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Have u ever been lost and wished there was an easy way to find out which way u
needed to go? How about finding yourself out hiking and then not knowing how
to get back to your camp or car? Ever been flying and wanted to know the
nearest airport?
Our ancestors had to go to pretty extreme measures to keep from getting lost.
They erected monumental landmarks, laboriously drafted detailed maps and
learned to read the stars in the night sky.
GPS is a satellite based radio navigation system which provides continuous, all
weather, worldwide navigation capability for sea, land and air applications. So
things are much, much easier today. For less than $100, you can get a pocket-
sized gadget that will tell you exactly where you are on Earth at any moment. As
long as you have a GPS receiver and a clear view of the sky, you'll never be lost
Navigation in three dimensions is the primary function of GPS. Navigation
receivers are made for aircraft, ships, ground vehicles, and for hand carrying by
individuals. Precise positioning is possible using GPS receivers at reference
locations providing corrections and relative positioning data for remote
receivers. Surveying, geodetic control, and plate tectonic studies are examples.
Time and frequency dissemination, based on the precise clocks on board the SVs
and controlled by the monitor stations, is another use for GPS. Astronomical
observatories, telecommunications facilities, and laboratory standards can be set
to precise time signals or controlled to accurate frequencies by special purpose
GPS receivers.


GPS, which stands for Global Positioning System, is the only system today able
to show you your exact position on the Earth anytime. The Global Positioning
System is a constellation of satellites that orbit the earth twice a day, transmitting
precise time and position (latitude, longitude and altitude) information.
Cavemen probably used stones and twigs to mark a trail when they set out
hunting for food. The earliest mariners followed the coast closely to keep from
getting lost. When navigators first sailed into the open ocean, they discovered
they could chart their course by the stars. The next major developments in the

quest for the perfect method of navigation were the magnetic compass and the
sextant. The needle of a compass always points north, so it is always possible to
know in what direction you are going. The sextant uses adjustable mirrors to
measure the exact angle of the stars, moon, and sun above the horizon. GPS is
one of the most fantastic utilities ever devised by man. GPS will figure in history
alongside the development of the sea-going chronometer. This device enabled
seafarers to plot their course to an accuracy that greatly encouraged maritime
activity, and led to the migration explosion of the nineteenth century. GPS will
affect mankind in the same way. There are myriad applications that will benefit
us individually and collectively.
The technology evolved from, Mr. Marconi’s transmission of radio waves. This
was applied for society during the 1920's by the establishment of radio stations,
for which you only needed a receiver. The same applies for GPS- you only need
a rather special radio receiver. Significant advances in radio were bolstered by
large sums of money during and after the Second World War, and           were even
more advanced by the need for communications with early satellites and rockets,
and general space exploration. The technology to receive radio signals in a small
hand-held, from 20,000kms away, is indeed amazing.
Throughout the 1960s the U.S. Navy and Air Force worked on a number of
systems that would provide navigation capability for a variety of applications.
Disadvantages of other navigation systems
Landmark: Only work in local area. Subject to movement or destruction by
environmental factors.
Dead Reckoning: Very complicated. Accuracy depends on measurement tools
which are usually relatively crude. Errors accumulate quickly.
Celestial: Complicated. Only works at night in good weather. Limited precision.
OMEGA: Based on relatively few radio direction beacons. Accuracy limited
and subject to radio interference.
LORAN: Limited coverage (mostly coastal). Accuracy variable, affected by
geographic situation. Easy to jam or disturb.
SatNav: Based on low-frequency doppler measurements so it is sensitive to
small movements at receiver. Few satellites so updates are infrequent.
Many of these systems were incompatible with one another. In 1973 finally, the
U.S. Department of Defense decided that the military had to have a super precise
form of worldwide positioning. And fortunately they had the kind of money ($12
billion!) it took to build something really good. In short, development of the
GPS satellite navigation system was begun in the 1970s by the US Department

of Defense. The basis for the new system was atomic clocks carried on satellites,
a concept successfully tested in an earlier Navy program called TIMATION. The
Air Force operated the new system, which it called the Navstar Global
Positioning System. It has since come to be known simply as GPS.
Why did the Department of Defense develop GPS? In the latter days of the arms
race the targeting of ICBMs became such a fine art that they could be expected
to land right on an enemy's missile silos. Such a direct hit would destroy the silo
and any missile in it. The ability to take out your opponent's missiles had a
profound effect on the balance of power but you could only expect to hit a silo if
you knew exactly where you were launching from. That's not hard if your
missiles are on land, as most of them were in the Soviet Union. But most of the
U.S. nuclear arsenal was at sea on subs. To maintain the balance of power the
U.S. had to come up with a way to allow those subs to surface and fix their exact
position in a matter of minutes anywhere in the world.
The first GPS satellite was launched in 1978 and a second-generation set of
satellites ("Block II") was launched beginning in 1989. Today's GPS
constellation consists of at least 24 Block II satellites. A full constellation of 24
satellites was achieved in 1994. The U.S. Air Force Space Command (AFSC)
formally declared the GPS satellite constellation as having met the requirement
for Full Operational Capability (FOC) as of April 27, 1995. Since then, the
system has been taken into full use. In 1995 an agreement was made between the
US-DOD and the US Department of Transportation regarding wide area
broadcasts. With the modernized Block IIF satellites nearing launch—and the
GPS III program now in its planning stages—the technology is poised to reach
new levels of sophistication unimagined just a few years ago.
GPS was originally intended for military applications, but in the 1980s, the
government made the system available for civilian use. After the downing of
Korean Flight 007 in 1983 -a tragedy that might have been prevented if its crew
had access to better navigational tools- President Ronald Reagan issued a
directive that guaranteed that GPS signals would be available at no charge to the
world. That directive helped open up a commercial market.
Deployment of GPS continued at a steady pace through the 1990s, with growing
numbers of civilian and military users. GPS burst into public awareness during
the Persian Gulf War in 1991. GPS was used extensively during that conflict, so
much so that not enough military-equipped GPS receivers were available. To
satisfy demand, the Department of Defense acquired civilian GPS units and

temporarily changed GPS transmissions to give civilian receivers access to
higher-accuracy military signals.
When the system was created, timing errors were inserted into GPS
transmissions to limit the accuracy of non-military GPS receivers to about 100
meters. This part of GPS operations, called Selective Availability, was
eliminated in May 2000.
The system's dominant roles are in intelligent transportation systems,
telecommunications, and precision delivery of military munitions. Moreover, its
use in supporting both critical civil infrastructure and military operations has
received new attention since September 2001. The GPS signals are available to
an unlimited number of users simultaneously, and there is no charge for using
the GPS Satellites either.
The Soviet Union also developed a satellite-based navigation system, called
GLONASS, which is in operation today.


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    that
continuously transmit coded information,
which makes it possible to precisely identify
locations on earth by measuring the distance
from the satellites. The satellites transmit very
low power specially coded radio signals that
can be processed in a GPS receiver, enabling the receiver to compute position,
velocity and time thus allowing anyone one with a GPS receiver to determine
their location on earth. Four GPS satellite signals are used to compute positions
in three dimensions and the time offset in the receiver clock. The system was
designed so that receivers did not require atomic clocks, and so could be made
small and inexpensively.
The gps system consists of three pieces. There are the satellites that transmit the
position information, there are the ground stations that are used to control the
satellites and update the information, and finally there is the receiver that you
purchased. It is the receiver that collects data from the satellites and computes its
location anywhere in the world based on information it gets from the satellites.

There is a popular misconception that a gps receiver somehow sends information
to the satellites but this is not true, it only receives data.


The principle behind GPS is the measurement of distance (or "range") between
the receiver and the satellites. The satellites also tell us exactly where they are in
their orbits above the Earth. It works something like this-If we know our exact
distance from a satellite in space, we know we are somewhere on the surface of
an imaginary sphere with radius equal to the distance to the satellite radius. By
measuring its distance from a second satellite, the receiver knows it is also
somewhere on the surface of a second sphere with radius equal to its distance
from the second satellite. Therefore, the receiver must be somewhere along a
circle which is formed from the intersection of the two spheres. Measurement
from a third satellite introduces a third sphere. Now there are only two points
which are consistent with being at the intersection of all three spheres. One of
these is usually impossible, and the GPS receivers have mathematical methods
of eliminating the impossible location. Measurement from a fourth satellite now
resolves the ambiguity as to which of the two points is the location of the
receiver. The fourth satellite point also helps eliminate certain errors in the
measured distance due to uncertainties in the GPS receiver's timing as well.


 There are basically two types of GPS positioning
   Single Point Positioning
   Relative Point Positioning
Single Point Positioning is also known as autonomous or absolute positioning. In
this type, the position of an unknown point is determined based on known
positions of GPS satellites in space.
In Relative Positioning, the position of unknown point is determined with
respect to another known point (base or reference station).
The term Differential Positioning (DGPS) is often used interchangeably with
Relative Positioning. Infact DGPS is a specific type of relative positioning.
 Again in GPS positioning there are two types
   Static
   Kinematic
In Kinematic differential positioning one receiver is at known station referred to
as base/reference (stationery) while second receiver referred to as rover is moved

over path to be positioned.
 Accurate positions in DGPS can be accomplished through two methods
   Real time processing
   Post processing
Real time processing requires a data link to transmit corrections from a base
receiver to a rover receiver. Real time processing yields low accuracy as
compared to post processing.
In post processing, observed data from all the receivers is processed using
special software.
~Thus all GPS positioning can be classified as static or kinematic, single point or
relative, real time or post processing.


The GPS signal contains ephemeris and almanac data.
Ephemeris data is constantly transmitted by each satellite and contains important
information such as status of the satellite (healthy or unhealthy), current date,
and time. Without this part of the message, your GPS receiver would have no
idea what the current time and date are. This part of the signal is essential to
determining a position, as we’ll see in a moment.
The almanac data tells the GPS receiver where each GPS satellite should be at
any time throughout the day. Each satellite transmits almanac data showing the
orbital information for that satellite and for every other satellite in the system.


The satellites transmit very low power signals (20- 50 watts) allowing anyone
with a GPS receiver to determine their location on earth. GPS receivers passively
receive satellite signals; they do not transmit. The signals travel “line of sight”,
meaning it will pass through clouds, smoke, glass and plastic but not through
solid objects like buildings and mountains. So GPS receivers require an
unobstructed view of the sky, they are used only outdoors and they often do not
perform well within forested areas or near tall buildings. GPS operations depend
on a very accurate time reference, which is provided by atomic clocks at the U.S.
Naval Observatory. Each GPS satellite has
atomic clocks on board.

Each satellite transmits a message which
essentially says, "I'm satellite #X, my position is currently Y, and this message
was sent at time Z." All GPS satellites synchronize operations so that these
repeating signals are transmitted at the same instant. The signals, moving at the
speed of light, arrive at a GPS receiver at slightly different times because some
satellites are farther away than others. Your GPS receiver reads the message and

saves the ephemeris and almanac data for continual use. The distance to the GPS
satellites can be determined by estimating the amount of time it takes for their
signals to reach the receiver. When the satellite is generating the pseudo random
code, the receiver is generating the same code and tries to match it up with the
satellites’ code. The receiver then compares the two codes to determine how
much it needs to delay (or shift) its code to match the satellite’s code. This gives
the travel time. To determine your position the GPS receiver compares the time a
signal was transmitted by a satellite with the time it was received by the GPS
receiver. The time difference tells the GPS receiver how far away that particular
satellite is (Range=travel time*velocity of light). Now that we have both satellite
location and distance, the receiver can determine a position. If we add distance
measurements from a few more satellites, we can triangulate our position. This is
exactly what a GPS receiver does. Let’s say we are 11000 miles from one
satellite, our location is some where on an imaginary sphere that has the satellite
at the center with radius 11000 miles. Then let’s say we are 12000 miles from
another satellite; the second sphere would intersect the first sphere to create a
common circle. If we add a third satellite, at a distance of 13000 miles, we now
have two common points where the three spheres intersect. With a minimum of
three or more satellites, your GPS receiver can determine a latitude/longitude
position - what's called a 2D position fix. With four or more satellites, a GPS
receiver can determine a 3D position, which includes latitude, longitude, and
altitude. With this calculated position the exact location of the receiver can be
pinpointed on a digitized map with the use of the proper GIS software tools. By
continuously updating your position, a GPS receiver can also accurately provide
speed and direction of travel (referred to as 'ground speed' and 'ground track').
The satellites, operated by the U.S. Air Force, orbit with a period of 12 hours.
Ground stations are used to precisely track each satellite's orbit.


The satellites transmit on two L-band microwave carrier frequencies
    LINK1 or L1 = 1575.42 MHz
    LINK2 or L2 = 1227.60 MHz
Pseudo Random Number (PRN code)
The pseudo-random code identifies which satellite is transmitting - in other
words, an I.D. code. Pseudo Random Number (PRN code) which is used to code

the signal from that satellite is a string of numbers that would look random to
anyone who do not know what the formula used to create it is. Characteristics of
PRN code are –
   No repetition: The sequence does not cycle around and repeat itself.
   Good numeric distribution: If the formula is producing random numbers
between 0 and 9, the number of zeros, ones, twos, etc. that it produces should be
roughly equal over a long period of time.
   Lack of predictability: You have no way to predict what the next number will
be unless you know the formula and the seed (the initial value).
Three pseudo-random noise ranging codes modulate the L Band carrier phase.
     coarse/acquisition (C/A) code
     precision (P) code
     Y-code
The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. The C/A
code is a repeating 1 MHz pseudo-random noise code. This noise-like code
modulates the L1 carrier signal, "spreading" the spectrum over a 1 MHz
bandwidth. The C/A code repeats every 1023 bits (period of one millisecond,
1.023 MHz rate). There is a different C/A code PRN for each SV. GPS satellites
are often identified by their PRN number, the unique identifier for each pseudo-
random-noise code. The C/A code that modulates the L1 carrier is the basis for
the civil SPS and is used primarily to acquire the P-code.
The precision (P) code is the principal navigation ranging code. The P-Code
(Precise) modulates both the L1 and L2 carrier phases and so the P-code is
available on both L1 and L2. Each one-week segment of this code is unique to
one GPS satellite and is reset each week. The P-Code is a very long (a period of
7 days, 10.23 MHz rate) 10 MHz PRN code. In the Anti-Spoofing (AS) mode of
operation, the P-Code is encrypted into the Y-Code. The encrypted Y-Code
requires a classified AS Module for each receiver channel and is for use only by
authorized users with cryptographic keys. The P (Y)-Code is the basis for the
PPS. The C/A code is available on the L1 frequency. The Navigation Message
also modulates the L1-C/A code signal.
The Navigation Message is a 50 Hz signal transmitted by each satellite
consisting of data bits that describe the GPS satellite orbits, clock corrections,
system time and other system parameters. In addition, an almanac is also
provided which gives the approximate data for each active satellite. This allows
the user set to find all satellites once the first has been acquired.

The various satellites all transmit on the same frequencies, L1 and L2, but with
individual code assignments. Due to the spread spectrum characteristic of the
signals, the system provides a large margin of resistance to interference.
The main purpose of these coded signals is to allow calculation of the travel time
from the satellites to the GPS receiver on earth. This travel time is also called the
Time of Arrival. The travel time multiplied by the speed of light equals the
satellite range (distance from the satellite to the GPS receiver).


     GPS elements:
   Space Segment
   Control Segment
   User Segment

Space Segment
The GPS technology is based on the NAVSTAR (NAVigation Satellite Timing
And Ranging) constellation composed of 24 satellites in space, the space
segment of the GPS system. There are often more than 24 operational satellites
as new ones are launched to replace older satellites. The satellite orbits repeat
almost the same ground track (as the earth turns beneath them) once each day.
These 24 satellites (21 navigational satellites and 3 active spares) are in 6
circular orbits (with nominally four SVs in each), equally spaced (60 degrees
apart), at an inclination angle of 55 degrees. These satellites weigh 1900 lbs in

orbit, travel at speeds of about 14,000
kilometres per hour or 8700 miles per hour with
a 12hr period (precisely 11hr 58 min). It is at
roughly 25,000 kilometers from the earth's
centre or 20,000 kms above the earth's surface.
The satellites are high enough to bypass the
problems encountered by land-based systems
they    send   wireless   radio   signals   from

Their configuration provides the user with between 5 and 8 space vehicles
anywhere on the earth. The spacing of satellites in orbit is arranged so that under
normal conditions a minimum of five satellites will be in view to users
worldwide, with a position dilution of precision (PDOP) of six or less. In
practice there are usually many more than this, sometimes as many as 12.
The satellites are generally allowed to "float" in their orbits and aren't rigidly
held in position. The orbital paths of these satellites take them between roughly
60 degrees North and 60 degrees South latitudes. What this means is you can
receive satellite signals anywhere in the world, at any time. As you move closer
to the poles, you will still pick up the GPS satellites. The NAVSTAR satellites
can see from the northernmost and southern most parts of their orbits. These
satellites provide 24-hour-a-day coverage for both two-and three- dimensional

positioning anywhere on Earth. They also continuously broadcast position and
time throughout the world. Currently there are 27 total satellites in the sky and it
is possible that there could be as many as 31 or 32.
Each satellite contains a supply of fuel and small servo engines so that it can be
moved in orbit to correct for positioning errors. With update control from the
ground units it can maintain an essentially circular orbit around the earth. It also
contains a receiver to get update information, a transmitter to send information to
the gps receiver, an antenna array to magnify the weak transmitter signal, several
atomic clocks to accurately know the time, control hardware, and photoelectric
cells to power everything. They are powered by solar energy and each satellite is
expected to last approximately 10 years. If solar energy fails (eclipse, etc.) they
have backup batteries on board to keep them running.
Each satellite transmits on two L band frequencies, L1 (1575.42 MHz) and L2
(1227.6 MHz). Each satellite transmits on exactly the same frequency; however,
each satellites signal is doppler-shifted by the       time it reaches the user. L1
carries a precise (P) code and a coarse/acquisition (C/A) code. L2 carries only
the P code. A    navigation data message is superimposed on these codes. The
same navigation data message is carried on both frequencies. The P code is
normally encrypted so that only the C/A code is available to civilian users;
however, some information can be derived from the P code. When encrypted, the
P code is known as Y code.
The current series of GPS satellites broadcast data using two distinct signals of
accuracy. The first is for the standard positioning system (SPS). The second one
is for the precise positioning system (PPS). The SPS signal is at the L1
frequency, which is 1547.42MHz. The L2 frequency carries the PPS signal and
is at 1227.60MHz.
There have been three distinct groups of NAVSTAR satellites so far, with one
sub-group. The groups are designated as blocks. The block I satellites were
intended for system testing. The block II satellites were the first fully functional
satellites, including cesium atomic clocks for     timing as well as the ability to
implement selective availability. They also have radiation-hardened electronics,
allowing for longer lifetimes in space. In addition, the block II satellite can
detect certain error conditions, automatically sending a code indicating that it is
out of service. Block II satellites can operate for 3.5 days between corrections
from the ground. The block IIa satellites are identical to the standard block II but

continue to operate for 180 days between uploads from the ground.
The latest satellites, the block IIR versions, include autonomous navigation.
These satellites can operate for 180 days between uploads like the block IIa.
Unlike the block IIa, they can generate their own navigation information. Thus,
the accuracy of the system can be maintained longer between uploads.
With the modernized Block IIR and Block IIF satellites nearing launch—and the
GPS III program now in its planning stages—the technology is poised to reach
new levels of sophistication unimagined just a few years ago.
Each satellite has two identifying numbers. First is the NAVSTAR number,
which identifies the specific satellite hardware. Second is the Space vehicle (SV)
number. This number is assigned in order of launch.
The third method to identify a satellite is by the Psuedo-random noise code
number. This is a unique integer number, which is used to code the signal from
that Satellite that would look random to anyone who does not know what the
formula used to create it is.
Some receivers identify the satellites that they are listening to by SV, others by

Control Segment
It consists of a system of tracking stations located around the world. The control
segment is composed of all the ground-based facilities that are used to monitor
and control the satellites. This segment is usually unseen by the user, but is a
vital part of the system. The NAVSTAR control segment, called the operational
control system (OCS) consists of 5 monitor stations, a master control station
(MCS) and 3 uplink antennas. The satellites send down subsets of the orbital
ephemeris data. The monitor stations track GPS satellites in view, collect and
send information from the satellites back to the master control station that
computes the precise orbits. The master station uploads the data which is
necessary for proper operation of the satellite, like ephemeris and clock data to
the satellites. Then the information is formatted into updated navigation
messages that are transmitted through ground antennas.
The MCS is located at Schriever Air Force Base (formerly Falcon AFB) in
Colorado. and is managed by the U.S. Air Force's 2nd Space Operations
Squadron (2nd SOPS). The MCS receives data from the monitor stations in real
time 24 hours a day and uses that information to determine if the satellites are
experiencing clock or ephemeris changes, and to detect equipment malfunctions.

New navigation and ephemeris information is calculated from the monitored
signals and uploaded to the satellites once or twice per day.
There are several remote monitor stations, which send their information to the
master control station. These stations are able to track and monitor each satellite
for 21 hours a day, resulting in 2 periods of 1.5 hours when the satellite is on the
other side of the earth out of reach for that ground station. These passive monitor
stations are nothing more than GPS receivers that track all satellites in view and
thus accumulate ranging data from the satellite signals. There are five passive
monitor stations, located at Colorado Springs, Hawaii, Ascencion Island, Diego
Garcia and Kwajalein. The monitor stations send the raw data back to the MCS
for processing.
The information calculated by the MCS, along with routine maintenance
commands are transmitted to the satellites by ground-based uplink antennas. The
ground antennas are located at Ascencion Island, Diego Garcia and Kwajalein.
The antenna facilities transmit to the satellites via an S-band radio link.
In addition to its main function, the MCS maintains a 24 hour computer bulletin
board system with the latest system news and status. The civilian contact for this
is the United States Coast Guard’s (USCG) Navigation Center (NAVCEN).
User segment

                                             GPS 72 handheld rxr

The user segment is composed of GPS receivers composed of processors and
antennas that allow for sea, land and airborne operators to receive the broadcast.
The receivers convert space vehicle signals into position, velocity and time. A
total of 4 satellites are required to compute these calculations. In order to make
this simple calculation, then, the GPS receiver has to know two things:
•The location of at least three satellites above you
•The distance between you and each of those satellites.
The GPS receiver figures both of these things out by analyzing high-frequency,
low-power radio signals from the GPS satellites. Better units have multiple
receivers, so they can pick up signals from several satellites
simultaneously. Most modern receivers are parallel multi-channel design.
Parallel receivers typically have five to twelve receiver circuits, each devoted to

one particular satellite at all times. Parallel channels are quick to lock onto
satellites when first turned on and they are able to receive the satellite signals
even in difficult conditions such as dense foliage or urban settings. If you want
to have continuous real-time position measurements, then the receiver has to
have at least four channels. If it does, then it can devote one channel to each of
the four satellites at the same time. Most of the time, this kind of accuracy is not
needed, so some receivers have only one channel. Older single-channel designs
were once popular, but were limited in their ability to continuously receive
signals in the toughest environments. One of the problems with this type of
receiver is that it doesn't always do a good job of monitoring velocity. Also, if
there is any movement of the receiver while it is collecting the four
measurements, the accuracy of those measurements will be affected. A
compromise that is used quite often is the three channel receiver. One channel
can be collecting the data from one satellite while the other two channels are
locking in on the satellites where the next measurements are going to come
from. This type of receiver doesn't waste time between measurements, because
they can instantly switch to the next satellite's data. Another benefit to this type
of receiver is that it can track up to eight satellites, so if one satellite is blocked,
it can switch to another one. Thus, the three channel receiver is more
economical than a four channel receiver, and it is more accurate than a one
channel receiver. Position, velocity and time are needed for marine, terrestrial &
aeronautic applications.
A standard GPS receiver will not only place you on a map at any particular
location, but will also trace your path across a map as you move. If you leave
your receiver on, it can stay in constant communication with GPS satellites to
see how your location is changing. With this information and its built-in clock,
the receiver can give you several pieces of valuable information:
•How far you've traveled (odometer)
•How long you've been traveling
•Your current speed (speedometer)
•Your average speed
• A "bread crumb" trail showing you exactly where you have traveled on the
• The estimated time of arrival at your destination if you maintain your current


     GPS provides two services
   SPS-Standard Positioning Service
   PPS-Precise Positioning Service

The Standard Positioning Service is a positioning and timing service, which will
be available to all GPS users on a continuous, worldwide basis with no direct
charge. SPS will be provided on the GPS L1 frequency, which contains a coarse
acquisition (C/A) code and a navigation data message. SPS provides a
predictable positioning accuracy of 100 meters horizontally and 156 meters
vertically and time transfer accuracy to UTC within 340 nanoseconds. The SPS
accuracy is intentionally degraded by the DOD by the use of Selective
The Precise Positioning Service is a highly accurate military positioning,
velocity and timing service which will be available on a continuous, worldwide
basis to users authorized by the U.S.P (Y) code capable military user equipment
provides a predictable positioning accuracy of at least 22 meters horizontally and
27.7 meters vertically and time transfer accuracy to UTC within 200
nanoseconds. PPS will be the data transmitted on the GPS L1 and L2
frequencies. PPS was designed primarily for U.S. military use. It will be denied
to unauthorized users by the use of cryptography. PPS will be made available to
U.S. and Allied military and U.S. Federal Government users. Limited, non-
Federal Government, civil use of PPS, both domestic and foreign, will be
considered upon request and authorized on a case-by-case basis.


Geometric View
In order to understand how the GPS satellite system works, it is very helpful to
understand the concept of trilateration. Let's look at an example to see how
trilateration works.
Let's say that you are somewhere in the United States and you are TOTALLY
lost -you don't have a clue where you are. You find a friendly-looking person
and ask, "Where am I?" and the person says to you, "You are 625 miles from
Boise, Idaho." This is a piece of information, but it is not really that useful by
itself. You could be anywhere on a circle around Boise that has a radius of 625
miles, like this:

                       If you know you are 625 miles from Boise,
                          you could be anywhere on this circle.

So you ask another person, and he says, "You are 690 miles away from
Minneapolis, Minnesota." This is helpful - if you combine this information with
the Boise information, you have two circles that intersect. You now know that
you are at one of two points, but you don't know which one, like this:

                       If you know you are 625 miles from Boise
                       and 690 miles from Minneapolis, then you
                         know you must be at one of two points.

If a third person tells you that you are 615 miles from Tucson, Arizona, you can
figure out which of the two points you are at:

                      With three known points, you can determine
                       that your exact location is somewhere near
                                   Denver, Colorado!
With three known points, you can see that you are near Denver, Colorado!
Trilateration is a basic geometric principle that allows you to find one location
if you know its distance from other, already known locations. The geometry
behind this is very easy to understand in two dimensional space.
This same concept works in three dimensional space as well, but you're dealing
with spheres instead of circles. You also need 4 spheres instead of three circles
to find your exact location. The heart of a GPS receiver is the ability to find the
receiver's distance from 4 (or more) GPS satellites. Once it determines its
distance from the four satellites, the receiver can calculate its exact location and
altitude on Earth! If the receiver can only find three satellites, then it can use an
imaginary sphere to represent the earth and can give you location information
but no altitude information.
For a GPS receiver to find your location, it has to determine two things:
      The location of at least three satellites above you
      The distance between you and each of those satellites
The gps receiver measures the length of time the signal takes to arrive at your
location and then based on knowing that the signal moves at the speed of light it
can compute the distance based on the travel time.
Measuring Distance :
   1.Distance to a satellite is determined by measuring how long a radio
   signal takes to reach us from that satellite(transit time or
   TDOA-Time Difference Of Arrival).
   2.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.
  3.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. This is the transit time.
  4.Multiply that travel time by the speed of light and you've got distance.

Now, armed with the satellite location and the distance from the satellite we can
expect that we are somewhere on a sphere that is described by the radius

(distance) and centered at the satellite location. By acquiring the same
information from a second satellite we can compute a second sphere that cuts the
first one at a plane. Now we know we are somewhere on the circle that is
described by the intersection of the two spheres. If we acquire the same
information from a third satellite we would notice that the new sphere would
intersect the circle at only two points. If we know approximately where we are
we can discard one of those points and we are left with our exact fix location in
3D space. Now, what would happen if we were to acquire the information from a
fourth satellite? We should expect that it would show us to be at exactly the
same point we just computed above. But what if it isn't? Before we can answer
that question we need a little more background.
A more basic question is, "How does the gps know the travel time(TDOA) so
that it can compute the distance?" The satellite sends the current time along with
the message so the gps can subtract its knowledge of the current time from the
satellite time in the message (which is the time that the signal started its descent)
and use this to compute the difference. Measuring the time would be easy if you
knew exactly what time the signal left the satellite and exactly what time it
arrived at your receiver, and solving this problem is key to the Global
Positioning System. One way to solve the problem would be to put extremely
accurate and synchronized clocks in the satellites and the receivers. The satellite
begins transmitting a long digital pattern, called a pseudo-random code, as part
of its signal at a certain time, let's say midnight. The receiver begins running the
same digital pattern, also exactly at midnight. When the satellite's signal reaches
the receiver, its transmission of the pattern will lag a bit behind the receiver's
playing of the pattern. The length of the delay is equal to the time of the signal's
travel. The receiver multiplies this time by the speed of light to determine how
far the signal traveled. If the signal traveled in a straight line, this distance would
be the distance to the satellite.

The only way to implement a system like this would require a level of accuracy
only found in atomic clocks. This is because the time measured in these
calculations amounts to nanoseconds. To make a GPS using only synchronized
clocks, you would need to have atomic clocks not only on all the satellites, but
also in the receiver itself. Atomic clocks usually cost somewhere between
$50,000 and $100,000, which makes them a little too expensive for everyday
consumer use!
The Global Positioning System has a very effective solution to this problem - a
GPS receiver contains no atomic clock at all. It has a normal quartz clock. The
receiver looks at all the signals it is receiving and uses calculations to find both
the exact time and the exact location simultaneously.
To simplify our explanation, we're going to talk in two dimensions. Just keep in
mind that in real-life GPS is in three
dimensions.     Let's assume that our
receiver clock isn't perfect, but it's
consistent. Pretend that it's a little
fast. Let's say that we are four seconds
from receiver A and six seconds from
receiver B, so "X" is where we really
are (keep in mind that it would take three points in three dimensions). If we
use the receivers that are a second fast, the two circles would intersect at a
different point, "XX". This would seem like the right location, because we
would have no way of knowing otherwise using just our calculations.

We will add another measurement to
our calculation. Let's say that we are
eight seconds away from satellite C. If
all of the receivers are working
properly, then all three circles would
intersect at X, because that is where
all three measurements intersect. If we
add the third measurement where the
clocks are one second off, then we will
still   have   satellites   A   and   B
intersecting, but satellite C's reading
will be nowhere near that. The little
computers in the GPS receivers are

programmed so that when they get a series of measurements that cannot
intersect, they realize something is wrong. They assume that their receivers are
off, so they start subtracting (or adding) the same amount of time off of each
receiver's time. By trimming the times, the computer "discovers" the amount of
time it needs to subtract to find where all of the points intersect. Thus, by adding
another measurement, we can cancel out any consistent clock error the receivers
may have. To move into three dimensions, we will need a fourth measurement
to cancel out any clock errors.    There are twenty-four satellites placed around
the earth so that four measurements can be taken from any place on earth. The
need for four measurements has had a huge impact on how GPS receivers are
designed. When you measure the distance to four located satellites, you can draw
four spheres that all intersect at one point, as illustrated above. Four spheres of
this sort will not intersect at one point if you've measured incorrectly. Since the
receiver makes all of its time measurements, and therefore its distance
measurements, using the clock it is equipped with, the distances will all be
proportionally incorrect. The receiver can therefore easily calculate exactly what
distance adjustment will cause the four spheres to intersect at one point. This
allows it to adjust its clock to adjust its measure of distance. For this reason, a
GPS receiver actually keeps extremely accurate time, on the order of the actual
atomic clocks in the satellites! So we will then have a good position fix and as a
side effect we will also have the correct time to about 200 nanoseconds or so.
One of the applications of gps technology is to provide the correct time even
when                we                 don't               care about our position.
One problem with this method is the measure of speed. As we saw earlier,
electromagnetic signals travel through a vacuum at the speed of light. The earth,
of course, is not a vacuum, and its atmosphere slows the transmission of the
signal according to the particular conditions at that atmospheric location, the
angle at which the signal enters it, and so on. A GPS receiver guesses the actual
speed of the signal using complex mathematical models of a wide range of
atmospheric conditions. The satellites can also transmit additional information to
the receiver.

To get the best possible measurement, a GPS receiver will take into account a
subtle principle of geometry called "Geometric Dilution of Precision" (GDOP).
While this sounds really complicated, it's not.
This term deals with the fact that the
measurement from one satellite will be better
than the reading from another. Because we now
know that any measurement from a satellite isn't
totally accurate all the time, we will let the distance of the satellite be
represented by a fuzzy line. The area created by the intersection of the two
circles makes a little box. Because of the uncertainty due to error, we can say
that we are located somewhere in the box. Depending on the distance between
the two satellites, we can either have a small box or a very long and large box.
The closer the angle of the satellites to one
another, the larger the box will be. Therefore, it
is better to get a reading from satellites with a
wider angle. Good receivers will calculate the
relative angles between satellites and pick the
readings where the measurements will be best,
which reduces (and in some cases totally minimizes) the GDOP error.
Maintaining the fix means that we need to continuously recalculate the
information based on the moving satellites. Once we have a number of fixes we
can derive much more information than just location data. For example a gps can
compute the travel direction (compass heading) by comparing current location to
previous location. Similarly the gps can keep track of travel distance, compute
speed, record travel time and other valuable data.
This view is simplified. In addition to the data already mentioned the unit uses
Doppler data from the moving satellites, almanac data to figure out the
approximate positions of all the satellites, and ephemeris data download directly
from the satellite that can be used to compute its position in the sky. The
problem is that the receiver clock is not perfectly in sync with the satellite clock.
This would cause a major error—the uncorrected reading is called a pseudo
range. The receiver clock is not top quality, and the military intentionally dithers
the satellite clock and coordinates.

Mathematical View

Another way to understand the operation of a gps system is to look at the math
that goes into calculating a position. From Pythagoras we have:
  R + Es = sqrt{(Xs - Ux)^2 + (Ys - Uy)^2 + (Zs- Uz)^2}
Where Ux, Uy, Uz are the positions we are trying to find. The terms Xs, Ys, Zs
are the satellite positions that can be calculated from ephemeris information sent
from each satellite. Transit time is the time taken by the signal to reach the
receiver. The Es term is a lump sum of all the modeling errors considered by the
gps. These include such things as troposphere and ionosphere errors, clock errors
from the satellite and any other error the gps receiver thinks is significant enough
to model. R is the approximate (pseudorange) distance from the receiver to the
satellite. R=c(T-dT);Here T is the transit time and dT is the time error at the
receiver. Since we can calculate the pseudorange and satellite positions
independently and we can factor in modeling information from hardcoded data
we are left with four unknowns, X, Y, Z, and dT. Therefore we need 4 equations
to solve for the 4 unknowns. Mathematically this is a standard least squares
problem. One approach is to use guesses of our current position to calculate
delta's from what we would expect and then iterate towards a converged
solution. This is the reason that the unit requires an estimate of our current
location to compute our position. Once we have the delta's down to an
acceptable level we have a solution.
A GPS receiver measures the amount of time it takes for the signal to travel from
the satellite to the receiver. Since we know how fast radio signals travel -- they
are electromagnetic waves and so (in a vacuum) travel at the speed of light,
about 186,000 miles per second -- we can figure out how far they've traveled by
figuring out how long it took for them to arrive.

The four satellites’ ephemeris data provide the satellite’s X, Y, and Z positions.
The range, R, is the receiver measurement made by calculating the time it took
for the signal to reach the receiver. The user’s position, (Ux, Uy, Uz), and the
time error dT at the receiver is then calculated (see figure).
Four equations are:
           (X1-Ux)^2+ (Y1-Uy) ^2+ (Z1-Uz) ^2= (R1 +E1) ^2
           (X2-Ux)^2+ (Y2-Uy) ^2+ (Z2-Uz) ^2= (R2 +E2) ^2
           (X3-Ux)^2+ (Y3-Uy) ^2+ (Z3-Uz) ^2= (R3 +E3) ^2
           (X4-Ux)^2+ (Y4-Uy) ^2+ (Z4-Uz) ^2= (R4 +E4) ^2
X, Y, Z - the positions coordinates of the satellite.
R - the measure range =C(T-dT)
C -velocity of em signal
T - transit time of the signal
dT-time error at the receiver
U- the output, user’s position
So there are four equations and four unknowns. In actual practice a receiver
calculates a set of equations with 7 unknowns. In addition to the 3 positions and
time they have added the Doppler data dx, dy, and dz which represents the
relative speed between the satellite and the receiver. These terms are needed
because our solution is based on moving objects and dx and dy can be used as
part of the receiver velocity calculation (dz is discarded). Four equations will

compute a full 3D solution but new 12 channel units can use additional satellites
to perform an overdetermined solution that will offer more accuracy. Older
multiplex units pick the best 4 satellites based on their DOP. As satellites move
out of view or get blocked from the receivers view by buildings, trees, and other
objects the receiver will switch to other satellites to maintain a location fix. If the
number of tracked satellites drops to three then a 3D solution is no longer
possible and the receiver will use the last available altitude and
compute a 2D fix for horizontal position.
Finding the Satellites
The other crucial component of GPS calculations is the knowledge of where the
satellites are. This isn't difficult because the satellites travel in a very high, and
predictable orbits. The satellites are far enough from the Earth (11,000 miles)
that they are not affected by our atmosphere. The GPS receiver simply stores an
almanac that tells it where every satellite should be at any given time. Things
like the pull of the moon and the sun do change the satellites' orbits very slightly,
but the Department of Defense constantly monitors their exact positions and
transmits any adjustments to all GPS receivers as part of the satellites' signals.
More Detail on Calculating a Receiver Position
The steps involved in calculating a position are:
    1. Sync with an available satellite and download the navigation information.
        Convert the messages to internal format for calculation. These include
        clock information, ionosphere data, and ephemeris (orbit) data.
    2. Calculate the exact satellite position. This will include both the elevation
        and azimuth data so we can apply troposphere modeling corrections that
        are dependent on how far above the horizon the satellite is.
    3. Calculate the pseudorange data and then correct for ionosphere and other
        modeling errors. (Note that consumer units may not compensate for
        ionosphere or tropospheric errors.)
    4. Repeat these steps for each available satellite. Correct the SV position for
        earth's rotation based on the time it takes for the signal traversal using the
        pseudo range data. (If the internal clock is close this can be done once,
        otherwise it will have to be repeated after the receiver position is
    5. Correct using differential data if available. (This may have to be done
        after the initial position is computed as part of the refinement step if the
        internal clock isn't accurate.) If the differential station is near the gps
        receiver it will be able to skip the corrections for modeling errors since

       this is part of the correction data available. Using dgps corrections leads
       to accuracy considerably beyond the capability of a standard receiver.
   6. Calculate the initial receiver position as described in the prior section.
   7. Convert the data based on whatever datum and grid system you have
       chosen and display the answer on the position page. Altitude is also
       corrected for geoid height prior to display.
   8. Add in the leap seconds and time offset from UTC time to the computed
       time data and converts it for display.
   9. Refine the position based on additional satellites and the correct time to
       obtain a 3D fix and subsequently improve the fix based on choosing SV's
       with a better DOP, applying an over determined solution, etc.


Satellites are equipped with very precise clocks that keep accurate time to within
three nanoseconds - that's 0.000000003, or three billionths, of a second. This
precision timing is important because the receiver must determine exactly how
long it takes for signals to travel from each GPS satellite.
Satellite clocks
Each Block II/IIA satellite contains two cesium (Cs) and two rubidium (Rb)
atomic clocks. Each Block IIR satellite contains three Rb atomic clocks.
GPS time information
GPS time is given by its Composite Clock (CC). The CC or "paper" clock
conforms to all Monitor Station and satellite operational frequency standards.
The system time is referenced to the Master Clock (MC) at the USNO from
which system time will not deviate by more than one microsecond.
A direct reference to UTC (U.S Naval Observatory, Master Clock) can be made
automatically by most timing receivers. These receivers can be commanded to
take the two constants, A0 and A1, from the NAV message for a linear
extrapolation to the USNO MC.


Each of the 24 satellites transmits a set of signals using spread spectrum
technology. Spread spectrum technology enables low-powered satellites to
produce signals that can be detected at very low received-signal levels.
Essentially, the carrier signal is modulated by a unique coding sequence, which

has the effect of spreading the signal’s frequency spectrum. Using a replicated
code sequence, a GPS receiver searches that spectrum looking for a match. The
signal can then be "unspread" and decode. Several signals are transmitted over
the same spectrum, but using distinctly different coding.


The accuracy of a position determined with GPS depends on the type of receiver.
Most hand-held GPS units have about 10-20 meter accuracy. Other types of
receivers use a method called Differential GPS (DGPS) to obtain much higher
accuracy. DGPS requires an additional receiver fixed at a known location
nearby. Observations made by the stationary receiver are used to correct
positions recorded by the roving units, producing an accuracy greater than 1
When the system was created, timing errors were inserted into GPS
transmissions to limit the accuracy of non-military GPS receivers to about 100
meters. This part of GPS operations, called Selective Availability, was
eliminated in May 2000.


With simultaneous data received from four satellites, one’s position (e.g.
latitude, longitude, altitude and time) can be calculated. More the number of
satellites visible, better the accuracy. Under ideal conditions, the location is
precisely and accurately determined. However, under real conditions, there is
always some degree of error. Despite the opportunity for error, positioning can
be calculated to within a few hundred feet or less in most cases.
Errors can be caused by
        Selective Availability or SA
The degradation applied by the US DOD to the satellite signal. The SA process
induces an error; however, using data from more than four satellites can mitigate
that error. Nevertheless, the SA-induced error is presently a fact of life in each
position calculation. Fortunately, SA will hamper very precise positioning
accuracy, but not to a point where it undermines the requirements for personal
        Ionosphere and troposphere delays
The GPS assumes that signals will be traveling between satellite and receiver is

in a straight line. The signal will actually be delayed upon going through the
ionosphere and troposphere.
       Receiver clock errors
Since it is not practical to have atomic clocks in the receiver, the receiver timing
references will have some small error.
       Multipath error

Multipath error can produce very large deviations. Multipath is caused by
satellite signals that arrive at the receiver after having bounced off some nearby
structure (e.g. a tall building), or the ground. Because the path is not straight, the
time delay will be longer, and the distance from the satellite will also seem to be
longer (see figure 2). This can produce location errors that are unacceptable,
particularly in urban automobile navigation applications.
       Signal attenuation
Non-restricted GPS signals are transmitted at 1.575 GHz, a microwave
frequency. Such signals are blocked by steel and concrete structures (e.g.
buildings and tunnels), and attenuated by passing through trees and leaves. The
GPS specification for minimum detectable signals renders reception marginal
when the signal is attenuated by foliage. Denser the foliage, more marginal the
signal. As such, receivers that just meet this specification are not reliable for use
in forests or even tree-lined streets. To ensure being able to detect signals in a
forest, the receiver must provide sensitivity that exceeds the current standard.
For example, the receiver must be able to detect signals whose power has been
attenuated to a level of about 5 percent of the initial level.
       Orbital errors
Also known as ‘ephemeris errors’, these are inaccuracies in the satellite’s
reported position.

16. DGPS

The idea behind all-differential positioning is to correct bias errors at one
location with measured bias errors at a known position. A reference receiver, or
base station, computes corrections for each satellite signal.
Differential GPS or “DGPS” removes common-mode errors, those errors
common to both the reference and remote receivers (not multipath or receiver
noise). Errors are more often common when receivers are close together (less
than 100 km). Differential position accuracies of 1-10 meters are possible with
DGPS based on C/A code SPS signals. That improved accuracy has a profound
effect on the importance of GPS as a resource. With it, GPS becomes more than
just a system for navigating boats and planes around the world. It becomes a
universal measurement system capable of positioning things on a very precise
Differential GPS involves the cooperation of two receivers, one that's stationary
and another that's roving around making position measurements.
The stationary receiver is the key. It ties all the satellite measurements into a
solid local reference.
The receiver used gets extra information through:
   A local support signal
         This option can only be used for professional applications. The low cost is
an advantage and several airports, to improve the control over flight traffic.
   FM-transmitters with RDS
         The use of this system is relatively simple, because a simple FM-receiver
can be designed and build for use with this system. It does require a small
number of DGPS transmitters.
   Long wave transmitters with AMDS
         For the AM frequency band a RDS system was designed also, called
AMDS. The range of an AM-transmitter is much larger; therefore the use of this
system is a good option for cheap use of DGPS.
   Long wave services
         Next to public DGPS services there is a possibility to use a signal created
by a private company, for use with their equipment only.


Block I Satellites
The Block I satellites were built by Rockwell International. They can operate for

3.5 days between navigation message uploads from the ground.
The L-band antenna array on the Block I satellite is composed of twelve
elements arranged in two concentric rings. The elements are hollow cylinders,
approx. 51 cm long and 7 cm in diameter. The elements are composed of.05 cm
thick fiberglass. Each of the elements protrudes from a 15 cm diameter launcher.
The launchers for the inner elements are approx. 26 cm tall, while the launchers
for the outer rings are approx. 19 cm tall. The inner ring, approx. 15 cm in
radius has four equally spaced elements. The outer ring, approx. 44 cm in radius,
has the remaining eight elements in an octagon configuration. The inner and
outer rings are fed 180 degrees out of phase with each other, with 90% of the
power supplied to the outer array. This antenna configuration results in a
"dimpled” pattern. This pattern is designed to supply even power across the face
of the earth.
Block II Satellites
The Block II satellite is designed to provide reliable service over a 7.5 year life
span. The satellites were built by General Electric Astrospace. The satellite
design requires minimal interaction with the ground and allows all but a few
maintenance activities to be performed without interruption to the signal
broadcast. Periodic uploads of data from the control segment cause no disruption
to service.
The L-band antenna array on the Block II satellite is also composed of elements
arranged in two concentric rings. The elements are approx. 62 cm long but are
tapered over the remaining 13 cm. The launchers for both inner and outer rings
are 25 cm long cones. Dimensions for the inner and outer ring launchers have
identical dimensions. The improved Block II antenna provides slightly greater
gain than the Block I antenna.
Block IIA Satellites
The Block IIA satellites are essentially identical to the standard block II with one
exception. In the event the ground stations are unable to upload new navigation
information, the satellites will continue to transmit the same navigation message
for up to 180 days. The orbit of the satellite will change over this period of time.
Without periodic navigation updates from the ground, the accuracy of the system
will degrade over time. However, the system would at least still be operational.
Block IIR Satellites
The block IIR satellites feature autonomous navigation. That is, they can create

their own navigation messages without uploads from the ground. This allows the
system to maintain system accuracy for much longer periods between contacts
with the ground.
SAT TRACK- SAT TRACK is a real time satellite tracking and orbit prediction
program for UNIX/Linux platforms running color X-windows.
SAT TRAK IV- I don't know who makes SAT TRAK IV yet, but here it is on a
web page maintained by William Roth, N7RYW. It's for IBM PC compatibles.
The positioning system works with software disks that provide geographic
information about different areas of the country. Monaco currently uses the
Carin Navigation System, which incorporates the use of a software package
consisting of seven CD-ROM disks. Each disk contains detailed travel
information with highways, local streets and points of interest for a particular
region. Each disk is equipped with the entire United States interstate highway
system, allowing users to travel cross country without the need of additional
The software was developed by a California-based company by the name of
Navtech. The company provides navigational software to a majority of the GPS
manufacturers in the U.S.
Activating the system is easy. Using an infrared remote control, simply enter
your destination and select your preferred route, whether it be the shortest
distance, via highway, or if you prefer, a more scenic route. The system can even
help you avoid tolls and construction by offering alternate routes.
The system allows you to select your destination in several different ways. You
can use either the on-screen keyboard function, locate the destination with the
on-screen map and cursor, or you can call up the destination from the system’s

                               The CD-ROM includes map information as well
                               as destination categories such as restaurants,
                               museums and other areas of interest. The system
                               also features a personal destination memory that
can store up to 100 of your favorite destinations. The 10 most recent destinations
are stored automatically.Once your destination is entered, the GPS directs you
with turn-by-turn verbal and visual guidance. The system is so advanced, the
delivery of verbal instructions is even timed to your vehicle speed. If your

vehicle’s speed increases, the instructions are delivered earlier, giving you
enough time to move to the correct lane to prepare for a turn or exit

Above:          An example of a few menu screens you might see on your GPS

The system shows your vehicles position on a full-color, scalable map, through a
LCD screen. Visual guidance is provided by detailed graphics showing upcom-
ing intersections and the direction in which you should travel. The graphics
illustrate all the surrounding streets at each junction, making it easy to find your
way. The system also allows you a variety of views of your traveling area. You
can zoom in tightly for close-ups of museum locations, hospitals and a variety of
other points along the way, or you can zoom out for a broader, more generalized


    GPS in the air
        GPS offers an inexpensive and reliable supplement to existing navigation
techniques for aircraft. Civil aircraft typically fly from one ground beacon, or

waypoint, to another. Pilots on long distance flights without GPS rely on
navigational beacons located across the country. With GPS, an aircraft's
computers can be programmed to fly a direct route to a destination. The savings
in fuel and time can be significant. A GPS-based navigation system will increase
the number of airports that are able to help a well-equipped plane to land in low-
visibility conditions. In the near future in the USA it will even be allowed to use
GPS as the primary form of navigation.
    GPS on land
      Everyone who has the proper equipment can use it. The user of the GPS-
system uses the satellite system to locate where he/she is, and with the help of a
CD-rom or another large database that contains the GIS-map the car's computer
is able to calculate the exact position of the car. Delivery trucks can receive GPS
signals and instantly transmit their position to a central dispatcher. Police and
fire departments can use GPS to dispatch their vehicles efficiently, reducing
response time. GPS helps motorists find their way by showing their position and
intended route on dashboard displays. Railroads are using GPS technology to
replace older, maintenance-intensive mechanical signals.
    GPS in sea
      GPS is a powerful tool that can save a ship's navigator hours of celestial
observation and calculation. GPS has improved efficient routing of vessels and
enhanced safety at sea by making it possible to report a precise position to
rescuers when disaster strikes.
    Military Uses for GPS
      With GPS, the soldiers are able to go places and maneuver in sandstorms
or at night when even the troops who lived there couldn’t. It is used also for
troop deployment, artillery fire etc. GPS has become important for nearly all
military operations and weapons systems. It is used on satellites to obtain highly
accurate orbit data and to control spacecraft orientation. Picture the desert, with
its wide, featureless expanses of sand. GPS receivers were carried by foot
soldiers and attached to vehicles, helicopters, and aircraft instrument panels.
    GPS in scientific research
GPS has made scientific field studies throughout the world more accurate and
has allowed scientists to perform new types of geographic analyses. Geologists
use GPS to measure expansion of volcanoes and movement along fault lines.
Ecologists can use GPS to map differences in a forest canopy. Biologists can
track animals using radio collars that transmit GPS data. Geographers use GPS
to define spatial relationships between features of the Earth's surface. Scientists

use GPS for a wide range of applications. Scientific analysis that formerly had to
be conducted in a laboratory can now be done quicker and easier in the field.
    Applications for your business
By use of GPS an insurance company will be able to track down a stolen vehicle
in every situation. A transport company which has GPS installed enables her
drivers to take the shortest route, avoiding traffic jams, to the delivery point
using GPS and GIS, thus offering better and faster service. For a transport
company using boats for transport, GPS can be of excellent use to locate a ship
with a specific cargo. The captain of a ship can use GPS to directly locate his
ship, and also the use of a beacon to locate a drowning person is a good option
for use of GPS.
    Monitor Nuclear Explosions
Nuclear explosions emit an X-ray flash lasting less than 1 microsecond. This
flash can be seen by the X-ray flash detectors on several satellites. By measuring
the time delay of arrival of the flash at several satellites, the location of the
explosion can be determined. Several of the GPS satellites carry background X-
ray radiation detectors to provide an accurate record of the X-ray environment
around the earth.
    Every Day Life
During construction of the tunnel under the English Channel, British and French
crews started digging from opposite ends: one from Dover, England, one from
Calais, France. They relied on GPS receivers outside the tunnel to check their
positions along the way and to make sure they met exactly in the middle.
Otherwise, the tunnel might have been crooked. With GPS we would be able to
help ships avoid icebergs by zeroing in on their position and notifying the ship of
the location and possibly bypass a disaster.
    Surveying and map making with GPS
Surveying that previously required hours or even days using conventional
methods can be done in minutes with GPS.
    GPS for Horticulture
In orchards, GPS is used mainly for orchard mapping or electrical mapping. The
GPS system allows orchardist's to accurately keep records of chemical
applications, which is extremely important where the government is concerned.
It can keep track of orchard costs, record and track yields. GPS also allows for
the fine-tuning of orchard management techniques for the grower.
    Set Your Watch!
Because GPS includes a very accurate time reference, the system is also widely

used for timekeeping. GPS receivers can display time accurate to within 150
billionths of a second.


This remarkable system was not cheap to build. Development of the $10 billion
GPS satellite navigation system was begun in the 1970s by the US Department
of Defense, which continues to manage the system, to provide continuous,
worldwide positioning and navigation data to US military forces around the
globe. Ongoing maintenance, including the launch of replacement satellites, adds
to the cost of the system. Amazingly ,GPS actually predates the introduction of
the personal computer. Its designers may not have foreseen a day when we
would be carrying small portable receivers, weighing less than a pound, at a
price as less as $300, that would not only tell us where we are in position
coordinates (latitude/longitude), but would even display our location on an
electronic map along with cities, streets and more. A commercial receiver used
for navigation purposes will be able to measure only the coarse pseudo range
distances coded on one of the two frequencies. Such receivers are available from
1500 FF or 300 USD. On the opposite, dual frequency receivers able to measure
both pseudo-range and phase data on both carrier waves cost up to 150,000 FF or
30,000 USD. There is an intermediate category of receivers which allow
relatively precise positioning without being excessively costly. Those are the
single frequency receivers, which measure pseudo-range and phase data on only
one of the two wavelength. Acquiring data only on the frequency with the higher
signal/noise ratio, those receivers are built with relatively cheap electronic.
There are no subscription fees or set up charges to use GPS.The designers
originally had military application in mind. Fortunately, an executive decree in
the 1980s made GPS available for civilian use also. Now everyone gets to enjoy
the benefits of GPS. The capability is almost unlimited. There are no
subscription fees or set up charges to use GPS. (Well, its your tax money that
paid for it)So we could just break out a GPS receiver, put the batteries in and
dive right into the fun!


Imagine being an archaeologist on an expedition to the Yucatan Peninsula in
Mexico. After preparing for your trip for months, you are certain that somewhere
close by are the ruins of villages once populated by Mayan Indians. The forest is
dense, the sun is hot, and the air is humid. The only way you can record where
you have been, or find your way back to civilization, is by using the almost
magic power of your GPS receiver.
Or let's suppose you are an oceanographer for the International Ice Patrol. You
may be responsible for finding icebergs that form in the cold waters of the North
Atlantic Ocean. Some of these icebergs are 50 miles long. They are a major
threat to the ships that travel those waters, and more than 300 of them form
every winter. Using a GPS receiver, you are able to help ships avoid disaster by
zeroing in on the position of the icebergs and notifying ship captains of their
locations, perhaps averting disaster.
There will probably be a time soon when every car on the road can be equipped
with a GPS receiver, including a video screen installed in the dashboard. The in-
dash monitor will be a full-color display showing your location and a map of the
roads around you. It will probably monitor your car's performance and your car
phone as well. Systems as amazing as this one are already being tested on
highways in the United States.
GPS is rapidly changing the way people are finding their way around the earth.
Whether it is for fun, saving lives, getting there faster or whatever use you can
dream of, GPS navigation is becoming more common everyday. GPS will figure
in history alongside the development of the sea-going chronometer. This device
enabled seafarers to plot their course to an accuracy that greatly encouraged
maritime activity, and led to the migration explosion of the nineteenth century.
GPS will affect mankind in the same way. There are myriad applications that
will benefit us individually and collectively.



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