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A Guide to Understanding World GPS Part I Early GPS - Man's History and Fixed Stars Solar Flares and Sun Spots World Wide GPS Systems - Named GPS Systems Throughout The World Today (GLONASS, Bei-Dou, Galileo, and More) Navigation by GPS Satellite GPS & Weather Prediction Systems - GPS Weather Satellites Cartography: GPS and Map-making | GPS & Urban Development GPS & City Infrastructure (Urban Engineering) Robotic Vehicles & GPS (Nissan & More) In Summary Part II Errors & Error Correction in GPS Satellite The Most Common Causes of GPS Error Are (A Quick Overview) How GPS Works | A Simple Explanation GPS & Signal Interference Satellite & Atomic Clock Error Multi-Path Errors Caused by Large Obstacles (GPS Refraction) The Importance of Satellite Geometry & Placement The Effect of Earth’s Gravitational Pull on GPS In Summary

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A Guide to Understanding World GPS

If before man navigated by the sky and stars (a shared sky - then as now), now, we navigate through a shared technology - specifically, through a network of manmade "stars" high in the firmament - GPS satellites. China's satellite system, Bei-Dou pays homage to the origins of star-sky navigation, calling its system Bei-Dou, translating roughly as "Northern Dipper", and refers to the seven brightest stars in the sky of Ursa Major (The Great Bear Constellation). Galileo and Fixed Stars When we think of navigation, it's almost impossible not to think of Galileo a true pioneer who helped develop the first military compass. Galileo's eyes were fixed on the skies. Galileo's primary assertion (and the one that got him into the most trouble: in fact, he was put under house arrest for the reminder of his life for this discovery), was that the sun does not circle the earth, but the other way around. Galileo posited that the earth's rotation (and other planets) had a large influence on tides as the earth revolved on its axis. Of course, he was right, but that didn't stop the Roman Inquisition from placing him under house arrest.

By 1608, Galileo developed a telescope with a (then astonishing) 30x magnification that could be used to observe the sky. What Galileo saw were satellites or moons clearly orbiting Jupiter in an elliptical pattern. He also noted the full phases of Venus, observed Neptune, Saturn, and most of the Milky Way. It's worth noting that Galileo was the first to see sunspots. Sunspots appear through a telescope lens as darker or shaded areas on the sun; each of these areas is an area of heightened magnetism. Why are sunspots important? For one, most solar flares have their origin in these magnetically active regions on the sun (or groupings of

sunspots). For us today, solar flares can impact our own GPS, causing outages. Weather GPS systems monitor for solar flares, among other weather phenomenon. A note on sun-spots and solar flares: why are they important?

 

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- Solar flares appear as darker spots on the surface of the sun - Galileo was the first astronomer to note these dark spots on the sun as "sun spots" - Sun spots are important because they are highly magnetized areas of the sun's surface - Highly magnetized areas of the sun's surface cause interference with GPS and can often affect our own weather, as well as cause GPS system black out (as well as GPS weather satellite black-out such that we cannot forecast the earth’s weather).

It was with these observations of Jupiter's satellites and copious notes that Galileo (correctly) asserted regarding the orbits of the satellites - Galileo deduced that one could use the positions of the planets as a sort of universal clock. Many clocks today are set by GPS standards (atomic clocks within the satellites). In fact, there are two measurements for Earth time: there is GPS time (GPST) and there is Coordinated Universal Time (UTC). The key difference is that GPS time is not adapted or altered to match the rotation of the earth - hence, GPS time does not contain leap seconds that are added to UTC (civil time). Unfortunately for him, Galileo's discoveries and assertions only caused him grief: he was put under house arrest and it is said that as he was being lead away from the Roman Inquisition, "But it still moves…" holding steadfast to his observations. Contemporary GPS: Navigation by Satellite Worldwide Fortunately today, the world has reached general agreement about the basic principles of the Universe - for the most part. And the Worldwide development of GPS and sharing of readymade GPS devices ushers in an era unmatched by any other in history. While it is true that GPS has been used in conflict (notably, the Gulf War - 1990-91), there is a greater hope that GPS and similar global satellite positioning systems will do more to unify the world than divide it. We are seeing cooperation between Russia as it develops its satellite system, China, Europe, and India. Such cooperation would have been unthinkable, let alone imaginable, some twenty years ago. The Russian system, known as GLONASS (Globalnaya Navigatsionnaya Sputnikovaya Sistema) is operated by the Russian government and run by the Russian Aerospace Defense Forces. Development of GLONASS began in the Soviet Union in 1976. Under Putin, GLONASS became a top priority and it became the most expensive program in the Russian Federal Space Agency, using approximately one-third of the budget in 2010. By 2010, GLONASS had full coverage of Russia's territory. By October 2011, the full

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GLONASS constellation of all twenty-four satellites gave the Russian's full worldwide coverage. The American system of GPS was created by the United States Department of Defense as a means of satellite navigation (which is what all of these systems are). It grew out of distance-measuring techniques (Doppler, for one) as well as radio navigation. GPS became fully operational in the United States in 1994. The original purpose of GPS was remarkably the same for all countries: initially intended as a means of defense and strategy. But as time went on, GPS has seen a far greater use. It has been used for everything from predicting weather to cartography to helping develop better infrastructure of urban areas. It eventually became clear that GPS had a broad-base civilian use, when in 1983, a civilian jet - Korean Airlines Flight 007

carrying 269 passengers was shot down after it accidentally flew into Russian prohibited airspace. After this tragedy, then President Reagan issued a directive that would make GPS available for civilian (read: non-military) use for the "common good" in an effort to prevent such disasters in the future. This use was affirmed in 1996 by President Clinton, who declared GPS a dual use system - a system whose power could be harnessed by the military but that also had practical implications for civilians and the business sector. Clinton's declaration (following in the footsteps of Reagan's), made GPS what it is today, a "national asset". By 1998, GPS was developing rapidly. Al Gore (see Al Gore and GPS) made plans to upgrade the system to GPS II for improved accuracy and reliability, particularly in regards to aviation safety. Congress immediately approved the initiative calling it GPS III. The 5.5 billion dollar upgrade to GPS will not only make for a stronger signal, but will allow our system to be more interoperable with that of other systems worldwide. At present, Lockheed Martin has developed the first satellite for GPS III, which is going through testing (testing requires that the satellite be exposed to extremes in temperature, ranging from extreme heat to extreme cold just like it will experience in orbit). The new satellite is scheduled to orbit in May 2014. All nations worldwide, whether as a union, regional, or as a single country, now employ GPS technology. Here are the names of each GPS system: - GPS - United States of America - GLONASS - Russia

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- Bei-Dou and Compass - People's Republic of China (limited to Asia and the West Pacific)

- Galileo - A global system being developed by the European Union and partner countries. - IRNSS - India and the Northern Indian Ocean. - QZSS - Japanese regional system covering Asia and Oceania. The Russian GPS system and its development and history is not unlike our own. Initially developed for military use, it soon became clear that GLONASS had clear implications for civilian use. In 2006, Defense Minister Sergei Ivanov ordered GLONASS be made available for civilian use (just as Clinton and Reagan had ordered, for the "common good" thereby making GPS a national asset), so then did the Russians. Still, despite the success of the Russian program, Russian consumers were not quite as eager to gobble up the new technology, as were their American counterparts. To wit, the first Russian-made GLONASS navigation device for cars - Glospace SGK-70 was introduced in 1970, but it lacked the utility of similar of similar American receiver. For one, it was bigger and not as elegant. More it was not as cost-effective. Now, Ivanov has been actively promoting civilian use of GLONASS devices. More, the Russian government has ordered that all cars that are manufactured (or even partially manufactured) in Russia, must be equipped with GLONASS beginning in 2011 - this includes companies that have parts companies in Russia like Toyota and Ford. In fact, the Russian GLONASS has come so far that it is actively on a par with the American GPS system and many smart phones sold both in the United States and Russia are equipped with both county's systems. The following companies have dual GPS systems (both Russian and American): - Sony Ericsson - Samsung - Nokia - ZTE Huawei - Motorola - Apple iPhone 4S

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Since original development of GLONASS, there have been two more iterations of the system (two more improvements). Currently active is the larger satellite version, GLONASS M. Following on the heels of this is GLONASS K which has a longer lifetime (ten years as opposed to seven) and has improved navigational systems and accuracy. All told, the GLONASS system requires: - Eighteen satellites in perpetual orbit for reliable accuracy. - At present, there are eighteen satellites that cover the Russian Federation - 24 satellites enable GLONASS one-hundred percent worldwide coverage. Right now, GLONASS is really the only world satellite system on a par with that of the United States. Yet other nations are catching up and this is what we mean by a never before seen worldwide unification and cooperation. First, think a while about the United States and Russia now having the same GPS system on telephones sold on both countries - and cars as well! This would have been unthinkable twenty years ago. It's clear that the Cold War is quite over. China's system, which we spoke about a little earlier - the aptly named Bei-Dou, has only three satellites and very limited coverage. It has been offering coverage for Chinese customers and neighboring regions since 2000. The next generation of Bei-Dou, known as COMPASS or Bei-Dou 2, is far more ambitious and includes thirty-five satellites. It became operational in China in December of 2011, first with only ten satellites. It is planned to be fully operational for customers in the Asia-Pacific region by 2012 (launch story here) (in fact, within a matter of days). The entire Chinese global system should be complete by 2020. China had originally signed on to collaborate with the European Union's development of GPS, aptly named Galileo. However, China was not satisfied with their role in the development of the system and finally dropped out and decided to compete with Galileo (in the Asian market) on their own terms and thus Bei-Dou was born. Note that Bei-Dou is slightly different from other GPS systems - the American and Russian and European which use medium Earth orbit satellites. China's Bei-Dou uses satellites positioned instead in a geostationary orbit. Other systems that have been developed include QZSS, the Japanese system covering Asia and Oceania. IRNSS, India's regional system which is planned to be fully operational by 2012 and will cover India and the Northern Indian Ocean. So the world is covered. It is covered by Russia and the United States alone - but that other nations have joined in with their own systems can only be a good and useful thing, both to their own countries and to the rest of the world.

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How's the Weather Down There? | GPS & Weather Prediction Systems How is GPS helping us in other ways? Well, GPS is of myriad use. It's more than getting in your car and navigating from point a to point b (though admittedly, that is extremely useful and some of us would be quite lost without it). GPS has helped us with more accurate weather forecasts, and some smaller GPS are available for cars that also give weather as well as navigation making driving doubly safe. For example: let's say you are making a road trip through a mountainous region. The shortest route may be over the mountain range, which follows logically. What you may not know - being from another state or country - is that a storm is predicted for that region or that over that range the weather is often treacherous and you could well get stranded. A weather GPS along with your navigation GPS will tell you this. You can then make the decision to take the longer route - around the mountain's base - knowing that you will avoid the storm and extreme weather as well as the chances of getting stranded with no available help. Weather GPS can save lives in cases such as this. But knowing the weather in any case can, and has, save(d) lives. Having a more accurate weather forecast - and a more accurate long-term forecast - is of great benefit to those who make their living based on the weather (think aviators, fishermen, coastguard, cruise ships, fleet ships and the like). Before GPS, Doppler radar was the standard used in tracking radar. Today, there are specially designed weather satellites that use GPS technology that detect atmospheric conditions and then relay that information back to earth. So, weather masses can be determined from orbit, but GPS can also collect information relating to electron density, air density, and the amount of moisture in the air (general / relative humidity). These weather satellites are placed over various regions all over the country and the information collected on a daily basis and updated to the corresponding GPS weather receiver. First introduced in 2006, GPS weather-enabled satellites are called The Constellation of Observing System Meteorology Ionosphere and Climate (or COSMIC). The new technology allows for more accurate weather, but more, it allows us to gather the information more quickly

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and weather changes updated more promptly. COSMIC allows forecasters to pretty reasonably predict the weather for up to about a week with a reasonable amount of reliability. So this is how we can use GPS to track the weather. But conversely how can the weather affect GPS? Weather can cause GPS system blackouts bringing shipping vessels, fleets and the like to a dead stand still. Researchers at the University of Florida have developed a camera-like instrument called an ultraviolet imaging

spectrograph that, once launched into orbit, will continually monitor the earth's upper atmosphere for trouble spots. Most of the information is based on solar weather (again, solar flares - those areas of high magnetism that we discussed) - the single biggest factor in causing GPS blackouts. NASA is currently reviewing the Florida project and the team hopes to have a

launch by 2017. Why is this important? Well, GPS can only work well to track weather (or for any other use for that matter), if it is actually working and not interfered with by solar weather. With the University of Florida's camera, the two technologies would ideally work in concert to bring us a steady stream of reliable information without interruption in the GPS operating system. In summary, GPS weather satellites and GPS bring us: - Electron density - Air density - The amount of moisture in the air (relative humidity) - Major weather systems accurately predicted

- Safer driving routes, even if not the shortest route, quite possibly the safest if a storm is predicted.

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GPS, Map-making, & Urban Development Map-making, or cartography, began many ages ago. The first maps were made by the Babylonians and date from the 9th century BCE and are carved on clay tablets. Of special note to us however, would be early maps of India which include the Pole star and other constellations - indicating that man knew that the stars were fixed in the sky (albeit changing throughout the year), but that they could be a dependable source of navigation, the Pole star and Polaris (the North star), as fixed points. Chinese scientist, astronomer, poet, and polymath Su Song (1020-1101 AD) was among the first to create "star maps" which he created through the use of the telescope (much like Galileo). Again, Su Song noted the Pole star and Polaris as fixed points. Since then, map-making changed, but surveyors often depended on telescopes to measure the North Star and the sun's position at noon for accurate latitude. The stars, it seems, have always been important in map-making. Once everyone agreed the world was round (a much debated issue) circa 350 BC, map-making became a little simpler, but how to measure the universe around it? We know about Galileo and he helps bring us largely up to date. GPS & Infrastructure (Urban Engineering) With the advent of GPS, map making and developing urban infrastructure has become more efficient than ever before. We can know the shortest distance between two points, for example, we can know of better routes, better byways and highways and we can build. But what is perhaps more interesting and more intriguing is how GPS is being used to develop a more efficient urban infrastructure in cities. How is this being done? In short, a GPS camera takes a satellite picture of the movements of traffic and people in say, a given area of the city that the governor or development committee wishes to be developed. Let's say for example the city has a great deal of congestion whenever there are events and they wish to clean up this problem somehow - make it easier for people to get in to, and out of, the city. How do they know the routes that people are taking and how to best funnel them back into and out of the city again? Well, a GPS satellite high over head takes an image that to you or I looks rather like a thermal overlay on a map. What we see is what looks like an ordinary map but on those days when there is an event, certain roads or routes are thickly lined in a sort of thermal blue (thickly lined). On other days, that thermal color is thinner, indicating fewer people are trafficking that route. This tells the engineers that the route is simple bottle-necked. Something must be done. And so they can set about a plan to build and solve the problem. This is especially useful for rush-hour problems and other patterns (called "movement patterns").

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What's more, GPS will not only tell the engineers how the people are getting into and going out of the city, but where they go, where they park their cars (if they drive that is), and whether or not there is enough parking to accommodate them. It will also show us where people gather (or loiter), where maybe we need to create more public space or parks where people naturally gather and congregate. Another instance of GPS development has been the introduction of "tracking devices" to willing participants in urban areas or densely populated areas

in households that meet a pre-determined set of requirements (depending on what the company is looking to build). The occupant of the house must agree to take the small tracking GPS unit with them whenever they leave the house and then re-charge it when they return. Each journey counts as a round-trip. These round-trips are logged and sent to the engineering firm and then evaluated. This information tells the engineers the movements of the people in that area - where they go, how they spend their time, the movements that they make. It may sound a bit Big Brother but actually it's not; it helps people lead better lives in the final account. First, one has to agree to the GPS tracker, but more, why wouldn't you if it's going to help build a better city for you and for your family in the long run? None of us want to sit in traffic. We all want more parking, more parks, better roads, more greenery, better streets, better routes and so on. GPS combined with urban engineers help bring us and build us quite literally a better world to live in. In summary:

- Urban development benefits by showing us satellite images of heavily congested areas in cities - With this information, engineers can build more routes, more parking garages, parks, byways and etc.

- GPS tells us how people move in and out of the cities in which they live - GPS technology is used in mapmaking to measure distance and the most efficient route between two points.

- Some engineers are now using GPS trackers (with volunteers) to track people's movements in their environments to improve those areas.

- GPS ultimately can help bring us better cities with more greenery, more efficient routes in and out of the city.

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Robotic Vehicles & GPS Most of us are already familiar with how GPS can enhance our driving experience, helping us navigate through unfamiliar territory or even the city in which we live. What most of us are not yet familiar with are the other uses for GPS that are currently in development, like those being developed at Nissan which include "robotic GPS". It sounds very futuristic - a robotic car that almost, but not quite, drives itself (although some companies are on the leading edge of that technology as well, notably a British firm). Nissan, however, has developed a robotic vehicle that works in concert with GPS, which means that the robotics onboard the car and the enhanced GPS enable the car to slow down for a turn and then resume regular cruising speed once the turn has been made. This new "on-board" intelligent GPS is not unlike early forms of (what some may remember) "cruise control", only much smarter, thus dubbed "intelligent cruise control". There are other robotic vehicles and these work by interpreting (or otherwise sensing) the vehicle's surroundings. How do they do this? This is accomplished through the use of equipment mounted on (or in) the vehicle itself. On-board cameras, ground and range sensors, and radars all work in an effort to sense the car's surrounding environment so that the car can safely navigate a path. One such robotic vehicle is the Wildcat built by B&E Systems. Unlike Nissan's robotic GPS car, the Wildcat vehicle was built to remove the reliance on GPS (so quite the opposite of Nissan). The end-goal of the Wildcat is "hand's free driving" in a car that is able to sense the road, track road risks, read and interpret road signs and traffic signals, navigate around boulders and trees and so on. Anything a driver could do, essentially, the Wildcat could do. Nissan's goal in creating their robotic GPS vehicle was not to decrease reliance on GPS (clearly, since their vehicle incorporates GPS), but to reduce road accidents by

having the car slow down and speed up at the appropriate times where they may be road slippage (turns). The Nissan Robot (robot BR23C) is modeled on the behavior of bees. According to a press-release from the company, bees move in an oval shaped dance (think: a bee dance, which is a dance of echolocation, which is how bees navigate their way from flower to flower and show other bees the way to the sources of pollen by creating little "dance

Illustration:  Honeybee.  Note  the  large  compound  eyes  after  which  Nissan  modeled  some  of  the  features  of  its  vehicle.  Other  features  are  modeled  on  the  way  bees  move  or  “dance”  to  show  other  bees  the  source  of  nectar.    

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maps"). Bees also use their compound eyes for a full 360-detail view of their environment. In short:

- GPS enhanced Robotic vehicles, like Nissan's are intended to improve safe driving - GPS enhanced Robotic vehicles work by sensing the road and picking up speed or slowing down at the appropriate time - Other robotic vehicles in development (not GPS vehicles) use sensors that are mounted on the vehicle (cameras, sensors, radar; literally 'feelers' that sense the environment') and are intended for an eventual "hands-free" driving experience. These are not to be confused with GPS enhanced robotic vehicles.

In Summary From the stars we have created our own stars. Where we once navigated by stars and had to keep our eyes fixed on the sky, we now have a computerized system that is based on real astronomy and astronomical concepts which is why were it not for the work of early astronomers and scientists, there would be no GPS at all. One civilization, ideally, builds and grows on another and we learn from those that came before us rather and we hopefully evolve rather than devolve. GPS continues to aid us in urban planning, see new and safer vehicles, more accurately predict weather which no doubt saves lives every year as well as just every day practical use (getting from point a to point b). Some of us would be lost without it. What is most striking however is the agreement now between nations that really is unprecedented about the stars and satellites. Astronomy was for hundreds of years a most controversial topic (remember, for hundreds of years, mankind could not even agree that the earth was round!). That we finally agreed enough to navigate by the Pole star and Polaris was a giant step - that was quite some time ago, but that we are essentially sharing a technology through digital devices and automobiles and satellites high in our firmament is a global union of sorts and speaks volumes to just how far we have come.

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Part II

Errors & Error Correction in GPS Satellite The Most Common Causes of GPS Error Are (A Quick Overview): - Incorrect placement of satellites / satellite geometry. - Atmospheric conditions in the ionosphere and the troposphere that may affect how the rays travel between satellite and receiver. - Atomic clock error - this is the clock built within the actual satellite. It is adjusted for some margin of error, but if the clock is too far off, even by a nano-second, this can translate into GPS errors in distance on the ground of up to several feet. - Large buildings or topographic interference resulting in refraction errors (the signal is blocked or bounces back). This results in a "multi-path" errors causing two signals. - Angle at which the satellites are placed to each other: Ideally, satellites should be placed equidistant and at a 90 degree angle for the best communication. Minor deviations can result in large problems. - Gravitational shift - satellites affected by the earth's gravity, according to the theory of general relativity: these errors are adjusted using the Lorentz theory. - Solar flares - these are eruptions on the surface of the sun, areas that are highly magnetized that wreak havoc on GPS.

We know how GPS works and how well it can work, but what can go wrong and why? There are myriad factors when considering how well a GPS signal is transmitted and received. It helps to keep in mind that the GPS signal is really not so different from any other wave that travels through the air (say, the speed of light for example). Knowing this and understanding this, we know that radio waves can meet with interference, and likewise, light-waves. Therefore, we know that the same rules apply to GPS but are a little more complex because we want to fix

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a large piece of machinery (man-made) which we are bending to our will (or trying to), which is not that simple. How GPS Works | A Simple Explanation If a light-ray is blocked or bent, it's a little simpler, we can most often remove the source blocking the ray or create an additional light source. It may help to think of a GPS signal traveling through the ionosphere and troposphere much the way a ray of light travels (and we know too that GPS signals travel at the speed of light). Well, it can meet with much of the same interference; sources that block the wave and prevent it from meeting the intended receiver. More, GPS rays are subject to gravitational pull, solar flares - these can wreak havoc on the system. But before we even get to those sources of interference, the most important aspect of GPS efficacy is the exact placement of the satellites in the firmament. This is essential for proper and accurate functioning. How does a GPS signal work? Simplified, one GPS satellite sends a signal from a ground location, which is called the unknown point of origin. This signal is then relayed to one or more GPS satellite(s) in orbit. The information is then computed based on various factors: a. The time of the signal (when it was sent, when it was received - these are exact measurements). b. Once the receiver satellite knows the exact time the signal was sent, that time is then multiplied by the Speed of Light (satellite signals travel at the speed of light, 186,000 per second). The answer to the equation is the distance. GPS & Signal Interference For GPS to work accurately, certain variables must be known: when the signal left the first receiver and b., when it was picked up by the second receiver. Any interference in between this process can cause GPS error or failure. So what can really go wrong? Well for one, the first premise, we know is that the speed of light (note: the signal for GPS which is approximately the same value) are only constant in a vacuum and we are not operating in a vacuum. Instead, we are dealing with constant variables. The equation for correct GPS is as follows; The time the first signal leaves the GPS transmitter, the satellite position at the time of transmission (reception), multiplied by the speed of light (186,000 miles per second).

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However, it's not quite that simple: GPS first sorts out a "pseudo-range" which is an approximation of the distance from satellite to receiver. This in turn defines a certain sphere (up for three or four satellites can be used to determine one position). With this information, knowing the speed of light and accounting for margin of error, the GPS transmits back a signal. But there are many things that can interfere with a GPS satellite signal. Just as light itself can be refracted, scattered, altered and sometimes even obscured, so it is with a GPS signal. With GPS, we have to make adjustments for atmospheric conditions as the signal travels through the ionosphere and troposphere. Sometimes, during the signal's journey, the signal is refracted (again, the way light can be refracted by a tall building or a boulder - many things can cause this refraction) - even weather system could cause some inaccuracy in GPS or humidity. More troublesome however are sunspots (again, Galileo first noted these), which are highly magnetized and can create sun-flares that create interference making it difficult to get an accurate GPS read. Satellite Atomic Clock Error Other errors relate more to the actual GPS satellite and its inner-workings/mechanics. For example, even a minor variation in the atomic clock (each satellite must have a

clock to function properly to relay time in the necessary equation), can result in quite a large error. How? A seemingly minor clock error of, say, a single nanosecond translates into a distance between one and three meters on the ground; that's a significant margin of error. Multi-Path Errors Caused by Large Obstacles (GPS Refraction) Because GPS is essentially a wavelength (just as light and radio are), the signal can be blocked by large buildings (often a problem in high-density urban areas

where there are large structures that may interfere with the signal. More, the signal may encounter another reflective surface before it reaches the partner satellite's antenna and bounce off of that surface. When this happens, we see what is known as a multi-path error. Roughly translated this means that there are two signal responses when there ought only be one (a direct line between receiver and satellite). When a

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third object is introduced, it creates another line (the second line). When both signals (lines) are relayed at the same time then we have "multi-path error" which looks like an overlay of two images (one correct, the other a sort of "ghost image") - a duality. The Importance of Satellite Geometry & Placement Most of how GPS operates comes down to geometry and physics (if you thought geometry was not important, think again.) GPS relies heavily on geometry and exact placement of the satellites in our firmament. A satellite tipped at the wrong angle will cause many errors. Of utmost importance for proper GPS functioning is the exact layout of the whole network of satellites. Imagine a web or cage of satellites that surround the globe and are in orbit, each relaying signals. How far these satellites are spaced apart from each other is critical (this is called "satellite geometry"). The satellites need to be evenly distributed over the network. The wider the angle between satellites, the better the result will be. Distribution of precision by satellites or satellites angled incorrectly will relay a poor signal or an incorrect signal. For the best coverage, we need even coverage (again, think evenly-spaced network) and with the specific angle that has been proven to work best (generally a 90 degree position). When the satellites are incorrectly placed in their orbit, scientists call this "Dilution of Precision". Re-positioning the satellites (redistributing them evenly) is the best solution, however there are mathematical models that help sort out the margin of error and the satellite then makes the necessary adjustments, generally related to its atomic clock. The Effect of Earth’s Gravitational Pull on GPS Finally, there is one last thing to consider when looking at GPS margin of error and that is the Earth itself which, depending on where the satellite is (closer or farther away), will create a notable gravitational shift which will affect time (the single biggest factor in accuracy). A clock closer to a large object will be slower than a clock farther away due to the theory of general relativity. This means that GPS satellites in orbit (and their atomic clocks, which is really what we are looking at), will be faster than those that are closer to the earth. There is a calculation that can be made for the adjustment that is based on the Lorentz transformation, which in part factors in the fact that a satellite in orbit is elliptical (not circular), which fundamentally changes the equation.

 

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In Summary As GPS continues to develop, both within the States and worldwide (as well as with increasing worldwide cooperation), it is likely that these errors will become fewer. Some however, are bound to remain: the sun isn’t going anywhere (not right now anyway) and so solar flares are here to stay, or will perhaps even increase; obstacles of refraction will remain and so forth. Despite all of this, however, the most remarkable thing at all is that GPS works at all when one considers what could go wrong (and often does) and just how far we have come in correcting and adjusting for those margins of error. Physicists and scientists of the past (including Einstein, Lorentz, Galileo, among many others) helped set our man-made "stars" in motion. So what has changed? We're still navigating by the skies, only our skies now have a little help from mankind.