GLOBAL POSITIONING SYSTEMS
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Seminar Report
on
GLOBAL POSITIONING SYSTEMS
Submitted by
Prateek Rastogi
(05IT6013)
M.TECH (IT)
School of Information Technology
Under the Guidance of
Dr. Arobinda Gupta
(Associate Professor)
Department of Computer Science and
School of Information Technology
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Abstract
GPS, which stands for Global Positioning System, is the only
system today able to show you your exact position on the
Earth anytime, in any weather, anywhere. GPS uses satellite
technology to enable a terrestrial terminal to determine its
position on the Earth in latitude and longitude. GPS
satellites, 24 in all, orbit 11,000 miles above the Earth.
Ground stations located worldwide continuously monitor
them. The satellites transmit signals that can be detected by
anyone with a GPS receiver. Using the receiver, you can
determine your location with great precision.
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Contents
1.
Introduction
2.
History
3.
GPS
4.
Modes of Operation
 Standard Positioning Service [SPS]
 Precise Positioning Service [PPS]
5.
Segments of GPS
 Space Segment
 Control Segment
 User Segment
6.
Signals from GPS
7.
Calculations
8. Causes of Errors
9.
Applications
10. Conclusion
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1. Introduction
Since prehistoric times, people have been trying to figure out a reliable way to tell where they are and
to help guide them to where they are going. 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 following the stars.
The ancient Phoenicians used the North Star to journey from Egypt and Crete. The next major developments in
the quest for the perfect method of navigation were the magnetic compass and the sextant.
About 50 years ago, when the space technology was born, it was actually giving birth to an entirely
new method of tracking and guiding, employing man made stars, now known as Satellite Navigation Systems
and Global Positioning Systems is the latest of its out come. The Global Positioning System (GPS) is a
worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. GPS
uses navigation satellites 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 meter! In a sense it's like giving every
square meter on the planet a unique address. GPS receivers have been miniaturized to just a few integrated
circuits and so are becoming very economical. And that makes the technology accessible to virtually everyone.
2. History
The era of ‘man-made stars’ started in the early 1950s, with the shock launching of sputnik-1 in the
lower orbits by USSR. But it was noticed that the frequency of the radio transmitter on board of sputnik shown
considerable dopler effect when observed from any fixed point on earth. It was soon found out that this dopler
phenomenon could be exploited to create a truly accurate global positioning system, free from many of the
constraints of existing earth-bound systems. During that period of cold war, within no time, USA also joined the
race for Space supremacy.
The first commercially available system to be developed was Navy Navigation Satellite System
[NNSS].In 1973 a combined US Navy and task force set out to develop a new and much more accurate
positioning system NAVSTAR also known as GPS. Space Vehicles [SVs] launched for GPS are called
Navigation Technology Satellites [NTS] and NTS-1 went into orbit in 1974 and become the embryo of the
system. The original SVs known as Block1 (numbered from 1 to 11), have ceased operation. The next
generation of SVs, which are designated as Block2 (numbered from 13 to 21) and Block2A (numbered from 22
to 40) were launched between February 1989 and November 1997 is now in operation. GPS was declared fully
operational by US Air Force Space Command on 27 th of April 1995.The NNSS was later called back and it
ceased its operation at midnight on 31 December 1996, and gave way to GPS.
3. GPS
The nominal GPS Operational Constellation consists of 27(24 in operation and three extras in case one fails).
The U.S. military developed and implemented this satellite network as a military navigation system, but soon
opened it up to everybody else. These satellites orbit the earth once in 12 hours. There are often more than 24
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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. The orbit altitude is such that the satellites
repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each
day). There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and
inclined at about 55 degrees with respect to the equatorial plane. This constellation provides the user with
between five and eight SVs visible from any point on the earth.
This Calculation of distance is based on a simple mathematical principle called trilateration. That is,
finding the location, if distance from a few number of fixed points are known. Below figure will help us to know
the various terminologies used in satellite navigation such as Orbital planes, inclination etc.
GPS has 3 parts, the space segment, the user Segment, and the control segment. The space segment
consists of 24 satellites, each in its own orbit 20,200 km s above the Earth. The user segment consists of
receivers, which you can hold in your hand or mount in your car. The control segment consists of ground
stations (five of them, located around the world) that make sure the satellites are working properly. It can work
in two modes of operations, which is explained in the next section.
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4. Modes of Operation
GPS can work in two modes of operations.
They are:
1.
Standard Positioning Service [SPS]
2.
Precise Positioning Service [PPS]
The main difference between these modes is in accuracy. Standard Positioning Service is of lower
accuracy while Precise Positioning Service is of much higher accuracy.
4.1. Standard Positioning Service [SPS]
Civil users worldwide use the SPS without charge or restrictions. Most receivers are capable of
receiving and using the SPS signal. The SPS accuracy is intentionally degraded by the use of Selective
Availability
SPS Predictable Accuracy

100 meter horizontal accuracy.

156 meter vertical accuracy

340 nanoseconds time accuracy
These GPS accuracy figures are from the 1999 Federal Radio navigation Plan. The figures are of 95%
accuracies.
Selective Availability (SA):
Selective Availability is an intentional degradation of accuracy, intended to prevent "the enemy" from
making tactical use of then full accuracy of GPS. To guard against fake transmission of SV data, a system called
Anti-Spoofing was used, in which the P-Code sent from SVs are in encrypted form.
4.2. Precise Positioning Service [PPS]
Military receivers can use the encrypted P code to get 20 meter accuracy, or better, regardless of the state of SA.
Authorized users with cryptographic equipment and keys and specially equipped receivers use the Precise
Positioning System..
PPS Predictable Accuracy

22 meter Horizontal accuracy

27.7 meter vertical accuracy

200 nanosecond time (UTC) accuracy
5. Segments of GPS
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Global Positioning System consists of three interacting segments:

The Space Segment -- satellites orbiting the earth

The Control Segment -- the control and monitoring stations

The User Segment -- the GPS signal receivers owned by users.
5.1.The Space Segment
The space segment consists of a constellation of 24 active satellites orbiting the earth every 12 hours.
There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at
about fifty-five degrees with respect to the equatorial plane. Four satellites are located in each of six orbits. The
orbits are distributed evenly around the earth. The satellites orbit at an altitude of about 20,200 km at a velocity
of 26.61 km per second. Satellites are positioned so that we can receive signals from six of them nearly 100
percent of the time at any point on Earth. You need that many signals to get the best position information. This
constellation provides the user with between five and eight SVs visible from any point on the earth. These
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 know
exactly how long it takes for its signal to get to each satellite and return. By knowing the exact amount of time
the signal has taken to get back from each satellite, it can calculate its position. Each SV contains four atomic
clocks (two cesium and two rubidium). SV clocks are monitored by ground control stations.
5.2. The Control Segment
The GPS control, or ground, segment consists of unmanned monitor stations located around the
world (Hawaii and Kwajalein in the Pacific Ocean; Diego Garcia in the Indian Ocean; Ascension Island in the
Atlantic Ocean; and Colorado Springs, Colorado); These stations track and monitor the GPS satellites. The
Master Control facility is at Falcon Air Force Base in Colorado Springs, Colorado; and four large ground
antenna stations broadcast signals to the satellites. These monitor stations measure signals from the SVs which
are incorporated into orbital models for each satellites. The models compute precise orbital data and SV clock
corrections for each satellite. The Master Control station uploads orbital data and clock data to the SVs. The
SVs then send subsets of the orbital data to GPS receivers over radio signals.
5.3. The User Segment
The GPS User Segment consists of the GPS receivers and the user community. GPS receivers convert
SV signals into position, velocity, and time estimates. Four satellites are required to compute the four
dimensions of X, Y, Z (position) and Time. GPS receivers are used for navigation, positioning, time
dissemination, and other research. Astronomical observatories, telecommunications facilities, and laboratory
standards can be set to precise time signals or controlled to accurate frequencies by special purpose GPS
receivers.
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6. Signals from GPS
The SVs transmit two microwave carrier signals. The L1 frequency (1575.42 MHz) carries the
navigation message and the SPS code signals. The L2 frequency (1227.60 MHz) is used to measure the
ionospheric delay by PPS equipped receivers.
6.1. The C/A Code and Pcode
Civilian receivers to locate position use the C/A code. The code is used to determine psuedorange (the
apparent distance to the satellite), which is then used by the GPS receiver to determine position.
The C/A code is "coarse". The P code is more precise, but it is encrypted into the Y code (by something called
Anti-Spoofing), and it cannot be decoded without having a key to the encryption. This is not available to civilian
users. Military GPS receivers first use the C/A code to acquire a position, then they use the P code to determine
the position more precisely. The P code is ten times as fast, which means it can determine the psuedorange ten
times more accurately. However, it is much more difficult to search, which is why even the military needs the
C/A code.
The C/A and the P codes are psuedo random noise (PRN) codes. This means that they have the
characteristics of random noise. But they are not random. They are very precisely defined. Out of all the
possible random sequences, these have been very carefully selected.
There are 37 PRN sequences used for the C/A code. Each satellite has its own. Each satellite broadcasts a
different code, repeating it over and over again. It contains no data; it is simply an identifier. However its timing
is very precisely determined, and that timing is used to determine the psuedorange.
The codes are a sequence of zeros and ones (binary). Each zero or one is called a "chip". They are called chips
instead of bits to emphasize that the zeros and ones do not carry data. It is a fixed pattern, with a fixed length,
which is repeated indefinitely.
The C/A code is 1023 chips long, and it is broadcast at 1.023 Mega-chips per second. That means that it repeats
every millisecond, and that each chip is 293 meters (or 0.978 microsecond) long. The whole sequence is about
300 km long.
The P code is 2.3547 x 10**14 chips long, so even though it is broadcast at 10.23 Mega-chips per second, it
would not repeat for 266.4 days. It is actually broken into 37 pieces (one for each satellite) and restarted every
week. The chip length is 29.3 meters.
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The job of the correlators in the GPS is to determine which codes are being received and to determine
their exact timing. A good correlator can determine the timing with an accuracy of about one percent of the chip
length. This would provide an accuracy in the psuedorange measurement of about 3 meters for the C/A code, or
about 0.3 meters for the P code.
6.2. Navigation Message
The GPS Navigation Message contains parameters that describe the location of the GPS satellites, their clock
offsets, and various other system parameters. The Navigation Message consists of 25 data frames, each divided
into five sub-frames. The sub-frames are 300 bit sequences. Bits are transmitted at 50 bits per second. Each subframe takes 6 seconds, each frame 30 seconds, and the entire set of 25 frames takes 750 seconds (12.5 minutes)
to complete.
Sub-frames one, two, and three contain messages pertaining to the satellite that is transmitting them
(complete orbit and clock descriptions) , while sub-frames 4 and 5 contain system data relating to all the
satellites (abbreviated orbit and clock data for all the satellites and common system).
Within a GPS receiver the received data bits are aligned, checked for errors (with a parity algorithm),
separated into sets of bits representing each individual parameter, scaled, converted into various numeric
formats, and then converted where required into specific units. These units include meters, meters squared,
semi-circles, radians, seconds, seconds per second, seconds per second per second, and weeks
. Normally, a receiver gathers new Orbital data each hour, but can use old data for up to four hours
without much error. The accurate orbital parameters are used with an algorithm that computes the SV position
for any time within the period of the orbit described by the orbital parameter set. The approximate orbital data is
used to preset the receiver with the approximate position and carrier Doppler frequency (the frequency shift
caused by the rate of change in range to the moving SV) of each SV in the constellation. Each complete SV data
set includes an ionosphere model that is used in the receiver to approximate the phase delay through the
ionosphere at any location and time.
The GPS receiver produces replicas of the C/A and/or P (Y)-Code. Each PRN code is a noise-like, but
pre-determined, unique series of bits. The receiver produces the C/A code sequence for a specific SV with some
form of a C/A code generator. Modern receivers usually store a complete set of precompiled C/A code chips in
memory.
The C/A code generator repeats the same code sequence every millisecond. PRN codes are defined for
37 satellite identification numbers. The receiver slides a replica of the code in time until there is correlation with
the SV code. If the receiver applies a different PRN code to an SV signal there is no correlation. When the
receiver uses the same code as the SV and the codes begin to line up, some signal power is detected. As the SV
and receiver codes line up completely, full signal power is detected. A GPS receiver uses the detected signal
power in the correlated signal to align the C/A code in the receiver with the code in the SV signal. Usually a late
version of the code is compared with an early version to insure that the correlation peak is tracked.
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7. Calculations
A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each, and use
this information to deduce its own location. Calculation of position is based on a simple mathematical principle
called trilateration.
7.1. Trilateration
If a point ‘X’ in a 3-dimension space is at a distance of ‘a’ from another point ‘A’ in space, the point
could be anywhere on the surface of a huge, imaginary sphere with radius ‘a’ and point ‘A’ as its centre. Hence
there is infinite probable location for point ‘X’. If the same point at a distance of ‘b’ from another known point
‘B’, we get a overlapping of the first sphere with second sphere. The spheres intersect in a perfect circle. Hence
the location of point ‘X’ could be any where on the circle. Here also the probable location remains infinite. If
you know the distance to a third point ‘C’, you get a third sphere, which intersects with this circle at two points.
Here probable location get reduced to 2.Hence now we need only one more sphere to find the exact location of
point ‘X’. Points A, B, C, are called reference points.
Conclusion: To locate a point in space we need to know the distance of that point to at least 4 reference points in
space.
The same concept is used in GPS. Here the un-known location is the location to be tracked (location
of
user).The reference points are taken as the positions of GPS satellites, with which the receiver can
communicate at that instance. The Earth itself can act as a fourth sphere -- only one of the two possible points
will actually be on the surface of the planet, so we can eliminate the one in space.
If the receiver had a perfect clock, exactly in sync with those on the satellites, three measurements,
from three satellites, would be sufficient to determine position in 3 dimensions. Unfortunately, you can't get a
perfect clock that will fit (financially or physically) in receiver, so a fourth satellite is needed to resolve the
receiver clock error. Three spheres will intersect, but the fourth spheres will not intersect at one point due to
clock error. Since the receiver makes all its distance measurements using its own built-in clock, the distances
will all be proportionally incorrect.
Now the receiver can easily calculate the necessary adjustment that will cause the four spheres to
intersect at one point. Based on this, it resets its clock to be in sync with the satellite's atomic clock. The receiver
does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the
satellites. The receiver will adjust its clock until they do, providing very accurate time, as well as position.
In order to make this simple calculation, the GPS receiver has to know two things:

The location of at least three satellites above you

The distance between the receiver and each of those satellites
The GPS receiver figures both of these things out by analyzing high-frequency, low-power radio signals sent by
SVs.
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7.2. Position of satellites
In order for the distance information to be of any use, the receiver also has to know where the
satellites actually are. This isn't particularly difficult because the satellites travel in very high and predictable
orbits. The GPS receiver simply stores an almanac that tells it where every satellite should be at any given time.
7.3. Distance between the receiver and satellites
At a particular time (let's say midnight), the satellite begins transmitting a long, digital pattern called a
random code. 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 signal's travel time. The receiver measures the time required for
the signal to travel from the satellite to the receiver, by knowing the time that the signal left the satellite, and
observing the time it receives the signal, based on its internal clock. The receiver multiplies this time by the
speed of light to determine how far the signal traveled. Assuming the signal traveled in a straight line, this is the
distance from receiver to satellite.
7.4. Calculation of parameters
1.
Position:
The GPS satellites (normal navigation) can be used to determine three position dimensions and time.
Position dimensions are computed by the receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ)
coordinate system.
Position in XYZ is converted within the receiver to latitude, longitude and height above the surface
using appropriate conversion factors.
2. Velocity:
Velocity is computed from change in position over time,
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3. Time
Time is directly got from the SV signal.
8. Errors:
No system is free from errors. GPS too has no exception. Various errors are:

Satellite Clock Errors

Ionospheric Errors

Tropospheric Errors

Multipath Errors

Relativity Errors
8.1. Satellite Clock Errors
SV clock frequency stability is one of the important problems. SV passes through a hostile
environment where temperature can vary as much as 300 degree Celsius. There is little protection from sun’s
radiations. But the clock is continuously offseted from the ground control stations.
8.2. Ionospheric Errors:
(10 meters). The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized
air. The transmitted model can only remove about half of the possible 70 ns of delay leaving a ten meter unmodeled residual. The speed of wave is decreased by the refraction of ionic clouds in the ionosphere.
8.3. Tropospheric Errors
(1 meter). The troposphere is the lower part (ground level to from 8 to 13 km) of the atmosphere that
experiences the changes in temperature, pressure, and humidity associated with weather changes. Complex
models of tropospheric delay require estimates or measurements of these parameters.
8.4. Multipath Errors
(0.5 meters). Multipath is caused by reflected signals from surfaces near the receiver that can either
interfere with or be mistaken for the signal that follows the straight line path from the satellite. Multipath is
difficult to detect and sometime hard to avoid.
8.5. Relativity Errors
It’s an entirely predictable error, caused due to the time compression by the mass of earth. These errors
are generally offseted from the ground stations
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8.6 Error Correction
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.
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.
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. 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.
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.
9. Applications
GPS has become important for nearly all military operations and weapons systems. They were carried
by foot soldiers and attached to vehicles, helicopters, and aircraft instrument panels. GPS receivers were used in
several aircraft, including F-16 fighters, KC-135 aerial refuelers and B-52 bombers; Navy ships used them for
rendezvous, minesweeping, and aircraft operations.
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.
GPS is also helping to save lives. Many police, fire, and emergency medical service units are using
GPS receivers to determine the police car, fire truck, or ambulance nearest to an emergency, enabling the
quickest possible response in life-or-death situations.
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9.1. Main Application Fields of GPS
Industry
Transportation
Agriculture
Mapping & GIS Data Collection
Public safety
Surveying
Telecommunication
Aviation
Fleet Tracking
Marine

Science

Military
Archaeology
Atmospheric Science
Environmental Science
 Geology & Geophysics
Oceanography
Wildlife
Intelligence & Target Location
Navigation
Weapon Aiming &Guidance
10. Conclusion
The future of GPS is as unlimited. New applications will continue to be created as technology
evolves. The GPS satellites are like handmade stars in the sky.
But no technology last forever. In the future GPS will also get replaced with more advanced
technologies. But it had really played a great role in its journey for more perfect and more convenient
Navigational Systems.
11. References
 www.trimble.com
 GPS primer “A Student Guide”(Aerospace Corporation).
 An Introduction – Global Positioning system S. K. Upadhyaya, G. S. Pettygrove,
J.W. Oliveira, B. R. Jahn1.
 GPS Guide for Beginners (GARMIN Corporation).
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