Eyes on Earth Episode 26 – Satellite Constellations

STEVE YOUNG:

Hello everyone. Welcome to this episode of Eyes on Earth. 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. Iím your host, Steve Young.

We have a fascinating discussion today about satellite constellations, which basically are a group of satellites working together as a system.† Satellite constellations can be used to provide television service, mobile communication, or even the ubiquitous GPS that helps us navigate our cars and track our exercise.†Constellations also play a part in studying the Earth through satellite remote sensing. 

Our current Landsat satellites, Landsat 7 and Landsat 8, operate as a system providing the same measurements over the same piece of land every 8 days.†Now individually, by themselves, each one sees the same piece of land every 16 days.

But there are also satellites in orbit that resemble trains following each other closely in the same path.†438 miles above Earth, these series of satellites line up on the same orbital track and chase each other at some 17,000 miles per hour-plus.† NASAís Earth Observing System, or (EOS), is an afternoon constellation thatís an example of that.

Here to talk about these constellations is Jim Lacasse, the USGS Landsat mission operations project manager based out at the Earth Resources Observation and Science Center near Sioux Falls, SD. Jim has worked with Landsat in various roles since 2003, so he knows this stuff.

Welcome, Jim.

JIM LACASSE:

Hello, Steve.

YOUNG:

So, we have two Landsat satellites orbiting the Earth up at 438 miles above the surface of the planet. What makes that distance, that 438 miles, so special?

LACASSE: 

Well, thereís a few aspects to really answer that question. You know, we have to look at what the satellites are trying to do. When you look at the altitude, you have to look at the resolution of the images. Thereís a direct relationship between the altitude of the spacecraft and the resolving power of the instruments that youíre using that determines how small of an area that can be imaged. Other characteristics of the instruments on the satellites determine how broad a range can be imaged. The combination of those things for Landsat means that they image areas on the ground that are about 185 kilometers, or 115 miles wide, with the ability to pick out features on Earth with dimensions of about 30-by-30 meters, which is about the size of a baseball infield.

YOUNG: 

So, the two Landsat satellites are at 438 miles. There are other satellites in that same orbit?

LACASSE: 

Well, Iím aware of about 10. The majority of them are NASA, and other international NASA partners. I also found one ... the Peruvian Space Agency has one up there.

YOUNG: 

And why are they all sharing the same space with us?

LACASSE: 

They all have similar requirements in terms of Earth remote sensing. You know, not only looking at the land, but some of them are also looking at the atmosphere and the ocean.

YOUNG: 

Apparently they want to capture kind of the same broad-scale resolution of the Earth at that altitude?

LACASSE: 

Exactly.

YOUNG: 

How close are they to each other? I mean, are they right on each otherís bumpers, or ?

LACASSE: 

Well, for the A Train, the afternoon constellation that we mentioned, theyíre as close to 40 seconds to 400 seconds is the range.

YOUNG: 

Do they need to be that close?

LACASSE: 

Yes, theyíre trying to get concurrent observations of different phenomena. If you fly too far apart, things will actually change in time. So, you want to have those measurements as close as possible together to remove the effects of change.

YOUNG: 

Well, I think one of the things that weíre interested in is sun angle, and how the sun illuminates the land below. So, I mean, the closer satellites are together, the more likely theyíll capture similar images with similar sun angles, right?

LACASSE: 

That is true.

YOUNG: 

Yeah, how many (satellites) are at the same altitude as Landsat, and who do they belong to? Do they all follow in ... one right after the other, or are there gaps between those groups of satellites?

LACASSE: 

Besides Landsat 7 and Landsat 8, thereís also NASA Earth Science Missions operating at that level, and youíve referred to before, the afternoon constellation, which is known by the A-Train. That actually was designed as a constellation and launched starting back in the late 1990s. Within that constellation, the whole purpose was that there would be synergy gained by flying the satellites in close proximity to each other. Within that, there were six satellites, the first one being Aqua that was launched, followed by Aura. There was a French mission called Parasol, which included lidar (Light Detection and Ranging) for detecting atmospheric properties. CloudSat followed that, and then a joint French mission called Calipso, that was another lidar mission to measure the role that clouds and atmosphere play in regulating the Earthís weather. Following that, NASA worked with the Japanese Exploration Agency, JAXA, to launch a satellite called GCOM-Weather1, which was a global change observation mission studying the circulation of water in the atmosphere. Parasol exited the train in 2013. And then the final satellite in the constellation is called OCO, the Orbiting Carbon Observatory, and that was to quantify sources in sinks of carbon dioxide at regional scales.

YOUNG: 

So those all are in the A-Train?

LACASSE: 

Yes, yup. And those all, just based on the science parameters they were looking at, have an equatorial crossing time, each time they pass over the Equator, about 1:30 in the afternoon.

YOUNG: 

And so, how many are in the morning train, and does the morning train have a name, or is it ...?

LACASSE: 

Yeah, the morning train doesnít follow one after the other. Theyíre rather in the same altitude, but theyíre offset such as they actually cross the same path on Earth on different days. All these satellites, even if theyíre not following each other in a train, they do intersect at the poles regularly.

YOUNG: 

It sounds to me like youíre saying that they arenít necessarily all capturing the same data. They all have different things they want to acquire as far as information, right?

LACASSE: 

Exactly, yes. Theyíre different sensors with different wavelengths for different purpose. Some are capturing their reflected light off the surface of the Earth, or long wavelength temperatures, or theyíre sounding for particles in the atmosphere column below them. Some of them can even measure wave heights out on the open ocean.

YOUNG: 

Even though what theyíre getting is different, I mean collectively, all that information, all that data thatís being gathered, does that provide a picture of whatís happening on the Earthís surface, or happening in the atmosphere?

LACASSE: 

Exactly. And thatís the chief goal of all this data collection. And over time, itís really matured to where the systems are being designed to do exactly that. In the case of the A-Train, those instruments, when they put them together, the data was meant to be included in the algorithms of the other instruments. And thatís used in a modeling of the Earthís processes. Those models are quite complex and have a lot of data inputs. In the case of the A-Train, that was a specific design. But even in the case of Landsat, data from other remote sensing satellites is used. Right now, we have data that indicates the energy received at the sensor in space. What we really want are the measurements on the Earth. But since weíre looking through the atmosphere, we want to be able to remove those effects. And the USGS is developing products to do that right now. Some of those inputs we get from other satellites.

YOUNG: 

So, other satellites have given us information about whatís in the atmosphere that we can use to correct and remove that when the time comes.

LACASSE: 

Thatís correct.

YOUNG: 

I think itís well known that thereís a lot of space junk up there, and every now and then satellites have to maneuver out of the way. If Landsat has to get out of the way of some space junk, do we in any way communicate with the other systems to let them know, or is everybody on their own when it comes to worrying about what might be in their path?

LACASSE: 

Yeah, each satellite, each operator is concerned with whatís in their path. We do get common information from the Air Force, whose charged with monitoring space debris, and the orbit of all satellites, and then providing notification to satellite operators when a conjunction is predicted.

YOUNG: 

They would provide that information not only to us, but share it with other agencies, other countries?

LACASSE: 

Yes, the Air Force provides notifications to satellite operators.

YOUNG: 

Itís up to each operator to make a determination of what they want to do about it?

LACASSE: 

They tell us when a predicted conjunction is going to happen, the probability of collision, and basically the quality of data that went into calculating those numbers. And then the operator has to make a decision based on that. And it can be a difficult decision. The satellites only have a limited amount of propellant on board. Every maneuver reduces that propellant and basically takes away from the length of the mission. You canít maneuver for every predicted collision. It is a very difficult decision sometimes.

YOUNG: 

Sure, let me ask you as a last question, Jim, we have all these different systems orbiting at the same altitude. Do we ever sit down with each other, get together, meet somewhere and talk about things like, what the mission is weíre trying to accomplish? How long weíre going to be up? When weíre going to move out of that orbit? I mean, are there conversations about?

LACASSE: 

Yeah, we do regularly meet with NASA, who has both the A-Train and flies some other Earth Science missions. And we meet two times a year. Since weíre operating at the same altitude, we have to basically be good citizens. If we do something to be stuck in orbit and not being able to maneuver, we affect all the rest of the people in that orbit. Things like on-orbit anomalies that could potentially leave us at 705 (kilometers) without the ability to move, or decommissioning in terms of when weíre planning on getting out of that orbit. Those are critical items that we need to share and stay abreast of as a group.

This is a timely conversation in that there are, as people know, more and more satellites being launched. And the commercial industry is getting much more involved. Youíll hear stories of some rather large constellations being launched. One of the big ones ... I wonít give any names ... but to provide Internet around the world. And those constellations will add not only to the capabilities that weíll have here on Earth, but theyíre going to add to the congestion that we as space operators have. So, itís going to be more and more important that we stay aware of whatís being launched and keep open communications so that Earth remote sensing can continue as it has for decades now.

YOUNG: 

We have to be good neighbors, is what youíre saying?

LACASSE: 

Exactly.

YOUNG:

Weíve been talking to Jim Lacasse, the USGS Landsat mission operations project manager, about orbital constellations, what they are, how they are comprised, and how they might benefit humankind.

Thanks for joining us Jim

LACASSE: 

Yeah, certainly Steve.

YOUNG:

We hope you come back for the next episode of Eyes on Earth. This podcast is a product of the U.S. Geological Survey, Department of the Interior. Thanks for joining