Global Positioning Systems GPS

hello everybody today we're focusing on global positioning systems but which are more commonly known as gps and this lecture is really gonna focus more on the idea of kind of how the whole system works as opposed to the the use of the actual receiver instrument so that will be the focus of lab so for the lecture we're really focusing on the system as whole and how it works one too far let's back that up there we go so little history of it so gps was developed for the military developed during the cold war and basically the the space race so the russians decided to launch sputnik in 57 and then there's just this focus of getting into space and space exploration and so one of the big things that we then focused on was the idea of of this gps system and being able to to to help help with location lot so the big focus at first became the us navy so in 1960 we had the transit system which allowed navy ships to fix their position once an hour so just think about how it's gone in in the 60 years from 1960 to now in terms of navy position navy ships had to check their position or were able to check their position once an hour whereas you now have gps in your phone you get you can get gps just receiver and it takes seconds to to get that information transit was replaced by timeation in 1967 which started using atomic clocks and we'll talk about those little bit later in the lecture no did it again there we go so gps was exclusively used by the military until 1983 the reagan administration then opened up gps for civilian use and that became extremely important for natural resources because it's something that has become really useful tool for natural resources now that now that's not the the military use only always do like to think about what else is out there because i'm sure the military has come up with better system since then and always kind of wonder what what will we what will gps look like in 10 years or so gps originally was about 11 satellites but then with the new system the network that's been set up now you need minimum of 24 satellites and currently there's 31 active satellites in the gps constellation and so if we take look at just kind of the development so there were 13 satellites launched between 97 and 2004 eight satellites launched between 2005 and 2009 12 satellites between 2010 and 2016 10 satellites are under contract and they're they're getting ready to go so the the big thing here is to just understand and see there there are some slight differences between the satellites i'm not going to expect you guys to know all that i'm just trying to give you basic history of it and really more of trying to paint the picture that it is so much more complex and there's so much that goes into it more than just the idea of there's something in space that beams signal to this thing in my hand it's there's lot that goes into into figuring it out and so let's look at where we're at today with gps kind of myself out of colorado real quick so the u.s gps system is managed by the air force the 50th space wing at the shriver air force base in colorado springs colorado one of the reasons for it it being in colorado springs colorado is it's it's high plains so as you can see by this picture here on the right of the gps facility it's nice and high above everything else but also flat so there's not lot of things that can interfere with the system nav star is the system that gps falls under is really expensive 1.2 million dollars of maintenance every year and that's because if you're managing 31 satellites and all that goes into them and creating satellites and doing all that it gets expensive the original accuracy was limited to 100 meters but in 2000 clinton turned off that selective availability and now you can get down to about meter or you know it kind of depends on what other things what other sources of errors you have but you can get you can get pretty accurate information these days so what kind of systems are there out there all sorts of things mostly they're used for navigations we have them in our phones or our tablets or our cars also handheld systems mostly in natural resources it's handheld receivers some people just use gps for to have fun so there's geocaching which the video for your lab we'll discuss little bit but just the idea of people hiding stuff and then you taking gps receiver and trying to go find it and kind of having little little adventure there's there's all sorts of different ways to use it i've always liked putting this picture up here of this toad with the little gps capture on it because it's just it's it just kind of it makes me really think about the history of gps to gps now and how how amazing it is the way that the technology has has changed that we're able to attach this tiny little receiver to this little toad and be able to monitor everything that this toad does and it's you know we're talking about system that's only 60 years old so pretty amazing in my mind there's three segments to any gps system the space segment the control segment and the user segment the space segment that is the satellites out in space the control segment is either control stations or base stations or part of that control segment is also that nav star base in colorado springs and then your user segment is going to be the receiver and then if you use any mapping software for the information that you get that would also be part of the user segment so three segments of the gps system space segment control segment user segment slide myself back up here so navstar gps they're going they're at about almost 13 000 miles above the earth and they orbit every 12 hours they are not geostationary so they are constantly moving now in order to move around in space and in order to stay in that constant orbit they're actually moving extremely fast probably somewhere in the 17 000 mile range per hour that's about how fast those satellites are going in space there are 31 satellites given the rotation of the earth satellites repeat exact ground tracks once day that's the that's the goal there are six orbital planes so if you look at this graphic on on the right here these red lines they represent the six orbital planes that are available for the satellites and the reason that they set up six orbital planes with at least four satellites on each orbital plane was so that you could get good satellite coverage for anywhere in the world because the idea is the whole point of gps is want to know where am at any part of the day and the system is going to work off of triangulation which we'll talk about in little bit but you need satellites in different places at different angles in order to be able to get the best the best version of your location and so the other part of the six orbital planes is you don't want just satellites going all over the place in any direction that they want because you don't want them banging into each other so you set it up to where if you know where the satellites are going to be and you know that it's going to be on consistent track then you can set up these different tracks and then make sure none of these satellites are ever running into each other the great benefit of doing the system this way is that you get at least six satellites within line of sight from almost anywhere on earth all the time which is that's gets to the idea of this is why we want gps because we want to be able to find out where we are anytime we can here's good good look at those six orbital planes and then the satellites the gps satellites that are that are orbiting so just kind of look at what that what that looks like and then the six orbital planes with different satellites moving at about 17 000 miles an hour 13 000 miles above the earth really impressive when you think about it so these satellites so each prn pseudo random noise or sorry each satellite has prn number pseudo-random noise number which codes the signal the gps receivers call satellites svs or space vehicles sometimes more likely with the basic receivers that you get they'll just say satellites and the satellite just transmits radio waves to earth so each satellite or each signal contains the orbital elements the clock behavior the system time and the status and almond mac is also transmitted which will provide the data for the the active satellite in the system so it's basically it helps the the satellite find the other the other satellite signals as well so lots of information being transmitted at all times from these satellites so let's break it down and let's make it more more obvious so three basic pieces of information that gps satellite is going to transmit an ephemeris which is its own location the almanac which is the approximate location of the other satellites to it so precision of the satellites and then time signal and that's going to help enable triangulation of the position so an ephemeris an almanac time signal or its location its precision in terms of the other satellites and then the time signal so that we're able to triangulate location so the satellite clocks sorry there we go so each gps satellite has four atomic clocks on it atomic clocks rely on the natural resonance of atoms to provide steady tick the master clock right now runs off of cesium it's determining true time is really hard and very complicated super math heavy but it basically comes down to this one second is defined as 9 billion 192 million 631 770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields that is what is happening to figure out one second on these super super accurate atomic clocks and this is look at the master cesium atomic clock at the national institute of standards and technologies in boulder so if you've ever wondered about time and how time is calculated it is extremely complicated it's it's kind of amazing to me to be to be honest little more information there we go so atomic clocks are incredibly precise but not perfect and that's just because you know you've got satellites moving around at these incredible speeds you've got things that can interfere you've got orbital instability so all that could cause problems however these clocks are actually pretty stable so it's estimated it would take an atomic clock 32 000 years to lose one second at time and the satellite will give up way before that 32 000 years of that atomic clock gives up so so actually very stable very consistent way to to tell time the clocks are frequently adjusted or they're adjusted two clocks used in the ground control stations the control is really gonna be it really it's just kind of the fine tuning of the satellite and space segment there can be slight variations in clock times because of the the signal speed and transmission that's why every once in while you'll get slight inaccurate position but most of the time it can it can correct later and then so the big thing is the gps receiver clock and the satellite clock need to be synchronized in order to give you the most accurate location so every once in while your location will be off but the receiver will sync up with the satellite's clock again and then your location will be good so that's the big thing though because the location is your location and the distance is going to be based on the time between the the signal from the satellite and your receiver so if that time is linked up you get an accurate location if that if there's any variation in that time you get less accurate location so control stations and base stations so this is your control segment of your of the system the control stations specifically ground-based facilities that monitor and control satellite movement operated by the us government so the nav star base in colorado the shriver air force base that manages the system that is control station there are other ones the the shriver air air base is the master control station your base stations though are are not associated with control stations these are they are transitory you can move them place to place usually you install them at locations that have been surveyed to very high level of accuracy so we talked about having those monuments on the ground that show location usually if it's something like that that's been surveyed and we know for sure that that is exactly where that point is that's the type of place you'd set up base station so that you could then check out what the gps signal is and see do we get that same location and then that way you can really make sure to pinpoint what the error is that you might have and be able to try and reduce the error as much as possible and so here's an example of using gps base station so you've got these satellites up here and then you would set up your base station so there'd be gps receiver radio antenna and then basically your you've got your base station set up and then you might set up an unknown point too and then you just try and check and you say all right so if this is known point know for sure this is exactly where that point is and know my amount of error then when set up this point i'll be able to figure it out and be able to understand what this point is because of my knowing this point that's basically how base station is going to work it's your it's your correction it helps you just really figure out your amount of error and then be able hopefully be able to reduce it or if not just be able to understand it so that you can give as accurate of location as possible you when you if you use one of the just handheld receivers like the ones we have at school are the garmin etrex 20s you'll you'll see when you bring up the satellite screen that you'll see it might say when you're looking at the the strength of the satellites and that means differential correction and it just means that there has been this process of basically trying to trying to improve the signal that's that's this idea of differential correction where you they've done they've done extra work to try and make sure that that you're getting more accurate location so the gps receiver here on the right are some examples of them i've used trimble i've used this garmin 6d cx and at my old school we used to actually have these ones this is very old version of garmin etrex but the size is similar to what we have at school and then just kind of going over how does the receiver work so it's going to work off of triangulation so you want to get at least three satellites but preferably four if you're trying to do anything with elevation involved and it's kind of basic four-step process each satellite will broadcast signals with their location status and precise time information or their ephemeris almanac and and time location the the radio signal will travel at the speed of light and then the gps device will receive that radio signal it'll note its exact time of arrival and then it'll use that to calculate the distance from the satellites and then that will help it triangulate the location and so it's just going to be it's going to use basic geometry to determine that exact location so i've mentioned triangulation couple times so let's talk about triangulation so the location just depends on triangulation so to understand the horizontal position you need three signal distances from the orbiting satellites the receiver will calculate how long each signal took in conjunction with the almanac to determine the distance to each satellite and because you figured out the distance to each satellite from the location you're able to to precisely get the the location because rather than just saying here's the signal from one satellite that could be off but if you get different satellites from all these different angles telling you this is exactly where your location is the better off you're gonna be now the other thing is it takes minimum of three satellites to do this but what you'll see is if it's good time of the day and you've got nothing in the way you might have 20 of those satellites or you are 10 or 15 and the more you get the more you get signal from satellite saying this is your location this is your location this is your location the more it's going to make it easier to find that exact location and then just going back to something mentioned earlier as well fourth signal is needed if you want to get elevation but if you have anything more than four elevation and location the more you more signals you have you have better hardly ever will you will you get 31 it'd be pretty amazing if you're in some sort of location where all the satellites that are focusing on the whole world would be able to you'd be able to pick up signal from them but there are places where you can get quite bit of the satellites to to to pick up so just another look at the whole system so number one here tracking stations use radio to determine the orbits of the gps satellites the command center will transmit orbital data time corrections and locations of other satellites in the gps constellation gps satellites simultaneously transmit synchronized time and orbital data back to earth the gps receivers compute the location using the orbital data and the difference in arrival times of the signals for at least the four satellites and the whole system works together all the way from shriver air force base to the satellites all the way to whatever location you're at and gives you signal in your receiver so what can be what could be problematic in terms of using the gps atmospheric interference is big deal so your atmospheric interference is least when the satellite is directly overhead and so what that means is think about the idea of up the if you have if you're outside and you're trying to get gps signal if the satellite is directly above you with nothing in the way that's going to be the easiest way to to get that signal but if the satellite is all the way out on the horizon in either direction or off at some weird angle there could be all sorts of other things that get in the way in terms of of what's out there in the atmosphere space dust and you know other satellites and other other things flying around in the atmosphere so it's it's important to just kind of just think of the idea that straight up would be the easiest way to get signal ephemeris it's only valid for about four hours if the satellite isn't where it was suggested to be in the almanac it will cause location determination so just making sure that the that that the ephemeris the the position it's it's only good for little bit because those things are moving so fast you could get atomic clock errors mostly it be in the signal relay of the air not in the clock itself and then your gps receivers clock so it every once in while it won't be synced up with every single satellite because of all these other things going on and more likely this is much more likely to happen as source of error than the atomic clock error for you though these last three are the ones that i'd say are really the most important sources of error because they're really the ones that we can actually do something about so signal to noise ratio so satellite signals do not penetrate buildings or trees so really try and stay away from objects especially any object that's going to be overhead because that's where our best source of signal is going to be multi-path error and multi-path is the signal bouncing off of landscape features so just here's an example right here if was standing right here in the middle of this of this big this big combine and this house and i'm trying to get gps signal and then the gps is the satellite is sending signal but it's bouncing off of this building and it's bouncing off of the combine and then i'm trying to get it it's very possible get kind of garbled signal or not as accurate signal and then shadowing so the idea of the signal being reduced in forested conditions basically having so many different objects and so many other things in the way to make sure that there's not good signal now the big thing to think about is if you're in heavy forested conditions and you cannot do anything about this you know you just have to understand that you're gonna have that you're gonna be dealing with air and so you really just kind of want to plan for that like for instance if i'm down in valley that's in the shadow of mountain or hill and know that there's going to be that's going to interfere with the signal especially at the time of day where the satellites might be really want to just think about okay my locations might not be as accurate as want them to be so just have to think about all right well if that's the case then if i'm trying to walk to this specific point i'll walk there but i'm gonna have to assume that once get to where the gps tells me it might be somewhere within that 20 feet or 30 feet and might have to do some looking around so it's just important to to understand that to just avoid frustration so just some important points with the gps horizontal gps positions typically are two to five times more accurate than vertical positions so in terms of your elevation so distance is much easier for gps to calculate rather than elevation elevation is little bit harder but we kind of have really mean you most of you have probably used google maps or waze or apple maps for quite bit of time so distance we've kind of nailed that one but elevation is still little bit trickier the multi-path error that we spoke of that one is the largest source of inaccuracy in natural resources due to dense vegetation or severe topographical relief or just basically lots of trees and lots of plants for the signal to bounce off of especially if it's taller than us or the receiver that we're holding and then topographical relief if we're in mountains and lots of up and down or we're in valleys or saddles might get really hard to get signal here's just an idea of those sources of error and how much you can see how much worse of problem multi-path is as opposed to some of the other stuff so some important questions whoops when is the best time to collect data it varies by satellite position and time of day the best thing is to just kind of try it bunch of times and you'll just kind of start getting used to which satellites are going to be around at which times how much data do need to collect really just think about most of the time you're just going to need to take receiver out there and just go and get data but you really want to just think about your project how much work is involved how long is it going to take and how how accurate do you need to be because if you need to be really accurate maybe you need to set up base stations and and go through all that if it's just it's okay that it's somewhere around there and we can figure it out you don't need to spend as much time so just how accurate do you need to be do need to differentially correct data it's probably needed so just when you look at the receiver you're using if you can get that differentially cleric to data if you can get the the on there you're great if it's not available then maybe the place you're working at needs to invest in base stations and learn how to go through that process will the base station have data covering my time period the base stations don't collect data continuously so you kind of just have to go off of whatever base station you're using it and figure out how to best use it do need to remove multi-path error if you can great you might have to delete some points that are just terrible but really it's much more of an idea of how do you reduce it then how do you get rid of it and then will gps data be integrated with other gis data absolutely but you just need to know how to do that process and we'll talk about that with gis labs and classes and that's it so hopefully that's just quick overview of gps and then yeah we'll get as close to hands-on as we can with the lab