Inventions That Changed The World: GPS

it's the 19th of july 1977 at cedar rapids iowa it's 20 to 2 00 in the morning and three engineers at rockwell collins are working methodically trying to receive the weakest of weak radio signals from a satellite called nts2 this is a hellish task they're using every trick in the book to try and pull the signal out of the noise so they're working late at night because the radio environment is often quieter then but also because the us air force has set up a competition to be the first to decode the signal there's a race to be the first contractor uh to decode the signal and the first retractor contractor to receive something the team are rather deflated all they seem to receive is a stream of aids it's so mundane they assume their equipment's faulty and they go home yesterday the day after that it becomes evident that they have received the first gps signal ever sent and thus a technology that transformed the world has begun you say gps to the average person they may not know what it stands for but they know what it is it's about finding your way around in the world the technology was developed in 1972 and it's still valid today you can see that in today's day and age when everything moved so fast it was a very robust technology the technology of gps has enabled us to be far more productive in how we grow food and how we deliver food which allows us to be significantly more sustainable and to do far more with far less and start to tackle some of the really big problems as engineers of feeding the world with restricted resources in what is rapidly becoming a trickier climate it's for security personal security personal utility confidence business it's ubiquitous it's everywhere it's amazing traffic can be controlled trucks and transportation can be directed planes can fly in a much safer way i'm rather glad gps exists when i'm in a plane i think gps is fantastic because it's it's touch everywhere for example i work in the panama canal and we use it daily to transit ships in the new locks that we just built in 2016 we use it on our surveys every day for maps for dredging operations the gps makes every activity of human beings much more productive much more useful much more comfortable and access is free access is free and that's an absolutely critical uh feature of uh gps so that was the judges of the 2019 queen elizabeth prize for engineering raving about uh gps and there were several important clips in that the technology has been very robust and it's sort of evident from that in the clip it's free to use so what i want to do in this talk is talk about how it works but before we get to there i feel a need for a bit of a preamble and um map making and uh location finding is really a very ancient technology you know um ptolemy uh the and the egyptians knew about how to measure angles and how to use that to find your position on the world and the key technology is called triangulation try try because it's about triangles and just because peeps was an enthusiastic supporter of um gresham college i've picked a chart that was um that was dedicated to peeps as a sort of example of that this is also a chart showing where i live so i have a personal interest in this and they've done a pretty good job i think measuring angles i would say part of the coastline this is the uh uh english coastline around the port of harrich ports of harrich and what is now felix dough uh parts of this look a little bit distorted i do wish that woodbridge haven was that close to langard point but i suspect it isn't but it's not a bad attempt at the job i think it's and i should say that if you were a sort of if you're of a certain age and you're a boy scout or a girl guide then probably you are familiar with the idea of triangulation measuring angles to things and working out your um position that said i think it's probably not fair if we're talking about gps to sort of range over such a wide range of navigational experiences so it's certainly true that there are astronomical observations are have a very long history um as soon as satellites were put up into the sky people started measuring uh doppler signals from them sputnik that was done with sputnik but the critical feature of gps is time of flight timing the time it takes for a signal to travel from one place to another place or propagation delay as we might call it that is the critical feature of gps and that's what i want to talk about in this lecture and uh that uh technique is known as trilateration for three distances or multilateration um now the principle is very easy to explain um let's see what i've got lying here here we go right so if we had a fixed point um uh let's say it's the head of this microphone here and i was able to measure very accurately the distance to that fixed point then i could be very confident that i'm lying somewhere on a sphere around this fixed point here so this fixed point could be a satellite that gps is um seeing in the sky and this is my other satellite here so all i have to do is intersect these spheres and bing that's where i sit and i'm sure your you know hyper geomet your geometry is good enough to work out how many satellites you need in order to get a reliable fix and all those sorts of things um so that's the that's the that's the sort of big idea if you like behind um gps and if you're going to find four things in this case they are latitude longitude altitude and time then you need a minimum of um uh four satellites in order to solve that problem uh the nice thing about radio waves which travel at about 300 000 uh meters a second in a vacuum is that when they're traveling in air or some other medium the speed doesn't change that much so if you can measure delay then you can measure distance so that's the whole that's the sort of basic principle behind gps but where did it start okay well um i don't think there's much doubt about this actually um it started probably with the british royal air force the raf and in world war ii the raf had a pressing need for accurate navigation they discovered that daylight bombing raids were very very expensive and you know they were suffering huge losses so they needed to bomb at night and bombing at night is tricky because you can't look out of a cop and know where you are so they had a solicit they needed a system that helped them navigate across what was then enemy territory so that's continental europe so they developed this system that was known as g and a typical g setup used three transmitters uh one of those transmitters which in this case was in daventry in the center of the slide was called the master or transmitter a and the others of which there might be two or three collectively they are called a chain were the slaves b and c in this case by the way the terms master and slave might sound a bit awkward to modern ears it's i'm afraid it's just standard terminology in electronics so i'm gonna sort of stick with it because i you know i don't want to not use the right words what happens is the master transmitter sends out to blip so it goes bip and uh it does that at regular intervals so if we were just looking at the master signal it will go well that's meant to be regular it's not very regular is it but it's my attempt to do that and uh what you would do in the aircraft is you would have an oscilloscope and the oscilloscope would be synced up on that blip from master a uh slave b for example waits until it receives the blip from master a and it says ah i've got ac i will now send out my blip usually waits a fixed period i think it's a millisecond so it says i've just heard a blip and so and c does the same thing it waits and sends that its blip as well so if you think about it you've now got this system which is all timed from the master and when you hear those blips will tell you something about where you are in relation to all of those systems so i think i can illustrate this uh reasonably effectively here's my uh bomber straying into enemy territory as it was then and we send out a blip from uh daventry say and i note that g doesn't measure absolute times it doesn't know when daventry went blip so that's going to form its sort of base if you like and then we'll pick another one that transmitter it too sends out to blip and then we measure the difference in blips between those two times between those two systems and that helps us work out where we might sit now just to take a very simple case you can see let's take the case where the two blips arrive at exactly the same time so you can see that on the slide they would the aircraft would either be here in this case or up here and in fact if the aircraft sat anywhere on the line between these transmitters it will receive the blip from here at the same time as the blip from here so the time difference between the two blips is zero so you can use that idea to create a map so there's a line so for example one measurement which is no time difference between the two blips gives you a line on a map which you can use to localize yourself now i've picked the easy case which is when the blip is zero when the when the sorry when the difference is zero when the difference is non-zero positive or negative the curve is a hyperbola on the chart but it's possible to work all of that this out in advance so the navigator in the aircraft doesn't need to be sort of drawing complex hyperbole on charts you issue them with a chart and then they look up with the time differences they're measuring where they are on the earth's surface so this is a g chart and i don't know if you can see carefully or often a bit difficult to read but there's a sort of set of hyperbole coming out here and here says a hyperbole centered on the left around one of our transmitters and there's one over here so what you would do is you will make measurements using this system and the aircraft and then you would get out your chart and the the measurements were made in normalized units and you would say well i must be on this line red 15 and this line here green 12 ah right well i must be here then sounds easy doesn't it um i'll just show you the technology this is the typical look of a g screen showing simulated pulses from the a master station along with those from slaves b and c note that once the screen has been synchronized to the pulse chain by using the large rotary control to the bottom right these will appear stationary well it all looks fairly easy although i have to tell you i'm not sure i would fancy being up in a lancaster bomber while uh nasty people were shooting at me having only an angle poise and a pencil and some these two rather heavy and uh difficult to use oscilloscopes in order to navigate with but it was the start of what is called hyperbolic radio navigation so g begat other systems and the most obvious uh sort of son of g if you like was a system called loran lauren a b and c uh which was essentially an american a us hyperbolic navigation system one interesting thing about g was the design decisions the designers were making when they were sort of cracking on with it the transmitters were based on land so um transmitter power doesn't really matter you know so they were using um i think they were about 300 kilowatts of power these transmitters huge great things i i've never known how they survived being bombed to death actually i mean it must have been painfully obvious if you ever captured one of these maps where where the transmitters were and they were giving out 300 kilowatts of power so it can't require very much uh effort to find them but anyway it was kept secret quite successfully i think um and they transmitted around uh 50 megahertz the system they were using to stop collisions is called time division multiplexing these blips are being sent at different times so you're using a single frequency that makes for a sim sync simple receiver in the aircraft but using time in order to help you transmit the information one of the challenges of course is that measuring time differences in analog electronics is really tricky and it's probably it was it's all right for a military system where the cost isn't really super important but if you wanted to be picked up by commerce time difference measuring with analog electronics is very has remained a big challenge so what about phase could you use phase instead yeah well there's a british system and that was called deca the decca navigator system and it used phase it used multiple frequencies and you look for phase nulls phaser a lot easier to work with use frequencies of about 100 kilohertz so it's quite long wavelengths about 3000 meters so if you were using a decker system you'd sit there twiddling knobs you know listening for phase nulls and used one of these charts this is the thames estuary i'll just blow it up for you so we can and you can see this chart is crisscrossed with green lines each with a unique label this one says f-39 and red lines here this one says e6 e7 and so on so it's pretty much the same principle you bought these charts looked up the locations of the knobs on your machine and off you go so very interesting design decision the decades the decker designers have made a decision that they really don't want to impose cost on the receiver end and so they've come up with this rather ingenious brackets complicated uh system in order to uh sort of manage the cost you know and that that's often what they say about an engineer you know an engineer could make for ten cents what any fool could make for ten dollars by the end of world war ii uh this was standard practice in the royal air force lorann was in good use across the u.s there were 72 loran stations and sort of tens of thousands of uh receivers and um lorenz c which was the later version of this and deca were pretty sort of comparable in localization accuracy you could get to within um yeah a couple of hundred meters in good conditions the problem was in bad conditions that could easily rise to many kilometers of error that wasn't so good and you're using ground based propagation and when you're using ground-based propagation you've got to think very carefully about the frequency of the transmitting of the wave of the waves that you're going to use because if they're too high frequency they are not going to go around the curvature of the earth and the curve the earth i'm sure there's no one in the audience who disputes the fact that the earth is curved but um it uh it curves rather more rapidly than you might think so uh your the range that you were getting on these things wasn't very impressive and so you got into this usual position with ground-based systems of the ground-based tower being higher and higher and higher and using as lower frequency as you dared to try and get it to refract around the earth's surface the lower the frequency the more difficult it was to do the timing measurements the more power you had to use the bigger the antennas you had to use and so on and so on and so on so that was the that was the sort of challenge that you were in now as soon as you move to satellites you can consider you've only got line of sight propagation to the satellite so you can get back up to using very high frequencies that's a big big help it's a big help because you get very small antenna and these are the frequencies that are still in use today so gps uses primarily a frequency in the what's called the l1 band uh perhaps this is a good time to say um i'm going to be talking about a system here which has military origins american military origins so um most texts on this and the standard sort of undergraduate textbook on this runs to about 500 pages of which at least 30 i think are acronym explanations as a glossary to explain all the horrendous alphabet soup that is gps i'm going to sort of try and keep it in control so you're not going to get an alphabet soup you'll get some of it it'll be an alphabet soup salt okay um l1 is the standard band and that's the one that most of your uh if you're in the habit of using uh gps daily and almost everyone in the audience will be doing that we'll be using l1 there's also an l2 band which originally was reserved for the military the l2 band is used by something called dual uh frequency sets and i'll try and remember to say a little bit about that later if i've got time so gps then 1977 was the situation i set up at the start of a lecture that was when the first signal was received uh it wasn't until 1993 that there was a full constellation of 24 satellites up there so that's quite a slow progression this sort of slow evolution of technology over the design life of a system is very unusual in the civilian world you know it's very common in the military world you know a sonar set for example will probably have a 20 or 30 year life you know the designer will be will be and so it's very common to be designing systems that you know cannot actually be built right now but you think the technology will be available in five years time to implement this thing and gps was certainly like that okay well how does it work then um right well it's very simple to log into gps.gov and download all of the doc public information documents and you could certainly download all of those and in principle you could decipher all of the gps standards and build a receiver it's a hell of a job to actually read all of that stuff so and the part of the problem is it's a mixed system it's part military part civilian and another part of the difficulty is the system has changed over the years so what i'm going to do is i'm going to do richard's sort of cod explanation for basic gps and then i'm going to add texture and sort of filigree to it to try and bring us up to date as we as we talk about it so just a first bit of technology in gps satellite which is where the transmitters are is usually called a space vehicle so if i start referring to space vehicles they they're usually satellites has a great advantage by the way if you're writing about gps that no one can spell satellite and space vehicles are very easily spelt so uh i shall try and do and it's highly likely that i have at some point uh misspelt uh satellite in this talk so let's have a look at this um slide and i will try and talk you through it so this is a simplified block diagram of what a gps transmitter looks like and um don't worry if you're feeling alarmed i'm going to going to decomplex this as we look at it on the left hand side is a critical thing of a gps satellite and it's an atomic clock it's a very accurate non-drifting or as non-drifting as you can possibly make clock and each satellite has one of these clocks as it passes over the ground stations of which the main one is somewhere in the center of the united states it is re-synced up with all of the other satellites and it is not locked to ground time but the internal clock that gps is using is given a conversion factor to get into um your you know set earthling time the sort of time that we use you know hours minutes seconds leap years corrected for and all that sort of stuff there's no absolute reason for your gps receiver to know the exact time where you are on earth but us earthlings find it very convenient to know the time so the transmitter carries a conversion table in it which you which you will use but the critical thing is it is a reasonable assumption that all of the clocks in all of the satellites are going tick tock tick tock together at exactly the same time now that feels sort of improbable if you're an electronics engineer and i'm still slightly surprised that they thought they could get away with it but um it has turned out to be possible to achieve that and all yes all of the gps systems that we're going to do will will do that and they have an absolute view of time that's very useful because it means when it goes if it was to go ping we would know what time it sent the ping because it can stamp it with a time stamp so that's that's important thing one the next thing we're going to do is we're going to transmit at a given frequency that was that l1 frequency that i referred to earlier and that is multiplied up from this very accurate clock so this thing is going tick tock tick tock this thing is going it's actually a sine wave going exactly that frequency now just as your radio well if you're of a certain age you will remember uh one form of modulation of your wave of your radio was amplitude modulation where you squeeze the amplitude of the carrier wave up and down or you could change the frequency i wouldn't recommend doing that because the frequency is carrying the clock which we we want to keep constant what gps does is it changes the phase so if you're sending a digit that's one you just send the carrier boo and if you send the if you're sending zero then you just flip the phase and you send it upside down okay so that's called binary phase shift keying so that's that bit okay and then it goes out to the antenna which looks like um a sort of uh looks like a smarty tube on the satellite i'll point it out when we see one this bit here these two bits here are the bits that carry the information this bit here is the possibly the most interesting bit it's the um ca code the conditional access code this was um sort of designed uh originally for to allow a course sort of sync up in these delays that we're trying to measure between the two clocks remember it was originally designed as a military system so the idea is you send out this rather sort of course code and then there's another channel which the civilians don't get to access with a similar cobra much longer and that's what's going to give you the precise timing as it happens the co ca code has turned out to be uh pretty good this code is unique for each satellite so it's a it's a new unique code and it's quite long there's 1023 bits to this code and we're going to send out that and that's going to be the thing that's going to allow us to lock up on a particular satellite and work out the propagation delay and then down here is side data that sort of covers um other stuff to do with um it tells you things like um it sends you an almanac of where this satellite is going to be so it's a future uh prediction it's uh it tells you the satellite ephemeris that's its motion parameters remember you need to know where the satellite is very accurately because it's a moving thing so you need to know where it was when it sent your when you received this signal you need to know where were you when you sent me the signal so you've got to compensate for all that it sends you corrections that allows you to get back to your local time signal um what else synchronization signals um oh under ionosphere corrections the ionosphere is a bit of the atmosphere that's full of ions it has a slight effect on propagation delay it can make the signal wander a bit and you can do some correction from that just by knowing a little bit of stuff from the ground stations i'll come back to that because it turns out to be quite a significant uh problem and there are better ways of of uh correcting it which give you very accurate um it can give you very very accurate um gps signals now the one i've just sort of skipped over a bit was this ca code so i'm now just going to sort of zoom in on the ca code bit um largely because it's so technically interesting and challenging i remember this as an undergraduate and i thought this is one of the most beautiful bits of um uh signal theory i've ever seen in my life it's a stunning thing so um this is an example of uh well this is not the full code this is a small segment of ca coders just looks and it's a pseudo random sequence so what that means in layman's terms is if you look at the code it looks random you couldn't easily predict it but it is cyclical so it comes back after 1023 of these things it comes back and repeats and we would convert that into an equivalent waveform so where it's zero we would make it minus one and where it's one we would make it plus one so that's the transmitted signal that's going to get used to control the phase of our carrier now then we're going to see that at our receiver and it should look the same i'm fairly confident is the same because i just dragged it from one place to the other on powerpoint but it's delayed but we don't know what the delay is so how can we work out what the delay is well we can use a very standard bit of technology to do that which is we can compare it against a reference so if we know all the codes that the satellite could send and your gps receiver generally does know the codes that are being used by all of the satellites they're not very long just has a record of them then what we could do is try and do what's called a cross-correlation comparison between my reference code which is the green one and the one i just received now exactly how you do that is probably a little bit intricate but let's see if we can do it so the idea is that we are going to sort of overlay the pink and the green code and let's see if i can use the pointer to show this so for example here this is a minus one and here this is a plus one so i'm going to multiply those two bits together minus 1 times plus 1 minus 1 times plus 1 which equals minus 1. and i'm going to do the same along here which is plus 1 times well you can't really see it but i'll say -1 and that also gives you a minus one and you can imagine that when they're not aligned you'll get some plus ones you get some minus ones on average you get as many minus ones as plus one so when you add them up you'll get zero no correlation between my reference code and my received code so what we would normally do then is we would then plot that point i've said got zero correlation at zero delay for my uh reference code and now i will just the reference code about a bit and i'll do that very precisely i'm sort of sort of like claude like moved it across the screen here but in practice i would go chip chip it's quite a laborious process this so i'm going to do this with every single code in my library against this transmitted thing so you can imagine the computing to do this i've just got the code try this one sort of reach into your pocket get this one plot the correlation look for the peak oh i found the peak it will be big it's as big as the code is long that delay there is the propagation delay that's one satellite now i'm going to do the next satellite now i'm going to do the next satellite and do that for every single satellite that i can see on earth now at this point you might have spotted what looks like something really odd in the sense that all of these satellites are transmitting at the same frequency why the hell don't they cause a mess with each other you know it's as if you're hearing all of these codes at the same time and the answer that is there a special sort of code called a gold code named after mr gold and these codes are specially chosen so that they do not correlate with any of the other codes and they only correlate with themselves at when they're exactly lined up if you choose the code wrong i hate to tell you this but say this but you know you can get spurious correlations called side lobes it's a nightmare so the choice of these codes is very very carefully chosen indeed they're not secret you can easily download them and they're very easily created using shift registers so once we got this thing synced up with the uh these satellites we know for each satellite what the propagation delay is and we've got we know that all of their clocks are synced and we are synced with it so it's a beautiful system it's as if i've you know it's as if i my sort of mickey mouse watch on my wrist here was sort of magically synced up with the cesium clock in well i don't know where it's kept but the national physical laboratory for the sake of argument and it does that by constantly sort of locking it uh together so that's why your gps receiver tells the time okay you can always trust the time on your gps receiver in fact i know a system um that was built by a friend of mine where he doesn't use his gps receiver for any of the location data it's used for a static system he's not interested in the space he knows where it is he put it in the ground there he's only uses it to get accurate timing so very um important feature of this now why is this beautiful because the um the satellites need to transmit on only one frequency which means you can build them very efficiently and it's the same frequency put another satellite in the air we just need to give it another code right which is all pre-declared and we don't need to change the antenna we don't need to have broadband antennas the receiving technology is also simple we just need one antenna it's at that frequency it's a declared frequency that's nice and beautiful this mechanism gives you gain this peak here is a thousand times larger than the background so it doesn't matter if we got a lot of noise with gps that's another beautiful thing this system is called code division multiplex it doesn't use a frequency that's fdma frequency division multiplex it doesn't use time it uses codes they're used in other places but they're relatively relatively uncommon another beautiful thing about it is it degrades gracefully so as we'll see in a moment the galileo system for example which is the european version of gps also uses this frequency it uses different codes so as you add more and more satellites your signal to noise ratio goes down and it gets it looks a bit more noisy because these codes look like noise but it doesn't just stop working it's a graceful degradation now that said you know on paper a gps receiver is a complex thing no question about it and i somewhat hesitate to devote many more minutes to describing it but broadly speaking you've got this high frequency front end here as it's called on the top left of the slide and that's got the antenna and it mixes it down to an intermediate frequency and that looks fairly conventional you've got this thing here often called the signal processor on the top right and that's doing all of this matching of codes working out which codes are in use what the delays are and so on and then it passes its estimates or sometimes called pseudo ranges because they're not quite ranges yet but they're estimates of primitive estimates of range to a computer and every gps system has a computer in it because there's a computer that handles all of your interaction with it or all the whatever purpose you're using that location information there's another computer and all that computer does is solve hyperbolic equations and even that is yeah it's more than a student project to solve those uh equations and then the rest doesn't matter you know so um it's a sort of remarkable well let me show you um the first gps receiver and you can you can sort of feel um feel my pain if you like so on the left hand side is the first known gps receiver those are seats there ladies and gentlemen so two large presents well i mean they're so large they might be small but they're they're substantial members of the u.s air force no doubt sat there receiving these highly secret uh signals as we know that would now fit into something about this the size of a pencil tip so gps has been an amazing recipient of miniaturization frankly i would never have guessed that it could be miniaturized um so effectively partly because it's got this hot it's got a front end that's quite analog you know it's difficult to miniaturize analog stuff and by the way over on the right um is a um is a satellite this is a boeing block 2f uh satellite so comparatively recent these are the antenna up here these ones are the civilian antenna i think and these mysterious ones up here are doing other things for the military not shown here but mounted on the side are the solar panels and um the they have a design life about 10 years these things so you sling them up there they open up and they're quite heavy and that's because they operate near the van allen belts which are full of um bad radiation so they're shielded i think so they they weigh between one and a half and two tons so it's quite a bit of the earth's resources are expended throwing two and a half tons up um that high so it's a good thing that they're useful um and this even this got fairly soon uh miniaturized so this is a picture from the smithsonian museum of one of the very early um sort of standard gps uh receivers um it's about this size it's called a man pack um yeah again perhaps perhaps perhaps a word that we wouldn't use nowadays but it is still called a man pack and um cost about forty five thousand dollars they're quite affordable in military terms and it weighed about nine kilograms and the of course the designers thought well you just sling it on the back and use it and the soldiers soon discovered nine kilograms was far too much thank you very much so they strapped it to the side of jeeps and all sorts of things and we're all perfectly well aware of how large a current gps receiver is right that was richard's simple gps system i can see some people the audience are sort of fainting away at the complexity of it even at this point there is a bit more complexity i wish which i apologize but it's kind of cool so i'm going to not apologize too much so we now have to introduce you to some more of the fiendish world of us military acronyms uh the various generations of uh gps are known as blocks for reasons that no one knows so block 2a which was the first generation satellite there are none of these up in the sky at the moment had these two channels right l1 and l2 and i've talked about one of those codes which i've shaded in green for civilians and i haven't talked about p y so let's just briefly mention that p is just a longer version of the ca code you have to have bigger registers but because it lasts longer you get more signal processing gain and more accuracy so it's exactly the same principle and it runs over the top of your ca code how does it do that it's code division multiple access so it just looks like noise as far as the other code is concerned l2 is a separate channel which only sends or only sent p a version which is the military code and the advantage of having two channels a twin channel receiver is that you can offset some of the effects of the ionosphere because you're using two different frequencies so the difference in reception times you get across those two frequencies allows you to backfit a model for that for one of the principal corruptions that affects your positional accuracy the acronym py refers to encryption the very early versions of these sent out unencrypted high accuracy signals they very soon realized that it was wise to encrypt the military signal so a military grade uh gps receiver has to go away and be have a key put into it which allows it to unlock the uh encrypted uh code if you're like you and me you do not have access to that code so we have only to work on the ca system now it soon became this was you know pre-2005 it soon became fairly obvious that the civilian uses of gps were far exceeding the military uses and uh several u.s presidents were involved in

basically adjusting this service to be more use and more help to the to the civilians of the world gps is a system that spans across the world or whether you're an enemy of the united states or a friend of the united states everyone gets access to the gps system it's worth just pondering that i mean i know there's a lot of america anti-american feeling around the planet at the moment well it has been for some time but those lovely americans they did give us gps you know it's a marvelous marvelous uh donation to the to to humanity incredibly useful um thing now it's worth saying very early on in gps's history there was a deliberate corruption that was applied to the ca code it's called selective availability uh to make it a little less accurate and you might remember in one of the early golf wars i think um there was a temporary shortage of gps military-grade gps receivers so they turned off selective availability and all of our gps receivers suddenly started working very much better it was a beautiful moment and it's never gone back and in fact i think it is now they're not allowed to put it back under u.s laws that's there with us forever well as we've gone through various generations of uh gps um so to our satellites there are some 2r satellites still up there what you can imagine what's happened if my clicker will work is that we will we have more and more channels in more and more bands but the principle so anything in pink is military and everything in green is available to you and me so we've now got to this highly desirable situation of three bands uh offering signals to us in green which offers the potential to all of us to correct for these ionospheric um errors which are one of the principal causes of error in gps so the satellite depending on how enthusiastic user of technology you are you you may not be using well i'm confident you won't be using all of these but some of you will be if you if you're fond of buying the latest absolutely the latest latest mobile phone um you might be using more than one of these bands mine does oh better not tell my wife how much i had to spend on it my my view was it was all part of research for this lecture so i'm putting it on expenses um now gps is only one system i entitled this lecture gps because it's the acronym everybody knows it stands for the global positioning system and it's become a generic word hasn't it for a machine that tells you where you are but the whole system is called gnss and gns comprises a number of different satellite systems devised by different countries so the the daddy of them all is gps which currently has 27 satellites up there and has worldwide coverage but we also ought to mention glonass which is the russian system glonass doesn't use cdma it uses frequency different frequencies from different satellites if i remember rightly uh which is a bit odd although it's rumored to be moving too um the next gen is river to be moving to six cdma it's very good in polar regions uh glonass because it's got some satellites with high uh angles it's up to sort of 65 degrees up from the equator and then there's galileo which is the european system which has 22 satellites and that is worldwide and then there are the ones that people know a bit less about but if you've been to china you might have been using a baidu um which is a is a bit of a spin-off of galileo there were some early collaborations between the chinese and the galileo team they do is the was it the first yes it was the first system i think to use geostationary satellites so most um of these systems the satellite whizzes around the earth uh like that and those deliberate design decision um because if it whizzes you can make sure it whizzes over the united states or whatever the home territory is and get its update from a secure master station that is not easy to spoof and hack so there's a good reason for having a whizzing satellite also i'll talk about this later but it's diff it's more difficult to shoot it down you know because it's moving it makes it easier to work when you've only got a few satellites because although you might not have a satellite in view right now you will have one because there will be something passing as it zooms around the earth but if you're only doing one area of the world and baidu does asia you could just pop a satellite up above asia and it will just sit there and that's also true of navic the um the indian system irnss there's a lot of rumors circulating about navic if you read the newspapers or the web it is asserted that the indian government decided to invest in navic because the american government during the 1990 cargill war between pakistan and india what he said is that u.s government denied access to gps data i don't know what that means because gps is free right i mean so i'm not sure what data they were expecting i mean possibly it was the military grade decryption that they couldn't get possibly selective availability had finished long before then so i don't really know i suspect that was a sort of urban myth but um anyway you know the indian government decided to build its own system which also has geostationary systems and then the japanese system is most interesting the japanese system is obviously over japan and covers northern australia and what's so interesting about it is it not only it works with gps so it provides infill to give you additional signal coverage in some of the difficult bits of japan very high you know urban environments deep values and so on but it also provides augmentation and that's an interesting topic in its own right so the current one of the current interesting uh innovations are satellite based augmentation systems called um s basses now probably the easiest way to explain this is an old system called differential gps so one of the tricks you could do with differential gps is i'll build a ground station and i know precisely where that ground station is i'll measure the local gps it's receiving which wanders around and has got errors and i'll just correct i'll send out a correction because if the correction works here then it probably worked for you in the surroundings so that's a differential gps or a ground based correction system okay so let's do that but it's a bit difficult to transmit the difference locally so what i'll do is i'll use a satellite ground station i'll send it back up to a satellite a geostationary satellite and it will beam it down so this is augmented set of satellites that sit alongside your constellation a gps or glonass or whatever satellites and give you additional positional accuracy now so great idea not many people need it i mean you and i we i don't need to know where i am to within you know meter accuracy but if you're an aircraft then you're very enthusiastic about sbas systems because um i'm sure you know this but when an aircraft's coming to land in poor visibility uh the pilot either has to sort of take a chance which isn't approved of in the aircraft industry or they use some radio waves that are directional and on the ground and you sort of fly in on those radio waves quite an old system but it's expensive to do what you can do now is you use your very high accuracy waas or egnos system that's in your cockpit and it tells you your altitude and position very accurately and you fly in by gps instruments and this is approved in almost well quite a few western countries for for use britain is the exception um i don't want to give a lecture on brexit but i will point out that we're now no longer part of egnos and we're no longer liable you can try and get the egnos data if you like but you can't at the moment because it's blocked and if you're landing in britain at the moment you're not allowed to use eggnos to land because we're not part of that system but back to queue data q0s is one of these systems that provides both augmentation of the gps constellation and that additional side data that gives you super super accurate positioning right brief observation about resilience i mean obviously the gps system and the gns has had this fantastic and amazing resilience that you could see people sort of raving about in the leading film to this that said it is vulnerable to hacking and some of these have come out in the uh in the recent future i'll sort of briefly comment but not say very much about the two at the bottom obviously space warfare is is now a possibility and nato in i think it's quite recently maybe as late as 2018 declared that space was a new domain of warfare which means i think they don't quite know what it will involve but there might be something bad going on so that's one possibility there is also the possibility of ionospheric and other variations which might possibly affect the ability to transmit radio waves effectively for a bit the best known of these was the carrington event of 1859 which caused a lot of disturbance in telegraphs and various other things so the two things that are common the commons probably the wrong word but are building in importance are jamming jamming is where you use a high power transmitter to essentially block the receiver from working at all and spoofing let's talk about the first of those jamming this is a paper which was using the international space station to look for any transmissions upwards in the l1 band clearly anyone who's got a high power l1 transmitter on the ground probably has some nefarious intent and if it's transmitting ca codes there's no reason to be doing that from the ground so this is an air force base on the west coast of syria probably being used by the russians and it is transmitting uh high power l1 carrier waves in an attempt to disturb local gps receivers solution well you could use a very directional antenna antennas are very omnidirectional at the moment you could increase the power in your satellites and current gps and galileo satellites are getting more and more powerful so sort of battle of the titans you know i shout you shout louder i shout you shout lola that might be the way around um jamming jamming needn't be that high power um trinity house which is the organization in the united kingdom that looks after uh atons aids to navigation on the sea did an experiment with one of their ships where they got a tiny little uh jammer with less less power output than a mobile phone and managed to move all of the gps's on their ship such that they were way way out of position so there isn't a lot of resilient design in gps receivers at the moment so that's jamming and the other one is spoofing uh spoofing is uh more evil if you like that's that's not only sending out a signal to capture the thing but with a spurious position and there have been several um examples of that in recent times most of them have taken place in russia or the crimea and some enterprising intelligence analysts in washington dc have correlated these appearances and they're fairly easy to spot because it's often with ships and ships are very um very disciplined about writing down their position in the log you know so they'll look at the gps position they'll write it down and log it because they're required to do that as part of their their code of practice you can also use a system called ais where ships automatically transmit their uh position and you can spot ships leaping about it was a good example of all of the ships leaping suddenly into the middle of an airport in russia you know instantly strangely these events seem to correlate with the appearance of president putin at these places so the accusation is that the russians have developed an effective spoofing technology i think it's probably unfair to call out the russians i think it's an obvious thing for people to do but obviously it's intensely dangerous and i would not i certainly wouldn't recommend trying it at home there's been a little bit of work in the united kingdom on what would happen if gps would fail and there's this report which you can read which is called a blackit review which was commissioned by actually another um gresham professor professor chris whittie when he was in a different post to look at what would what would happen to us and there have been reports that look at the economic impact of that and they draw it around and at one point the minister who was at caroline knox was persuaded that we had to have a backup system that the uk was too dependent upon gps and she said i believe that we should have a ground-based system which he calls in this letter e lauren well i i really can't get my head around this i mean this letter was written two years after we withdrew e lauren so it's the most peculiar sort of observation and frankly i i have a suspicion that isn't the way forward um we have a gnns and it is got multiple political actors in it surely that is the way to give ourselves redundancy to make sure that we're never reliant on one of them on a brief political point i would say britain i think is the only country that is a member of the permanent u.n permanent but there's a permanent member of un security council and does not have sovereignty over any of the gnss right so just talking very parochially for the time being because this is a lecture being recorded in britain we have put ourselves in the position of needing gps for almost everything we do but having no political power over any of it and that was a deliberate decision to remove ourselves from the galileo program which was one of the fallouts of the brexit negotiations i i assume it was just too expensive so that's an interesting observation what i think in terms of this story this series that i'm talking about which is sponsored by the worship or company of information technologists is all about invention and i've tried to pick inventions that perhaps haven't not so well understood by people that have had this most enormous impact and i'm confident myself that gps sort of meets the bar you know it's had this most enormous and important societal impact and it's detailed and intricate and fascinating and thank you very much for joining me in the explanation of it thank you so the first question is how do all of the detailed information for example speed limits road delays construction gets sent to my gps provider and then to me so quickly yeah so what this correspond what your correspondence is talking about here is the computer that we call a gps in our car and contained within that is the gps that i've been talking about and around it is another computer that knows all of the roads in say the united states or all of the charts in france or europe or whatever and how are they put in well it used to be done by a team of people typing them in so you would very laboriously digitize all of the roads in the united kingdom from aerial photographs and then you would type in where all the speed signs were and where all that was and indeed all of the road works which you collect from the main control and then you would use a separate radio trail a terrestrial radio channel to tell you what was going on it's changed a bit but not that much so it's basically two systems in one there's the knowing where you are and that's the gps i know we all call it a gps but that's the gps and then but it might be galileo now remember and then there's all of the navigational software and all of that stuff which of course now as we all know now sits easily on your mobile phone thank you um and one more from the online audience can gps be hacked by malevolent organizations or countries it would seem so is the simple answer to that so the first question might be can it be hacked by non-malevolent people right just by accident well there is a danger you know it's a very poor signal-to-noise ratio at the receiver so i think it is possible to overwhelm your gps and you can see the evidence of that because when you're indoors it doesn't work the just doesn't get transmitted so the the the answer that wasn't to the question that wasn't asked is it is quite sensitive to jamming the answer to malevolent powers is sort of contained in the latter part of the lecture i think thank you um so i have a question around the laws for building one so you know you did show that there have been new gps's made what's the law around that of course the law is like you know intricate but can can can i build one um am i able to build my own gps or are there laws which stop me or laws which stop other countries from building their own what can you briefly describe yeah good question so um let's talk about the usa right in the usa certain um activities are subject to prohibition uh so anything that interferes with gps is basically you'll get a knock at the door and the boys in blue will will will deal with you and there are laws stopping you interfering with gps there are laws in this country about interfering with official transmissions there is nothing to stop other countries building additional systems if they do it right what they do is they form a treaty so the galileo for example which came after gps was enshrined in a treaty between the u.s and all of the european countries and what he said it's very simple what it says is we agree to use these frequencies we agree to use these ca codes we agree not to interfere with you um there's one interesting thing it says right at the beginning um so long as it's not wartime so if it's wartime all bets so in virtual reality how is gps and virtual reality could have mixed in a creative way in the future right that is a complicated question i knew you'd pick the last question to be a fiendishly complicated question but they are intimately linked technology so the lady if you didn't hear the question was asking to what i think you were asking to what extent are vr and gps intertwined and vr doesn't work unless you know where you are right and one of the challenges when i was a kid really of getting vr to work was to narrow out know where you were and gps just wasn't good enough to do it now it is anyone here play pokemon go just looking at the more antique of the audience oh yeah well there is a bit of an age split here but a few few people my age have had a go at it okay so um one of the things that happens in pokemon go is pokemon pokemon go for you those who don't know is a vr game where you you go and find jewels out in the real world and of course it's very dangerous and probably get run over by all sorts of cars when you're trying to you're absorbed in your in your screen um there is a small niche area which is pokemon go hacking where you spoof your gps position back to the game so people think you're somewhere else right and there's an example of now i haven't talked about that sort of hacking that because that's a that's a whole different form of attack on the gps system where people are pretending to others that they're not where they say they might be it's also it's not done quite as maliciously but fishermen often turn off their ais receivers so their competition don't know where all the sea bats are to be found okay thank you very much once again please join me in thanking professor harvey [Applause]

2021-10-26 01:20

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