Serious DX: The Deep Space Network

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.

Deep Space Three

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.

Space Flight Operation Facility, “The Center of the Universe” and home of the DSN control center. Source: JPL

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.

Listening In

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.

Cassini calling home from Saturn. Source: UHF-Satcom [M0EYT] (click to enlarge)
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.

M0EYT’s 1.8-meter dish, now fully steerable.

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

26 thoughts on “Serious DX: The Deep Space Network

  1. Awsome!

    These systems are really incredible. Don’t forget they have to track that spacecraft and account for doppler not only while it is moving, but while the planet is rotating and circling the sun. Not a small challenge.

    I got to work peripherally on the systems that are used to steer the dishes. It’s a very cool system.

  2. I’m not sure we really qualify as being a spacefaring species yet. I mean not disrespect or devaluing of the accomplishments that many have worked very hard for. Our astronauts, cosmonauts, taikonauts, *nauts (really, do we need separate words for this single concept?!?!) and all the ground based people that support them deserve tons of admiration and credit.

    But.. we are a species of 7 Billion, going on 8. Out of those only a handful have ever been to another world, that ended a couple generations ago and it was our own planet’s moon! Other than that.. we usually maintain a few people in low earth orbit.

    Sorry, I just don’t think we deserve the title ‘spacefaring species yet’. I say this not to be offensive but as a ‘kick in the pants’. I want us to be a spacefaring species. Do you [dear comment reader]?

    1. We are spacefaring, not because we are putting on a spacesuit and running down to the spacemarket to buy some spacemilk… We are spacefaring because so much of our society depend on the things that we put in space, like gps. I just googled it, and we have 2,271 operational satellites orbiting the planet.
      You’know if you go out into the ocean, its not like there are people everywhere all over the place; ships, though many in number, are relatively spread out so its less likely to bump into one another out on the open ocean. But, we are a seafaring people because our society relies on fishermen, shipping, and oil-rigs (sadly), not because we all are literally living out on the sea…
      It is the same idea with space.

  3. Are these things only detectable as carriers which cross the waterfall in predictable ways due to their doppler shift (which is cool enough)? Or is there actually enough signal there for an amateur to have a shot at demodulating something and pulling some information out?

    1. That’s how I understand it, only carriers on a waterfall.
      There is really no limit how small antennas you can use. In theory you can just use smaller filters and integrate over long timescales to detect weak signals but calculating the correct doppler shift and stearing the antenna can be tricky.

  4. Very Very impressive!
    How I wish I had the time.
    There is so so much we can do today. And it not just electronics, its in all the fields of science or Magic.
    When I really think of it I get so overwhelmed.

  5. ” 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.”

    Kind of like “expired” drugs, it may still have some life pass that point.

    1. Well.. considering that radioactive substances decay with a half-life…

      The rate that ‘fuel’ is ‘consumed’ reduces with time so it never really totally runs out although as the rate of consumption reduces so does the power output.

      I would imagine that at some point one of three things happens.
      1 – the amount of radioactivity reduces to the point where whatever they are using to collect it and turn it into electricity no longer can function
      2 – it no longer generates enough power to run some critical function such as the computer or the radio

      or

      3 – It no longer generates enough heat to keep the electronics from freezing and therefore breaking.

      Either way.. can they predict the exact moment? I’m guessing no. And which of these three is the eventual death? Any experts?

      1. It’s loss of power. 2020 isn’t a total shutdown date: it’s the date when (currently useful) science instruments have to start shutting off. By ~2025 the Voyagers won’t be able to power any of their instruments at all – that’s when it’s really “dead”.

    1. Yep I watched these videos right before I came to this article. You’ll notice that the -154.27dB figure in the article is actually wrong since the real data read as ~159dB.

  6. Too bad the dish at Delaware Ohio (USA) was torn down (i.e. Ohio State University’s Big Ear radio telescope). It was the one that detected the WOW! signal on 15-August-1977 for 72 seconds by Dr. Jerry R. Ehman. It was a hydrogen line signal (1420.4056 MHz) that originated in the Sagittarius system some time around 18th century A.D. The signal was so powerful we here on Earth could never produce it. A creepy person with a specious college degree (and claims to be a professor) is trying to sell that it was from a passing comet Dr. Ehman could not have known about. The truth being we can not know about who this so-called “professor” really is and what his game is about.

    Here is a link to the incident: http://www.bigear.org/wow20th.htm

    1. Is this a moderation queue hiccup? The message I posted (below), timestamped 1:18pm, appeared on the site about 3 hours before Mike’s message, but Mike’s is timestamped an hour earlier.

  7. Picking nits, or janskys, whichever: Error in first paragraph: GPS satellites are not in geosynchronous orbit. The system wouldn’t work at all if they were there (think about it…). They’re about halfway out, way above LEO and the belts, doing about 2 laps/day around us. Watch how they move on your own GPS receiver’s satellite map.

    1. > GPS satellites are not in geosynchronous orbit. The system wouldn’t work at all if they were there (think about it…)

      I’m thinking about it, and there’s no reason why GPS couldn’t work with satellites in geosynchronous orbits. Geostationary orbits would be problematic for users near the poles, but using inclined orbits rather than geostationary would fix that. (Since we’re nitpicking, note that geostationary is a subset of geosynchronous)

      1. Nitpicking accepted — yes, a GPS-like system could work with geosynchronous orbits, specifically the subset of those with inclined planes. Geostationary (i.e. confined to the equatorial plane) orbits would have problems at the poles, as you say (due to no visibility), but also the north/south hemisphere ambiguity (resolved by antenna patterns, perhaps). But by far the worst effect is the dilution of precision caused by having all the satellites residing in the same plane: the volume of the tetrahedron formed by the receiver & four satellites (which is commonly taken as a surrogate estimate of precision — the larger the better) is *zero*.

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