Satellite Navigation Systems

A satellite navigation or sat nav system is a system

of satellites that provide autonomous geo-spatial
positioning with global coverage.
It allows small electronic receivers to determine
their location (longitude, latitude, and altitude) to
high precision (within a few metres) using time
signals transmitted along a line of sight by radio
from satellites.
The signals also allow the electronic receivers to
calculate the current local time to high precision,
which allows time synchronisation.
A satellite navigation system with global coverage
may be termed a global navigation satellite system
or GNSS.
WHAT IS SATELLITE NAVIGATION?
Navigation- art or science of plotting, ascertaining,
or directing of movements (knowing your whereabouts
and being able to find your way around)
Celestial Navigation Direction and distances determined with
timed sighting of stars
Wandering Technique used by most of us while at new place
Piloting Fixing position and direction wrt familiar and significant
landmarks
Radio / Electronic Navigation Position is determined by
measuring the travel time of radio waves.
Terrestrial Systems such as Decca, Omega, Loran etc.
Satellite Systems such as Navstar GPS and Russian Counter
Part,.
 GNSS Overview
GNSS (Global Navigation Satellite Systems) started with the
launch of the U.S Department of Defense Global Positioning
System (GPS) in the late 1970s

GNSS systems currently include

GPS (United States)
GLONASS (Russia)
IRNSS (India)
Galileo (European Union)
BeiDou (China)
 As of now, only the United States NAVSTAR Global
Positioning System (GPS) and the Russian GLONASS are
global operational GNSSs.

China is in the process of expanding its regional Beidou

navigation system into the global Compass navigation
system by 2020.
The European Union's Galileo positioning system is a
GNSS in initial deployment phase, scheduled to be fully
operational by 2020 at the earliest.
France, India and Japan are in the process of developing
regional navigation systems.
 GPS
The United States' Global Positioning System
(GPS) consists of up to 32 medium Earth orbit
satellites in six different orbital planes, with the
exact number of satellites varying as older
satellites are retired and replaced. Operational
since 1978 and globally available since 1994,
GPS is currently the world's most utilized
satellite navigation system.
 GLONASS
The formerly Soviet, and now Russian,
Global'naya Navigatsionnaya Sputnikovaya
Sistema (GLObal NAvigation Satellite System),
or GLONASS, was a fully functional navigation
constellation in 1995. After the collapse of the
Soviet Union, it fell into disrepair, leading to
gaps in coverage and only partial availability. It
was recovered and fully restored in 2011.
 Indian Regional Navigational Satellite System
The Indian Regional Navigational Satellite System (IRNSS) is an
autonomous regional satellite navigation system being developed by the
Indian Space Research Organisation (ISRO) which would be under
complete control of the Indian government.

It will consist of a constellation of 7 navigational satellites. All the 7

satellites will be placed in the Geostationary orbit (GEO) to have a larger
signal coverage and lower number of satellites to map the region. It is
intended to provide an all-weather absolute position accuracy of better
than 7.6 meters throughout India and within a region extending
approximately 1,500 km around it.

The requirement of such a navigation system is driven by the fact that

access to foreign government-controlled global navigation satellite
systems is not guaranteed in hostile situations. The IRNSS would provide
two services, with the Standard Positioning Service open for civilian
use and the Restricted Service, encrypted one, for authorised users
(military).
 Seven satellites with the prefix "IRNSS-1" will constitute the space
segment of the IRNSS.
Satellites
IRNSS-1A
IRNSS-1B
IRNSS-1C
IRNSS-1D
IRNSS-1E
IRNSS-1F
IRNSS-1G
A network of 21 ranging stations located across the country will
provide data for the orbit determination of the satellites and
monitoring of the navigation signal, with the master control facility
(MCF) at Hassan, Karnataka.
The total cost of the project is expected to be INR1420 crore, with
the cost of the ground segment being INR 300 crore and each
satellites costing INR125 crore .
 IRNSS-1A : The satellite was launched on-board PSLV-
C22 on 1 July 2013 from the Satish Dhawan Space
Centre at Sriharikota, while the full constellation of 7
satellites is planned to be placed in orbit by 2015. ISRO
needs to launch at least four of the seven satellites to
start the operations of the IRNSS.

IRNSS-1B was launched recently on April 4, 2014 from

the Satish Dhawan Space Centre at Sriharikota.

IRNSS 1C and IRNSS 1D are planned to be launched by

the end of this year, and the next three IRNSS satellites
are planned in the beginning of 2015. So, by mid of
2015 all the seven satellites will be in orbit .
 What is GPS?
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.
 First developed by the United States Department of Defense (DOD).

The first Satellite was placed in orbit on 22nd February 1978, and
there are currently 28 operational satellites orbiting the Earth at a
height of 20,180km.

GPS receivers are used for positioning, locating, navigating,

surveying and determining the time.

Provides reliable positioning, navigation and timing services to civilian

user on a worldwide basis.

It is freely available to all.

It provides accurate location and time information for an unlimited

number of people in all weather, day and night, anywhere in the world.
 GPS Satellite Vehicle
Weight
2370 pounds
Height
16.25 feet
Width
38.025 feet including
wing span
Design life10 years
Block IIR satellite vehicle
assembly at Lockheed
Martin, Valley Forge, PA
 Popular Companies Manufacturing GPS:

Garmin
Trimble
Tom Tom
Magellan
Navico
Motorola Inc.
 Principle
In order to calculate ones exact location, the signal transit
time is measured between the point of observation and four
different satellites whose positions are known.
To get the distance to each satellite, the GPS transmits a
signal to each satellite.
The signal travels at a known speed.
The system measures the time delay between the signal
transmission and signal reception of the GPS signal.
The signals carry information about the satellites
location.
Determines the position of, and distance to, at least three
satellites, to reduce error.
The receiver computes position using trilateration.
 WORKING

GPS satellites broadcast signals from space

that are picked up and identified by GPS
receivers.
Each GPS receiver then provides three-
dimensional location (latitude, longitude,
and altitude) plus the time.
 GPS satellites provide specially coded satellite signals
that can be processed with a GPS receiver, enabling
the receiver to compute position, velocity and time.
A minimum of four GPS satellite signals are required to
compute positions in three dimensions and the time
offset in the receiver clock.
Accuracy and precision of data increases with more
satellites.

The three parts of GPS are:

Satellites
Receivers
Software
 Satellites

There are quite a number

of satellites out there in
space. They are used for a
wide range of purposes:
satellite TV, cellular
phones, military purposes
and etc. Satellites can
also be used by GPS
receivers.
 GPS Satellites
The GPS
Operational
Constellation
consists of 24
satellites that orbit
the Earth in very
precise orbits twice
a day. GPS satellites
emit continuous
navigation signals.
A visual example of the GPS constellation in motion with the Earth
rotating. Notice how the number of satellites in view from a given
point on the Earth's surface.
 Receivers and Satellites
GPS units are made to
communicate with GPS
satellites (which have a
much better view of the
Earth) to find out exactly
where they are on the
global scale of things.
 STRUCTURE
The GPS is made up of three parts:

Space Segment(satellites orbiting the Earth, GPS

satellite vehicles, or SVs)
Control Segment(monitors stations on Earth,
tracking stations: A network of earth-based
facilities)
User Segment(GPS receivers owned by users,
Users & Receivers)
 Three Segments of the GPS

Space Segment

User Segment

Control Segment

Ground
Antennas
Master Station Monitor Stations
GPS Communication and Control
 COMPONENTS OF SPACE
SEGMENT
24 satellite vehicles
Six orbital planes
Inclined 55o with respect to equator
Orbits separated by 60o
20,200 km elevation above Earth
Orbital period of 11 hr 55 min
Five to eight satellites visible from
any point on Earth
 SPACE SEGMENT
GPS satellites fly in circular orbits at an altitude
of 20,200 km and with a period of 12 hours.
Powered by solar cells, the satellites
continuously orient themselves to point their
solar panels toward the sun and their antenna
toward the earth.
Orbital planes are centered on the Earth
Each planes has about 55 tilt relative to Earth's
equator in order to cover the polar regions.
 SPACE SEGMENT (Continued)
Each satellite makes two complete orbits each
sidereal day.
Sidereal - Time it takes for the Earth to turn 360
degrees in its rotation
It passes over the same location on Earth once
each day.
Orbits are designed so that at the very least, six
satellites are always within line of sight from
any location on the planet.
 Space Segment
The space segment is an earth-orbiting constellation of
24 active and five spare GPS satellites circling the earth in
six orbital planes. Each satellite is oriented at an angle of 55
degrees to the equator. The nominal circular orbit is
20,200-kilometer altitude. Each satellite completes one
earth orbit every twelve hours (two orbits every 24 hours).
Each satellite has a design life of approximately 10
years, weighs about 2,000 pounds, and is about 17 feet
across with its solar panels extended. Older satellites still
functioning are equipped with 2 cesium, and 2 rubidium
atomic clocks. Newer satellites are equipped with rubidium
atomic clocks. All satellites also contain 3 nickel-cadmium
batteries for backup power when a satellite is in earth
eclipse (out of view of the sun).
 Each satellite transmits as part of its signal to ground
stations and all users the following information:
-Coded ranging signals (radio transmission time
signals that allow receivers to triangulate their positions).
-Ephemeris position information (a message
transmitted every 30 seconds containing precise
information on the location of the satellite in space).
-Atmospheric data (necessary to help correct signal
interference from the satellites to the receiver).
-Clock correction information defining the precise
time of satellite signal transmission (in GPS Time)/
 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.
An almanac containing information on the GPS
constellation, which includes location and health of the
satellites. Whenever a GPS receiver is operating outdoors it
automatically downloads an almanac from the satellites.
This almanac is stored in the receivers memory until the
next time it is turned. The stored almanac allows a receiver
to more quickly acquire GPS satellite signal because it
already knows the general location, and other information,
about the satellites in the constellation.
 More than 300 miles while turned off (or turned on but
not picking up satellites), the stored almanac may not be
of any use to the receiver when it is finally turned back
on. A new almanac will be need to be downloaded to the
receiver for it to be able to quickly acquire satellite
signals again.

Why the need for a new almanac if the receiver is moved

more than 300 miles while turned off? Beyond 300 miles
from its last used location the receiver is presumed to be
using different GPS satellites, and therefore should
download a new almanac to reflect the new PRN codes. If
the receiver is turned on and collecting satellite signals
while moving over 300 miles, its almanac is automatically
updated.
 CONTROL SEGMENT
Composed of a master control station, an
alternate master control station.
It also includes a host of dedicated and shared
ground antennas and monitor stations.
 Control Segment

US Space Command

Cape Canaveral
Hawaii
Kwajalein Atoll

Diego Garcia
Ascension Is.

Master Control Station Monitor Station Ground Antenna

 Control Segment
The Master Control Station, or MCS) is located at the US Air Force
Space Command Center at Schriever Air Force Base (formerly Falcon AFB)
in Colorado Springs, Colorado. It's responsible for satellite control and
overall system operations. The Control segment is made up of a Master
Control Station (MCS), four monitor stations, and three ground antennas
(plus a reserve antenna at Cape Canaveral used primarily for pre-launch
satellite testing) used to uplink data to the satellites.

Monitor Stations continuously receive GPS satellite transmissions, and relay

this information in real time to the Master Control Station in Colorado. The
user segment also receives these same transmissions. Monitor stations
(MS) are located at Schriever Air Force Base, Hawaii, Kwajalein Atoll, and
Diego Garcia, and Ascension islands.
These stations are unmanned remote sensors that passively collect raw
satellite signal data and re-transmit it in real time to the MCS for
evaluation. Monitor stations basically function as very precise radio
receivers, tracking each satellite as it comes into view.
Ground antennas are remotely controlled by the MCS. They
are also located at Ascension, Diego Garcia, Kwajalein Atoll,
as well as Cape Canaveral, Florida. Ground antennas
transmit data commands received from the Master Control
Station to GPS satellites within their sky view. They also
collect telemetry data (the transmission of data from space
vehicles to receiving stations on the ground) from the
satellites.
The MCS uplinks data to GPS satellites which includes:
-Clock-correction factors for each satellite; necessary
to insure that all satellites are operating at the same
precise time (known as GPS Time).
-Atmospheric data (to help correct most of the
distortion caused by the GPS satellite signals passing
through the ionosphere layer of the atmosphere).
 -Almanac, which is a log of all GPS satellite positions and health,
and allows a GPS receiver to identify which satellites are in its
hemisphere, and at what times. An almanac is like a schedule
telling a GPS receiver when and where satellites will be
overhead. Transmitted continuously by all satellites, the almanac
allows GPS receivers to choose the best satellite signals to use to
determine position. The almanac is automatically downloaded
from satellites whenever a receiver is collecting a GPS signal. An
almanac can also be downloaded from a computer, a base
station or other archived almanac.
-Ephemeris data is unique to each satellite, and provides
highly accurate satellite position (orbit) information for that GPS
satellite alone. It does not include information about the GPS
constellation as a whole. Ephemeris information is also
transmitted as a part of each satellites time signal.
By using the information from the GPS satellite
constellation almanac in conjunction with the ephemeris data
from each satellite, the position of a GPS satellite can be very
precisely determined for a given time.
 Remember
The monitor stations track the navigation signals from all
the satellites and continuously send the data to the
Master Control Station (MCS) for processing. The MCS
computes orbit position projections for each satellite in
the constellation, as well as corrections to the satellites
on-board clocks.
The MCS sends this updated orbit and clock data to the
four ground antenna stations, from which it is uploaded
to each satellite three times per day to maintain system
accuracy. The ground stations also transmit commands to
the satellites for routine maintenance, software updates,
and orbit adjustments.
Every satellite is always in view of at least two ground
stations and usually three.
 Remember
GPS signals include ranging signals, used to
measure the distance to the satellite, and
navigation messages. The navigation messages
include ephemeris data, used to calculate the
position of each satellite in orbit, and
information about the time and status of the
entire satellite constellation, called the
almanac.
 The GPS Message
The navigation message is a continuous
stream of data transmitted at 50 bits
per second. Each satellite relays the
following information to Earth:
System time and clock correction Physically the signal is
just a complicated
values. digital code, or in other
Its own highly accurate orbital data words, a complicated
sequence of on and
(ephemeris). off pulses.

Approximate orbital data for all other

satellites (almanac).
All GPS satellites synchronize operations so that these repeating
signals are transmitted at the same instant.
 USER SEGMENT
The user's GPS receiver is the US of the GPS system.
GPS receivers are generally composed of an
antenna, tuned to the frequencies transmitted by
the satellites, receiver-processors, and a highly-
stable clock, commonly a crystal oscillator).
They can also include a display for showing
location and speed information to the user.
A receiver is often described by its number of
channels this signifies how many satellites it can
monitor simultaneously. As of recent, receivers
usually have between twelve and twenty channels.
 Components of the System
User segment
GPS antennas & receiver/processors
Position
Velocity
Precise timing
Used by
Aircraft
Ground vehicles
Ships
Individuals
 Four Primary Functions of GPS
Position and coordinates.

The distance and direction between any two waypoints,

or a position and a waypoint.

Travel progress reports.

Accurate time measurement.

 Waypoints
Waypoints are locations or landmarks that can be
stored in your GPS. Waypoints may be defined and
stored in the unit manually by inputting latitude and
longitude from a map or other reference.

Or more usually, waypoints

may be entered directly by Direction
taking a reading with the unit of
waypoint
at the location itself, giving it Waypoint
Your
location
a name, and then saving the Date Latitude and
point. and
Time
Longitude
 The Four Basic Functions of the GPS
The primary functions of the GPS fall into four categories:
1) Position and waypoint coordinates. Using the GPS a
receiver can provide position or waypoint information for its
current location or for any remote location on the earth, and
display that information in a variety of coordinates.
2) The distance and direction between a receivers
position and a stored waypoint, or between two remote
waypoints.
3) Velocity reports: Distance to and between waypoints;
tracking to a waypoint; heading (direction of travel); speed;
estimated time of arrival.
4) Accurate time measurement: GPS has become the
universal timepiece, allowing any two receivers (as well as any two
clocks or watches) to be precisely synchronized to each other
anywhere in the world.
Position is Based on Time
Signal leaves satellite at
time T

Signal is picked up by the

T+3 receiver at time T + 3
Distance between satellite and
receiver = 3 times the speed of
light
 Receiver Position is Based on Time
The Global Positioning System allows a GPS receiver to
determine its position by using a simple formula: Velocity x
Time = Distance
GPS satellites continuously transmit digital radio pulses
at precise, known times. So by measuring the exact instant
when the pulses arrive, the receiving GPS equipment can
determine the distance to each satellite. But theres a
problem. The clocks on board the satellites are all
extremely accurate, while the clock in the GPS receiver is
not. So a GPS receiver calculates what are called pseudo-
ranges (false ranges), which are approximate calculated
distances (as a measurement of time) to every satellite the
receiver has acquired.
 Pseudo Random Noise Code

Time
Difference

Satellite PRN

Receiver PRN
if receiver applies different PRN code to SV signal
no correlation
when receiver uses same code as SV and codes begin
to align
some signal power detected
when receiver and SV codes align completely
full signal power detected

usually a late version of code is compared with early version

to insure that correlation peak is tracked
 Eventually, in order for the GPS receiver to determine
a precise position it will have to get its own clock
synchronized with the satellite clocks. A clock error in
the GPS receiver of as little as a few nanoseconds
(billionths of a second) can translate into a position
error on the ground of as much as 300 meters.
For example, if a GPS satellite transmits its signal
at precisely T+ 0 nanoseconds, travels at 186,000 miles
per second (light speed) to the earth, and arrives at
the receiver at precisely T+ 645,16000 nanoseconds
later, then the signal traveled 12,000 miles (12,000
miles at 186,000 miles per second).
 Calculating Distance
Velocity x Time = Distance
Radio waves travel at the speed of light, roughly 186,000
miles per second (mps)

If it took 0.06 seconds to receive a signal

transmitted by a satellite floating directly
overhead, use this formula to find your distance
from the satellite.

186,000 mps x 0.06 seconds = 11,160 miles

 Atomic Clocks
Atomic clocks don't run on atomic energy. They get the name
because they use the oscillations of a particular atom e.g cesium
and rubidium i.e. oscillations between the nucleus of an atom and
the surrounding electrons. This form of timing is the most stable and
accurate reference man has ever developed.
But what about our receivers here on the ground?
Remember that both the satellite and the receiver need to be able to
precisely synchronize their pseudo-random codes to make the system
work.
If our receivers needed atomic clocks (which cost upwards of $50K to
$100K) GPS would be a lame duck technology. Nobody could afford it.
Luckily the designers of GPS came up with a brilliant little trick that
lets us get by with much less accurate clocks in our receivers. This
trick is one of the key elements of GPS and as an added side benefit it
means that every GPS receiver is essentially an atomic-accuracy
clock.
 Atomic Clocks
GPS satellites use Atomic Clocks for accuracy, but
because of the expense, most GPS receivers do not.
The GPS receiver uses a clever technique to calculate the
precise time it took for the GPS signal to reach it.
By shifting its own generated copy of the satellites PRN
code in a matching process, and by comparing this shift
with its own internal clock, the receiver can calculate how
long it took the signal to travel from the satellite to itself.
By comparing the time difference between the two, and
multiplying that time by the 186,000 miles per second
travel speed of the signal, the receiver can roughly
determine the distance separating it from the satellite.
This process is repeated with every satellite signal the
receiver locks on to.
 The distance between satellite and receiver derived from
this method of computing distance is called a pseudo-range
(false range) because the receivers clock is not synchronized
with the satellites clocks. Pseudo-range is subject to several
error sources, such as delays caused by the atmosphere, and
multipath interference.
Heres a simplified example of how a GPS receiver
synchronizes itself to GPS Time for precise positioning. Imagine
that the GPS satellite PRN signal is a song being broadcast by a
radio station. The GPS receiver is a record player which is
playing the same song, but its not synchronized to the
broadcast song. Both songs are playing, but at different places
in the song and at different speeds. By speeding up or slowing
down the turntable, the two songs can be precisely matched.
They become synchronized. Similarly, the GPS receiver
synchronizes its digital signal to match that of each satellites
signal.
Receivers and satellites must have some way of "syncing up" so
that the receiver can figure out how much time it took for the
radio signal from the satellite to reach earth. They sync up
using a repeating code called the Pseudo Random Code
(PRC). This code is a series of "on" and "off" pulses.

Physically the signal is just a

complicated digital code, or in
other words, a complicated
sequence of on and off
pulses.

The satellite propagates this code and sends it out on the radio
signal. The receiver is also propagating this code
within itself. They both start the code at the same time, but
when the satellite's code reaches the receiver, it is delayed,
since it had to travel the 11,000 miles from outer space.
When the receiver gets the satellite's version of the
code, it can tell that there has been a delay or "lag" in
the code. The size of the lag or how much the
receiver must shift back to match up, indicates the
radio signal's travel time from the satellite to the
receiver.

Time Difference
 Remember
A GPS receiver determines the travel time of a
signal from a satellite by comparing the
"pseudo random code" it's generating, with an
identical code in the signal from the satellite.
 How a Receiver Determines Its Position
Traveling at the speed of light each satellite PRN signal takes a
brief, but measurable amount of time to reach a GPS receiver. The
difference between when the signal is sent and the time it is
received, multiplied by the speed of light, enables a GPS receiver to
accurately calculate the distance between it and each satellite.

When a GPS receiver is turned on it immediately begins

searching the sky for satellite signals. If the receiver already has a
curent almanac (such as one acquired on a previous outing), it speeds
up the process of locating the first satellite signal. Eventually it
locates and acquires its first signal. Reading this signal the receiver
collects the Navigation Message. If the receiver does not have a
current almanac, or was moved more than 300 miles while turned
off, it must collect a new almanac, which will take about 12-13
minutes after the first satellite signal is acquired.
 Triangulation
The basic principle inherent in GPS is to determine with the best
possible accuracy a point in space, as defined by three coordinates,
here geographical latitude and longitude, as well as elevation above
sea level. For sailors, the elevation is not relevant!
This is done by means of triangulation, that is measurement of
triangles. In practice, this involves determining the distances to at least
three GPS satellites from the user's GPS receiver. The positions of the
satellites in space are known all the time by means of various
observational methods and orbital computational methods.

Geometric Principle:
You can find one location if you
know its distance from other,
already-known locations.
 A GPS receiver uses the signals transmitted by a satellite to
determine its distance from that satellite.

If you know your distance from one satellite, you could be

anywhere on a sphere around that satellite.

If you add distance information from a second satellite,

you narrow your location to the intersection of the two
spheres around those satellites, which puts you
somewhere on a circle.

Addition of a third sphere narrows your position to two

points, one of which can be eliminated because it is
nowhere near the earth's surface.
 Signal From One Satellite

The receiver is
somewhere on
this sphere.

In the above graphic, the GPS receiver calculates a rough location

somewhere on this three dimensional sphere, which is actually
thousands of miles in diameter. All the receiver can really do at this
point is collect system data and search for more satellites.
Signals From Two Satellites
Triangulation
How a Receiver Determines Its Position (cont.)
In a perfect world, where both satellite and receiver
clocks were perfectly synchronized with each other, an
accurate position could be determined from just two
satellites. However, most receivers are incapable of
calculating an accurate position using just two satellites. The
dot in the example represents the approximate location of
where the receiver thinks it is based on the information
provided by two satellites. At least now the receiver knows
that it is somewhere at the intersection of those two satellite
signals. But thats the only improvement in its position
calculations.
The satellite signal spheres should intersect at precisely
the receivers location, but dont because the clock in the GPS
receiver isnt yet synchronized with GPS Time. So the receiver
estimates a pseudo-range to each satellite.
Three Satellites (2D Positioning)
Triangulation
How a Receiver Determines Its Position (cont.)

3 satellites can provide only a two-dimensional (2D) position.

Without manually entering the receivers exact elevation, the rendered
2D position may be off by several kilometers on the ground. If the exact
elevation of the GPS receiver is known, entering that elevation into a
receiver with this capability replaces the need for a fourth satellite
signal to allow a receiver to triangulate a precise position. The receiver
essentially uses elevation in lieu of a fourth satellite, and makes the
appropriate adjustments to trilaterate a reasonably good 3D position.

But without manual elevation correction most GPS receivers

must rely on a fourth satellite to provide the final clock correction
information necessary to calculate a 3D position. Until a fourth satellite
signal is acquired the receiver will not be able to determine x and y
horizontal, and z vertical positioning (a true 3D position). This is because
the fourth satellite signal is used by the receiver not to provide more
position data, but, rather, the final time correction factor in its ranging
calculations.
 Note
As a rule, 2D positions should always be avoided whenever
possible. Use 2D positioning only when a 3D position is not
possible, but be aware of the horizontal error inherent in any 2D
position.

The inability of a GPS receiver to triangulate a 3D position may be

due to a variety of factors, including user error, poor satellite
geometry, and harsh landscape conditions (tall buildings, canyons,
and dense tree cover among others).

Its up to the user to be aware of the errors associated with 2D

positioning.
Triangulating Correct Position
How a Receiver Determines Its Position (cont.)

Accessing only two or three satellite signals, the clock

in the GPS receiver cannot yet be synchronized precisely with
GPS Time.

The pseudo-range spheres (the diagram here shows only two

satellites for simplification), as interpreted by the GPS
receiver, will either be just a little too large (if the receivers
clock is running faster than GPS Time) or too small (if the
receivers clock is slower than GPS Time). The spheres will not
intersect with each other. In this example, the do not could
be the false pseudo-range position if the GPS receivers clock
is running faster than GPS Time, or the dot is the position if
the receivers clock is slower than GPS Time.
Three Dimensional (3D) Positioning
Determining Position In 3D
How a receiver determines its position (cont.)
For a GPS receiver to achieve three-dimensional (3D) positioning it
needs to acquire four or more satellite signals.

A 3D position is comprised of X and Y (horizontal), Z (vertical) positions,

and precise time (not varying more than a few hundred nanoseconds).

The receivers processor uses the fourth satellite pseudo-range as a

timing cross check to estimate the discrepancy in its own ranging
measurements and calculate the amount of time offset needed to bring
its own clock in line with GPS Time (recall the radio station and record
player simultaneously playing the same song).

Since any offset from GPS Time will affect all its measurements, the
receiver uses a few simple algebraic calculations to come up with a
single correction factor that it can add or subtract from all its timing
measurements that will cause all the satellite spheres to intersect at a
single point (x, y, and z).
 That time correction synchronizes the receiver's clock
with GPS Time. Now the receiver essentially has
atomic clock accuracy with the time correction factor
needed to achieve precise 3D positioning.

The pseudo-ranges calculated by the GPS receiver will

correspond to the four pseudo-range spheres
surrounding the satellites, causing the four spheres to
intersect at precisely the receivers location (the dot in
the diagram).
 Note
It is unavoidable that the precision of the clock in the receiver is much less
than that of the atomic clocks in the satellites. The receiver clock may be some
fractions of a second off.

But how can the time delays then be measured?

The trick lies in the fact that the time offset of the clock in the GPS receiver
is considered as the fourth unknown (the first three being the three space
coordinates of the receiver). If a fourth satellite signal is received and a
fourth distance is measured, it will also be possible to determine with high
precision this time offset and then to find the correct space coordinates.
Said in other words, the four distances to the four satellites will only fit and
determine one particular point in space, if the time offset has a certain
value. This calculation is done automatically by the software in the GPS
receiver.

This is the reason that the acquisition of three GPS satellites does not give a
very high precision, and that at least four are needed for a satisfactory
measurement.
 Remember
The coordinates are calculated according to the
World Geodetic System WGS84 coordinate system.
The satellites are equipped with atomic clocks.
Receiver uses an internal crystal oscillator-based
clock that is continually updated using the signals
from the satellites.
Receiver identifies each satellite's signal by its
distinct C/A code pattern, then measures the time
delay for each satellite.
The receiver emits an identical C/A sequence using
the same seed number the satellite used.
 Remember
By aligning the two sequences, the receiver can
measure the delay and calculate the distance to the
satellite, called the pseudo-range.
Orbital position data from the Navigation Message
is used to calculate the satellite's precise position.
Knowing the position and the distance of a satellite
indicates that the receiver is located somewhere on
the surface of an imaginary sphere centered on that
satellite and whose radius is the distance to it.
When four satellites are measured at the same
time, the point where the four imaginary spheres
meet is recorded as the location of the receiver.
GPS Signals
 GPS Signals & Codes
Each GPS satellite transmits a unique
navigational signal centered on two L-
band frequencies of the
electromagnetic spectrum.
At these frequencies the signals are
highly directional and so are easily
reflected or blocked by solid objects.
Clouds are easily penetrated
 GPS Signals & Codes
The satellite signal consists of the following
components :
The two L-band carrier waves.
The ranging codes modulated on the carrier
waves.
The so-called "navigation message".
Signal: Electromagnetic Spectrum

GPS:
VISIBL
X-RAY MICRO L1,L2
E
UV IR
GAMM RADIO
A

10-11 10-9 10-7 10-5 10-3 10-1 10 103 cm

7.5x1014
3x1019 3x1017 3x1012 3x109 Hz
4.3x1014

85
 The satellite signals basically consists of :
The two L-band carrier waves.
The ranging codes modulated on the carrier waves.
The Navigation Message.
The carrier waves provide the means by which the ranging
codes and Navigation Message is transmitted to earth (and
hence to the user). The primary function of the ranging
codes is to permit the signal transit time (from satellite to
receiver) to be determined.
The Navigation Message is modulated on both carrier
frequencies and contains the satellite ephemeris, satellite
clock parameters, and other pertinent information such as
general system status messages and an ionospheric delay
model, necessary for real-time navigation to be performed.
 GPS SIGNAL
Each satellite continuously broadcasts low-power radio
signals that identify it and provide information about its
location in space, as well as system timing and other data.
The signals are broadcast using two carrier frequencies in
the L band of the ultrahigh frequency.

These are radio frequency waves capable of transmission

through the atmosphere over great distances, but which
cannot penetrate solid objects.

All GPS satellites transmit carrier waves at the same two L-

band frequencies.
 The GPS currently uses two frequencies to accomplish data
transmission, L1 and L2.
- L1 (1575.42 MHz) -The primary navigation signal
- L2 (1227.60 MHz) - The secondary navigation signal
The signal contains two complex patterns of digital signals:
Precise (P) code and Coarse/Acquisition (C/A) code
A long period modulation broadcast data as SV# or ephemerides.
Frequency Wavelength
(MHz) (m)

C/A code 1.023 293

P-code 10.23 29.3

L1 1574.42 0.19

L2 1227.6 0.24

L1 freq. (1575.42 Mhz) carries the SPS code and the navigation
message.
L2 freq. (1227.60 Mhz) used to measure ionosphere delays by PPS
receivers.
Although each satellite transmits the same frequencies (L1
and L2), each satellite encodes the frequency with a
different C/A code and different P code. The receiver uses
the code to determine how far it is from the satellite.

The L1 frequency is transmitted twice, once with the

C/A code and once with the P code
The L2 frequency is only encoded with the P code.
This is Used to correct for ionospheric delays
Primary use of the P code is to remove the unknown
atmospheric delays and refine the pseudorange to a very
accurate true range.
 Ranging Codes

All of the NAVSTAR satellites transmit carrier waves at the

same two frequencies, known as the L-Band, which however,
do not carry any information and must be modified.

Modulated onto the carrier waves are the PRN ranging

codes and navigation message for the user.
The primary function of the ranging codes is to permit the
signal transit time (from satellite to receiver) to be
determined.
The transit time when multiplied by the velocity of light
gives a measure of the receiver-satellite "range.
Pseudo-Random Noise Code (PRN)

The PRN is a serial sequence of binary 1s and 0s

that appear to occur at random but are actually
generated by a digital circuit and have a beginning
and an end
A pseudo-random binary sequence might appear as
0001100111010110000101011100010110001100
This very complicated code identifies which satellite is
transmitting and looks like random electrical noise
The signal is simply a timing signal that repeats
itself every 1,023 bits
The L-band carrier waves are modified by two ranging
codes:
The clear/access or coarse/acquisition (C/A) code
The Private or precise (P) code
The original GPS design contains two ranging codes:
1. The Coarse/Acquisition (C/A) code
"clear/access" or "coarse/acquisition" code
(sometimes also referred to as the "S code"), which is
freely available to the public, and
2. The restricted Precision (P) code, the "private"
or "precise" code usually reserved for military
applications.
C/A (Coarse Acquisition) Code
Sometimes also called the Standard Positioning Service (SPS). Available
to all GPS users (not classified by military).
- Modulates the L1 carrier phase. The C/A code is generated at
a rate of 1.023 MHz Pseudo Random Noise (PRN) Code, and the entire
C/A code repeats itself every millisecond.

C/A code is 1023 chip long code or in other words Sequence Length =
1023 bits. Each bit (0/1) in the sequence is called a chip.

2 bits count to 3 (2^2)

3 bits count to 7 (2^3)
4 bits count to 15 (2^4)

10 bits count to 1023 (2^10)
 The C/A code is an easy code to acquire and
normally the first code locked onto before
transferring to the P code (if so allowed).
The starting point for each code generated by each
satellite is unique so no two satellites have the same
start point (or epoch)
There is a different C/A code PRN for each SV
(space vehicle).
GPS satellites are often identified by their PRN
number, the unique identifier for each pseudo-
random-noise code.
This C/A code that modulates the L1 carrier is the
basis for the civil SPS (standard positioning service.
Receiver replicates the C/A code to correlate with the
measured signal.
 P (Precise) Code
P-Code (Precise) modulates both the L1 and L2 carrier
phases. The P-Code is a very long, 10 MHz PRN code.
The resolution of this code is ten times the resolution of the C/A
code, and is repeated every week as compared to C/A code which is
repeated every millisecond.
Typically, receivers need to acquire the C/A code before switching to
the P code.
The Precision (P) code, sometimes called the Precise Positioning
Service (PPS), is modulated onto the L1 and L2 carriers allowing for
the removal of the first order effects of the ionosphere. The P code is
referred to as the Y code if encrypted. Y code is actually the
combination of the P code and a W encryption code and requires a
DoD authorized receiver to use it. Originally the encryption was
intended as a means to safe-guard the signal from being corrupted by
interference, jamming, or falsified signals with the GPS signature.
P code is known, but encrypted by unknown (secret) W code into the Y-code
 Precise (P) Code

P-code rate is the fundamental frequency (provides the

basis for all others)

P-Code (10.23 MHz) /10 = 1.023 MHz (C/A

code)

P-Code (10.23 MHz) X 154 = 1575.42 MHz (L1).

P-Code (10.23 MHz) X 120 = 1227.60 MHz (L2).

 Remember that we defined two ranging codes C/A code
and P code. Accordingly, there are two levels of accuracy
standards:
SPS: Standard Positioning Service is the level of accuracy
available with your civilian receiver.
PPS: Precise Positioning Service provides special
capabilities for users with military receivers; civilian
receivers cannot access this service.
The military maintains exclusive access to the more accurate "P-code"
pseudo random code. It's ten times the frequency of the civilian C/A
code (and so potentially much more accurate) and much harder to
jam.
SPS broadcasts on one frequency, while PPS uses two. This means
military users can perform ionospheric correction, a technique that
reduces radio degradation caused by the Earth's atmosphere. With
less degradation, PPS provides better accuracy than the basic SPS.
 NAV/SYS Code or
Navigation Message Code

Navigation Message modulates both the

L1 and L2 signals. The Navigation
Message is a 50 Hz signal consisting of
data bits that describe the GPS satellite
orbital information, clock corrections, and
other system parameters.
 Navigation Message

In order for a GPS navigator to derive real-time position

(and to make the task of the GPS surveyor easier when he
comes to reduce his data), a Navigation Message is
transmitted on both L-band frequencies, containing the
following information
Predicted satellite ephemerides.
Predicted satellite clock correction model coefficients.
GPS system status information.
The GPS system ionospheric model.

Rain, fog and snow have no effect on these signals, making

GPS an all-weather system.
GPS Signal Characteristics
 Selective Availability (SA)

SA is an intentional degradation of the accuracy of

GPS horizontal positioning and vertical positioning for
SPS Users.

To deny high-accuracy realtime positioning to potential

enemies, DoD (Deptt. of Defence) reserves the right
to deliberately degrade GPS performance

Selective availability is Only on the C/A code

By far the largest GPS error source.

 Selective Availability
Because of the P code's higher measurement resolution
it was expected that the accuracy of positioning using
the P code would be much better than that possible
using the C/A code.

However, it was found that the performance of C/A code

positioning was often no worse than that of P code
positioning by a factor of two.

As a result of the demonstration of a surprisingly good

level of SPS accuracy, the policy of "Selective
Availability" (SA) was endorsed in order to artificially
widen the gap between the two levels of positioning.
 Selective Availability

SA is implemented through an encryption of the

navigation message whereby a part of the
transmitted ephemeris and satellite clock data is
falsified (the so-called "epsilon" effect) and the
satellite clock is "dithered" (the so-called "delta"
effect).

SA does not affect PPS users who have user

equipment able to decipher the correct
ephemeris and clock error data.
 Anti-spoofing: A technique used by military to encrypt P-
code so that it is only available to authorized personnel.

Implemented in 1994 to make P-code unavailable to non-

military users. Encrypted P-code is referred to as Y-code.

Under the policy of Anti-Spoofing the P code is encrypted

through modulation by a further secret code (the "W
code") to produce a new "Y code".

The encrypted Y-Code requires a classified Module for

each receiver channel and is for use only by authorized
users with cryptographic keys.

P (Y)-Code is the basis for the PPS. Military GPS

receivers use the P (Y) Code on both L1 and L2 frequencies to
compute positions.
GPS Errors
 Sources of GPS Error
Standard Positioning Service (SPS ): Civilian Users
Source Amount of Error
Satellite clocks: 1.5 to 3.6 meters
Orbital errors: < 1 meter
Ionosphere: 5.0 to 7.0 meters
Troposphere: 0.5 to 0.7 meters
Receiver noise: 0.3 to 1.5 meters
Multipath: 0.6 to 1.2 meters
Selective Availability (see notes)
User error: Up to a kilometer or more
Errors are cumulative and increased by PDOP.
 Sources of GPS Error
1. Satellite clock errors: Caused by slight discrepancies in
each satellites four atomic clocks. Errors are monitored and
corrected by the Master Control Station.
- Clock Errors can occur when, for example, a GPS
satellite is boosted back into a proper orbit. The receiver's
calculation of the satellite's position will be incorrect until it
receives another ephemeris update.

2. Orbit errors: Satellite orbit (referred to as satellite

ephemeris) pertains to the altitude, position and speed of
the satellite. Satellite orbits vary due to gravitational pull and
solar pressure fluctuations. Orbit errors are also monitored
and corrected by the Master Control Station.
3. Ionospheric interference: The ionosphere is the layer of
the atmosphere from 50 to 500 km altitude that consists
primarily of ionized air.
Ionospheric interference causes the GPS satellite radio
signals to be refracted as they pass through the earths
atmosphere causing the signals to slow down or speed up.
This results in inaccurate position measurements by GPS
receivers on the ground. Even though the satellite signals
contain correction information for ionospheric interference,
it can only remove about half of the possible 70
nanoseconds of delay, leaving potentially up to a ten meter
horizontal error on the ground.
Fortunately, error caused by atmospheric conditions is
usually less than 10 meters.
Signal Refraction
Signals from satellites can be like
light. When they hit some
interference (air patterns in the
atmosphere, uneven geography,
etc.) they sometimes bend a little.

Signal Interference
Sometimes the signals bounce
off things before they hit the
receivers.
 Line of Sight Transmissions
Line of sight is the ability
to draw a straight line
between two objects
without any other objects
getting in the way. GPS
transmission are line-of-
sight transmissions.

Obstructions such as trees, buildings, or natural formations may prevent

clear line of sight.
4. Tropospheric interference: The troposphere is the lower
layer of the earths atmosphere (below 13 km) that
experiences the changes in temperature, pressure, and
humidity associated with weather changes. GPS errors are
largely due to water vapor in this layer of the atmosphere.
Tropospheric interference is fairly insignificant to GPS.

5. Receiver noise is simply the electromagnetic field that

the receivers internal electronics generate when its turned
on. Electromagnetic fields tend to distort radio waves. This
affects the travel time of the GPS signals before they can be
processed by the receiver. Remote antennas can help to
alleviate this noise. This error cannot be corrected by the
GPS receiver.
6. Multipath interference is caused by reflected radio
signals from surfaces near the GPS receiver that can either
interfere with or be mistaken for the true signal that
follows an uninterrupted path from a satellite.

What is Multipath
Multi-path occurs when signals are reflected off of objects
such as trees or buildings. These reflections delay the
signal before it reaches the antenna and throw off the
range calculations. Since accurate timing is necessary for
accurate positioning, this can cause significant error. Multi-
path is the greatest source of error in forestry settings and
the most difficult to combat.
What Affects a GPS Signal

Atmospheric
Error

Multi-Path Error
Human Error
 Sources of Signal Interference

Earths Atmosphere

Solid Structures

Metal Electro-magnetic Fields

Multipath is difficult to detect and sometimes impossible for the user
to avoid, or for the receiver to correct. Common sources of multipath
include car bodies, buildings, power lines and water. When using GPS
in a vehicle, placing an external antenna on the roof of the vehicle will
eliminate most signal interference caused by the vehicle. Using a GPS
receiver placed on the dashboard will always have some multipath
interference.
7. Selective Availability : Discussed earlier

8. Control Segment blunders due to computer glitches or

human error can cause position errors from several meters
to hundreds of kilometers. Checks and balances by the Air
Force Space Command virtually eliminates any blunders in
the Control and Space segments of the GPS.

9. User mistakes account for most GPS errors. Incorrect

datum and typographic errors when inputting coordinates
into a GPS receiver can result in errors up to many
kilometers. Unknowingly relying on a 2D position instead of
a 3D position can also result in substantial errors on the
ground. A GPS receiver has no way to identify and
correcting user mistakes.
Even the human body can cause signal interference.
Holding a GPS receiver close to the body can block some
satellite signals and hinder accurate positioning. If a GPS
receiver must be hand held without benefit of an
external antenna, facing to the south can help to
alleviate signal blockage caused by the body because the
majority of GPS satellites are oriented more in the
earth's southern hemisphere.
GPS Dilution of Precision and Its Affects
On GPS Accuracy
GPS Satellite Geometry Affects the Level of Precision on the
Ground
Satellite position geometry can affect the quality of GPS
signals and accuracy of receiver trilateration.
Dilution of precision (DOP), or geometric dilution of
precision (GDOP), is a term used in satellite navigation to
specify the additional multiplicative effect of navigation
satellite geometry on positional measurement precision,
and reflects each satellites position relative to the other
satellites being accessed by a receiver.
DOP indicates the quality of the geometry of the GPS
satellite constellation at a particular time. A higher DOP
indicates poor satellite geometry and a potentially less
accurate position than a lower DOP..
There are five different kinds of DOP:

GDOP: Geometric Dilution of Precision (refers to

GPS satellite orbit anomalies)
VDOP: Vertical Dilution of Precision (imperfect
height measurements on the ground)
HDOP: Horizontal Dilution of Precision (imperfect
horizontal measurements on the ground)
TDOP: Time Dilution of Precision (refers to slight
anomalies in time measurements)
PDOP: Position Dilution of Precision (three-
dimensional position errors)
 There are several expressions of DOPfor example,
horizontal DOP (HDOP) and time DOP (TDOP)but position
PDOP (PDOP) is the most commonly used. The higher the
PDOP, the poorer the measurement. Generally, a PDOP value
from 1-3 is considered very good, 4-5 good, 6 fair, >6 poor.
Typical accuracies are between 10 to 30 meters. By only
capturing GPS positions with a low DOP, you tend to
capture more accurate positions.

Position Dilution of Precision (PDOP) is the DOP value used

to determine the overall quality of a receivers position, and
the quality of the data collected by the receiver. PDOP is
specifically the calculated likelihood of position error based
on the present position of the satellites being tracked,
including horizontal and vertical errors. In other words, its
HDOP and VDOP combined.
 Satellite Distribution
When the satellites are all in the same part of
the sky, readings will be less accurate.

Positional Dilution of Precision

GPS receivers get satellite position information from the ephemeris
message sent as part of the data stream from each satellite.
Itsusually up to the GPS receiver to pick satellites which
provide the best position triangulation.
More advanced GPS receivers can filter out poor DOP values.
Poor Satellite Geometry
 Satellite Geometry
Most of the time there are more satellites available than a
GPS receiver needs to triangulate a position fix, so the
receiver can pick satellites which will provide reasonably
good geometry for the receiver to triangulate a precise
position.
However, if a receiver happens to acquire satellites that are
bunched close together in the sky, the intersecting circles
that define a position will cross at very shallow angles. This
increases the potential area of error around a position. In
the diagram above, two satellites close together will emit
radio signals which cross at a shallow angle, producing a
large area of potential error on the ground (the horizontal
diamond in the above diagram). Add several more satellites
with the same tight pattern, and the resulting PDOP will
be very high.
 Topography on the ground also affects
satellite geometry. A receiver inside a vehicle,
near tall buildings, under dense canopy, or in
mountainous terrain can be affected by
blocked signals. GPS receivers require clear
line of sight to every satellite being acquired.
Good Satellite Geometry
 Good Satellite Geometry
The best satellite configuration for optimum receiver
positioning is one satellite directly above the receiver, and three
or more satellites equally spaced around the horizon (as shown
in the above diagram). In this configuration satellite geometry is
very good since distance measurements are from all directions.
The area where all four calculated pseudo-ranges intersect will
be much smaller, providing a more precise position (as shown
above). The farther apart the satellites are from each other
(better geometry resulting in low PDOP) the smaller the red
diamond will be in this diagram. Low PDOP results in a much
smaller area of position uncertainty (the diamond in the
diagram).
A GPS receiver will choose a minimum of four satellites
spread out across the sky rather than four satellites bunched
close together in one quadrant of the sky.
 Note that many receiver types (Garmin
included) always try to provide a position,
even a poor position, regardless of satellite
geometry. The reasoning is that any GPS
position information provided by the receiver
is better than no position information. This
also explains why GPS receivers will provide a
2D (two-dimensional) position even though
that position may be off by a considerable
distance on the ground.
Receiver Errors are Cumulative!

System and other flaws = < 9 meters

User error = +- 1 km
Errors in GPS are cumulative, and are compounded by
position dilution of precision (PDOP) (covered later). It is
the users responsibility to insure the accuracy of the data
being collected with the GPS.

As shown in the above diagram, the Global Positioning

System can contribute system errors of up to about nine
meters on the ground.

However, user errors can dramatically increase that to a

kilometer or more. Among the most common user errors
include using a 2D position instead of a 3D position,
mismatched datums between a GPS receiver and a users
map, and high position dilution of precision (PDOP).
 Receiver Errors are Cumulative!
In the case of two dimensional positioning, the error occurs
because the GPS receiver isnt acquiring enough satellite
signals (usually four or more) to provide a precise position.
The difference can be as much as several kilometers from a
users true location.

In the above example, the dark stars represent the potential

positions a GPS receiver can triangulate for a user depending
on the amount error incorporated into that receivers
triangulation calculations. The large circle is a gross example
of poor user control over the GPS receiver, and the smaller
circle is the uncontrollable circle of error possible through
system errors. The centered star represents the hypothetical
true location of the user.
 GPS Position vs. Stored Waypoint

In GPS, positions and waypoints are two distinctly different

things.

A GPS position is: Is based on real-time satellite tracking

(usually the receiver must be receiving 3 or more satellite
signals). Represents only an approximation of the receivers
true position (due to inherent errors, as described earlier).
Has no name or other reference information.

Is not static, and constantly changes as the receiver moves

over ground, or can even change while the receiver
remains stationary.
 Position Fix
A position is based on real-time satellite tracking.
Its defined by a set of coordinates.
It has no name.
A position represents only an approximation of the
receivers true location.
A position is not static. It changes constantly as the GPS
receiver moves (or wanders due to random errors).
A receiver must be in 2D or 3D mode (at least 3 or 4
satellites acquired) in order to provide a position fix.
3D mode dramatically improves position accuracy.
 Waypoint
A waypoint is based on coordinates entered into a GPS receivers
memory.
It can be either a saved position fix, or user entered coordinates.
It can be created for any remote point on earth.
It must have a receiver designated code or number, or a user
supplied alphanumeric name.
Once entered and saved, a waypoint remains unchanged in the
receivers memory until edited or deleted.
Can represent any remote location on earth.
A waypoint can also be created from coordinates derived other
than by GPS. Coordinates on a map can become a waypoint.
Coordinates radioed from person in a remote location to another
person can also become a waypoint once they are programmed
into a GPS receiver.
 To turn a position fix into a waypoint is simply a
matter of saving the receivers current position
as a waypoint.
The receiver must be receiving signals from
enough satellites to provide at least a two-
dimensional (2D), or three-dimensional (3D) fix.
The receiver will give the position coordinates an
alpha-numeric name, or the user can designate a
unique name. Once this happens the position fix
becomes a waypoint with static coordinates
saved in memory.
Applications
lat=31.398191 long=73.11676
 Applications
Military.
Search and rescue.
Disaster relief.
Surveying.
Marine, aeronautical and terrestrial navigation.
Remote controlled vehicle and robot guidance.
Satellite positioning and tracking.
Shipping.
Geographic Information Systems (GIS).
Recreation.
 APPLICATION EXAMPLES OF GPS
A space shuttle that navigates by itself using GPS
Most cell companies can already pinpoint callers
locations using GPS
An airplane that lands itself using GPS
A football coach who tracks players on the field
using GPS
A hiker who loses her way and returns to safety
using GPS
Tracking a species of animal using GPS
 APPLICATION EXAMPLES OF GPS
Navigation (very important for ocean travel)
Zero-visibility landing for aircraft
Collision avoidance
Surveying
Precision agriculture
Delivery vehicles
Emergency vehicles
Electronic maps
Earth sciences (volcano monitoring; seismic hazard)
Tropospheric water vapor

ANYTHING THAT INVOLVES LOCATION, MOTION, OR NAVIGATION

 Applications Military
Military GPS user equipment has been integrated
into fighters, bombers, tankers, helicopters, ships,
submarines, tanks, jeeps, and soldiers
equipment.
In addition to basic navigation activities, military
applications of GPS include target designation of
cruise missiles and precision-guided weapons and
close air support.
GPS satellites also can contain nuclear detonation
detectors.
 Applications Civilian

Automobiles are often equipped with GPS

receivers.
They show moving maps and information about
your position on the map, speed you are traveling,
buildings, highways, exits etc.
Some of the market leaders in this technology are
Garmin and TomTom, not to mention the built in
GPS navigational systems from automotive
manufacturers.
 Applications Civilian (contd)
For aircraft, GPS provides
Continuous, reliable, and accurate positioning
information for all phases of flight on a global basis,
freely available to all.
Safe, flexible, and fuel-efficient routes for airspace
service providers and airspace users.
Potential decommissioning and reduction of
expensive ground based navigation facilities, systems,
and services.
Increased safety for surface movement operations
made possible by situational awareness.
 Applications Civilian (contd)
Agriculture
GPS provides precision soil sampling, data
collection, and data analysis, enable localized
variation of chemical applications and planting
density to suit specific areas of the field.
Ability to work through low visibility field
conditions such as rain, dust, fog and darkness
increases productivity.
Accurately monitored yield data enables future
site-specific field preparation.
Mapping, Scouting, and Sampling in Agriculture

Map and navigate

around field boundaries,
point, line, or area
features
Map hard-to-reach
features from a vehicle
or airplane
Accurately scout crops
and navigate to weeds,
pests and diseased areas
 Applications Civilian (contd)
Disaster Relief
Deliver disaster relief to impacted areas faster,
saving lives.
Provide position information for mapping of
disaster regions where little or no mapping
information is available.
Example, using the precise position information
provided by GPS, scientists can study how strain
builds up slowly over time in an attempt to
characterize and possibly anticipate earthquakes
in the future.
 Applications Civilian (contd)
Marine applications
GPS allows access to fast and accurate position,
course, and speed information, saving navigators
time and fuel through more efficient traffic
routing.
Provides precise navigation information to
boaters.
Enhances efficiency and economy for container
management in port facilities.
DGPS
 Relative positioning (DGPS)
DGPS (Differential GPS) is an enhancement to Global Positioning
System that provides improved location accuracy.

Differential correction is a technique that greatly increases the

accuracy of the collected GPS data.
It involves using a receiver at a known location - the "base station
and comparing that data with GPS positions collected from
unknown locations
Corrections are transmitted from the base-station to various users.

Since we know this receivers exact location, we can determine the

errors in the satellite signals.

DGPS gives accuracy of 3-5 meters, while GPS gives accuracy of

around 15-20 mts.
To obtain more accurate measurements than is possible
from a single GPS unit, a GPS receiver broadcasts the
signal it receives from a known position. The GPS unit in
the field simultaneously receives data from the GPS
satellites and the other GPS receiver on the ground
through a radio signal. The GPS error from the known
position is compared to that of the GPS receiver in the
unknown location.

Measurements takes place in three phases:

1. Determining the correction values at the reference
station
2. Relaying the correction values from the reference station
to the GPS user.
3. Correcting the pseudo-range measured by the GPS user.