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Global Positioning System
Anurag MishraDeputy Director
Forest Survey of India, Dehradun
Outline for Today
Today, we will review the basics of the GPS system and
• Its history• Key components• Functioning• Applications etc.
Trying to figure out where you are and where you're going is probably one of man's oldest pastimes
Latitude & Longitude
78°01’31.2” E and 30°20’01.6” N
135° 7’45.9” W and 8°37’24.4” S
A Little Bit of History
• In the past, humans had to go to pretty extreme measures to keep from getting lost.
• They erected monumental landmarks, laboriously drafted detailed maps and learned to read the stars in the night sky.
• For centuries, only way to navigate was to look at position of sun and stars.
Things are much easier today
Starting Rs.10,000/- you can get a pocket-sized gadget that will tell you exactly where you are on Earth at any moment. As long as you have a GPS receiver and a clear view of the sky.
Why GPS ?
•Accurate & Precise
•Efficient, Economical
•Easy to Operate
•Portable
•Navigation
•Works Everywhere
•Additional Information
What is GPS?• Satellite-based
navigation system
• Continuously transmits coded information
• Precisely identify locations
• Measuring distances from the satellites
Man-made stars
IntroductionDeveloped by US Department of Defense in 1978
24 Satellites in 6 orbits
Situated at an altitude of 20,200 km
Life of Satellite is about 7.5 to 10 years
12 hours period and orbit is precisely predictable
Contd.
• Satellite clock: Atomic (Rubidium, Cesium)
• Powered by solar energy
• There are no subscription fees or setup charges to use GPS
• No restriction in using GPS signals
• Doesn’t work under dense canopy, covered areas
• GPS works in all weather conditions
Global Positioning System (GPS)
NAVSTARNAVigation Satellite Timing And Ranging satellites (NATO)
GLONASSGLObal NAvigation Satellite System (Russian)
Galileo To be operational by 2012 (EU)
NAVSTAR
• The only fully functional Global Navigational Satellite System
• Constellation of at least 24 Medium Earth Orbit Satellites that transmit precise Microwave signals, the system enables a GPS Receiver to determine its Location, speed/direction, and time
• The cost of maintaining the system is approximately US$750 million per year, including the replacement of aging satellites, and research and development
GPS CONSTELLATION
What does a GPS receiver do?• Position and coordinates.
• The distance and direction between any two waypoints
• What direction you are heading
• Some models can show you:• how fast you are going• your altitude• a map to help you arrive at a destination
How does the GPS work?
Using satellites in the sky, ground stations on earth, and a GPS receiver, the distances between each of these points can be calculated.
The distance is calculated based on the amount of time it takes for a radio signal to travel between these points.
This allows the GPS receiver to know where you are, in terms of latitude and longitude, on the earth.
Triangulation• A GPS receiver's job is to locate four or more of these
satellites, figure out the distance to each, and use this information to deduce its own location.
• This operation is based on a simple mathematical principle
called triangulation or trilateration.
• Triangulation in three-dimensional space can be a little tricky, so we'll start with an explanation of simple two-dimensional trilateration.
Triangulation
3D Triangulation
• Fundamentally, three-dimensional trilateration is not much different from two-dimensional trilateration, but it's a little trickier to visualize.
• Imagine the radii from the examples in the last section going off in all directions. So instead of a series of circles, you get a series of spheres.
GPS Triangulation
• If you know you are 10000 miles from satellite A in the sky, you could be anywhere on the surface of a huge, imaginary sphere with a 10000-mile radius.
Earth
10000 miles
GPS Triangulation (Cont’d)
• If you also know you are 15000 miles from satellite B, you can overlap the first sphere with another, larger sphere. The spheres intersect in a perfect circle.
10000 miles15000 miles
GPS Triangulation (Cont’d)
• The circle intersection implies that the GPS receiver lies somewhere in a partial ring on the earth.
PossibleLocations of GPS Receiver
Perfect circle formed fromlocating two satellites
GPS Triangulation (Cont’d)
• If you know the distance to a third satellite, you get a third sphere, which intersects with this circle at two points.
GPS Triangulation (Cont’d)
• The Earth itself can act as a fourth sphere -- only one of the two possible points will actually be on the surface of the planet, so you can eliminate the one in space.
• Receivers generally look to four or more satellites, however, to improve accuracy and provide precise altitude information.
Calculating Distance
Distance = Speed x time
Control Segment
Space Segment
User Segment
Three Segments of the GPSThree Segments of the GPS
Monitor Stations
GroundAntennas
Master Station
The Space Segment• Arranged in the orbits in such a way
that at least 4 satellites are always available
• Circle earth once every 12 hours
Functions
• Receive and store information from ground control segment
• Maintain very accurate time
• Transmit signal to the earth
Kwajalein Atoll
US Space Command
The Control SegmentThe Control Segment
Hawaii
Ascension Is.Diego Garcia
Cape Canaveral
Ground AntennaMaster Control Station Monitor Station
The User Segment
• Military.• Search and rescue.• Disaster relief.• Environment, Forestry & Wildlife• Marine, aeronautical and terrestrial navigation.• Remote controlled vehicle and robot guidance.• Satellite positioning and tracking.• Shipping.• Geographic Information Systems (GIS).• Recreation.
User Segment
GPS Receivers
• Better units have multiple receivers, so they can pick up signals from several satellites simultaneously.
• Radio waves travel at the speed of light (about 186,000 miles per second, 300,000 km per second in a vacuum).
• The receiver can figure out how far the signal has traveled by timing how long it took the signal to arrive.
Downloading of GPS Data
• Data Cable
• Mapsource
• Pathfinder
Standard Positioning System (SPS)
Provided on the GPS L1 frequency. Contains a coarse acquisition (C/A) code and a navigation data message.
The P-code and the L2 frequency is not unavailable to SPS users.
Accuracy
100 m in horizontal position 156 m in the vertical component
Precise Positioning System (PPS)
Available to authorized military users and users with PPS receivers
This consists of the SPS signal plus the P code on L1 and the carrier phase measurements on L2
Accuracy
22 m in horizontal position 27 m in the vertical component
DGPS is used for higher accuracy
Differential GPS
• There is no such thing as a Differential GPS
• It is the Differential capability
• Geodetic GPS
Differential GPSUses the point position derived from either the C/A or P-codes
Applies correction to that position.
These corrections, difference of determined position and the known position, are generated by a reference receiver, whose position is known and is fed to the instrument.
Used by the second receiver to correct its internally generated position.
DGPS Site
x+30, y+60
x+5, y-3
True coordinates = x+0, y+0
Correction = x-5, y+3
DGPS correction = x+(30-5) and y+(60+3)
True coordinates = x+25, y+63
x-5, y+3
Real Time Differential GPSReal Time Differential GPSReal Time Differential GPSReal Time Differential GPS
DGPS ReceiverReceiver
Causes of Errors• Ionosphere and troposphere delays
• Signal multipath
• Orbital errors
• Number of satellites visible
• Satellite geometry/shading
• Intentional degradation of the satellite signal
Sources of Signal InterferenceSources of Signal InterferenceSources of Signal InterferenceSources of Signal Interference
Earth’s Atmosphere
Solid Structures
Metal Electro-magnetic Fields
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• User error: Up to a kilometer or more
Introduced Errors in GPS
Selective Availability
To reduce horizontal positioning capabilities from approximately 20 m to 100m
Anti Spoofing
Encryption of the ‘P-Code’
Receiver Errors are Cumulative!Receiver Errors are Cumulative!Receiver Errors are Cumulative!Receiver Errors are Cumulative!
User error = +- 1 km
System and other flaws = < 9 meters
Ideal Satellite GeometryIdeal Satellite GeometryIdeal Satellite GeometryIdeal Satellite GeometryNN
SS
WW EE
Good Satellite GeometryGood Satellite GeometryGood Satellite GeometryGood Satellite Geometry
Poor Satellite GeometryPoor Satellite Geometry
Planning a Navigation RoutePlanning a Navigation Route
Start= Waypoint
Applications in Forestry
• Location of Plantations
• Area and Perimeter
• Areas
• Assessment of TOF Resources
Wildlife Management
• Wildlife Census, Habitats
• Direct/Indirect sightings
• Wildlife offenses
• Settlements
Habitations & Encroachments
• Forest Villages
• Encroachments
• Settlements inside forests
• Delineation of Areas
Boundary Pillars
• Location of Pillars
• Bearings
• Distance between pillars
• Track between the pillars
Use of GPS by FSI
• Ground truthing
• Forest Inventory
• Assessment of TOF
• Monitoring of FDAs
2½’
5’ 5’
5’ 5’
2½’
2½’
2½’ 1¼’
1¼’
TWO SAMPLE PLOTS
ARE SELECTED BY TAKING
CENTER OF 1¼’X 1¼’ GRID
2½’
5’ 5’
5’ 5’
2½’
2½’
2½’ 1¼’
1¼’
2½’
5’ 5’
5’ 5’
2½’
2½’
2½’ 1¼’
1¼’
TWO SAMPLE PLOTS
ARE SELECTED BY TAKING
CENTER OF 1¼’X 1¼’ GRID
Important• Battery Life
• Size & Weight
• DGPS Capability
• Price
• Specifications
• Requirements