How GPS Works, And How It Got Better Than The Designers Ever Imagined

foreign Scott Manley here as you may have heard I'm a bit of a fan of space and space flight you know when I meet regular people on the street I like to talk to them about space and space light and they think well that's all very interesting but they say I don't have to worry about this stuff in my everyday life I mean you don't exactly have to do orbital mechanics to figure out say how to get to work at least that's what they think because if you're using a phone to navigate it is using GPS and for GPS to work the receiver has to figure out the orbits of the satellites that it's receiving from so effectively all navigation we do these days is orbital navigation you see GPS has become a core part of Technology initially when GPS came along it was like low Precision it was actually thought the civilian GPS would have a Precision of 100 meters it was more like 30 to 50 meters but nowadays it is accurate enough that it is possible to land an aircraft on autopilot right I've been flying around an aircraft and I see GPS is everywhere like when I fly my storage you might have seen some videos from it and the display in front of me the primary flight display displays a 3D model of the terrain and that's used GPS and orientation data to show what I should be seeing if I would say inside a cloud and you know you can see that this is accurate enough that when I come down to the runway it's putting me in exactly the right position also you know like every instrument in a pro a lot of the instrument approaches these days are all uh GPS based and every aircraft flying around is using adsb instead of a secondary radar for their locations as I said it's everywhere so how did we go from something of this very low Precision to something that was so precise well that's what we're gonna tell talk about today right so GPS was conceived at the Pentagon over Labor Day weekend in 1973 so this is like the 50th Anniversary purely by accident yeah it was back then it was called defense navigation satellite system dnss and they took their inspiration from a couple of projects that already existed there was the Navy's thymation system which had high Precision clocks and satellites that would just orbit in low orbit and they would beam the time down to military ships or the Navy ships so it could be used for other navigation systems you either tracking against the stars or maybe talking to the loran system and there was another project that was a uh an abandoned idea for satellite-based navigation called project 621b so out of this meeting came the basic idea for GPS where you had a 26 satellites in 12 hour orbits you're high up they could see a lot of the surface so the first geostation GPS satellite was launched in 1978 by 1993 the network was fully operational and in the intervening years there were two very important events that uh sort of shaped GPS in the early years first there was the incident with Korea Airlines flight 007 which on flying from Alaska to Seoul made a navigational error with the autopilot and flew over uh you know Soviet airspace and was shot down with the loss of everyone on board and as a result of that uh Ronald Reagan essentially signed an order saying that the GPS system although it had been intended for the military would have a civilian component available to everyone to avoid these things going forwards and then 1990 you know the first Gulf War at that point the U.S military still hadn't been totally sold on GPS capabilities but the people on the ground they were finding that these GPS receivers that they had acquired from Marine work or you're from boats these were perfect for navigating the featureless Landscapes of Iraq out in the desert in the desert storm and so families back in the U.S were purchasing these things and getting them shipped to their people on the ground to help them out with their navigation needs and of course that then accelerated the Navies that are sorry the the Armed Forces adoption of GPS and nowadays we have an incredibly uh it's a pervasive system that is everywhere right so the basic way that the global positioning system works as you many of you probably know is that you have a number of satellites in these high orbits and they are transmitting signals with very accurate clock codes and by reading these signals you can get a measurement of the distance to the satellite and by knowing the orbits of the satellite you can figure out where you are on the surface and you might think well okay I'm trying to get my position in three-dimensional space so I need three satellites not quite because the satellites have these incredibly accurate incredibly expensive clocks and they wanted a system which didn't require incredibly accurate and Incredibly expensive clocks on the ground so what you have is a not quite as good clock and that means you can't really accurately measure the distance to the satellite but you can measure the differences in arrival times from these signals and use that to figure out roughly how accurate your clock is so what you're really measuring is a four-dimensional value your x y z and your delta T time correction that means you need a minimum four satellites for GPS to work now the the system they initially launched used two different frequencies it used there the L1 frequency which was 1575.42 megahertz and in other words the L2 frequency at 1227.6 megahertz and more modern satellites also have the L5 signal and we'll talk about that now the initial satellites had the standard positioning system which was provided by the course acquisition code on the L1 frequency and then it had the precise positioning system used by the military and that used the code on L1 and L2 so the way this worked was uh it used CDMA right what is CDMA well um GPS was actually one of the first systems to actually use this so CDMA stands for code division multiple access if you if you're like tuning your car radio to different frequencies that is frequency division right you're using a separate frequency for each transmitter there's another tdma is time division so that's where you have a series of things cooperating on the same frequency and they all choose to transmit at different times and they coordinate amongst themselves CDMA is code division multiple access and this is where it gets cool so what you have is a binary code and in the case of What's called the course acquisition signal you have a a 1023 bit sequence which is unique to each satellite there's up 32 different codes and what you do is you take the signal and you essentially convolve it we multiply it by the signal that's coming in and if you get the things lined up to within one bit you will see a very strong correlated signal and if you're not quite there you'll have a like a week you know less than 10 correlated signal so once you get these things lined up you can you multiply them out and you can start to extract the actual underlying bits so this chord is just a way of finding the signal and identifying the specific signal and the beautiful thing is you can have two or three different satellites broadcasting on the same battery you could have like dozens of satellites on the same band and while they're talking all over each other by multiplying it by this code you basically extract out the specific frequency there's a specific data for that satellite and that's how code division works so with the geostation the GPS satellites the code is transmitted a thousand times a second 1023 bit sequences transmitted a thousand times a second right and that in turn encodes a 50 bit per second data signal so you have like 20 repetitions of the 1023 bit sequence and then the bit may flip or not uh and you have like 50 bits per second each data frame is like a 300 bits and it takes a five data frames to transmit a whole sequence so you have a 30 second block of data that is being transmitted on the course acquisition frequency and what this uh what this contains is uh synchronization words to actually make sure that the you know you're actually synchronized with the underlying data stream the timing of each frame it contains the orbital parameters for the satellite in question it also then contains our parameters for correcting for things like uh ionosphere and uh you know solar weather and finally it will contain in each 30 second block orbital parameters for at least one other satellite for one other satellite not at least and so it iterates through all the other satellites and over 12 and a half minutes it will transmit the positions or the orbital elements of all the other satellites and that's called startups so it takes about 12 and a half minutes to fully warm up a GPS receiver but once it's warmed up it can store that stuff in memory and put object forward so the next time it turns on it knows what satellites it thinks it should be able to see and therefore can quickly bootstrap into this find your try the relevant code and uh you know synchronize them and hopefully get started a whole lot faster so anyway that's roughly how this works I would like to go into a whole lot more detail but frankly uh that is like a task for somebody that wants to talk about you know software-defined radio so anyway coming back you have this sequence so you're transmitting about a thousand bits or chips as they're called per second and you're transmitting these time signatures so you can tell exactly on the bit boundary what time things are so you can imagine if the speed of light is 300 kilometers 300 000 kilometers per second then one million samples per second should get you about 300 meter accuracy but actually Hardware can go better than that because they can actually measure the transitions between bits and get the position estimate down to you know like 30 meters or thereabouts now the military code it's also transmitted at the same time but the military code doesn't repeat the same 1024 bits in sequence it has a for each satellite it has a week-long sequence and what you do is you get you sort of tune in you synchronize using the course acquisition code which will let you know what time it is and then from that code you can then figure out where you are in the big long week sequence and then synchronize to that and now now you're in on the Military Channel and the military channels use 10 times the bit rate so they can get shorter more accurate measurements but more importantly they can or they also work on two different frequencies the L1 and the L2 and by having two different frequencies you can compensate for one very important source of error the delay due to the ionosphere so the ionosphere is up above the atmosphere it's a region where you have free electrons that have been pushed all you're kicked off their atoms by things cosmic rays and more importantly ultraviolet light from the sun and these electrons and protons floating around they interact with the radio waves and slow them down a little so because of ionospheric delay that translates to a delay in the signal equivalent to about 10 to 20 meters so you lose about you know 10 to 20 meters of accuracy just because of ionospheric delay and as I said the basic civilian signal that's transmitted includes a model of the ionosphere which is you know specific to that time and so it changes over time they can update it and so they can cut that error in half using this very simple model but the military system because it has two different frequencies and because the effect of the ionosphere is dispersive that is affects different frequencies by different amounts you can look at the differences between the L1 and the L2 frequencies and compute the actual electron density and therefore what the actual delay should be and almost remove it entirely just by using the two different frequencies so yeah the military signal was a lot more accurate originally the US when they designed it they thought that they would have 100 meter Precision for the civilian system and that was what they wanted it to be kept at the problem was then that people that looked at the signal and worked with their you know smart human brains on it they were able to get higher and higher Precision so another system had been designed into GPS to make it less accurate for civilians it was a feature called selective availability and that's where they would introduce very subtle clock delays the clocks on the satellites would wobble back and forth by less than a microsecond and they would do this in a way that could be calculated by the military site but not by the civilians and so that meant that when selective availability was turned on the civilian gear would uh variance location by about 50 to 100 meters and you know this uh this was well this is what the military wanted but it turned out that the US government is not a monolithic entity and there were other agencies that really wanted to use this global positioning system there was a U.S Coast Guard the Department of Transport the FAA they all wanted better Precision for their particular things and even though the Department of Defense and these guys were all on Team USA they couldn't get special treatment so they had to come up with their own Solutions independent of the Department of Defense the U.S Coast Guard wanted really accurate navigation data to help shipping around the course since they came up with something called differential GPS I mean they didn't invent it but they began to deploy it the idea with differential GPS is you have receivers that are placed at known points where you know the Precision to within millimeters and they then take the GS the GPS signals they measure them and they compare their solution to what it should be and by then subtracting that error and telling everyone around them they can say well I'm measuring you know 10 meters east of where I really am so everybody nearby them says well I will adjust my position by 10 meters to the east that I'm actually in the correct location and this was great this meant they could get you know resolution of a few meters and that allowed the US postcard to work with shipping and to transmit correction data to ships that were working in and around the US now it wasn't just the U.S Coast Guard that did this countries all over the world took GPS and they built their own differential GPS systems I mean the difference is that the US you know well the US Coast Guard would obviously go to you know US Government Thanksgiving dinner and be all passive aggressive towards the Department of Defense where's my Precision come on so the fact that the U.S government was sort of competing against itself on this GPS Precision thing wasn't lost on the higher Ops in the US government and in May of 2000 they turned off selective availability forever and uh yeah there's actually great graphs showing the drop the or the increase in the Precision exactly at 4am on like May 2nd or whatever it's also interesting if you look at this graph you can see the horizontal error and the vertical error are separate on these and it's if you look carefully you'll see that the vertical error the error in your altitude is about twice as much as the horizontal error and this is important the reason why this happens is if you think about it you want to have the widest range of directions so that you can get best Precision right if you have everything All in One Direction then you've got almost no uh you've got a lot of errors but if you have things that like almost opposite sides of the sky that's like almost the best condition however while you can have satellites to the north the South to the east of the West you can't have satellites up and down because the ones below you can't be seen so you have only like half the space to work with in the vertical axis therefore you have half the Precision in the vertical axis see that vertical error was very important for one other government agency the FAA they wanted to revolutionize aerial navigation using GPS and GPS was not accurate enough for them again the original spec said 100 meters and that was definitely not good especially when you consider the vertical differences would be even larger so the FAA were working on their own differential GPS solution but instead of transmitting the uh the augmentation right directly to the aircraft via ground-based antenna they wanted to send everything up to satellites and geostationary orbit and then have those sand down to the aircraft using pretty much the same protocols that uh GPS you know systems actually used so this is what's called a satellite-based augmentation system you use a geostationary satellites and these aren't like US Government geostationary satellites these are just commercial GE dual satellites and U.S Federal Aviation they buy like some bandwidth on it they put in their software and they're just basically transmitting this data down and format that is usable by aircraft so it's called the wide area augmentation system or was and it is pretty much required on any new aircraft in fact indeed many old aircraft have it now despite it being a very kind of new system it was originally deployed in 2003 and Alec as of 2020 I think it was pretty much required in every aircraft that operates in the major air spaces in the US so if you want to fly within 30 miles of a class Bravo that is something like San Francisco Airport then you have to have uh adsb which requires uh was capable GPS system now Europe has a similar requirement they have their own network it's called eggnos the European geostationary navigation overlay service and it literally is the same technology using practically the same protocols to the extent that the same Hardware on the aircraft can talk to both systems and interoperate correctly so the goal of the FAA was to get the Precision down to about 7.5 meters of accuracy and but actually they did way better it turns out the measured performance is about one meter Precision at the 95th percentile and the other thing about this whole wise system is that it doesn't just augment the quality the accuracy of the satellite they can also detect when there is a misbehaving satellite and instantly tell all the aircraft to start ignoring this particular satellite and this happens there are cases where Like Satellites were moved on orbit and they weren't taken out of the network and so your aircraft or that we're using this satellite suddenly found their positions Drifting by about 100 meters which would be really bad if you were say on a very narrow approach into you know one of these mountain airfields and you couldn't see where you were going uh you know when you're flying with uh instrument rules you need the all the Precision you can get and these days if you look at the majority of uh instrument procedures they are all now coming up with their GPS based procedures because they work they are more diverse a lot of the old navigation Hardware like video Wars and non-directional being beacons and DME these old ground-based systems are either being disabled or they're being shut down or they're failing and nobody's wanting to repair them because they're not essential and so over time we've not only added a whole bunch of GPS based approaches to airfields which would never have them but we've also got rid of a lot of the old hardware and this is starting to dominate one of the important things to realize about these differential GPS systems is that the accuracy you know enhancement falls off as you get further and further from the ground station because there's just differences in the strength of the ionosphere over over time over you know distance so there's another thing we have is like a local augmentation system where you have a specific airport with a specific receiver and then that can transmit updates locally to those aircraft and then they can get again this sub meter Precision using GPS with the local augmentation and that's great if you want to get very accurate approach vectors for you know any Runway just say you have like two or three four or five runways and you don't want to have to build instrument Landing system antennas for all of them you just put in one GPS you know augmentation system and boom you've got the ability to Define approaches for all of these things and get your aircraft landed you can even Define approaches which would never have been possible using the the previous gear so yeah the success of this was system means that the terrestrial Network that had been developed by the Coast Guard was no longer being needed anymore and so it was actually shut down in 2020 and now even the Marine shipping is able to use these uh area augmentation systems and now the GPS satellites themselves those are also undergoing improvements modernization of the protocols so back in 2005 we got the first modern satellite it was launched there was a block 2R Dash M and this started transmitting in new civilian signals it was called the l1c and the l2c and that meant that we had a civilian signal on the L1 and the L2 frequencies so you could do those direct ionospheric measurement they also added the L5 frequency right this is another frequency range it was down in a range of frequencies which were supposed to be for Aviation navigation so this fit right in it transmits at higher power there's more resolution in the signal and that means there's less problems with interference right also new data formats so the l1c it transmits on the same frequency as the course acquisition and the thing is they didn't want to just shut down the old course acquisition system so they kept that running what you have is the new civilian signal is still transmitting it's using different codes different code sequences so by CDMA you can still pull this out but there's a neat trick that they did by uh carefully choosing their sequence they can shift like the spectral power around the center of the frequency so that they like so the original one had this like big peak in the middle and then you have a Minima and these but you know lobes that come out well they change the bit sequence in a very subtle way to make it wiggle more and now instead of having a big peak in the middle they've got nothing in the middle and the lobe comes up and so if you look at these two things on top of each other the two different civilian signals the old one and the new one the old one is strong in one place and where the old one is weak this new one is strong so they're complementing each other literally fitting the bits into the gaps in the Spectra just by very carefully doing the math on their Fourier series it's very clever and you know they do the same thing with the military signal the new military signal um pushes a lot of the energy far out into the lobes which actually then apparently makes it harder to jam because if you like jam the middle of the frequency sure you take out the old uh GPS but the military one is sitting Faro on the sides and it's sort of just working just fine while everybody else's GPS stuff isn't working this is it's all like very clever stuff and I wish I totally understood it now I should probably finish as well by saying that GPS isn't the only game in town the when we're talking about global navigation Satellite Systems there's at least three other major players there's glonass which is the Soviet which became a Russian system there's the European Galileo and the Chinese beetle and they're all actually pretty close in the design they all have their bands very close to each other the formats are largely software differences and uh I mean one thing that's is significantly different is that the old glonass actually uses different frequencies for the satellites and I think it's interesting because I think they have like eight frequencies available and they have 16 satellites so the way they fix this is they make sure that the same frequency is assigned to satellites that are on the opposite side of the Earth from each other so they will never interfere so they have these pairs that are opposing and on the same frequency but yeah other than that like all these different networks they all talking to them is largely a software change and that's important if you're say building the aviation you know the GPS systems or the global the navigation systems for aircraft because it's literally a software tweak to switch from one to the other and to transparently move from one country to another and provide consistent navigation so that you can make these accurate approaches and landings I'm Scott Manley fly safe [Music] [Music] foreign [Music]

Channel: Scott Manley

Views: 726,461

Keywords:

Id: qJ7ZAUjsycY

Length: 27min 19sec (1639 seconds)

Published: Thu Aug 31 2023