Global Positioning System Anurag Mishra Deputy Director Forest Survey of India, Dehradun

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