 Good afternoon, everyone. My name is Bill Crossley. I'm the J. William Uregg and Anastasia Voronas head of Aeronautics and Astronautics here at Purdue. And I'm pleased to kick off the first, if I understand correctly, first in-person pedals talk in almost two years. Here in the atrium, at least, at the atrium. So happy to have everybody here, see everybody in three dimensions. So thank you for joining us. My job today as Aeronautics and Astronautics, we're the host for our speaker. My job right now is to introduce our Dean and Executive Vice President of Purdue University for Strategic Initiatives. He's the John A. Edwardson Dean of College of Engineering. Make sure you get the title correct. And the Roscoe George Distinguished Professor, Electrical and Computer Engineering, Meng Cheng. So please welcome Meng Cheng. Thank you, Bill. Indeed, the Purdue Distinguished Lecture Series was started several years ago. We turned completely virtual for a while. And this year, we started coming back in person. But because of roof repair at the new Armstrong Hall of Engineering, where we are located, to those who are in person and those who are streaming online or watching later on our YouTube channel, which has, as I understand, become very popular from Purdue Engineering. I just want to let you know that, indeed, now we have brought this outstanding series back to the atrium of the Armstrong Hall of Engineering. And what a fitting place for AeroAstro, our school's recommended outstanding distinguished speaker today. Now, before I introduce today's lecturer, I just want to highlight that at this very moment, if you are watching it a month or years later, you may not fully appreciate the magnitude of Purdue getting into a sweet 16. So for the record, this is year 2022 in March. But I'm also proud to say that Purdue's College of Engineering, our graduate ranking, at least is already in the final four. So we are very proud of the faculty, students and staff's outstanding effort. And as a way to connect us with some of the most distinguished engineers throughout the country and the world, indeed, we initiated this series called the Purdue Engineering Distinguished Lecture Series. And today we're absolutely delighted and excited to have Professor Panina Exoret as the distinguished lecturer. And Panina is a distinguished professor at University of Colorado, Boulder. And she is world renowned as an expert on the subject she will introduce to us today on GNSS. She is a member of the National Academy of Engineering, a federal both Institute of Navigation and the American Institute of Aeronautics and Astronautics. After getting BSMS from MIT and PhD from Stanford and Air Astro for the past three decades, she's been on the faculty at University of Colorado at Boulder Department Chair over the five-year period from 2012 to 2017. And her research includes space domain awareness, technology and algorithms for GNSS-based position, navigation, timing in space, airborne, marine and land environment, multi-path characterization and correction, and remote sensing. This is truly an exciting time for anything that flies. And I always envy Bill. Bill here has just mentioned as the head of our School of Air Astro that anything from Jung right above our heads all the way out to Sis Lunar and in between, including satellite navigation, satellite systems. Delight, welcome today's distinguished lecturer to Purdue Engineering. Thank you, Panina. All right. Well, first of all, thanks very much to Vice President Chang and Professor Crossley for the gracious invitation to be at Purdue today. I also appreciate all the faculty who took time to meet with me and the students. It's a real honor and a pleasure to be here. So I've been working in GNSS for a really, really long time. It started in 1985 and it was kind of an accident. I had a choice as an intern of one of three projects to work on for the summer and I happened to pick the idea of using GNSS on a space station. Okay, one out of three chance happened to pick it. And it turned into a lucky coincidence when I was looking at my PhD to get an opportunity to continue working on GNSS with Brad Parkinson, who's considered the father of GPS. And at the time I wasn't really keen on GPS. I finished my master's degree and I was pretty sick of it. Didn't want to see the letters GPS together at all anymore. But he convinced me that it was worth continuing mostly by letting me know that I could get paid to work on it as a post, as a RA, whereas if I didn't want to work on GPS, there probably wasn't a position open. So it worked out quite well. And I didn't at the time really appreciate the magnitude of how much there was to learn. And so I've continued to work in GNSS ever since. It's not that I don't do anything else, but there are so many things that have been discovered and improvements that have been made to GPS since those days that it's really just been a interesting ride kind of driven by this particular system. So I wanted to share with you some thoughts about what makes it really game changing. You know, why is GPS the greatest invention ever? And what we might expect in the future, how things are going to go forward. So this is the classic GPS image, three segments, the space segment, the control segment and the user segment. Here, the user segment is focused on a military navigation application, which certainly was foremost in the minds of those putting together the system originally. The satellite segment is probably the most visible, right? We think about GPS satellites. The control segment, of course, is a bunch of 18 year olds down in Colorado Springs in front of monitors, you know, telling the satellites what to do. But also the stations around the world that track the satellites, the 18 year olds don't have to do very much most of the time, but they are there paying attention, which is good. There's a lot of misconceptions about this, right? People have this idea that GPS was military and was always intended only for that purpose. But in fact, from the beginning, people knew that it had to have civilian benefits, too, in order to get taxpayers to be willing to put in the money for this over a long period of time. And so it's been modeled or considered a dual use system from the very beginning. The initial focus, though, was really on navigation. Navigation meaning real time positioning, the idea that you could be anywhere, a soldier could be anywhere in the world, passively, right, without transmitting a thing, determine their location in the middle of a hostile environment, right, and not give away their location at all. They would know their location, they would know their time, but nobody else would. And so that was really powerful and an important driver of it. But of course, it didn't stay that way, right? So in 1978 was the first GPS satellite launched. The system was obviously designed many years before that. But that was the first satellite went up in 1978. In 1983, there was a shooting down of a commercial airliner by the Soviet Union. And at that time, President Reagan made an announcement saying that, and part of the thought was they had accidentally wandered into the airspace. And President Reagan came out and said GPS would be provided free of charge, user charges to anyone in the world to use to avoid such a thing. Okay, this is obviously sounds good, right? And it turned out to be true, though, that the U.S. would continue to provide GPS service indefinitely to the world. And if they ever were going to change it, the rules they'd give 10 years notice, okay? And of course, it hasn't changed, right? The U.S. has continued to provide GPS. We, the taxpayers, are paying for it for everyone to use anywhere they want. A colleague of mine once described this as spectrum dumping, because it made everybody dependent on our GPS system, right? And it really did. It was very effective. But in the course of doing that, at the time, of course, the Russians have their own similar system called GLONASS. And since then, the Europeans and the Chinese have developed their own global navigation systems. You can use any one of those systems to navigate anywhere in the world. You don't have to switch when you switch countries. And that was really important, because it made everyone able to go places they had never been before without stopping at a gas station to get a map, right? I mean, that came later. But the idea was you could figure out where you were if you had a receiver with no preparation, no effort. You didn't have to learn how to use a sextant. You would turn it on and it would show you the answer. And that really is kind of mind-blowing, right, for people who weren't used to it. In 1993, the official service became available, meaning that it had enough coverage and redundancy that it could be considered reliable all the time. 24-7. Didn't need good weather. Works great in snowstorms. My students in my class are always concerned that GPS won't work when the weather is bad. And specifically, the frequencies were chosen so that it does work when the weather is bad. Even when the weather is terrible, it still works. And that's, you know, important, which I'm sure you're all familiar with this. So I'm going to talk about the evolution of it and also the future. But before I do, I want to give you a heads up that I'm not a very good futurist. So when I was a grad student, I read this paper, Civil GPS from a Future Perspective, written by Tom Stansel, who's one of the anchors of GPS. He worked on the transit system that came before GPS. He was one of the strongest advocates for the civil signal called the L2. He's a fantastic guy. But I didn't know him at the time, but I read this paper that he wrote where he described how GPS was going to change the world. And in particular, the thing that struck me was this statement here. The very concept of being lost may cease to have meaning or come to mean that your navigation equipment stopped working. This man said this in 1983, where nobody, you know, I would go skiing and sit on his chairlift and people would say, what do you do? And I say, I work on GPS and they would say, what's that? Okay. I knew things had changed when everybody would know what that is, but in 1983, people weren't really that familiar with it. And this gentleman had this vision that this is what was going to be. Okay. And I just find that mind-boggling because at the time I thought, that's ridiculous. What kind of a stupid thing is that? But he was really visionary and as well Parkinson and the other people who worked on it, they didn't envision all the things that could happen with it, but they created an infrastructure, a signal thing and a constellation and a way of managing it in such a way that it really did change our knowledge of time, our ability to position. And what's more is we could not only position ourselves, but it turns out we could position scientific experiments and all kinds of other things. What I'm going to talk mostly about is the things that they didn't really envision, right, that still happened and are quite useful. So, again, this is another phrase from my past that I thought I would bring up, which is at the time we just said GPS, GNSS, Global Navigation Satellite Systems is more general. Okay. A tool looking for a problem. GNSS really is a tool looking for a problem. It provides signals that you can pick up anywhere. These signals probe the earth. They probe the atmosphere. We can use it for navigating and positioning and all kinds of things. This was a negative feedback I got on an NSF proposal where I had proposed using GPS for something and they're like, this isn't really science. This is a tool looking for a problem and I'm like, yeah, but you should fund it, but they didn't. But that was okay. So, you see here a variety of applications. Some people are really excited about the agricultural applications. We're an aerospace department, so we like the whole UAV space exploration thing. I think remote sensing is another really powerful thing that certainly wasn't anticipated. And there's certain aspects of it that really make it so multi-purpose, why it really is a Swiss army knife of everything. Okay. I'm not biased. All right. So, there are three key elements of GPS. The first is the constellations and it started with just GPS and GLONASS not long after that. But right now, there are over 100 active GNSS satellites. There are 31 in the GPS in the U.S. constellation, 24 Russian satellites, about 28 European Galileo satellites. I don't know whether these numbers are totally accurate because it's hard to tell if some of them are out of commission a little bit. But the Chinese system, BIDO, has 49 satellites. Their constellation is a little different and I'll come back to that later. There are also these subsets of satellites. Can you see my pointer yet? There's three subsets. So, the QZSS, which is a Japanese system, an Indian system called NAVIC, and this more generic term called SBAS. These are satellite subsets that don't cover the entire Earth, but they provide augmentation in regions where there's special needs why they would need extra satellites. So, for example, in Japan and the urban environments with regular GNSS, you don't always get good coverage. So, they have satellites in specifically designed elliptical orbits to dwell over Japan or over that region and so they provide better service. Same thing with the Indian system. So, this is one of the key things, right? We have constellations of satellites. They're in medium Earth orbit, which means they spend hours in the sky. Some of them are geo, but most of them are meo. They spend hours in the sky. They're always available 24-7. You've always got at least four visible from GPS and from each of the other ones. And so, it's important because there's a signal out there all the time ready for you to use whenever you feel like it, okay? Second key element are the pseudo-random noise code signals, or PRN codes, and that sounds really technical, but what's important about the PRN codes, it allows all of the satellites to transmit at the same frequencies without interfering with each other. It also allows you to determine range, okay? So, the PRN codes give you a ranging signal that you can use to measure, and it also provides you the ability to remote sense in many ways, because it gives you a very regular signal pattern that has noise-like properties, but you know it ahead of time. So, it's a lot easier than using a signal of opportunity, which you can do too, but here you know the profile that's coming in, and so you can look at what's coming out and better extract what's happened to it. So, that's really powerful as well. Most receivers just track these things and the types of measurements that they make are called pseudo-range, carrier phase, and signal-to-noise ratio, and it turns out that you can get useful information from all three of these factors. And then people, especially people like Professor Garrison, also capture the signal distortions when the signals interact with their environment and things that they reflect off of, and that's where a lot of the science remote sensing comes from in using GPS. Okay, the third key element are atomic clocks. And the atomic clocks are on board, there are two sets of atomic clocks. There's ones on board the satellites, and then they're also in the control segment, right? They have atomic clocks that maintain time. And these are really important, because what it means is that the satellites can generate very stable signals without any interaction from the ground. The ground sends them updates once in a while and reports corrections, and these stable signals allow long integration times, and they allow receivers to position by just using those signals. They can get all the data they need directly from the satellites. So there are ways you could have implemented it without the atomic clocks, but the atomic clocks give it tremendous robustness and allow it to be used for many other applications than you maybe originally had planned. So here are a couple pictures. It's just a box, but this is the rubidium atomic frequency standard on the GPS satellites. This is the passive hydrogen maser that the Europeans have developed for Galileo. And in particular, some of the other constellations have put more effort into developing more advanced clocks, and I'll come back to clocks a little bit later. So those, I would say, are the three most critical elements of the system, and I'm pausing because I'm trying to figure out what time it is, so I'm good. All right. All right. So those are the three parts that come from the GNSS systems themselves. The next part, which is not part of the system, is the proliferation of receivers. And so the constellation was up there, people were using it, and it was used mostly in the way that it was intended. The part that I think really changed things dramatically is the fact that it was this massive deployment of receivers. And all the way on the left are the scientific receivers. There's an organization called the International GNSS Service, IGS. IGS is a voluntary organization of scientific and government things all over the country, all over the world, that deployed receivers and they were primarily used for scientific purposes. But they created a database of measurements from all over the world that anybody could use. So when I started doing GPS, if you wanted to see how GPS would perform, or whether it was subject to rain or whatever, you had to collect your own data. IGS made it so these data sets were available, so if you wanted to know, how does rain affect GPS in Alaska, you just have to look up the Alaska site and you could grab data for years all at once without going out into the field. And that made a huge difference because people could now explore different scientific ideas without having to do their own experiments. You want to do something weird, of course you had to do your own experiment, but there's a lot of data out there. And then with the advent of smart phones and smart watches and GPS embedded in everything and the internet, suddenly now you have users of GPS not intentionally but with Google monitoring everything we do, suddenly you have positions reported all over the world that allow you, because they're using GPS to position, you have flow of people and transportation and anything that happens to have a GPS on board, not because of GPS. GPS is a passive system, it doesn't track you, but the fact that it's embedded in your cell phone or in your driving system of your car or anything else with a calm system means that now suddenly you not only have scientific receivers collecting data, but you have individual people and cars collecting data too. And so the fact that GPS is present in something that's telling somebody where you are means now we have much more crowd sourcing of what's going on in the world, right? We know where cars are, we know where planes are, we know where satellites are. And this proliferation of data and its integration in a common coordinate frame, everything's all in the same frame is a really powerful view of the world, right? Which we didn't have before GPS and I think that's really a profound advantage. It's not just that GPS tells you how to get anywhere, but it's the fact that you have scientific and just incidental data about what's going on, I think that's really exciting and world-changing, right? And here over on the right I have satellites, again, and then this one, I don't know if you... I tried to look this up for West Lafayette and this is the motion of taxis and it helps you to see where there's traffic. When I looked at West Lafayette, it was all green, so it wasn't very interesting, so I pulled up New York City. But it's really kind of cool, you can get these animations of where the taxis are in New York City and not individual ones, but just sort of the flow. If you think about that, right, it means without going to New York City, you can have a sense of how traffic and where people get picked up and where people get dropped off. Again, not because GPS is tracking you and telling the government or somebody where you are, but because the cell phones or in the cabs are doing it. So lots of really amazing things happen with the synergy of the three things that the system provided and then the fact that we all got addicted to it. We're all using it. Okay. So the IGS, the International GNSS Service, has a mission to provide this scientific data. So these are not, you know, cheesy little receivers. They put solid, you know, high-quality geodetic receivers in. They put high-quality antennas. Sometimes they put them in bad places, but usually they put them in a good spot. And they're over, I think this is 400, but I think they're more than 500 stations now contributing to the IGS. There's also these analysis centers, and the analysis centers independently process the data, not just their own data, but the whole, you know, sets of them. The analysis centers each come up with their own orbits and clocks, and they provide cross-checking then between the different analysis centers. There's one in the US at JPL. There's DLR. There's in China at Wuhan. There's different analysis centers all over the world. They all contribute, and then they combine. There's a coordinator who puts out a final product. But IGS also puts out these individual products. So you not only can get the raw data, you can get the solutions from each of these centers, and then you can get the combined data. Again, really powerful way to democratize the use of really high-precision data. You don't have to go there to do it. You just grab the data and work with it. So I think that's been really cool. All right. So I'm going to now talk about some science applications. These were definitely not intended when GPS was created, but they're very useful. So these are all, on this first slide, are all scientific applications that came about because we know where things are that have GPS on board. So for example, one of the earliest techniques was plate tectonics. So they put GPS on the Earth, and then they watched how it moved over the years, centimeters per year. They could detect that. This is a picture here of a much more rapid event where there are GPS receivers from the IGS, and during an earthquake, they moved very subtly, and they were able to, with GPS, determine some aspects of the earthquake that they couldn't tell with seismometers because the earthquake was so big that the seismometer saturated. So a seismometer is a more sensitive technique, but Professor Larson, who did it, Sue Boulder, as well, figured out how you could use the GPS carrier phase signals to detect and measure the motion of really large earthquakes. So again, not really intended for GPS, not like you could do it with your cell phone, but that was a cool thing. The bell picture here is the GRACE satellite. So this is a satellite in orbit. It co-orbits with another one, and it's used to measure the gravity field of the Earth, and they measure that through the perturbations of the orbits. They use not GPS, but laser connections and a higher frequency cross-link in order to extract the gravity field. But the fact that the GPS is on board, they can use the orbital information as part of that process. So these are in-situ science measurements. Here's a really fairly new one, is the idea of using CubeSats in orbit. So CubeSats nowadays all carry GPS on board. CubeSats are usually pretty low, except the ones go into the moon, most of them are pretty low, and the SPIR satellites have pretty high-quality GPS on board. They're affected by atmospheric density, and that's affected in turn by space weather. And through the motion of the CubeSats, you can actually extract the density variations that are influenced by space weather. So one of my students in particular is working on this, Shayla Muchler, where she's using SPIR satellites and looking at minute perturbations, differences in the acceleration that it experiences, and through that, she puts it through a physics-based model and infers what the density is. Okay. There are also direct effects on the GPS signals, and this allows you to monitor the ionosphere, the effect of the ionosphere directly, and also the effect of the atmosphere. And there are ways of getting this to be amplified more by using what they call radio-occultation signals. And this is where you put a lower-thorbit satellite in orbit, and instead of looking just up at the satellites, you look through the limb of the Earth, and so you are able to measure the effect of the atmosphere on those signals as well. At the bottom here, I have another unusual application, again, by Professor Larson, where she was able to use signal-to-noise ratio, which is not something scientists often look at in GPS. And this is not the right volcano picture, but there was a volcanic eruption. This is Mount Redoubt, I think, I'm not sure how to pronounce it. But looking through the ash plume, she could actually see attenuation of the GPS signals as they passed through this ash plume. And the receivers happened to be sitting there, and so they were able to detect this very rapidly deploying ash plume. The third one is Reflections, and Jim Garrison was a real leader in this. And in 1997, first, got the first ocean surface reflection using ocean surface reflections from estimating ocean surface winds using GPS signals that are reflected. And that's a picture from then. And this has progressed a lot, right? The Cygnus mission is now measuring ocean surface winds in really severe weather conditions all over the world, with a constellation of six large CubeSats. And he's also working, obviously, on not just GPS signals, but also other signals of opportunity. So as techniques that developed for GPS became more mature, people have started applying those same methods to other signals of opportunity, other transmissions that happen to be out there, and that allow you to probe the Earth in different ways. Okay, so what's next? I was going to say that GPS is not your grandmother's... GNSS is not your grandmother's GPS, okay? There's a lot of new signals out there. There are truly multi-GNSS constellations way over determined with new dual-frequency carriers for civilians. So that's a big change from when we first started, right? We used to have, you know, just barely four satellites. Now we've got 20, right? If you can use all the constellations, you've got a hugely powerful thing to look at. The other thing that has changed is more awareness of our inherent dependencies on GNSS. And this has raised a lot of concern in the media and people paying attention to it because people are concerned about denial of service. So either intentional jamming or spoofing or just interference from things. So that's an important factor. I think that the two things go together quite well, though, because of all this extra redundancy if receivers are designed well and if we take care, our governments take care to remove people who are interfering with it or devices that are interfering with it intentionally, we can preserve all these great benefits that it brings to us. There's also some new signals of opportunity and methods to use them that have been arising. And this is because people are starting to do more types of communication systems, especially from low earth orbit, but also terrestrial. And we can leverage those for the same kinds of purposes we use for GPS. So in the new GNSS satellite category, I have a picture here of the Beto system. And this one is interesting because regular GPS, all the satellites are in NEO and there's augmentations in Geo that provide corrections. In Beto, they've incorporated that from the very beginning and they're also doing something interesting which is they're using... So normally the satellite orbits are determined by tracking things from the ground, which is great. We know what ground stations are, it's all good. The downside is there's sort of this inherent beat period between the GPS orbits and the earth rotation where the ground stations are. It's called the draconitic period. And it can produce small errors that are systematic. And when you're trying to estimate things like reference frame and you're getting down to millimeters, suddenly these are a problem, right? So what they have done in Beto right now is they have lower-thorbit satellites that have different orbital periods and they can be used to observe the GPS or GNSS signals as well and that helps to reduce this break, this geometrical repeating pattern. And that's very powerful as well. And I think we're going to see more of that. There's also NTS-3 is an Air Force project to test new kinds of signals. This will be a Geosynchronous satellite that's supposed to launch in 2023. And they're going to be testing adaptive or variable signal types of signals. So they have a reprogrammable signal generator on board. They'll be able to try... We can never change GPS because it's up there. It stays there for a long time. It takes a long time to change it. They're going to be able to try different kinds of signal structures and use software-defined radios on the ground to track those. And this is particularly for military applications, but you can imagine that that's... What happened? There we go. And everybody's excited about right now is Leo GNSS. So low-earth orbit GNSS. So instead of... People in the early days decided that Leo was a terrible idea for GNSS, low-earth orbit, because the orbital period's really short and you need lots and lots more satellites to cover the Earth than you do from Neo. Way more. And so it was kind of written off as, yeah, not a practical thing to do. Because nobody thought you would put thousands of satellites in Leo. But of course we have and rich people have and there are Leo constellations proliferating all over the place. So this is the up-and-coming thing. In my opinion, and some of these might end up being designed specifically for GNSS purposes and some of them might be synergistic. They're designed for communications purposes and we will find ways to use these signals. In fact, there's a lot of work going on right now. I think Zac Cassis at Irvine is probably the leader in this of being able to use these systems. I keep wanting to say synergistically, but it doesn't benefit the systems. It's more like parasitically, but it doesn't hurt them either, where you can use it without any information from the provider. You listen to their signals and you're able to use it for positioning. So this is also pretty game-changing, how we're going to be able to use those signals as well. The next thing, and this is more deliberate in terms of where we're going. The next thing that's going to make a big difference, I think, are advances in atomic clocks. There are two main types. The first is this DSAC, the Deep Space Atomic Clock. This is a JPL advancement and it's currently flying in low-Earth orbit as a demonstration mission, but this clock has been able to demonstrate three parts intense the minus 15 at 23 days. So it's an incredibly stable clock and the advantage of that is that right now you can predict orbits really well. The current predictions are really limited by clocks. So with something like this, you can get much better long-term prediction of the clocks than you can with GPS. And they designed it specifically for a one-way Deep Space navigation, but it has implications that if you used a clock like this on a GPS satellite, it would make improvements there too. There are other types of atomic clock advances and these are optical clocks. So most clocks that we use right now, the fundamental ones, are based on microwave frequency transitions of atoms. You can do better with atomic clocks that are linked to optical frequencies. And so they're optical ion and lattice clocks that are being developed. This one here is specifically being worked on for space applications and so this is the performance down here. It's right now around 10 to the minus 15. It's going out for a long time, in the order of the same as the DSAC. But these optical lattice clocks have the potential to get to 2 times 10 to the minus 17 fractional frequency uncertainties. And this can make a huge difference for GPS if you put them on global navigation satellites. There's a lot of overhead that comes with going to an optical clock, which is you have to somehow be able you can't measure its phase directly, so it needs to be transitioned to other frequencies. And with that you can do it with an optical frequency comb. So I wanted to show this pic. I had to have some picture of Boulder. So this is a picture from NIST. It's called the Boulder Atomic Clock Optical Network or Bacon Collaboration. But this is not space born. These are huge lab clocks with tremendous amount of infrastructure that goes with them. But they were able to compare three clocks, all of them optical, reproducibility, reproducibility at below 10 to the minus 17, so 10 to the minus 18. And they did this across optical fiber and a free space link and using these optical frequency combs. So this is, you know, two orders of magnitude, three orders of magnitude better than what we currently have operating with GPS. You can't put them easily on a satellite, but this is going to redefine time and make a big difference. The part that's maybe more relevant about optical clocks is, so, again, NIST is working at femtoseconds, 10 to the minus 15 is a minimum. And that's going to be difficult to transition to navigation. But there's a secondary version of it, which is kind of like the clock equivalent of pseudo, of PRN codes where you can put PRN codes on these signals, and they've already demonstrated about a micro-savagant level time transfer. GPS sits on the order of nanoseconds, so this is like three orders of magnitude, two orders of magnitude better, because GPS can get sub-nanosecond, but not much better than that. So this is a big deal, and this is a future area where I think you're going to see tremendous improvements in GNSS through these optical links, again, or using optical communication with PRN codes. And then, the next version is quantum sensing. So GPS is a radio-nav system, so you don't need inertial sensors, but inertial sensors are nice, because it allows you to navigate or propagate between radio updates. So people use conventional accelerometers and gyroscopes. In normal navigation, you're often limited by your knowledge of gravity. So the gravitational field, people like to talk about INS system being unstable, because if you don't know the gravity field that well, if you guess you're too high, your estimate sort of goes off exponentially. But with these potential for quantum-based sensors, they're particularly useful for inertial navigation, because they have very good long-term properties. They don't have great short-term properties, conventional sensors are actually better, but quantum sensors, in particular, these quantum interference sensors, or atom interference interferometers have the ability to have very good long-term stability. And you could potentially integrate them with good short-term clocks, but this is a huge factor as well. And so the idea behind this, and in particular, there's a group at CU Boulder that's working on optical atomic lattices. I'm going to skip the steps of how it actually happens, and I'll jump to the end here. Well, so you have atoms and they're interfering in using wave-like properties, and it uses something called a Bose-Einstein condensate, like it's a complicated thing, but it works. But they haven't put it yet in a small enough package to use on a satellite or a handheld thing, but, you know, could get there. The point is that they can get these things down to spectral density stabilities of 10 to the minus 11 meters per second squared, so it's an acceleration per square root hertz. In particular, what's interesting about this is most accelerometers drift over time, like they're good in a short-term, but then they start to diverge. These are kind of the opposite. They have very good long-term properties, so you have to calibrate things because of the other electronics, but they have the ability to hold on to a value for a long time. And so they're not going to get rid of GPS, you'll see headlines that say, you know, it makes GPS irrelevant. That's not really true, but it does allow you to integrate and navigate in-between GPS updates, and so it provides a very good complementary benefit to how we normally use GPS. So, future prospects, I think we're going to see an integration of GNSS, so satellites designed intentionally for GNSS and ones that we use parasitically for GPS. We'll see a combination of Leos and Geos, things with different periods and different directions, different geometries. I think we're going to see more crowdsourcing of integrity. That is, we'll be able to bring in data from all kinds of receivers and different venues in order to get a better idea of when there's a problem with the GPS or when somebody's spoofing, things like that. In order to take this, I think we need to make advances in geodesy and improving our models of the relativistic effects under gravity, especially the gravitational ones, when we start to look at clocks with this kind of level of precision, so that's something that needs to be addressed. I think we'll see optical clocks evolving to where we'll be able to put them on GNSS satellites, and I think our user platforms will start to integrate these really high stability inertial systems, as well as clocks going in the future. So, thank you for your attention, and I'll be glad to try to answer questions. Sorry to run over. No, it's okay. I think it's okay. Thank you very much, Dr. Axrod. So, we have a couple of microphone runners, so if you have a question, please raise your hand. Usually the host of the moderator has to start with one, so I actually thought of an interesting one. It's kind of a softball. Were you an early adopter? Did you get a Tom Tom for your car right away, or did you actually wait, given where you were in the development of GPS? I didn't have a Tom Tom, but I bought a Garmin, you know, like one of the early Garmin handhelds, yeah. So, not the first wave, but pretty close. I mean, not the car, like, okay, I had a Dotson B210, so you can't really put that in there, but I did buy a handheld receiver when, as soon as they got cheap enough that I could afford one. Questions from the audience? Thank you so much, Petita. I'm just curious about what you mentioned on your work with Data from Spire on the atmospheric density. How is it looking, you know, are we, you know, is there a significant effect, you know, historically changing in density? We're just curious. What we're looking at there are space weather events, so we're not looking at long trends. What we were trying to look for is when there's a space weather event, when there's a, you know, something coming from the sun and there's an effect of the atmosphere, we're trying to see those short-term behaviors. Yeah, we're not set up to, like, look at historical or something like that. Are they legal? They're legal, yeah. Some of them are quite low, some of them are like 400, some of them are higher. The lower they are, the better we can use them to see what's going on with the atmospheric density. And then, you know, the space weather affects the higher part, but then there's propagation models. We use TIGCM as a way of understanding where their atmosphere actually is driven by the space weather. Hi, Carolyn. This one back. Okay, so you mentioned earlier on that Japan and India were developing kind of separate systems to augment. Could you explain a little bit more like why they needed the orientation? Some of it's just national sovereignty, you know, you want some of your own control, right? And so if you have your own satellite, you know you're getting that one. It's inconsistent with what's coming in from everybody else's. It gives you a sense that you get some idea that you can control and you also, not just a sense, but you really do have your own way to observe if there's something amiss, right? Like if the US is messing with it or whatever. It also increases the coverage in your region. So their satellites in particular are designed so that they dwell over those regions. So you have your normal Neo ones are going around, but then you always have an extra three to four satellites from the national system. So, yeah. Yeah, so that's a lot louder than I thought. I know it's really loud. So you mentioned trying to put more of the infrastructure for GNSS and Leo is a big push right now. Will that not make the requirements on the clocks and the gravity information even more stringent than they currently are in Neo? So you sort of have competing interests there? What's the... question? I think that people are interested there's pros and cons to Leo, right? The pros are that the signals can be quite a bit stronger because it's closer, right? So you can get a stronger signal. The orbital mechanic, you know, the predictability is not as good as in Neo, but you get a lot of visibility of it too, right? You can use, okay, in Leo you can use the Neo GPS to determine your orbits, right? And as long as you're transmitting at the same frequency then you can actually sort of leapfrog it or what do you call it bootstrap it to the Earth is one thing that can be done. They don't do that with Satellis and there's another system I can't think of the name of it right now. Something with an O. Do you remember? Yeah. Yeah, I can't think of the name right now, but there's another one. And I don't know if they're actually planning to use GPS. I think they're planning to use GPS for their orbits and then leapfrog that to the ground. So, you know, like you said, they're pros and cons. I actually don't think that the governments are going to take over the Leo. I think that the governments probably the Leo is pretty popular for communications and people like Elon Musk and others, Bezos will figure out how to make money from that. And so they're going to be the ones doing the Leo. What I think really makes sense is for governments to continue to support the GEO or MEO cases, which will serve as like fundamental references. And then the other ones can either leverage that or augment that and then we get this really rich framework of clocks and signals that we can rely on, right? If it's all commercial you never know if they're going to fold or start charging money or whatever. But this way we have an infrastructure that's designed and then we can hang extra things on it that provides substantial improvements. Okay. I think Caroline had a question. Very nice presentation. I have a question. So you showed a slide where it was a New York City. So I have an experience in the Times Square where the GPS dot was like all over the map and I had a similar experience in Chicago too. So I was wondering, is it an inherent weakness of GPS or is there a way to circumvent this problem? That's a great question. It depends on the receiver and your phone. Like if you have an iPhone, it's terrible. No, I'm just kidding. When you're in an urban environment, the GPS signal, some of them are going to be blocked by buildings and some of them are going to be reflected by buildings. And so there's a big push nowadays actually to try to incorporate urban building models. So digital elevation maps that include all the building infrastructure. So the receiver could actually create a map of where there's going to be signals and if it gets something that it shouldn't, it knows that it's coming indirectly. There's a method called shadow matching where you actually not just use the signals that you see, but you use the fact that you're not tracking something to tell you you must be here in Times Square because the building pattern matches the blockage of what you're not seeing. So it's being advanced right now. It's ridiculous, right? But Google has this built into their maps. So depending on what app you're using, how old your phone is or things like that, you may or may not be fully taking advantage of these other things that allow your phone to be a more advanced user of GPS. So rather than using GPS the way you would if you were out in the middle of nowhere, in an urban environment, your phone or other device that has information coming from other sources besides GPS can actually do a better job. But it is challenging. You have to know that that's something that is paying attention to. But there's some interesting articles about it recently. Yeah. Thank you for the great talk. So you talked about the IGS and how even very high quality data and data products are shared. So why do you think that is the case for the GNSS and that's very different from other satellite data we are not sharing? I know. I mean, I think it's that's a great question. It's because the data can't be controlled. The signals can't be controlled. So if you're I don't know in Antarctica and you set up a receiver, no one can stop you from picking up GPS signals and then if you put them out somewhere you can do that. So I think that the IGS really was a visionary group of scientists who said we should do this. We're all busy collecting our data for our purposes. We can gain a lot by actually sharing it. And I don't know the history of how that evolved but I think it started Boitler was one of the leaders of it in Switzerland and certainly the German research organizations. I don't know how they managed to pull it off honestly. But in the U.S. I know they support JPL to support it so our tax dollars pay for that too but not as much as we could in other countries as well. It's a good question. It's really a picture of scientific collaboration that is probably unmatched. So, yeah. I was curious if you've given any thought to some of the overlaps of aviation. A lot of people have read about the 5G signal interfering with the old-fashioned radio altimeters. So what are some of the next steps to let us increase density in the airspace with positioning like GNS? I mean GPS really has impacted air navigation quite substantially with the RNAV and things like that. I think the concerns are always with integrity. Right? How do you ensure that you're not being spoofed or jammed or interfered with? One thing you need to do is not let legato transmit. You know, that would be one thing. The 3G thing is certainly an issue. I think the only way around it is having inertial systems on board, right? So if you have high quality inertial, that would be a big help. People have talked about it before. I think it's important to make sure that you don't have to run in other things like that. I'm not convinced people want to add separate completely different frequencies to their aviation systems, do you think? It's hard to make changes across such a large set of aircraft. Especially until it gets desperate. If things are working okay, if it's not broken, you don't fix it. I think finding really smart ways to maintain integrity is something that we have to do. I should check my time real quick. I was going to ask because you mentioned legato, so someone should know what that is. But maybe not everyone is and it's an interesting sort of public policy rulemaking, interfering with the scientific endeavor. You don't have to give your opinion where you could. You sort of did, I guess. But explain just a little bit for people who don't know what legato is and what's going on with it. There are people who want to use spectrum that's very close to GPS. It's not at the GPS frequency, but it's close. And they want to use it for 5G, right? And the FCC, so it's kind of interesting. The FCC regulates a lot of things that seem not obvious, right? So when you think about whether someone's allowed to transmit a signal, the FCC is the one who decides to do that. It's the one who gets input from different government organizations about how it affects the military, how it affects the Department of Transportation, things like that. So they do that. But there's a lot of pressure to expand the spectrum for 5G because people want to have their data really fast. The problem is that it operates in a region which was allocated for space to ground links. And so the assumption was that the signals would always be very weak. The way your receiver is actually able to track is by integrating the signal over a period of time when the signal pops up out of the noise. If someone so many starts playing loud signals right nearby, it interferes with and leaks into the GPS band and it can disturb your receivers. Some receivers are okay, some of them are not. And so this company wants to get permission to set up ground stations to transmit at this frequency. And there's this big debate over how densely populated they're going to be and how strong they're going to be and things like that. So there were recommendations not to allow it, but the FCC decided to allow it. And so that's where we are. But there are concerns about it just like there are with the altimeter thing. Similar idea. Any other questions from the audience? We do have one, sorry. Is the research being done with transmitters? Are we doing? Yeah, any research being done for upcoming constellations new signals outside of the L band? Yeah, there are some of the modernized signals are a little bit outside of L band. They're not dramatically outside of L band. Actually, that's a really good question that you raised. So L band is really ideal for navigation because of its insensitivity to weather. And so when you're doing communication with a stopped signal or a delay that's a little bit big, I guess it's inconvenient you get a glitch on your screen, but it's different than when you're trying to actually measure the path length. And so in my mind, and again, I'll say I'm biased towards GNSS, I feel like preserving the spectrum where the signal path is really predictable under all conditions. It should be reserved for the one where it makes the biggest impact, I would think. So there are other cases, there are other navigation systems at different frequencies too going forward. But a lot of that just has to do with more politics and allocations in the U.S., but also at the international level. Thank you for the great talk. You mentioned the atomic clock and also quantum sensing, and I think these would be beneficial in the context of planetary exploration through the exploration. Do you have any prospects on how these advancements will impact on the navigation in the context of planetary exploration? I thought we were going to talk about that at the panel. I mean, I do think they will, right? So people have talked about putting a GPS-like system near Mars or other things like that. GPS, it's expensive to put that whole constellation up there, but I do think you could build one up adaptively, so you send satellites or spacecraft there, they have calm systems. Once they're there, they could potentially start creating a new network where they're transmitting signals. Initially, you won't have enough satellites to be able to position just instantaneously, and that's where the inertial sensors, sorry, just kind of hear these noises, having inertial systems and stable clocks will allow you to do orbit and trajectory estimation with fewer transmitters, right? So DS, a deep space network, the reason you can use it from the Earth to wherever you're going is because you have the ability to integrate dynamical models and using the inertial sensors and the clocks will help that, for sure. So Dr. Axelrod just kind of hinted at something, which is we do have a panel planned after this, and so what I'd like to do is let's thank Dr. Axelrod for the lecture. Very enjoyable and very interesting about GNSS. Thank you.