Eyes on Earth Episode 85 - Landsat 7 Extended Science Mission

TOM ADAMSON: 

Hello everyone, and welcome to another episode of Eyes on Earth. We're a podcast that 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, where we'll talk about what's going on with Landsat 7. Our guest today is Chris Crawford, USGS research physical scientist, USGS Landsat Project Scientist, co-chair of the Landsat Science Team and Landsat Science Data Acquisition Manager. Now Landsat 7 launched on April 15th, 1999. Its nominal science mission just ended back in April of this year. So, Chris, where is Landsat 7's orbit right now? How high up is it?

CHRIS CRAWFORD:

Thanks, Tom, for the opportunity to talk about Landsat 7 today. So back in late 2016, early 2017, the USGS realized that Landsat 7 was running low on fuel, and fuel is important for a Landsat observatory because it helps the observatory maintain its inclination and do drag makeup maneuvers over the course of its operational mission. And this is important to maintain its positional altitude as it acquires imagery. In August of 2017, USGS did one last flight operations maneuver to bring up Landsat 7 back to its final inclination. And then from that point on, the inclination began to drift. Now, it wasn't till roughly early 2020 that it started drifting outside its nominal mission requirement. Fast forward to April 6, 2022. A decision was made for Landsat 7 to exit its 705 kilometer altitude, or 438 miles above Earth. So we moved from 705 kilometers down to 697 kilometers. It took three different burn maneuvers, as they call it, to move to the lower orbit. And and that required fuel in order to do that, which is why back in 2017, the decision was made to do the final inclination maneuver in order to reserve enough fuel to be able to to move the Landsat 7 observatory out of its nominal orbit into a lower orbit. And so April 6 reflected the end of the nominal science mission. And then on May 5th, we resumed science imaging at the lower orbit at 697 kilometers in what we now call the Landsat 7 extended science mission.

ADAMSON:

One thing that's really important with Landsat is this consistency. And that's why you talk about these inclination maneuvers, make sure Landsat's at that consistent orbit. So is Landsat 7 data no longer consistent with Landsat 8 and 9, or is it still usable?

CRAWFORD:

When we moved out of the 705 world reference system orbit, which is our nominal ground track that both Landsat 8 and Landsat 9 are flying on today, they're both phased 180 degrees apart, which allows them to image every piece of earth every eight days. We moved Landsat 7 out of that 705 orbit, and Landsat 9 replaced Landsat 7 in that 705 orbit. And now we're flying at 697, but we're off the world reference system to orbit. And so it's continuing to image, but it's not imaging in a nominal way, meaning that you're not necessarily getting that regular repeat.

ADAMSON:

Yeah, Landsat 8 and 9 will take the same exact footprint with every orbit, so to speak, right?

CRAWFORD:

Correct.

ADAMSON:

And Landsat 7 - not doing that anymore?

CRAWFORD:

You can sort of think about Landsat 7 as wobbling back and forth. And currently what's unique about Landsat 7 at its lower orbit and continuing to image is that it's underflying Landsat 8 and Landsat 9. So it's not flying in the WRS-2 orbit, but it's flying 8 kilometers below in more or less the same orbit. But we're off that sort of nominal orbit that we maintain in order to adhere to the operational mission requirements. And so we're still getting data from Landsat 7, and we're underflying Landsat 8 and Landsat 9 on a regular basis as it moves back and forth under each observatory, which is a really neat thing about the Landsat 7's extended science mission.

ADAMSON:

Is the resolution still the same?

CRAWFORD:

The resolution is still the same. There's been quite a bit of work done in the Landsat calibration-validation team to verify the imaging geometry, to verify the imaging radiometry. And then the science portion of the project, we've done extensive analysis guaranteeing the quality of the products. And so at this point in Landsat 7's extended science mission, we have high confidence that the data is on par with what we were acquiring in imaging during its nominal mission.

ADAMSON:

Is the processing that happens here at EROS the same as it is for Landsats 8 and 9?

CRAWFORD:

Yes, the data is being processed as it was during the nominal science mission, and it's getting processed into Landsat Collection 2 right now.

ADAMSON:

Even though its equatorial crossing time is sort of regressing backwards and not in line with Landsat 8 and 9, that data can still be valuable.

CRAWFORD:

Yes, the data still can be valuable. And let me tell you why. So we operate in a sun-synchronous orbit. And so we are in a descending orbit, which means we start at the top of the poles, sort of in the Arctic, and we descend down over and cross over the equator. And then we continue down over and fly over Antarctica.

ADAMSON:

And it's always daytime during that descending part of the orbit, right?

CRAWFORD:

That's correct. It's always daytime during that descending orbit. And then on the ascending side, it's dark. And then Landsat 7 starts its orbit again over the poles, over the equator, back down to Antarctica. And so we cross over the equator, which is sort of used as the nominal mean local time. But we're still imaging with enough sunlight at the higher and mid-latitudes in order for the data to be still useful. And for me, it's really about the solar zenith angle, or the elevation of the sun. So the elevation of the sun obviously progresses between the Northern Hemisphere and the Southern Hemisphere. So it's overall getting darker as we image earlier and earlier in the day. But we acquire darker data at the poles all the time. And so we're already collecting sort of lower illuminated imagery at the poles. And so drifting to an earlier mean local crossing time, what it enables us to do is to actually acquire the same areas at a lower sun elevation than we would at the nominal science mission. The data is still high quality, and it just sets up a condition or a scenario where we can do intercomparisons between the effects of solar illumination on the data and then also how imaging is being conducted. So we're a fairly narrow swath imager plus or minus seven and a half degrees for a total field of view of 15 degrees.

ADAMSON:

The swath is 115 miles wide?

CRAWFORD:

Yes, or 185 kilometers. So this sets up a unique observational scenario to compare Landsat 7 with Landsat 8, and then also compare Landsat 7 data in the lower in a lower orbit extended mission with data that was acquired in the nominal science mission. And so we begin to understand the importance for things like bidirectional reflectance distribution or solar illumination corrections, things that really impact the observations. And so this is sort of one of the unique aspects of the mission. And then also operating three Landsats at one time is something that we've never done in the mission history. And so this is also another unique aspect of this extended science mission.

ADAMSON:

We've never had three Landsats operating at the same time before like we are doing right now?

CRAWFORD:

Yeah, conducting what I would say science imaging at once.

ADAMSON:

Well, the more data, the better.

CRAWFORD:

That's right. Yeah. More data, the better. And one of the unique aspects is that because we have three observatories, we're now collecting data, and we can begin to evaluate the timing of imaging and whether there's added value up there from an observational frequency perspective. So you can imagine a scenario where Landsat 7 is flying over, imaging a particular area, and then Landsat 8 is coming over an hour and 15 or an hour and 20 minutes later imaging the same area. And so because we know these observatories so well - we know their calibration, we know the data quality, we process it consistently - we can really use this data to understand in much greater detail the quality of the data so the quality assurance information, whether the clouds have changed.

ADAMSON:

Do we happen to know how many images Landsat 7 is acquiring every day?

CRAWFORD:

So we are acquiring up to 450 images per day in the extended science mission. And this is actually not much of a departure from what we were doing during the nominal science mission.

ADAMSON:

Same as always then.

CRAWFORD:

Same as always, pretty much. And so, you know, one of the things that we spent a lot of time doing in the weeks and months leading up to the decision to continue the extended science mission, is that the Operations Team and the planning teams, Flight Operations did a lot of work to understand if we could continue to image in a sort of full capacity mode during this extended science mission. So we're still imaging roughly 80% of the Earth's land surface area.

ADAMSON:

OK.

CRAWFORD:

Including coastal zones.

ADAMSON:

OK.

CRAWFORD:

And islands. And corals.

ADAMSON:

Well, there's still a lot that it's doing.

CRAWFORD:

It's been doing a lot. We've seen some very interesting samples during its extended science mission, being able to capture wildfires or fire fronts. And then also some of the hurricane impacts from Hurricane Ian down in Florida a couple of months ago. Landsat 7 got one of the first images after the hurricane cleared.

ADAMSON:

It still has the data gaps, right?

CRAWFORD:

It still has the data gaps. Can't get around the scanline corrector issue.

ADAMSON:

That scan line corrector issue that happened back in 2003. We lose, is it something like 22% of the data? There's these black stripes running through the data?

CRAWFORD:

Yeah, right. There's some black stripes running through that.

ADAMSON:

But the data that is recorded is the same high quality as we would expect.

CRAWFORD:

It is the same high quality. And I'd like to add that another aspect to Landsat observations is we oftentimes think about the reflectance data that we get from Landsat 7, but it's also carrying a thermal infrared capability on board as well. That's at 60 meter resolution. That's the finest spatial resolution that we've ever collected with thermal observations. So, for example, Landsat 8 TIRS and Landsat 9 TIRS collect data at 100 meters spatial resolution. So Landsat 7's sort of 60 meter thermal data is another strong element to the observations. And Landsat 7 has been a remarkably stable instrument over its lifetime.

ADAMSON:

How much longer can Landsat 7 go on like this?

CRAWFORD: 

The plan is to continue to image. Really, the only thing that's changing is the mean local crossing time. And so that's, you know, getting earlier I think today here, maybe towards the end of September and October, we were somewhere in the area of 8:35 mean local time at the equator.

ADAMSON:

And nominally it's what time?

CRAWFORD:

Nominally, Landsat 7 was 10 a.m. plus or minus 15 minutes. So we could wobble between 10 a.m. within a 15-minute envelope and still be within our mission requirements. So as I was speaking about earlier, is that at some point, I think it was in 2020, we started drifting outside that sort of plus or  minus 15 degree that centered on 10 a.m. But we've done analysis to show that we can continue to image up through 8 a.m., and that would result in continuing observations up through July of 2023, which gets us into the next Northern Hemisphere growing season. And so the decision was made to just do periodic evaluation from the scientific viewpoint, from the calibration and validation viewpoint, and then also the operations team would continue to monitor the observatory, and that we would we would periodically reevaluate whether we can continue without degrading the quality of the data or creating sort of a risk that would need to be mitigated. And so one of the things that that we sort of used, at least from the scientific viewpoint, back in 2000, right after Landsat 7 got launched, NASA launched the NASA Earth Observer 1 mission, which carried the Advanced Land Imager, as well as the first imaging spectrometer, spaceborne imaging spectrometer, known as the Hyperion Instrument. It flew alongside with Landsat 7 for early in its lifetime, early in the 2000s. And then its orbit began to drift after 2011. And so we've referred to the NASA Earth Observer 1 mission, which ended up flying all the way to 8 a.m. mean local time at the equator, as a pathfinder for understanding how we might operate Landsat 7 in an extended science mission mode. And we've found that to be very helpful.

ADAMSON:

You have something of a playbook to work with as you as you evaluate the Landsat 7 data.

CRAWFORD:

A little bit, a little bit. Back in 2015, prior to coming to EROS, I was working at NASA Goddard Space Flight Center, and I worked quite a bit with the Earth Observer 1 team there and got to understand a little bit more about that observatory. And so they've published a couple of good papers back in 2016, 2017 on the extended science mission for Earth Observer 1 on the value, and so we sort of used that to continue Landsat 7. We have a lot of confidence that the data can be used in sort of a nominal context up through 2020. When we get into 2021 and now 2022, and then, you know, as we move into 2023, we probably ought to employ a little bit more caution in terms of how we're using the data. But that's what we're hoping that the user community will sort of uptake this data and do some interesting analysis, and maybe we might learn something that we didn't know about Landsat 7, or, given the fact that it's flying underneath Landsat 8 and Landsat 9 right now, gives us a unique opportunity, a unique snapshot of Earth imagery. And I think that as we move forward, the USGS will determine how to best incorporate this extended science mission into that sort of long term collection based archive. That being said, we are processing the data into the Collection 2 products because that's our current specification for data product generation. But this is sort of in a continual evaluation, and we're hoping that the user community can help us understand the quality of the data during the extended science mission.

ADAMSON:

Is there anything else that you think we need to add?

CRAWFORD:

So I started using Landsat 7 data in 2005, when the USGS was still distributing the data on CD-ROMs. I was working on a project looking at the conversion of rural landscapes to impervious surfaces in the Greater Charlotte region, North Carolina, at that time. You know, Landsat 7 has a very special place with me in the fact that when I was doing my Ph.D. research at the University of Minnesota, I started at a time in 2008 when they opened up the Landsat archive for free and open access. And then I finished my graduate work in 2013, just right after the launch of Landsat 8. So when I was using Landsat 7 data, it was before we had collections, and it's before we had Landsat 8. So Landsat 7 and Landsat 5, when I was starting to use Landsat data, were the only game in town. And so since that time, you know, Landsat 7 then was operating with Landsat 8, and then we've now replaced Landsat 7 with Landsat 9, and we're now in this extended science mission. And these experiences with Landsat is, I've kind of grown up along with Landsat 7 as it's ended, as it works towards its end of its mission. And so I felt very strongly that the USGS should continue to image because there was a lot of value potentially there in flying at a lower orbit and acquiring data while it was underflying Landsat 8 and Landsat 9. And I'm very excited about what we've been able to achieve so far.

ADAMSON:

I'd like to thank Chris for updating us on the status of Landsat 7 and the extended science mission and the data that the user community can continue to use with Landsat 7. And thank you, listeners, for joining us on Eyes on Earth. You can find all our shows on the USGS EROS website. You can also follow EROS on Facebook or Twitter to find the latest episodes or to subscribe on Apple or Google Podcasts.

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