Friday, July 26, 2013
ABSTRACT
The Global Positioning System (GPS) is a
worldwide radio-navigation system formed from a constellation of 24 satellites
and their ground stations. In a sense it's like giving every square meter on
the planet a unique address.
This presentation demonstrates in brief
about GPS mechanism in 5 steps they are Trilateration, Measuring Distance, Getting
perfect timing, positioning Satellites, error correction, with a glance at
description of basic elements involving
in GPS satellites, garmin receivers, Signals which are connectors.
GPS technology is rapidly changing how
people find their way around earth. Whether it is for fun, saving lives, getting
there faster, or whatever we dream up, GPS navigation is becoming more common
everyday. I hope that this presentation will give you enough information.
Further more we tried to focus on
performance improving techniques such as modeling, dual frequency measurement
etc. This also debates on the increasing utility of GPS for
future generations.
BACKGROUND:
Trying to figure out where you are and where you're going is
probably one of man's oldest pastimes.
Navigation and positioning are crucial to so many activities and yet
the process has always been quite cumbersome.
Over the years all kinds of technologies have tried to simplify the
task but every one has had some disadvantage.
Positioning System:
Anything which helps to locate where we are on the globe.
Various positioning systems
include landmarks, dead reckoning, celestial, omega, loran and SATNAV each has
its own disadvantages.
In brief they are,
Ø LAND MARKS
Only work in local area.
Subject to movement or destructions
by environmental factors.
Ø CELESTIAL
Complicated.
Only
works at night in good weather.
Ø OMEGA
Based on relatively few radios
direction beacons.
As of 30th September 1997 , 0300
UT, the OMEGA Navigation System terminated.
Ø LORAN
Limited coverage (mostly coastal). Accuracy variable, affected by
geographic situation. Easy to jam or disturb.
LORAN systems were up and
running during World War II and were used extensively by the US
Ø SATNAV:
It is based on low frequency, Doppler measurements so it is sensitive
to small movement at receiver.
Birth of GPS
GPS took birth as
a super precise form of worldwide positioning by U.S. Department of Defense. It
took to build something really good in 1970’s.
GLOBAL POSITIONING SYSTEM
The Global
Positioning System (GPS) is a satellite-based navigation system made up of a
network of 24 satellites placed into orbit by the U.S. Department of Defense.
GPS was originally intended for military applications, but in the 1980s, the
government made the system available for civilian use. GPS works in any weather
conditions, anywhere in the world, 24 hours a day. There are no subscription
fees or setup charges to use GPS.
GPS uses these
"man-made stars" as reference points to calculate positions accurate
to a matter of meters. In fact, with advanced forms of GPS you
can make measurements to better than a centimeter!
In a sense it's like giving every square
meter on the planet a unique address.
SOME AMAZING ASPECT OF
GPS:
Here are some other interesting facts about the
GPS satellites (also called NAVSTAR,
the official U.S. Department of Defense name for GPS):
v The first GPS satellite was launched in 1978.
v A full constellation of 24 satellites was achieved in 1994.
v Each satellite is built to last about 10 years. Replacements are
constantly being built and launched into orbit.
v A GPS satellite weighs approximately 2,000 pounds and is about 17
feet across with the solar panels extended.
v Transmitter power is only 50 watts or less.
v GPS satellites are powered by solar energy.
WORKING
MECHANISM OF GPS
The 24 satellites that make up the GPS space
segment are orbiting the earth about 12,000 miles above us. They are constantly
moving, making two complete orbits in less than 24 hours. These satellites are
traveling at speeds of roughly 7,000 miles an hour.
GPS satellites transmit two low power radio
signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42
MHz in the UHF band. The signals travel 
by line of
sight, meaning they will pass through clouds, glass and plastic but will not go
through most solid objects such as buildings and mountains.
by line of
sight, meaning they will pass through clouds, glass and plastic but will not go
through most solid objects such as buildings and mountains.
A GPS signal contains three different bits
of information — a Pseudorandom code,
Ephemeris data and almanac data.
The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information.
We can view this number on your GARMIN GPS unit's satellite page, as it
identifies which satellites it's receiving.
Ephemeris data tells the GPS receiver where
each GPS satellite should be at any time throughout the day. Each satellite
transmits ephemeris data showing the orbital information for that satellite and
for every other satellite in the system.
Almanac data, which is constantly
transmitted by each satellite, contains important information about the status
of the satellite (healthy or unhealthy), current date and time. This part of
the signal is essential for determining a position.
The mechanism
takes place in five steps.
·
Triangulating
·
Measuring distance
·
Getting perfect timing
·
Satellites positions
·
Error correction
TRIANGULATION:
The basis of GPS is Triangulation from satellites.
We're using the word
"triangulation" very loosely here because it's a word most people can
understand, but purists would not call what GPS does "triangulation"
because no angles are involved. It's really Trilateration. Improbable as it may seem, the whole idea behind GPS
is to use satellites in space as reference points for locations here on earth.
That's right, by very, very accurately measuring our distance from three
satellites we can "triangulate" our position anywhere on earth.
Suppose we measure our distance from a
satellite and find it to be 11,000 miles. Knowing that we're 11,000 miles from
a particular satellite narrows down all the possible locations we
could be in the whole universe to the surface of a sphere that is centered on
this satellite and has a radius of 11,000 miles.
locations we
could be in the whole universe to the surface of a sphere that is centered on
this satellite and has a radius of 11,000 miles.
Next, say we measure our distance to a second satellite and find out
that it's 12,000 miles away. That tells us that we're not only on the first
sphere but we're also on a sphere that's 12,000 miles from the second
satellite. Or in other words, we're somewhere on the circle where these two
spheres intersected.
If we then make a measurement from a third satellite and find that
we're 13,000 miles from that one that narrows our position down even further,
to the two points where the 13,000 mile sphere cuts through the circle that's
the intersection of the first two spheres.
So by ranging from three satellites we can narrow our position to
just two points in space.
To decide which one is our true location we
could make a fourth measurement. But usually one of the two points is a
ridiculous answer (either too far from Earth or moving at an impossible
velocity) and can be rejected without a measurement.
MESURING DISTANCES:
Distance to a satellite is determined by
measuring how long a radio signal takes to reach us from that satellite.
- To make the measurement we assume that both the satellite and our receiver are generating the same pseudo-random codes at exactly the same time.
- By comparing how late the satellite's pseudo-random code appears compared to our receiver's code, we determine how long it took to reach us. Multiply that travel time by the speed of light and you've got distance
GETTING PERFECT TIMING
If measuring the travel time of a radio
signal is the key to GPS, then our stop watches had better be darn good,
because if their timing is off by just a thousandth of a second, at the speed
of light, that translates into almost 200 miles of error!
On the satellite side, timing is almost perfect because they have
incredibly precise atomic clocks on board.
Remember that both the satellite and the receiver need
to be able to precisely synchronize their pseudo-random codes to make the
system work.
If our receivers needed atomic clocks (which cost
upwards of $50K to $100K) GPS would be a lame duck technology.
The secret to perfect timing is to make an
extra satellite measurement.
SATELLITE POSITIONS
Knowing where a satellite is in space
We have been assuming that we know where the GPS satellites are so
we can use them as reference points.
But how do we know exactly where they are? After all they're
floating around 11,000 miles up in space.
That 11,000 mile altitude is actually a
benefit in this case, because something that high is well clear of the
atmosphere. And that means it will orbit according to very simple mathematics.
The Air Force has injected each GPS satellite into a very precise
orbit, according to the “GPS master plan.”
(The launch of the 24th block II satellite in March 1994 completed
the GPS constellation.)
On the ground all GPS receivers have an
almanac programmed into their computers that tells them where in the sky each
satellite is, moment by moment.
CORRECTING ERRORS:
Up to now we've been treating the
calculations that go into GPS very abstractly, as if the whole thing were
happening in a vacuum. But in the real world there are lots of 
things that
can happen to a GPS signal that will make its life less than mathematically
perfect.
things that
can happen to a GPS signal that will make its life less than mathematically
perfect.
To get the most out of the system, a good
GPS receiver needs to take a wide variety of possible errors into account.
Here's what they've got to deal with.
Factors that can degrade the GPS signal and
thus affect accuracy include the following:
·
Ionosphere and
troposphere delays —
The satellite signal slows as it passes through the
atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error.
There
are a couple of ways to minimize this kind of error. For one thing we can
predict what a typical delay might be on a typical day. This is called modeling and it helps but, of course,
atmospheric conditions are rarely exactly typical.
Another
way to get a handle on these atmosphere-induced errors is to compare the relative
speeds of two different signals. This dual
frequency measurement is very sophisticated and is only possible with
advanced receivers.
·
Signal multi-path: This occurs when the GPS signal is reflected off objects such as
tall buildings or large rock surfaces before it reaches the receiver. This
increases the travel time of the signal, thereby causing errors.
Sophisticated
receivers use a variety of signal processing tricks to make sure that they only
consider the earliest arriving signals (which are the direct ones).
·
Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic
clocks onboard the GPS satellites. Therefore, it may have very slight timing
errors.
·
Orbital errors — Also known as ephemeris errors, these are inaccuracies of the
satellite's reported location.
Receivers
maintain an "almanac" of
this data for all satellites and they update these almanacs as new data comes
in.
Typically, ephemeris data is updated hourly.
·
Number of satellites
visible — The more satellites a GPS receiver can
"see," the better the accuracy. Buildings, terrain, electronic
interference, or sometimes even dense foliage can block signal reception,
causing position errors or possibly no position reading at all. GPS units
typically will not work indoors, underwater or underground.
·
Satellite geometry/shading — This refers to the relative position of the satellites at any
given time. Ideal satellite geometry exists when the satellites are located at
wide angles relative to each other. Poor geometry results when the satellites
are located in a line or in a tight grouping.
Even though the satellites positions are
constantly monitored, they can't be watched every second. So, slight position or
ephemeris errors can sneak in between monitoring times.
ADVANCED
CONCEPTS
Differential GPS
A
simple concept can increase the accuracy of GPS to almost unbelievable limits.
- Differential GPS or "DGPS" can yield measurements good to a couple of meters in moving applications and even better in stationary situations.
- Differential GPS involves the cooperation of two receivers, one that's stationary and another that's roving around making position measurements.
- Many new GPS receivers are being designed to accept corrections, and some are even equipped with built-in radio receivers.
- Not all DGPS applications are created equal. Some don't need the radio link because they don't need precise positioning immediately.
PUTTING GPS TO WORK
GPS technology has matured into a resource
that goes far beyond its original design goals. These days scientists,
sportsmen, farmers, soldiers, pilots, surveyors, hikers, delivery drivers,
sailors, dispatchers, lumberjacks, fire-fighters, and people from many other
walks of life are using GPS in ways that make their work more productive,
safer, and sometimes even easier.
Location - Sometimes an
exact reference locator is needed for extremely precise scientific work. Just
getting to the world's tallest mountain was tricky, but GPS made measuring the growth
of Mt. Everest
Navigation - GPS helps you determine exactly where you are, but sometimes
important to know how to get somewhere else. GPS was originally designed to
provide navigation information for ships and planes. So it's no surprise that
while this technology is appropriate for navigating on water, it's also very
useful in the air and on the land.
Tracking - If navigation is the process of getting something from one
location to another, then tracking is the process of monitoring it as it moves
along.
Commerce relies on
fleets of vehicles to deliver goods and services either across a crowded city
or through nationwide corridors. So, effective fleet management has direct
bottom-line implications, such as telling a customer when a package will
arrive, spacing buses for the best scheduled service, directing the nearest
ambulance to an accident, or helping tankers avoid hazards.
Mapping - It's a big world out there, and using GPS to survey and map it
precisely saves time and money in this most stringent of all applications.
Today, Trimble GPS makes it possible for a single surveyor to accomplish in a
day what used to take weeks with an entire team. And they can do their work
with a higher level of accuracy than ever before.
Timing - Although GPS is well-known for navigation, tracking, and mapping,
it's also used to disseminate precise time, time intervals, and frequency. GPS
makes the job of "synchronizing our watches" easy and reliable.
Conclusion:
Imagine the
possibilities. Automatic construction equipment could translate CAD drawings
into finished roads without any manual measurements. Self-guided cars could
take you across town while you quietly read in the back seat.
