Department of Civil Engineering
JSSATE, NOIDA
GLOBAL POSITIONING SYSYTEM(GPS)
RUCHI SARASWAT
Asst. Prof.
CED
JSSATEN
♦ 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.
♦ 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
Accurate(precise)
• Precise
• Reliable Almost!
• All weather
• Anytime & anywhere
• Unlimited user capacity
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
1. Space Segment
A constellation
of24 satellites
Source:Trimble
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
+
Satellite A 11,000 miles
•At this point we could be located
anywhere on the specified sphere
+
Satellite B
•Next, let us take another measurement
from a second satellite, Satellite B
• Now our position is narrowed down to the
intersection of theses two sphere
Determining GPS Position
•Taking another measurement
+ from a 3rd satellite narrows our
Sat ellite position down even further, to
A the two points
+ C •These points are located where
+
Satellite B
Satellite
the 3rd sphere cuts through the
the intersection of first two spheres
•So by ranging from 3 satellites we can
narrow our position to just two points in space
How do we decide which one is our
true location?
•We could make a 4th measurement
from another satellite to determine
+ the true point
Sat ellite
A OR
+ • We can eliminate one of the
+ Satellite C
two points that gives a
Sat ellit
eB
ridiculous answer
• The ridiculous point may be
• However, GPS receivers too
use afar4thfrom
satellite to precisely
the earth
locate our position
Determining GPS Position
Methods of data collection
Three methods of positioning
• Autonomous
10-20 meters
• Differential
2-5 meters
• Phase Differential 10-20 m 2-5m cm
centimeter
Sources of error
Multipath
PDOP
SNR
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
To obtain more
accurate Differential GPS
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.
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
One particular negative characteristics of this technique is th thee roving receiver(s).
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.
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
Scroll Cycle
keypad through
pages
Goto PAGE
On/Off Save a
switch MARK
waypoint
Quit ENTER
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
