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22.5 Tiny Bytes: GNSS? GPS? What's the Difference?

• A Podcast for Precision Agriculture Geeks • Season 1 • Episode 22

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Our latest episode of Tiny Bytes dives into the fascinating world of satellite navigation. Discover the differences between GPS and GNSS, learn how satellite triangulation ensures precise positioning, and explore the transformative role of differential systems like WAAS and RTK in precision agriculture. With practical insights into multi-system receivers and the science behind satellite signals, this episode equips you with knowledge to navigate both the tech and trivia of satellite systems. Perfect for enthusiasts and professionals alike, tune in to elevate your understanding of the technology guiding our world! 

GK Technology, Inc.: https://gktechinc.com/

Jodi:

And now it's time for a Tiny Byte of knowledge. You know it's been really weird about 2024, Sarah, I don't, but I'm sure you're going to tell me. I feel like there's been so many Northern Lights this year, and every morning I'll open up Facebook and another person posts a picture of beautiful northern lights from that morning that I missed yet again.

Sarah:

Absolutely beautiful. Unfortunately, this also results in many phone calls complaining about wonky GPS signals. Have you ever thought about exactly what GPS is?

Jodi:

Yes and no, but I feel like we're going to talk about it.

Sarah:

I think we should. So what is GPS? Well, since you asked, the words that stand for GPS actually are global positioning system and, ironically, while we oftentimes misuse that term to indicate just overall guidance or use it incorrectly to identify what precision agriculture is, it really is actually a network of satellites that's owned by the United States Defense Department. It isn't the overall generic term for guidance or precision agriculture. So what is that generic term talking about satellite navigation systems?

Jodi:

Yeah, so that term is actually Global Navigation Satellite System or GNSS, and actually this has been probably the most enlightening part of researching this episode and getting prepared for this episode is really understanding that GPS, that terminology. That GPS, that terminology isn't just the technology, it's just one of the satellite constellations that give out and help us figure out exactly where we are on this earth. So, for example, gps is analogous to GLONASS from Russia and BeiDou, and Galileo, that was awesome.

Sarah:

Great Minds think alike.

Jodi:

Yeah, yeah. So like, even though, like we often refer to GPS as being something or a technology that helps us figure out exactly where we are on the earth, what we're actually referring to most of the time when we say GPS is actually GNSS, that global Navigation Satellite System.

Sarah:

That is the generic term for satellite navigation systems. So again, just to recap that a lot of countries own their own GNSS systems. China owns BeiDou, the United States owns GPS, Russia owns GLONASS, Japan owns Quasi-Zenith and the European Union owns Galileo.

Jodi:

Sarah, did you try to pronounce GLONASS?

Sarah:

I did not, but I'd be willing to give it a go. Or do you want to give it a go?

Jodi:

Is it Globaln aya Navigazionnaya Sputnikovaya Sistema?

Sarah:

I have no idea. Global Naya Navigacionaya Spnik. Nope Can't do it. Gesundheit.

Jodi:

Gesundheit, and this is why we call it GLONASS.

Sarah:

But what's important to know is that the two systems that we know the most about and that get used the most conventionally are probably GLONASS and GPS, and they are the only two fully operational GNSS systems at this point in time, and even though they are owned by different governments. Oftentimes higher quality GNSS receivers will be able to use satellites from multiple systems, so we can oftentimes use both the GLONASS satellites as well as the GPS satellites in our agricultural practice.

Jodi:

Let's get into a little bit more about the science behind global navigation systems and talk about and understand more about why it might be helpful to have more satellites to get a more accurate signal and a more accurate location.

Sarah:

Yeah, so a global navigation satellite system. It works where each satellite transmits a radio signal carrying digital information and, generally speaking, that digital information has two key pieces of information that it's broadcasting Ephemeris, ephemeris. So ephemeris, it's the satellite's orbital location, that's what that one is, ugh Gesundheit again and the timing code. So the two pieces of information transmitted are ephemeris and the timing code. Thank you, Jodi, I really appreciate your help there. So ephemeris is the satellite's orbital location and the timing code is used to calculate the distance between the satellite and the receiver. So when that signal is sent, it is timed how long it takes for the receiver to receive that signal and that helps us determine the distance. When we use triangulation between multiple satellites and the receiver, we are able to identify the location of the receiver.

Jodi:

Yeah. So, like every time you turn on a GPS system, or like what you would call a GPS system, you really your DNS system. What it's going to do is it's going to start receiving those satellites or start the triangulation process. Ideally, what you need is four satellites to accurately figure out where on earth you are. If, for whatever reason, if you're only using one constellation or more of the satellites are behind the area of what your GPS receiver can receive, you might only be able to pick up three of those satellites, and then you'll probably get a warning signal that you're only getting 2D position. And so, ideally, to get the best triangulation, get the most accurate location of where you are on the Earth, we'll need four satellites to give us the ephemeris and then that timing code to really calculate where on Earth we are with our receiver.

Sarah:

Another way that we can improve our accuracy is by using differential signal, which is actually quite normal, and the concept behind a differential. The concept behind a differential signal is that there is a base station on the ground someplace and that base station's location is precisely known, and so it will receive information from the satellite, which is obviously mobile in the orbit. And because it receives that data and the base station is stable, it can back-calculate any errors that might occur and communicate that to the rover receiver, which rover means the mobile receiver, like what you have in your car or in your tractor. That's the receiver that's moving.

Sarah:

Probably the most common correction system for a differential GNSS system that gets used in the United States and probably Canada is the WAAS system, the Wide Area Augmentation System, and what that is. Waas is made up of 38 wide area reference stations. These are base stations that have known locations, precisely known locations, and because of their precisely known locations they are able to determine differences in the orbital location of the satellites as well as the timing code that might be coming down to that base station, communicate that with a number of master stations, and those master stations actually generate out a correction code for any differences that might be occurring every second and because of that we are able to have widely available differential GNSS guidance. And there are actually 38 wide area reference stations, so 38 of those base stations that we think about for the WAAS signals, and they are located throughout the United States, some in Canada, some in Mexico, mostly throughout North America. So, Jodi, we're sitting here in Halstad, Minnesota. What are the closest wide area reference stations for the WAAS system to?

Jodi:

San Juan, Puerto Rico. Just kidding, that's not the right answer. There's actually a correction signal station in Winnipeg, Manitoba, and then also in Billings, Montana.

Sarah:

So we actually are pretty close.

Jodi:

But there are better improvements beyond just WAAS correction.

Jodi:

As all of you have probably experienced using a GNSS system in your life, the WAAS correction is pretty common right?

Jodi:

It's what you use in your phone, your phone's GPS setup really basic satellite receivers.

Jodi:

They're probably going to use the WAAS correction before they start asking for payment for a better correction, when you start getting into thinking about, okay, I want more accurate location for what I'm going and doing outside in the field, and especially if you're working on drainage or you really need elevation data, it's really important that you have something more than just the WAAS correction or the WAAS differential signal to determine your location.

Jodi:

And that's where the concept of RTK comes in. And what's really great about RTK is that typically in an RTK system, you're going to have a base station that's located a lot closer to you than just the known one of the 38 known reference stations in the WAAS system, and so ideally, especially if you're working on elevation, you're going to want to have an RTK base station within at least one mile of you, and so that's going to really help to improve accuracy of your signal and get you the best elevation data and location data for what you're doing out there in the field and again it's working on that same concept that we're working with with WASP. RTK base station has a known location.

Sarah:

We know exactly where it is on the ground. The RTK base station is the base station, so the whole purpose of it is to do the same kind of corrections just more exactly. But they do have improved algorithms and techniques within there, so they have very much so improved mathematics.

Jodi:

So one thing with an RTK base station, instead of having to rely on a reference station, like you do in WAAS, that might be hundreds of miles away, you can purchase an RTK base station and have that correction closer to you in the field that you're working on. And there are other benefits of RTK, too, that I'll let Sarah talk about.

Sarah:

So when you have that RTK-based station, the whole concept of RTK it is a differential GNSS system. That's what RTK is. But the difference between a normal differential global satellite navigation satellite system, an RTK, is that there are upgraded algorithms and mathematics and calculations and techniques that get used for making those corrections from satellite errors so that you have a more precise location. An RTK does get the accuracy down to like centimeters or inches rather than a couple of feet that you would normally see from a WAAS signal, and that's really important, especially when you're thinking about something like drainage.

Sarah:

In drainage we really need to make sure that we've got our elevation information accurate and if you're in the Red River Valley of North Dakota, northern Minnesota, like we are right now, where there isn't a whole lot of changes in elevation, you really need to make sure that you've got extremely accurate information. When you get farther away from your RTK base station, you can keep your guidance driving straight for quite a few miles away from your base station, but your elevation accuracy will drop off quicker the further away you go. In other words, you'll be able to drive straight down the field and stay on your row farther away from your base station as compared to when you're trying to do elevation work with your RTK. Ideally, when you're doing drainage work, you want to make sure that you've got your base station within a mile of your tractor. When you're doing drainage work, the accuracy for the up and down will degrade faster the further away that you get from the base station, as opposed to your ability to drive straight.

Jodi:

As you get farther away from RTK base station, it's going to be harder to get an accurate up and down value compared to the distance value or the compared to the left and right, left and right.

Sarah:

Yeah, it is.

Jodi:

Directions are really hard. That's why we have GNSS.

Sarah:

Directions are hard and GNSS is amazing. I feel like there's a lot of S's in there.

Jodi:

The whole concept of satellites and all these different global positioning systems et cetera. It's a whole word salad, it's an alphabet soup. It's really what it is. There are so many different acronyms and terms to describe how we're transmitting radio signals and then taking these signals and these codes and these data and translating it into our position on this earth.

Sarah:

It's a lot of letters, it's a lot of acronyms, but it's a really, really awesome technology that allows us to accomplish a lot in agriculture and especially in the precision ag world, I think when I'm done with my career, I'll look back and try to analyze what one of the greatest technological advances were in agriculture in my lifetime and I'm pretty sure that global navigation satellite systems, GNSS, will definitely be one of them and the use of that in agriculture and all of the opportunities that's made available for us, absolutely.

Jodi:

And I know we've had the conversation today that GPS and GNSS are not the same terms and that GNSS, or the Global Navigation Satellite System, is the correct term. Just know that you will probably be catching me on future podcasts referring to GNSS in general as GPS. So my apologies. I'll try my best, but I think that's been pretty beaten to our head to use the term GPS. But at least now us and you, our listeners, now know the difference between GPS and GNSS and you're going to have a super fun party piece of trivia for the next time. You get together with friends and you're going to be the life of the party, the life of the party.

Sarah:

Glad to help you out with that Tune. In. Next time for a tiny bite of knowledge from GK Technology where we have a map and an app for that.