The Global Positioning System (GPS) is a satellite-based navigation system that can be used to locate positions anywhere on earth made up of a network of 24 satellites placed into orbit

1. GPS

2. What is GPS
The Global Positioning System (GPS) is a
satellite-based navigation system that can
be used to locate positions anywhere on
earth 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

3. WHY GPS
Some of the benefits we might receive from GPS
Travel directions:
Utilizing a GPS system will give you accurate directions for travel. Weather you are
travelling locally, across country, or globally, you will get accurate direction and constant
monitoring of your current position. I wouldn't think of taking another family vacation
without a GPS system in my vehicle.
GPS Tracking:
If you run a courier business, rental business, or just plain delivery service, and need to
keep track of several vehicles, then GPS is what you need. Keeping track of your
vehicles will keep your overhead down. The other end of tracking is keeping track of a
family member or child, a GPS tracking system will aid in your efforts.

4. Recreation:
Weather you are camping, or hiking, a hand held GPS unit will benefit you. I can't imagine
going camping in the middle of the woods and not being able to find my way out. The news
is filled with daily reports of people (or kids) that go hiking in the woods or the desert, and
can't find their way back home or to their campsite. A GPS unit is the answer for these
people.
Maritime GPS:
If you have ever watched one of the popular fishing shows on T.V., or are a fisherman or
sailor yourself, then you know how a GPS unit will benefit you. Getting lost at sea or on a
large lake would not be fun. The other side of the coin is the fisherman, say there is a great
hotspot on any given lake in the world, that specific spot can be saved in your GPS system
and found again at any time. This could be beneficial to the recreational or professional
fisherman.

5. HOW GPS WORKS
1-Triangulating
2-Measuring distance
3-Getting perfect timing
4-Satellite Positions
5-Error Correction

6. 1-Triangulating
Global Positioning System (GPS) navigators use the
mathematical technique of trilateration to determine user
position, speed, and elevation. GPS navigators constantly
receive and analyze radio signals from GPS satellites,
calculating precise distance (range) to each satellite being
tracked.
Data from a single satellite narrows position down to a large
area of the earth's surface. Adding data from a second
satellite narrows position down to the region where two
spheres overlap. Adding data from a third satellite (see
illustration) provides relatively accurate position. Data from
a fourth satellite (or more) enhances precision and also the
ability to determine accurate elevation or altitude (in the
case of aircraft).
GPS receivers routinely track 4 to 7 or more satellites
simultaneously. If a GPS navigator is receiving insufficient
satellite data (not able to track enough satellites) it will notify
the user, rather than providing incorrect position information.

7. 2-Measuring distance
Measuring distance from a satellite
We saw in the last section that a position is calculated
from distance measurements to at least three satellites.
The Big Idea Mathematically:
In a sense, the whole thing boils down to those
"velocity times travel time" math problems we did in
high school. Remember the old: "If a car goes 60 miles
per hour for two hours, how far does it travel?"
Velocity (60 mph) x Time (2 hours) = Distance (120
miles)
In the case of GPS we're measuring a radio signal so
the velocity is going to be the speed of light or roughly
186,000 miles per second.
The problem is measuring the travel time.

8. -Timing is tricky
-We need precise clocks to measure travel time
-The travel time for a satellite right overhead is about 0.06
seconds
-The difference in sync of the receiver time minus the
satellite time is equal to the travel time
The timing problem is tricky. First, the times are going to be
awfully short. If a satellite were right overhead the travel time
would be something like 0.06 seconds. So we're going to
need some really precise clocks. We'll talk about those soon.
But assuming we have precise clocks, how do we measure
travel time? To explain it let's use a goofy analogy:
Suppose there was a way to get both the satellite and the
receiver to start playing "The Star Spangled Banner" at
precisely 12 noon. If sound could reach us from space
(which, of course, is ridiculous) then standing at the receiver
we'd hear two versions of the Star Spangled Banner, one
from our receiver and one from the satellite.

9. These two versions would be out of sync. The version coming from the satellite would be
a little delayed because it had to travel more than 11,000 miles.
If we wanted to see just how delayed the satellite's version was, we could start delaying
the receiver's version until they fell into perfect sync.
The amount we have to shift back the receiver's version is equal to the travel time of the
satellite's version. So we just multiply that time times the speed of light and BINGO! we've
got our distance to the satellite.
That's basically how GPS works.
Only instead of the Star Spangled Banner the satellites and receivers use something
called a "Pseudo Random Code" - which is probably easier to sing than the Star
Spangled Banner.

10. 3-Getting perfect timing
Although GPS is well-known for navigation, tracking,
and mapping, it's also used to disseminate precise
time, time intervals, and frequency.
Time is a powerful commodity, and exact time is more
powerful still. Knowing that a group of timed events is
perfectly synchronized is often very important. GPS
makes the job of "synchronizing our watches" easy
and reliable.
There are three fundamental ways we use time. As a
universal marker, time tells us when things happened
or when they will. As a way to synchronize people,
events, even other types of signals, time helps keep
the world on schedule. And as a way to tell how long
things last, time provides and accurate, unambiguous
sense of duration.

11. GPS satellites carry highly accurate atomic clocks. And in order for the system to work, our GPS
receivers here on the ground synchronize themselves to these clocks. That means that every GPS
receiver is, in essence, an atomic accuracy clock.
Astronomers, power companies, computer networks, communications systems, banks, and radio and
television stations can benefit from this precise timing.
One investment banking firm uses GPS to guarantee their transactions are recorded simultaneously at
all offices around the world. And a major Pacific Northwest utility company makes sure their power is
distributed at just the right time along their 14,797 miles of transmission lines.

12. 4-Satellite Positions
In this tutorial we've 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.
A high satellite gathers no moss
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.

13. GPS Master Plan
The launch of the 24th block II satellite in March of 1994 completed
the GPS constellation.
Four additional satellites are in reserve to be launched "on need."
The spacings of the satellites are arranged so that a minimum of five
satellites are in view from every point on the globe.
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.
The basic orbits are quite exact but just to make things perfect the
GPS satellites are constantly monitored by the Department of
Defense.
They use very precise radar to check each satellite's exact altitude,
position and speed.
The errors they're checking for are called "ephemeris errors"
because they affect the satellite's orbit or "ephemeris." These errors
are caused by gravitational pulls from the moon and sun and by the
pressure of solar radiation on the satellites.
The errors are usually very slight but if you want great accuracy they
must be taken into account.

14. 5-Error Correction
First, one of the basic assumptions we've been
using throughout this tutorial is not exactly true.
We've been saying that you calculate distance
to a satellite by multiplying a signal's travel time
by the speed of light. But the speed of light is
only constant in a vacuum. As a GPS signal
passes through the charged particles of the
ionosphere and then through the water vapor in
the troposphere it gets slowed down a bit, and
this creates the same kind of error as bad
clocks. Ionosphere The ionosphere is the layer
of the atmosphere ranging in altitude from 50 to
500 km. It consists largely of ionized particles
which can exert a perturbing effect on GPS
signals. While much of the error induced by the
ionosphere can be removed through
mathematical modeling, it is still one of the
most significant error sources.

15. Troposphere The troposphere is the lower part
of the earth's atmosphere that encompasses
our weather. It's full of water vapor and varies
in temperature and pressure. But as messy as
it is, it causes relatively little 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. Error Modeling Much of the delay
caused by a signal's trip through our
atmosphere can be predicted. Mathematical
models of the atmosphere take into account
the charged particles in the ionosphere and
the varying gaseous content of the
troposphere. On top of that, the satellites
constantly transmit updates to the basic
ionospheric model.

16. Getting the message out
Once the DoD has measured a satellite's
exact position, they relay that information
back up to the satellite itself. The satellite
then includes this new corrected position
information in the timing signals it's
broadcasting.
So a GPS signal is more than just pseudo-
random code for timing purposes. It also
contains a navigation message with
ephemeris information as well.
With perfect timing and the satellite's exact
position you'd think we'd be ready to make
perfect position calculations. But there's
trouble afoot. Check out the next section to
see what's up.

17. A GPS receiver must factor in the angle each signal is taking as it enters the
atmosphere because that angle determines the length of the trip through the
perturbing medium. 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. Dual Frequency Measurements Physics says that as light
moves through a given medium, low-frequency signals get "refracted" or slowed
more than high-frequency signals. By comparing the delays of the two different
carrier frequencies of the GPS signal, L1 and L2, we can deduce what the
medium (i.e. atmosphere) is, and we can correct for it. Unfortunately this requires
a very sophisticated receiver since only the military has access to the signals on
the L2 carrier. Civilian companies have worked around this problem with some
tricky strategies.

18. APPLICATIONS OF
GPS
The applications of the Global Positioning System fall into five categories: location,
navigation, timing, mapping, and tracking. Each category contains uses for the
military, industry, transportation, recreation and science.

19. Location
This category is for position determination and is the most
obvious use of the Global Positioning System. GPS is the
first system that can give accurate and precise
measurements anytime, anywhere and under any weather
conditions. Some examples of applications within this
category are:
Measuring the movement of volcanoes and glaciers.
Measuring the growth of mountains.
Measuring the location of icebergs - this is very valuable
to ship captains helping them to avoid possible disasters.
Storing the location of where you were - most GPS
receivers on the market will allow you to record a certain
location. This allows you to find it again with minimal effort
and would prove useful in a hard to navigate place such
as a dense forest.

20. Navigation
Navigation is the process of
getting from one location to
another. This was the what the
Global Positioning System was
designed for. The GPS system
allows us to navigate on water, air,
or land. It allows planes to land in
the middle of mountains and helps
medical evacuation helicopters
save precious time by taking the
best route.

21. Timing
GPS brings precise timing to the us all.
Each satellite is equipped with an
extremely precise atomic clock. This is
why we can all synchronize our
watches so well and make sure
international events are actually
happening at the same time.

22. Mapping
This is used for creating maps by
recording a series of locations. The best
example is surveying where the DGPS
technique is applied but with a twist.
Instead of making error corrections in
real time, both the stationary and
moving receivers calculate their
positions using the satellite signals.
When the roving receiver is through
making measurements, it then takes
them back to the ground station which
has already calculated the errors for
each moment in time. At this time, the
accurate measurements are obtained.

23. Tracking
The applications in this category are ways
of monitoring people and things such as
packages. This has been used along with
wireless communications to keep track of
some criminals. The suspect agrees to
keep a GPS receiver and transmitting
device with him at all times. If he goes
where he's not allowed to, the authorities
will be notified. This can also be used to
track animals.