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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
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.
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

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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

14. 14
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).

15. 15
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

16. 16
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.

17. 17
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.