchronous orbit, communications get a little dicier. The inverse square law and the limited power budget available to most interplanetary craft exact a toll on how much RF energy can be sent back home. And yet the science of these missions demands a reliable connection with enough bandwidth to both control the spacecraft and to retrieve its precious cargo of data. That requires a powerful radio network with some mighty big ears, but as we’ll see, NASA isn’t the only one listening to what’s happening out in deep space.
The need for a way to talk to satellites was recognized very early on in the US space program, and development of the space communication network that would come to be known as the Deep Space Network (DSN) paralleled developments in space technology that quickly pushed hardware farther and farther from Earth. The DSN was built specifically so that each new mission didn’t need to roll its own communications solutions and could just leverage the current network. Networks for the ESA and for other countries’ space programs have since been built as well, and cooperation between all the network operators is commonplace, especially during emergencies.
Three sites around the globe were selected for the DSN ground stations — Canberra, Australia; Madrid, Spain; and Goldstone, California. The sites are almost perfectly 120° apart, which means that their view of the sky overlaps at an altitude of about 30,000 km; anything farther away from Earth than that is always within view of the DSN regardless of the Earth’s rotation.
Each site has fully steerable parabolic reflector antennas ranging in size from 26 meters to a whopping 70 meters. Sensitive receivers and digital signal processing gear can still pick up the vanishingly faint signals from the Voyager spacecraft, currently more than 30 light-hours away from Earth. Uplink transmitter power vary depending on frequency. The S- and X-band transmitters generally have 20-kW amplifiers, but there’s also a 400-kW amplifier that sometimes gets called into service for the S-band transmitter on the Canberra 70 m dish with special coordination with aviation authorities so that no planes fly through the beam, and with dish elevation limited to 17° above the horizon to avoid frying anyone on the ground.
JPL has a very cool interactive page that lists the current status of all the antennas in the DSN and what each one is doing. While I’m writing this, the 70 m dish in Madrid is sending a 19 kW signal to Voyager 1 and getting back a -154.27 dBm signal. That’s about 370 zeptowatts, but still enough signal to pull out 159 bits/second of data.
With all this specialized hardware it would be natural to assume that there’s nothing the average hacker can do to listen to the inbound DSN signals. And indeed, the good folks at the JPL have answered that question with a definitive “No.” They appear not to have checked in with an avid bunch of radio enthusiasts who routinely turn homemade dish antennas to the sky and do their best to pull in the ultimate in DX signals.
One such hobbyist, amateur radio operator Paul Marsh (M0EYT), has enough deep space contacts to populate an active Twitter feed. Sprinkled in among the many images he routinely captures from weather satellites are waterfall displays showing the characteristic diagonal line caused by Doppler-shifted signals from spacecraft far from Earth. He recently bagged the Cassini probe, currently making its final orbits of Saturn before plunging into the gas giant in September. He and his cohorts have listened to plenty of other deep space probes too, including the Mars Reconnaissance Orbiter, the OSIRIS-REx mission in the asteroid belt, the STEREO-A and STEREO-B solar observatories, and the Juno mission to Jupiter.
The gear that Paul uses to accomplish all this is deceptively simple compared to the big rigs of the DSN. His dish is an off-the-shelf 1.8 m prime focus satellite dish with extender wings to bring it out to 2.4 meters in diameter. Until recently the dish was manually positioned, but is now fully motorized using a Royal Navy surplus altazimuth mount. His receive gear is what you’d expect in any microwave enthusiast’s shack — low-noise amplifiers, mixers, filters — but mostly custom built and optimized for deep space work. A spreadsheet inside the shack calculates the frequency to listen on for any given spacecraft based on its Doppler shift due to its relative velocity. From there, patience and experience lets him pull the faintest of signals.
And what of Voyager, that most remote outpost of humanity? Calling home from interstellar space with an 18 watt signal, it would seem that only the big ears of the DSN could possible pick that up. But Dr. Achim Vollhardt, DH2VA, actually heard Voyager 1 in 2007 at a distance of 9.5 billion miles using a 20 meter dish. So it’s possible, and other deep space listeners may well be able to replicate the feat, but they’d better hurry — Voyager 1’s radioisotope thermal generator is only rated for another three years.
As a ham and a shortwave listener, I can attest to the thrill of working a weak contact through noise and interference, chasing it up and down the dial until the stars align and you’re finally able to copy a callsign from the other side of the planet. How much more thrilling must it be to be able to point a dish at the right location and calculate the correct frequency to tune, and to see that diagonal line on the waterfall indicating a signal from across the solar system!
Title images: Jet Propulsion Lab
