Mars is more crowded than ever. With a successful arrival by the Perseverance rover in February, NASA alone is operating three orbiters, two rovers, and a lander on or around the planet. Other orbiters above the planet are being operated by the European Space Agency, Roscosmos, the Indian Space Research Organization, and others. The China National Space Administration hopes to successfully land its first rover, Tianwen-1, later this year. Radio spectrum, which never seemed in short supply on the barren world, is suddenly facing some of the same interference concerns that satellites around Earth already deal with.
Peter Ilott is the lead telecommunications system engineer for the Mars 2020 mission at the Jet Propulsion Laboratory. After more than a decade in the commercial spacecraft industry, he came to JPL in 2000. In the subsequent two decades, he’s had a hand in every Mars mission undertaken by NASA and JPL. Ilott recently took some time to talk with IEEE Spectrum about how interplanetary communications work, what happens when Mars gets more crowded, and how NASA will have to handle the consequences of that crowding.
Mars is millions of kilometers away. How do you make sure a rover like Perseverance stays in touch with Earth over that distance? How does it communicate?
It’s essentially the same process as we did on the Mars Science Laboratory [Curiosity], and pretty similar to what we did on the Mars Exploration Rover mission [Spirit and Opportunity]. When we launched MER, we had the X band system as the prime communication system with the earth. We had UHF [Ultra High Frequency], but it was more of an experiment.
We had Odyssey, which was the orbiter. We also had the Mars Global Surveyor at the time, and the European Space Agency’s Mars Express. They had a much lower capability for relaying data. But we considered the X band system as the primary system to communicate with Earth. As I said, UHF was kind of an experiment, and we’ll see when we got there. Very soon after we did the first couple of UHF passes, we realized, oh my god, this is the way to go. At the time, if we got, 80 megabits using Odyssey, we were thrilled. With Perseverance, we routinely get 10 times more than that in one pass. And since we have four to five passes a day, we often get way, way over a gigabit of data in one Sol.
Walk us through a day in the life of a rover like Perseverance. How does it stay in touch with Earth?
If you think of Mars time, the rover wakes up in the morning, and it will listen for an uplink from the ground at X band (7 to 11.2 gigahertz) using the high-gain antenna. The high gain points at the Earth, and we send up the sequences for the day. The sequences will typically be some data management and then some activities for the day—for example, driving to the place where we’re going to drop off the helicopter. That’s a typical activity.
When we send those commands, we sometimes get direct-to-Earth communication from the rover, we sometimes get telemetry from it on the X band. The rates aren’t very high, but there’s enough to get some basic information and health. We also get what we call beeps—there’s three possible beeps each day. One is the saving beep. If we get that one, we’ve got a problem. We’ve never gotten one of those yet on Perseverance. The second beep is the normal beep that tells us, “the sequences that you just send me are okay, and I’m running on the new master sequence for the day.” If we get that one, then we know things should go well the rest of the day. The third beep is the run-out beep. If we see that one, that means the sequences for today didn’t get in for some reason or another.
The rest of the communications with the rover until the next day is UHF (300 megahertz to 3 GHz). We get a bunch of afternoon passes with the relay orbiters. Included would be the priority data for the day that needs to be brought down so that the planning for the next day can occur. Overnight, while the rover sleeps, orbiters pass overhead and rover wakes up periodically to talk to an orbiter. And then the next day starts.
You’ve been at JPL for two decades now. How have communications—and the expectations—changed over time?
The radios are more advanced. The radios on the Odyssey orbiter and Spirit and Opportunity had fixed rates—it would go 8, 32, 64, 128, 256 kilobits. The maximum rate was 256 kilobits per second. And we had to choose which rate we were going to use for the day. We would typically use either 128 or 256. If we had the orbiter way low on the horizon, the best we could do is 32 Kbps. And if it was that low, then it probably wasn’t worth the power expenditure to wake up and try to send data, so we would focus on passes where we could get 128 or 256.
The Electra-lite radios on Curiosity, the Mars Reconnaissance Orbiter, MAVEN orbiter, and the [ESA’s/Roscosmos’] ExoMars Trace Gas Orbiter could go to much higher rates, up to 2 megabits per second. And we’ve developed adaptive data rates, where the data rate would be adjusted, depending on the signal strength. If you can get 2 minutes at 2 Mbps, that’s a hell of a lot more data than 10 minutes at 128 Kbps.
With Perseverance, we still use Odyssey as one of the relay orbiters, and so we have to choose either 128 or 256 Kbps as our max rate. But with MRO and TGO and MAVEN, we can run much higher rates. With TGO, for example, we regularly get over a gigabit of data per pass.
Now that there’s several active rovers, landers, and orbiters on or around Mars, it seems like NASA has developed something like a Martian network. How has the growing number of machines on or orbiting the planet impacted how they communicate—either with each other or with Earth?
We actually call it the Mars Relay Network. There’s a lot of things to take into account. One is storage on board. Every orbiter has a fixed amount of storage space. When it when it gets a relay pass with a rover, an important parameter would be how soon can the orbiter send that data to Earth? And how soon can it clean off its own solid-state drive? And that’s true of all the orbiters. Odyssey has the smallest capacity because it’s an older bird. In fact, Odyssey’s capacity is small enough that with Curiosity, we often overrode its buffer.
And then how do we coordinate it? Some of it is still done manually, but we also have this tool called MaROS. The rover looks at the different passes that it could possibly access and it decides which ones it would like to use. Then it puts a request in to the orbiters using MaROS. It’s kind of like an online scheduling tool.
As we add more landers and more orbiters, it becomes more and more complicated. At one point, we had Spirit and Opportunity, and the Phoenix lander. We now have two rovers and one lander again; the difference is that we’ve got two extra orbiters. And we’ve got a hell of a lot more capacity, both in relaying from the landers and in onboard capacity. So it’s the same problem but multiplied.
As more missions simultaneously explore Mars, will we start to see similar problems like interference that we already see here on Earth?
We’ve actually seen interference from our own orbiters. We’ve seen high order harmonics coming into our band from the MAVEN orbiter and the Mars Express orbiter. I mentioned looking for the beeps in the morning, and looking for the downlink; we’ve seen suddenly this tone appears off to the left in our spectrum and just kind of marches across because an orbiter’s doppler compensation isn’t the same as a rover’s. It’s strong enough to present a potential problem when we’re trying to see the beeps from the low-gain antenna.
Fortunately, we can tell that it’s not our beep, because first of all, it tends to move. But we have seen it march right down to the center of our spectrum and even sit right next to the center of our spectrum. It takes a little bit of base telecom knowledge to know and interpret that. In that sense, it’s not a problem yet, but it could potentially be a problem into the future.
How can future missions ensure that growing interference doesn’t cause a rover to lose contact, or worse, cause a mission failure?
I’ve been warning the next generation, look: The biggest problem my generation has is UHF. My cell phone, my Bluetooth headset are going to interfere at UHF, but nothing is really getting up in the X band yet. But it’s getting close. It won’t be long before we’re going to have regular interference from just the hardware onboard spacecraft. The next generation is going to have to deal with that.
The first thing to get is get the Electromagnetic Interference/Compatibility teams on board earlier. This is something I encouraged on Perseverance. I remember that they weren’t involved early enough and strongly enough on Curiosity. The first time we buttoned up the rover on the ground, the project leader wanted us to do an early test to get an idea of whether the situation was bad or not. We spent weeks setting up this test, we button up the rover, and we plan the test. We turn the UHF radio on, and immediately the radio locked up to a false signal. In five minutes, the test is over.
It turned out that a lot of it was coming from the avionics itself, that the rover itself just hadn’t been sealed enough. We spent a lot of time and money fixing the problem. We reran the test a year, year and a half later, and it passed—except that we found that there were certain instruments, if we turned them on, we would get false locks and leakage would still get out. So, we had certain sets of instruments and cameras that we weren’t allowed to turn on when we were doing a UHF pass. UHF took priority, because if you can’t communicate, there’s no point in doing anything. On Perseverance, we started earlier and I think the lab learned the lesson. When we ran the same test as we did for Curiosity, we had fewer problems.
There’s also passive intermodulation, which comes up on the commercial side a lot because there are these very large antennas with kilowatts of power being generated. You get two channels that interfere with each other, and they create a passive intermod tone that falls in another channel. It’s extremely difficult to deal with.
It’s not something that we’ve had to deal with yet on these exploratory birds, and I hope it never comes up. I might write something for the next generation: “Keep an eye on this, one day it may show up as a problem for you and it’s going scare the hell out of you.” There have actually been commercial missions where passive intermod just ruined the mission. It created havoc where the bird actually became useless. It’s not a trivial problem, and you can’t fix it after the fact—you can’t go up and wrap it in copper tape. But it’s being taken more seriously as we move forward.