Jim Green: NASA flies spacecraft all over the solar system, and orbits the Earth. How do we communicate with them when they’re so far away? Let’s find out from an expert.
Jim Green: Hi, I’m Jim Green. And this is a new season of Gravity Assist. We’re going to explore the inside workings of NASA in making these fabulous missions happen.
Jim Green: I’m here with Nacer Chahat, and he is the senior antenna and microwave engineer with NASA’s Jet Propulsion Laboratory out in Pasadena, California. He has worked on one of the most important challenges for NASA and that is, in designing a spacecraft, how do you make it communicate back and forth with Earth? So welcome to Gravity Assist.
Nacer Chahat: Thank you for having me here.
Jim Green: A lot of people are familiar with the idea of an antenna from a radio or TV set, for which they receive a signal. But when we look at a spacecraft, we see all kinds of different antennas. What’s their main function?
Nacer Chahat: Yes, so, the principle is the same. However, every single spacecraft have different instruments, So, depending on the requirement, whether they are instruments or communication, they come in different form, different aspects. So, most of the communication antennas will be the typical dishes that we see. So, those are the ones that you see the most often on spacecraft. But whenever we push the boundary of what we are trying to do, we have to come up with different innovative antenna solutions to address the needs of our scientists.
Jim Green: You know a lot of the antennas that you use, you use for what we call remote sensing. What’s that really mean?
Nacer Chahat: So, remote sensing allows us to get something from afar and so we’re using radio frequency to transmit pulses. These pulses are reflecting back from the surface of what we want to study, and then we’re processing this data to make conclusions.
Jim Green: Yeah, that’s everything from having the opportunity to bounce radio waves off surfaces to even penetrate those radio waves into surfaces like, like the ice caps or even the Sahara Desert.
Nacer Chahat Yeah, and that also allows us to get images of things we cannot see. So, for example if the weather is bad and Earth is completely covered with clouds, we can still see what’s happening under them.
Jim Green: Well, you know, communication with our surface assets on Mars is kind of complicated. You know, when we landed Curiosity, I don’t see it carrying a big truck with a big dish behind it. So how do we communicate back and forth with, with our surface assets, like InSight, like Perseverance, like Curiosity?
Nacer Chahat: Yes, so we have two concepts to do so. The first one is to communicate with the orbiter. So we have orbiters around Mars which we can, they are dedicated for science, but for a critical event like that, we can use them to relay the data to Earth. And we also have on these landers and rover, high gain antennas that allow us to communicate directly with Earth but at lower data rate. So it’s really a trade-off. When, when we should be using the orbiter or when we should be using the high gain antenna. Most of the time, we end up using the orbiter because it allows us to transmit the science much faster. So those are the reasons and most of the Mars mission have always worked this way. So we transmit the data to the orbiter and the orbit transmits back to the to the Deep Space Network on Earth.
Jim Green: Yeah, that sounds complicated, but it provides an infrastructure that allows any asset that’s on the surface to be able to be relayed through our orbiting satellites. It’s really an exciting time in in this particular field.
Jim Green: You know when the InSight lander arrived landed on Mars in November 2018 it carried with a two CubeSats and we call those CubeSats “MarCO.” What were they supposed to be doing and how important was communication for them?
Nacer Chahat: Yeah so MarCO was actually one of my first projects, and when I joined JPL the former director of the lab, Charles Elachi, challenged the lab to find a way to do real time communication during the lending of InSight. So with the existing orbiter we were able to collect all the information from InSight during the landing However because of the alignment of the orbiter, we couldn’t get the data right away.
Nacer Chahat: So the role of these two CubeSats was actually to receive data from the lander, InSight, and transmit these data in real time so the, the main challenge was to be able from these very, very tiny satellites which are the size of a shoebox, being able to transmit at the same data rate which was roughly around 8 kilobits per second. This was really difficult because we needed an antenna that was three times the size of the satellite itself. So we had to find a way to fold the antenna and deploy it. That’s also the first interplanetary CubeSat so we had to ensure that we could actually survive the flight to Mars, which we did very successfully.
Nacer Chahat: When we witnessed the landing of InSight we were all excited because for those who actually follow the landing you should remember when they were saying oh we are now 100 meter 90 meter 20 meter and all the excitement was coming up all I had in mind was oh that’s my antenna who’s transmitting all these data and then he landed we exploded with joy and we finally received these first pictures from InSight on the surface of Mars which also got relayed using the MarCO antenna.
Nacer Chahat: If we did not have the CubeSats we would have had to wait more than two hours after that.
Jim Green: Wow.
Nacer Chahat: Because the orbiter was not on the line of sight.
Nacer Chahat So, that’s such an amazing accomplishment that we’ve been able to do and that’s really what’s beautiful about working at NASA is that we’re able to do things that nobody has done before.
Jim Green: Yeah i remember that time I was head of planetary science and it was really riveting and i was just delighted that the MarCO spacecraft worked.
Jim Green: And in fact, it was really nailbiting for the simple reason that we were having problems with one of the MarCO spacecraft very close to the encounter time. Do you remember that? And what happened?
Nacer Chahat: I do, I do, and we were very worried about that. So one of them actually restarted? Like, I think it was a few hours before maybe a day before, I don’t recall exactly. So we were worried that one of the CubeSats would not be able to relate to data. But after it rebooted, it went in safe mode and rebooted in a nominal mode. And they both successfully related the data.
Jim Green: Well, you know, we’re flying a helicopter on Mars for the first time. And we call that Ingenuity. Did you get involved in that? And what’s your role?
Nacer Chahat: When I saw the first flight, I couldn’t help but think of all these hours spent working really hard to solve technical problems. All these people working on delivering hardware that you don’t necessarily see on TV. This achievement is absolutely historical and I am happy I got to contribute.
Nacer Chahat: My contribution was with the telecommunication subsystem, to ensure that the rover can send commands to the helicopter, and the helicopter can send images or telemetry back to the rover. So, I worked on the antenna design and also worked on the system engineering.
Jim Green: Wow, that sounds really difficult because this is a very small vehicle. What’s that antenna look like? I don’t remember seeing it. Does it stick out? Or is it part of… does it go up to the top or, where’s it at?
Nacer Chahat: Yeah. So on the top of the helicopter, there is a solar panel, which allows us to recharge our battery. And we decided to locate the antenna on this surface, because this is what provided the largest area to use as a reflective surface. So this is the simplest type of antenna that you can ever use. This is called a monopole. So monopole is basically a single wire, which is resonant at the frequency of operation, and located on top of a reflective surface. And the reflective surface in this case is the solar array.
Nacer Chahat: This wire allows us to operate at the frequency of interest. But this antenna, this type of monopole antenna are being used when you need to communicate what we call omni, omni directionally. Meaning, we need to communicate this with the same capabilities in any direction, because we don’t know where the helicopter or rover will be because it will constantly move when it’s flying. Right. So that’s the reason why we use such an antenna.
Nacer Chahat: It’s very small it’s about five to six centimeter which is basically a quarter wavelength or the frequency of operation.
Jim Green: Yeah, that makes a lot of sense. I mean, yeah, you move the copter and it comes back down. And it’s not necessarily in that same orientation.
Nacer Chahat: Exactly.