Eyes on Earth Episode 108 – Landsat 8’s 100th Drag Make-Up Maneuver

LARRY TORNABENE:
Our biggest goal is keeping these satellites safe and healthy up there, right? Once you put it in space, you really can't go back and fix it. So we have to take care of these things. So we double and triple check our homework every time.

TOM ADAMSON:
Hello everyone, and welcome to another episode of Eyes on Earth, a podcast produced at the USGS EROS Center, which celebrates its 50th anniversary this year. Our podcast focuses on our ever-changing planet and on the people here at EROS and across the globe who use remote sensing to monitor and study the health of Earth. My name is Tom Adamson, your host for this episode. 

Landsat 8 launched in February 2013. Its twin, Landsat 9, launched in September 2021. The older sibling has put on many more miles and orbits and recently completed an interesting milestone. On October 5th, 2023, the USGS Landsat Flight operations team executed the 100th drag makeup maneuver on Landsat 8. To help us understand what a drag makeup maneuver is and why it's important to the successful operation of the Landsat satellites, we're talking with Larry Tornabene, Flight systems manager for Landsat 8. Now, Larry, this conversation is going to involve a little bit of jargon, like atmospheric drag. Can you start with explaining what that is and how it affects Landsat?

TORNABENE:
Yeah, absolutely. So atmospheric drag is kind of there in the name of it. It's the result of the atmosphere causing frictional resistance on everything in it. If you remember back in Physics 101, whenever you were doing speed, acceleration and momentum calculations, you were probably being told that was in a vacuum. And that's because in real life, there's a lot of external forces acting on everything out here like gravity, friction and magnetic forces. Without turning this into another physics 101 class, your altitude is directly related to your speed. Basically, the slower you go, the lower you go. That's the effect of atmospheric drag on our satellite. It's just constantly slowing us down and bringing us down to Earth.

ADAMSON:
Ah, all right. So even though it's way up there in space, 438 miles in space feels like a long way, but there's still some atmosphere sort of tugging on it.

TORNABENE:
Yeah. Also really interestingly is another thing that affects the Earth's atmosphere is the Earth’s sun. There's a roughly 11-year solar cycle. So when the sun is very active, it's putting a lot of energy out into the solar system and when it gets to Earth, one of the things it does is heats up the Earth's atmosphere itself. As the earth atmosphere heats up, just like a balloon or anything else, it expands a little bit. And as the earth’s atmospheric expands, it gets a little bit thicker near our satellite, but also every other satellite, and creates extra drag on us. We're actually in a solar activity peak now, so we are experiencing more drag today than we were six years ago.

ADAMSON:
So why do we have to do these drag makeup maneuvers?

TORNABENE:
So Landsat 8, but also 7 and 9 operate at an altitude of 705 kilometers.

ADAMSON:
Also known as 438 miles.

TORNABENE:
Yeah. And we need to stay there. But we have to control our altitude pretty tightly and that's driven mostly by our science requirements. So when we say 705, what we're really talking about is like a three kilometer variance. So we're sticking within 702 to 708 kilometers. Another factor that does drive that too though is 705 is kind of a busy altitude. So we don't want to get too close to our neighbors either. As our altitude changes, our speed changes so we we speed up or we slow down, we might actually drift into other satellites’ orbits more than we want to, and that will force us to have to change our speed and therefore our altitude or force them to have to change their speed and therefore their altitude. So we, just as part of being a good neighbor, really want to stay right where we are.

ADAMSON:
I know that a big deal with Landsat is making sure that the data is consistent. A lot of the researchers around here refer to Landsat as the gold standard for Earth observing satellite data, so this is just one of those pieces that helps achieve that. So tell us how this works. How do EROS employees move Landsat 8?

TORNABENE:
Don't let the Air Force commercials fool you. There's no joysticks. The actual process, the first big step is we make sure we know where we are going is where we want to be. That sounds kind of silly when I say it that way, but we know the orbit we're in and we know that orbit is very safe. Whenever you maneuver your satellite, it always adds a little bit of risk, so we want to make sure the orbit we're shooting for is the right orbit, but also that the orbit we're trying to insert ourselves into is clear of debris and isn't going to put us at risk of running into anything else and is really the right orbit that's going to keep us in that right science box. Once we have a good set of flight dynamics ephemera, which is the fancy word for, you know, flight path basically, we will take that and give it to our mission planners who develop a command timeline. Satellite subsystem experts will then take that command timeline, validate that the commands are the right commands, that they're not, that they're in the right order, that there's no risk to the satellite of executing them. Then we have a meeting that's called a command authorization meeting where we sit down and again prove to ourselves that this is really the right series of commands, in the right order and we're really comfortable with doing this execute—with executing these commands and then we uplink that series of commands to the satellite, and then on the day of, for, let's say about 4 hours, we're basically watching all those commands execute and those commands are, you know, heating up the thrusters because everything in space is very cold, so you don't want to just shock it with heat when you're actually burning. So you heat up your thrusters, you open latch valves so you can have fuel actually flow through your pipes. You point your satellite in the direction you want it to point and then ultimately the actual burn itself, which is A) the highlight of the day, and B) so very short. Like it's almost nothing. You spend 4 hours getting ready for it and then it's done in a heartbeat and then you're watching your satellite turn everything off and get itself back to normal science.

ADAMSON:
It's amazing to me that you have all these meetings and planning for a maneuver that, as you said, just takes a heartbeat to actually do.

TORNABENE:
Oh yeah. Yeah, I mean, we don't do anything on board the satellite that we don't—that we aren't very confident is safe for the satellite. For most of our special commanding, we spend anywhere between a week to maybe even a month or two, planning it ahead of time. We have contingency plans on contingency plans. We are ready to keep the satellite safe if something goes wrong with any of our commanding. Even with these maneuvers, right, we have two or three exit points in the like in that 4 hour walk up where like if anything doesn't look right, we can jump out and replan it again at a later date.

ADAMSON:
You're the flight systems manager, but you have a team of flight operators.

TORNABENE:
Yes, I'm the Flight Systems Manager, so I'm the USGS civil servant out here. We actually at one point for Landsat 9 and also Landsat 8 and I think also every satellite that is not a military one had a NASA team with us for our launch period as well because NASA does all the launch services. They provided the launch and then they did a handover to us. Landsat 9 has been exclusively in USGS's hands. Landsat 8 also did a transition from NASA launch team to a USGS operations team. So that operations team consists of spacecraft specialists who are subsystem experts, power experts, thermal experts, attitude control system experts, people who are really focusing in on that subsystem and are both looking at the historic trends of the subsystem and are planning forward on the predicted health of those subsystems and making plans for corrections to keep the satellites as safe and healthy as for as long as possible. We also have mission planners who build those command loads that I spoke about earlier. So they're doing really just kind of developing these sequence of commands that do our day-to-day stuff, which is everything from setting up our telemetry and command contacts. This satellite has to be told to turn the transmitters on and off. The satellite has to be told where to point the actual dishes to talk to the ground stations. The satellite has to be told when to take imagery. The satellite has to be told when to do everything, so the planners are developing these timelines of command loads that we uplink regularly to the satellite. So it just keeps doing the thing we want it to kind of automatically up in space and we have to send commands from the ground which kind of looks like, for the older crew, Apollo 13. Like that command room thing does exist. We have one of those. So when we're doing that kind of commanding, that's our real-time OPS team.

ADAMSON:
OK, maybe not using slide rules anymore, but yeah, kind of like that.

TORNABENE:
There might be one, we have some old timers.

ADAMSON:
Nice. When commands are being sent up to the satellite, how exactly do they get there?

TORNABENE:
So we beam our command to the satellite. The satellite receives it and then, yeah, sends a confirmation back down to the ground that says I received a command and then also is constantly telling us its status when we are in a real-time contact with it. 

ADAMSON:
We have this 10-meter antenna out here behind the building at EROS. Are the commands going through there? But we also have other ground stations that are also sending commands to it as well?

TORNABENE:
Yeah. So that 10-meter is actually one of our primary satellite stations. We absolutely use that one to talk to our satellites.

ADAMSON:
How much fuel does one of these drag makeup maneuvers use?

TORNABENE:
Drag makeup maneuvers, usually 10s of grams, maybe even less than 10 grams. They're very, very small burns.

ADAMSON:
OK. So do you think there's enough fuel left on board for 100 more drag makeup maneuvers?

TORNABENE:
Yes. And I'm saying that because we have hundreds of kilograms of fuel on board, but that's also because drag makeup maneuvers are some of our smallest burns. We have a couple of other types of maneuvers that we do that take up a lot more fuel. The other types of maneuvers that we do are inclination maneuvers, which are a different kind of I want to call it makeup maneuver. Our satellite is in a polar orbit. Our satellite basically goes over the North Pole and then down around the South Pole. The orbital inclination of Landsat 8 is 98.2 degrees, so the orbital inclination is the angle at which a satellite crosses the equator.

ADAMSON:
OK, it doesn't cross the equator perpendicular at 90 degrees. It's a little bit off from that, right?

TORNABENE:
Yeah. Yeah. So if it were parallel, it would be on the equator. Nothing really does that. We're not exactly perpendicular, but we're pretty close to perpendicular. So anyways, so because we're pretty close to perpendicular, the way centrifugal force works is it kind of wants to make everything trend down towards the equator, right? If you think about like if you were spinning a bucket of water on string, nothing to make that bucket of water suddenly want to go up and down. If you're spinning it around parallel to the ground, right, you would have to do something special to it make it spin in a circle next to your body as opposed to over your head, right? 

ADAMSON:
Yeah, it would take a lot of arm strength to get it going over your head.

TORNABENE:
Yeah. So our satellite is in an orbit that is against where centrifugal force wants it to be. So about once a year, we have to do a pretty big burn just to push it back up to 98.2 degrees.

ADAMSON:
So that's a bigger burn that takes more fuel. 

TORNABENE:
Yes. 

ADAMSON:
And you said that was how often?

TORNABENE:
We do those pretty much annually.

ADAMSON:
So any of these maneuvers, the drag makeup maneuver or this inclination maneuver that you were just talking about, it's not a surprise that you have to do these, right?

TORNABENE:
No, no, no. We do get some surprise maneuvers, though. You know, close approaches with other things. Something that's a real big concern for the space community right now is space junk—orbital debris. Space debris. Junk feels too informal. I tend to call it junk.

ADAMSON:
Everybody calls it space junk.

TORNABENE:
Yeah, space junk. Yes, space junk then. Yeah. So we do these risk mitigation maneuvers (RMMs) because of space junk. There's a lot of stuff up there in space; a not insignificant portion of it is other satellites that can maneuver. But there's also things like dead satellites, right? The old philosophy for ending your mission was just turn it off. Someone else's problem in the future. Which wasn't a great philosophy, but that's what they did back in the day. 

ADAMSON:
So these risk mitigation maneuvers for space junk—orbital debris—these are the ones that will come along unexpectedly. But you've also had to do a few of them, right?

TORNABENE:
Yeah. So Landsat 8 has executed 12 RMMs at this point. And I'll also say Landsat 9 is on its 5th. And also we are doing a lot of preparation for maneuvers. And when I say that what I mean is every time we get a new analysis of what's going on out there in space, you know, we look at it and we say hey, we're still flying clear or hey, we're getting kind of close to some stuff. What do we want to do about that? So maybe once a month, I'd say we have something that is close where we have to, like, seriously consider doing a risk mitigation maneuver for it. And usually what winds up happening is as we get closer to the actual close approach, we get a better set of predictions on whether or not we're going to hit it, because when we're guessing a week in advance, you just have less certainty because it's farther out there into the future, and as you get closer and closer, you're more confident in where you and it are. So the certainty is the important thing there. So we get predicts about seven days in advance on whether or not on all the things that we might hit and usually somewhere around 3 days in advance is where we get a good sense of whether or not it's a real threat. So we want to start doing preparation 7 days in advance and if we're lucky, 3 days out, we find out that it's not a real threat. And we don't have to do the burn.

ADAMSON:
It's something that you constantly monitor though.

TORNABENE:
Oh, absolutely. It's literally the first and last thing I look at every day. Yeah, I wake up and I check my phone for this report and I go to sleep right after I check the report. It keeps me up at night.

ADAMSON:
At this point, I'll bet this is fairly routine. Or are there some other exciting moments?

TORNABENE:
They're all always exciting. One of the things we had to consider with our 100th DMU was we had a piece of debris that was, when we started planning, above us and we knew where we were and we knew where it was, and if we were going to execute this DMU, it was going to become below us and a little bit closer to us than we would normally like for a piece of debris to be. But ultimately, on the day of the burn, we got a good set of predictions that told us that the debris was going to be very clear of us. And we went ahead and executed.

ADAMSON:
It's also interesting that, incidentally, in talking about the increase in solar activity leads to an increase of these drag makeup maneuvers, the 101st drag makeup maneuver for Landsat 8 already took place. That was earlier today. Was that one as exciting?

TORNABENE:
Way more routine. This one was safe from the get-go. We had clean predicts the entire time.

ADAMSON:
And these drag makeup maneuvers—that was only two weeks ago. I mean, we're talking today, two weeks after you did the 100th maneuver. And you already did another one. They don't typically happen every two weeks. Sometimes they're more spaced out than that. It varies is what I'm saying, right?

TORNABENE:
Well, yes and no. But that's actually also again driven by that solar cycle. Now that we're at the solar peak, it's yeah, every two to three weeks feels about right for our drag make up maneuvers. We fly Landsat 8 and 9 out the same Control Center with the same flight OPS team. So we're really kind of just ping ponging between the two. And once the solar cycle gets closer to that trough, what we'll see is something that feels more like once every two to three months, right? Landsat 8 launched on the ramp down from the previous solar cycle. So it was doing a lot, but when it was down in that trough around year six, yeah, we're doing burns every, I said before, two to three months.

ADAMSON:
But it's pretty amazing that it has that much of an effect on it, these solar cycles. Well, I think you and your team should win a safe driver award or something because you're driving these satellites so incredibly carefully. It's impressive how much goes into what you might consider these minor maneuvers, but it takes a lot of planning to get them done.

TORNABENE:
Yeah. Our biggest goal is keeping these satellites safe and healthy up there, right? Once you put it in space, you really can't go back and fix it. So we have to take care of these things. So we double and triple 
check our homework every time.

ADAMSON:
What is your elevator speech when somebody asks, So, Larry, what do you do? What do you tell them?

TORNABENE:
I fly satellites.

ADAMSON:
I’ll bet it's fun to answer that.

TORNABENE:
Yeah, I mean, I'll say I'm married now, back when I was dating, that's how I would actually answer that question. You've seen Apollo 13? You know that room full of, like, skinny tie guys? I’m Ed Harris. I have worked with people who worked with Gene Kranz, the Ed Harris character.

ADAMSON:
Nice. Thank you, Larry, for joining us for this episode of Eyes on Earth where we talked about Landsat 8’s 100th drag makeup maneuver. And honestly, we thank all of your team for their work because they’re important pieces in making sure that Landsat data is accurate and benefits the world in monitoring water use, land change, urban growth, natural disasters, and so many more benefits. Now I don't want you to be a distracted driver. It's probably best if you keep your hands on the wheel so we'll let you go here. And thank you to the listeners as well. Check out the USGS EROS social media accounts to watch for our newest episodes, and you can also subscribe to us on Apple and Google Podcasts. 

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