Dr.S.Balakumaran,
Prof.in Geology

 The Global Positioning System (GPS) is a
satellite based radio navigation system
provided by the United States Department of
Defence. It gives unequalled accuracy and
flexibility in positioning for
navigation,surveying and GIS data collection.
 GPS is the shortened form of NAVSTAR
GPS.This is an acronym for Navigation system
with Time And Rangining Global Positioning
System.

 DOD sponsored project puts
satellites into orbit
 First Sat launched in 1978
 24 Sats by mid 1990s
 28 Currently in orbit, with
more coming
 A fundamental change in
how positioning is done
 What GPS has changed?

What is GPS ?
A very precise positioning system
• Developed and maintained by the
US Department of Defense (DOD)
• Satellite Based
* 24 satellites
* 20,200 km high orbit

The Global Positioning System (GPS) was
designed for military applications. Its
primary purpose was to allow soldiers to
keep track of their position and to
assist in guiding weapons to their
targets. The satellites were built by
Rockwell International and were
launched by the U.S. Air Force. The
entire system is funded by the U.S.
government and controlled by the U.S.
Department of Defense. The total cost
for implementing the system was over
$12 billion.
A GPS satellite. The GPS constellation of
satellites consists of at least 24
satellites – 21 primary satellites and 3
orbiting spares. They orbit the earth at
an altitude of20200 KM (10,900 miles) at
a speed of 1.9 miles per second between
60°N and 60°S latitude. Each satellite
weighs 1900 lbs and is 17 feet (5.81
meters) wide with solar panels extended.
The satellites orbit the earth twice a day.
This guarantees that signals from six of
the satellites can be received from any
point on earth at almost any time.

• The following are the conditions to fulfill the effective operation of
GPS.
• It should be suitable for all classes of platforms- aircrafts,ships, land
vehicles, space satellite and missles.
• It should be able to handle a wide variety of dynamics.
• It should provide the user with real time positioning, velocity, time
determination to an appropriate accuracy.
• The positioning results should be available on a single geodetic
datum.
• Highest accuracy should be restricted to a certain class of users.
• It should be resistant to jamming (blocking of radio signal) intentional
and unintentional.
• It should have redundancy provision to ensure the survivability of the
system.
• The system should be a passive positioning system ( one way ranging
system) that does not require transmission of signal from the user to
satellite.
• It should be able to provide the service to an unlimited numbers of
users.
• It should be low cost and low power consumption.

 The above conditions are fulfilled in the design of GPS with the
salient features.
 A one way ranging system in which the satellites transmit signals
but unaware of who is using the signal ( no receiving function) .
As a result, the user can not be detected by the enemy ( military
context), or the user is not charged for using the system.) Civilian
context.
 GPS signals are not affected by cloud or rain.
 Use of latest atomic clocks for time accuracy and microwave
transmission technology including spread spectrum techniques.
 A multiple satellite system which ensures visibility of sufficient
nos. of satellites anywhere on the world at any time ( four
satellites).

• System overview
• GPS configuration comprises of three distinct segements.
• The space segments-comprising satellites orbiting the earth.
• The Control segments-consisting of control stations positioned at
various locations to control the satellites.
• The user segments- Anybody who receives and uses the GPS signal
comes under the segment.
• The space segment- consists of a constellation of GPS satellites and
the signals broadcast by them. Which allows users to determine
position, velocity and time.
• Basic functions of satellites-1)To receive and store data transmitted
by the control stations. 2) to maintain accurate time by means of
several on-board atomic clocks. 3) to transmit information and radio
signals to users on two L bands frequencies. L1-1575.42 MHz and L2-
1227.60 MHz.Anoher additional frequency,L5-1176.45 MHz will be
use in future.
• 4) to provide stable platform and orbit for the L band transmitters.
• 24 satellites well placed approximately at an altitude of 20,200 KM
orbiting the earth every 12 hours.
• The satellites travel at a speed of 11,500 Km per hour which allows
them to circle the earth once every 12 hour.

 Satellites are powered by solar energy which lasts for 10years. If
solar energy fails due to eclipses, they have back-up batteries on
board to keep them running.
 Small rocket boosters to keep them orbiting in the corret path.
 Fist GPS satellite was launched in 1978- a full constellation 24
satellites was achieved in 1994.
 Six orbital planes with inclination of 55 degree with respect to the
equator
 4 satellites in each orbit.
 Space segment is so designed that there will be minimum of four
satellite visible above 15 degree cut off angle at any point on the
earth surface.
 4 satellites are the minimum that must be available for most
applications.
 Each satellite has several very accurate atomic clocks on
board.The clocks operate at a fundamental frequency of 10.23
MHz.This is used to generate the signals that are broadcasted
from the satellites.

 Control segment
 Also referred as ground segment –one master control station,six
monitor stations and three uploading stations. The main functions
of this segment are
 Estimate the on board clock status and define the corresponding
parameters to be broadcasted with refer. To constellation master
time.
 Define the orbit of each satellite in order to predict the ephemeris
(precise orbital information) along with the almanac ( coarse
orbital information)
 Determine the attitude (orientation) and location of the satellites
in order to determine the parameters to be sent to the satellites for
correcting their orbits and
 Uploading the derived clock correction parameters,
ephemeris,almanac and orbit correcion commands to the satellites.

 User segment-consists of GPS receiver- composed of an
antenna(internal or external) tuned to the frequencies transmitted
by the satellites, receiver-processors, and a highly stable
clock(crystal oscillator) and display system.
 Receiver clock is not as precise as the satellite clock.
 Receivers are classified by its no.of channels-this signifies signals
from how many satellites it can process simultaneously.(4
channels to 48 channels)
 Receiver can relay the position data to a personal computer or
devices.It can inerface with other devices using methods serial
connection ,USB or Bluetooth
 It is operated thro many terminals-boaters,ships,pilots, military,
land vehicles ect.

 The major tasks of a receiver are
 Select the satellites in view
 Acquire the corresponding signals and evaluate their health.
 Carry out the propagation time measurement
 Calculate the location of the terminal and estimate the error.
 Calculate the speed of the terminal and
 Provide accurate time.

Characteristics of GPS
• Free
• Precise
• Reliable
• Anytime & anywhere
• All weather
• Unlimited user capacity
Accurate(precise)
Almost!

Segments of GPS
1. Space Segment
A constellation of 24 satellites
2. Monitor Station
A network of earth-based facilities
3. Users & Equipment
Source:Trimble

Segments of GPS
Source:Trimble
1. Space Segment
A constellation
of 24 satellites

GPS Monitoring Station

How GPS Works ………
Uses measurements from 4+ satellites
Distance = travel time x speed of light
Source:Trimble

The GPS receiver and satellite generate the same pseudo-random
code at exactly the same time. When the code arrives from the
satellite, the time difference is compared to the same code
generated by the receiver. This difference is multiplied by the
speed of light (186,000 miles per second) to determine the
distance to the satellite.

Determining GPS Position
• Suppose the distance from
Satellite A to our position is
11,000 miles
• At this point we could be located
anywhere on the specified sphere
Satellite A
+
• Next, let us take another measurement
from a second satellite, Satellite BSatellite B
+
• Now our position is narrowed down to the
intersection of theses two sphere

Satellite C
+
Determining GPS Position
Satellite A
Satellite B
+
+
• Taking another measurement
from a 3rd
satellite narrows our
position down even further, to
the two points
• So by ranging from 3 satellites we can
narrow our position to just two points in space
• These points are located where
the 3rd
sphere cuts through the
the intersection of first two spheres

Satellite C
+
How do we decide which one is our
true location?
Satellite A
Satellite B
+
+
• We could make a 4th
measurement
from another satellite to determine
the true point
• However, GPS receivers use a 4th
satellite to precisely
locate our position
• We can eliminate one of the two
points that gives a ridiculous
answer
• The ridiculous point may be too
far from the earth
OR

Determining GPS Position

How accurate is GPS?
Depends on some variables
• Design of receiver
• Relative positions of satellites,
technically known as PDOP (Position
dilution of precision)
• Postprocessing
• Time spent on measurement

 D=RxT
 Rate is Speed of light
 Time is the key!
Technology made it
possible
 One you have distance, its
“easy”

Methods of data collection
Three methods of positioning
• Autonomous
10-20 meters
• Differential
2-5 meters
• Phase Differential
centimeter
2-5m10-20 m
cm

PDOP
Multipath
SNR
Sources of error
Source:Trimble

Multipath
• When GPS signals arrive at the receiver
having traveled different paths

What is a PDOP?
• Position Dilution of Precision
Good PDOP Poor PDOP

SNR (signal-to-noise ratio)
• SNR determines the signal strength
relative to noise
• GPS position is degraded if the SNR of
one or more satellites in the
constellation falls below certain range
Signal Strength
Indicators

 GPS has numerous advantages over traditional surveying
methods.
 1) Inter visibility between points is not required.
 2) can be used at any time, day, or night and in all weather
conditions.
 3) produces result with very high geodetic accurcy.
 4) more work can be accomplished in less time with less man
power.
 5) limited calculation and tabulation works required.
 6) large area can be surveyed in short time.
 7) network independent site selection hence sites can be placed
where needed.
 8) economic advantage arise from greater efficiency and speed of
survey.
 9) three dimensional coordinates are obtained.

 GPS positioning mode
 1) Absolute positioning mode-coordinates are
in relation to a well defined global reference
system.
 2) Differential or relative positioning –
coordinates are in relation to some other fixed
point.This is refered to as baseline
determinaion.
 3) Static positioning-coordinates of stationary
points is either in absolute or relative
mode.This is surveying mode of position.

 4) Kinematic posiioning- co-ordinates of moving points is either in
absolute or relative mode. This is generally navigation mode
based on pseudo range observation.
 Absolute positioning – use of single receiver at one station
location to collect data from min of 4 satellites.
 Not sufficient accuracy for precise surveying. Used for
commercial and some military purposes- standard positioning
service user can obtain 25 m accuracy- Precision positioning
service user with P- code can obtain 10-12 m accuracy.
 It can be divided in to two categories-Absolute positioning using
carrier phase and Absolute positioning using C/A code. In carrier
phase the reciver is tracking both C/A code and carrier phase-
tracking real time pseduo-range values with position accuracy of 3
meter.In Psedo-range , C/A code data alone used to calculate
approxi. Range.

 Differential Positioning( carrier phase tracking)-carrier phase
signals are tracked in order to obtain more accurate range
resolution due to shorter wavelength of L1 and L2 frequencies-
 Ability of receiver to get 2 mm accuracy. Hence, primary
applications in engineering, topographic an geodetic surveying.
 There are several techniques using DGP
 1) Static surveying
 2) Rapid static surveying
 3) Stop- and Go Kinematic surveying
 4) Real time Kinematic surveying
 5) Real time DGPS surveying.
 Some important points for a GPS survey solution
 The fundamental unit of GPS solution is a three dimentional
baseline vector joining the antennas of two GPS receivers that are
tracking same satellites simultaneously- GPS software in the

Differential GPS
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.

 To carry out the solution task
 One en d of baseline is fixed with known coorinates and
coordinates of other stations are determined relative to it.
 All results are obtained in WGS reference system
 Length of observation time , No of satellites tracked by
receiver,signal delay correction factors applied,software used for
processing the data.
 Static surveying-mostly used technique for control and geodetic
surveying-long observation time-1-2 hours to resolve ambiguities
bewteen satellite and receiver.
 Relative static positioning involves several GPS receivers collect
ing data simultaneously from at least 4 stallites for 30 min to 2
hrs.in stationary carrier phase mode-
 Requirements for static mode- 1) More than one receiver 2)chance

 Of getting direct signal from four or more staellites
 3) unobstructed sky above the stn
 4) enough power supplyto complete the observation
 5) enough memory space to store all data for post processing.

One particular negative characteristics of this technique is the
requirement that phase lock must be maintained by the roving receiver as
it moves from site to site. This requires special hardware mounts on
vehicles if the survey is carried out over a large area.
An additional requirement is that
the stationary reference receiver
must continue to track all the
satellites being tracked by the
roving receiver. The accuracy
attainable is about the same as
for the "rapid static" technique.
As with the "reoccupation"
technique, the receiver must have
the ability to handle data files
from several different sites. The
software then has to sort out the
recorded data for the different
sites, and to differentiate the
"kinematic" or "go" data (not of
interest) from the "static" or
"stop" data (of interest). It can
be implemented in real-time if a
communications link is provided to
transmit the "carrier-range" data
from the reference receiver to
the roving receiver(s).

GPS Receivers

GeoXT Versus Garmin
Mostly used
for GIS data
collection
Mostly used for
recreational
purposes

• Wide Area Augmentation System.
• It provides FREE GPS differential correction data for
visible satellites
• Developed & operated by the FAA (Federal Aviation
Administration) for flight navigation but it’s available
free to GPS users
• WAAS-enabled receivers can provide sub-meter level
accuracy anywhere in most locations of the US and
southern Canada.

• Differential corrections are computed from
ground stations and then uploaded to
geostationary satellites for broadcasting
• WAAS-enabled GPS receiver automatically
uses such correction data to enhance the
positional accuracy

Using Garmin12
Quit ENTER
Goto PAGE
MARK
Scroll
keypad
Cycle
through
pages
Save a
waypoint
On/Off
switch

Common use of GPS
A. GIS data collection & mapping
B. Navigation
C. Recreation

GPS for Navigation

GPS in recreation

GPS in Farmland

Future GPS
A. GPS in USA
B. GLONASS program from Russia
C. GALILEO from European countries

GPS in recreation

 :
 Advantages
 Higher accuracy than pseudo-range solutions
 Appropriate for many survey applications
 High productivity
 Similar procedures to modern terrestrial surveying
 Disadvantages
 Special hardware and software
 Susceptible to orbit, atmospheric multi-path disturbances
 Higher capital costs
 Ambiguity-fixed or continuous lock required
 Two negative characteristics of these modern GPS techniques are:
 They are susceptible to multipath disturbance to an axtent (affecting the receiver
signals during both the kinematic and static stages of the tracking) than the
conventional static technique. Multipath during the ambiguity resolution period is
especially dangerous, as wrong ambiguities may result.
 The results from short observation sessions are more sensitive to bad satellite
geometry (large GDOP) than the conventional static techniq