3.5 Tiny Bytes: Landsat vs Sentinel for NDVI and Variable-Rate Maps

We take a practical trip into satellite imagery and explain how Landsat and Sentinel support real agronomy work like NDVI analysis and variable rate mapping. We break down what makes these datasets reliable, how resolution changes what you can see, and why revisit timing matters when clouds get in the way. 

In this episode, we tackle:

• free government-operated imagery through NASA USGS and the European Space Agency 
• using multi-band satellite data to calculate NDVI for crop vigor mapping 
• why sun-synchronous orbits create consistent lighting and shadows 
• resolution explained as ground area per pixel and why 10 m shows more detail than 30 m 
• Landsat program timeline and the Landsat 7 data gap problem 
• constellations as a backup plan and a way to increase image frequency 
• Sentinel revisit cadence vs Landsat revisit cadence and what that means during the season 

Tune in next time for a tiny bite of knowledge from GK Technology, where we have a map and an app for that. 

Related episodes: 

• S1 20.5 Tiny Bytes: Choosing Quality Imagery https://www.buzzsprout.com/2297340/episodes/16135793
• S2 1.5 Tiny Bytes: Check Out GK Technology's Imagery Library https://www.buzzsprout.com/2297340/episodes/16524859

GK Technology Info on obtaining satellite imagery: https://gktechinc.com/data-links/

https://gktechinc.com/

Transcript

Tiny Bytes Takes On Satellites

Sarah 0:00

And now it's time for a tiny bite of knowledge.

Jodi 0:14

On this episode of Tiny Bytes, Sarah and I are heading out of this world. So buckle up because we're talking satellite imagery.

Sarah 0:26

It's all about Landsat and Sentinel imagery today. Here at GK Technology, we actually use a lot of imagery, satellite imagery for a lot of different reasons. A lot of NDVI data, which is a vegetative index. You can listen to other tiny bite episodes about that. But today we're going to actually compare and contrast a little bit of the satellites that we use the most, which are Landsat and Sentinel.

Jodi 0:56

So both of these satellite imagery sources are both government operated. So Landsat is owned and operated by the USA through NASA and the USGS. And then Sentinel is another satellite imagery provider that's operated by the European Union and the European Space Agency. So because of this, we get access to free imagery through both of these. Did you say free?

Sarah 1:26

There's free, free imagery for everyone. Yes, you could even make your own library of uh your own satellite imagery library from these data sources if you want. Here at GK, we actually do have a huge library, vast, from both of these data sources on our servers, um, ready to go. And if that is something that you would choose to use, we could certainly help you connect, connect you with that opportunity.

Jodi 1:54

Absolutely.

Why Agronomists Use NDVI Imagery

Jodi 1:55

And before we get into the details of like what are the big differences between this Sentinel and this Landsat, I do want to talk a little bit about why satellite imagery is such a great tool for use as an agronomist and as somebody making variable rate maps. So to Sarah touched on NDVI before, with the satellite imagery, the images that are collected, we're able to get multiple bands that help us make the calculation for NDVI. And this is a super cool thing about it. So A, both Landsat and Sentinel, their satellites cover the same place on Earth multiple times throughout the year. So it's not just once, it's not just twice. They're going through multiple times during the season. But here's the really, really big thing. What they do is they go in a sun-synchronous orbit. And what that means is that in North Dakota, Sentinel will typically take pictures at about 10:30 a.m. and Landsat 8 will take pictures at about 10 a.m. And meaning that in the mid-morning, they're taking photos that are early enough that A, clouds haven't fully developed and that like on those thunderstorm days, and B, because they're collecting those pictures at the same time when they're going across an area, they're able to get consistent shadows. So you don't have to worry about as you're putting multiple layers of imagery together, that you're gonna have shadows coming from different angles from different tree rows that you'll have to think about when you're putting images together. You just having that sun-synchronous imagery from these satellites is so crucial and takes another piece out of any sort of variability that might come with imagery. So super cool.

Sarah 3:38

It is important to remember with sun synchronous imagery that you know it does matter what time of the year we're actually taking these pictures because the season matters, right? In North Dakota, at uh we our sun rises and sets at different times of the year. If you have to work from sunup to sundown in June, those are long days because you're talking about like four o'clock in the morning until about 10 o'clock at night. Whereas in uh December, on the shortest day of the year, we have much less sun. So the time of the year makes a difference, but within that, within that time of the year in a specific spot, you're getting that imagery at the same time. So at the same time of the day. So that that sun-synchronous thing really is a big deal. It's a it's important to remember a couple of those things though.

Resolution Basics Landsat Vs Sentinel

Jodi 4:27

Talking about these two imagery types, one of the big differences I think about using Landsat versus Sentinel is just the resolution. And so when we're talking about resolution of imagery, what we mean by that is how uh basically how much area each pixel, quote unquote, of the image represents. And with with Landsat, it's a low resolution. So meaning that each of the pixels is 30 meters, meaning that it's you're not getting as much detail. But with Sentinel, what we're getting is a 10 meter resolution image. So we're able to get more detail. And remember, 10 meters, that's 30 feet. That's probably the width of or smaller than the width of most planters nowadays. Just that difference in resolution size is a big driver of when I'm looking at satellite imagery. A lot of the times, most of the time, I'm I'm choosing Sentinel. Um, but that doesn't mean that Landset doesn't have a use either.

Sarah 5:23

So within that, um, a pixel uh is a picture that we see on a computer is made up of pixels. It's little boxes, um, if you will, or little squares that have uh their own color assigned to them. And the more pixels you have, the more detailed your image is. So if you have one pixel represented by 30 meters of area on the ground, that has less detail than one pixel with 10 meters represented on the ground. That's kind of a resolution is kind of a difficult concept to wrap your head around. Maybe we should just have a whole podcast about that one day.

Jodi 6:04

That's actually a really good idea. Think about the first camera you ever had on your cell phone, right? Maybe a flip phone, you had a little camera on it, and now you look back at the pictures and say, man, why does it look like I was taking photos with a potato? You know, that's that's a low resolution image. And now, like with most iPhones, Androids, the cameras on them are so amazing. And that big difference is just resolution quality. So older phones, low resolution, newer phones, higher resolution, better images, more data per pixel. And that's what we're talking about with the 30-meter resolution with Landsat versus 10 meter for Sentinel. 10 meter, it's a lower number, but it's higher resolution and gets us some good data, better data.

Program History And Data Gaps

Sarah 6:49

So when I'm thinking about Landsat versus Sentinel, one of the things that I think about is when the satellites started flying and how the age of the actual programs. So the United States of America has a very old satellite program. Landsat 5 started flying in 1984. Now, the actual Landsat program was actually started back in the 1970s, but the Landsat 5, which is really the first satellite that we have access to, the imagery from it, started in 1984. And yes, on our servers, we actually have data that goes all the way back to 1984, and that's what it's from, Landsat 5. That satellite flew until 2013. That had a very long mission, a very long life, satellite uh Landsat 5. Landsat 7 was the next satellite that flew, which flew from 1999 until 2024, which was fine. Except someplace in that time frame, there was it was in 2003. In 2003, there's actually something that happened to that satellite. And so there's actually like lines going across the scenes. And if your field ended up underneath that line, you had big, huge gaps in the data. As a matter of fact, 22% of Landsat 7 scenes were missing because of that. So sometimes we had good imagery if you weren't underneath that, and sometimes you didn't. And as I've been making maps, I have actually used some Landsat 7 data, and then there's been fields that it just doesn't work. Landsat 8 was launched in 2013. It's still up and flying, and Landsat 9 was launched in 2021. So the Landsat program, very old. Sentinel was started in 2015, is when this was Sentinel 2A was launched. Sentinel actually has a series of like six um different missions up there, and each mission has at least two satellite constellations at a time. And satellite two or Sentinel 2 is the one that we use the most in in agriculture right now, and what we have our our data on the uh on our servers. 2A was launched in 2015, like I said, and 2B was launched in 2017. And so those are the baseline operations. The age of the program is one of the things that's different between these. Landsat is very old, and Sentinel is a lot newer. Landsat 9 was launched in 2021. There it is, there, and now is actually operating in constellation with eight. So um, which is which is a new thing, that constellation concept. And we can we can talk about that in a second. Landsat 8 has its mission slated to end in 2028. And one of the things that's happening with these programs is they're trying to get satellites launched before something actually goes wrong with them. I'm wondering, and I don't have any information on this, but I'm wondering if what happened with Landsat 7 and the lines in the data, I'm wondering if that created enough challenges that we wanted to make sure that we were still logging data, even if we couldn't, even if the constellation had to be interrupted so that we only ended up with one satellite logging at a time if something were to happen to the to the other one. There is actually another Landsat satellite slated to fly again. Um, and that's going to be called Landsat next mission.

Jodi 10:36

Ooh, fancy. Dun dun dun.

Constellations And Revisit Frequency

Jodi 10:41

And I think it's important to point out and just talk for a brief second about constellation. So having these two paired satellites together is what we consider a constellation. And it sounds like the goal of this is to um reduce any possibilities of one of them failing and not getting data. So there'll always be coverage from one of them in the Landsat constellation and the Sentinel constellation. There'll be at least one operating at all time to avoid issues that were presented with like Landsat 7 and that streak across.

Sarah 11:14

And in addition to that, what this constellation setup allows for us to also have is more imagery on a whole. So the Sentinel satellites revisit any given location every 10 days with a satellite on the opposite side of the earth. So they're offset by 180 degrees, opposite side of the earth, which means that once you have that constellation running and each satellite is logging every 10 days, you get to double how much imagery you're getting in that spot. So now you're revisiting every five days, and that's a really big deal. So we're getting more imagery. You have to remember that because of cloud cover, not all of that imagery that gets logged is going to be good. And so we increase our frequency of having good usable data because we're effectively doubling our imagery um gathering. And like Jodi said, it gives us a backup plan so that if something happens to one of the satellites, we're still getting data.

Jodi 12:21

And Sarah, what's so you mentioned that in Sentinel the average revisit is about five days between imagery. What's that like for Landsat? Landsat is eight days.

Sarah 12:36

So Landsat 8 and Landsat 9 are constellation together. And so they're doing a 16-day revisit. The Landsat ones are doing a 16-day revisit, which means in a constellation, then you're getting between the two of them, then either Landsat 8 or 9 is logging data every eight days. So effectively, what's happening, for example, with Landsat is when we only had Landsat 8, we were getting about 13 overpasses a year for any given location. But because we we have that second satellite up there operating in a constellation, we now are increasing those overpasses up to 26 per year with the Landsat. Pretty cool,

Big Takeaways And Sign Off

Sarah 13:22

huh?

Jodi 13:22

I'm glad we could take this trip up into space. And the thing that I love beyond the fact that this imagery is free and available for the public is the fact that I can learn about space as an agronomist, something I never thought that I would ever have to do.

Sarah 13:36

Imagine if our grandfathers knew that we had we were talking about farming and using satellites to farm something from space. Wow. Tune in next time for a tiny bite of knowledge from GK Technology, where we have a map and an app for that.