The Global Positioning System an Overview

Presented byGurubasavaraj MAIET Moodbidri

Dept of Civil EnggUSN 4al14cv403

Contents1. What is GPS ?2. Segments of GPS3. Satellite Constellation4. GPS Signals5. GPS: How does it work?6. Sources of GPS Error7. Characteristics of GPS8. Applications of GPS9. Conclusion10.Reference

What is GPS?A very precise positioning system

•Developed and maintained by the US Department of Defense (DOD)

• Satellite Based* 24 satellites in 6 orbital planes* 4 satellites in each plane* 20,200 km high orbit* 55 Degree inclination

Although it is a very precise geographic positioning system.•It is very easy to get yourself into trouble•Why?•Because you (probably) don’t understand how it

works•And that leads to garbage

GPS Segments

GPS has 3 Segments:

1. Space Segment : the constellation of satellites

2. Control Segment : control the satellites

3. User Segment : users with receivers

Control Segment

Space Segment

User Segment

Three Segments of the GPS

Monitor Stations

GroundAntennas

Master Station

1.Space Segment• System consists of 24 satellites

in the operational mode: 21 in use and 3 spares 3 other satellites are used for testing

• Altitude: 20,200 Km with periods of 12 hr.

• Current Satellites: Block IIR- $25,000,000

2000 KG• Hydrogen Maser Atomic

Clocks:These clocks lose one

second every 2,739,000 million years.

2.Control Segment

• The control segment tracks the satellites and then provides

them with corrected orbital and time information.

• The control segment consists of four unmanned control

stations and one master control station.

• The four unmanned stations receive data from the satellites

and then send that information to the master control station

where it is corrected and sent back to the GPS satellites.

Kwajalein Atoll

US Space Command

Control Segment

Hawaii

Ascension Is.

Diego Garcia

Cape Canaveral

Ground AntennaMaster Control Station

Monitor Station

3.User

Segment• 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.

• GPS Nominal Constellation

• 24 Satellites in 6 Orbital Planes

• The nominal orbital period of a GPS satellite is one-half of a sidereal day or 12hr.

• 4 Satellites in each Plane

• 20,200 km Altitudes, 55 Degree Inclination.

• The orbital radius is approximately 26 600 km.

Satellite Constellation

GPS Signals• Two signals are transmitted on carriers:

L1 = 1575.42 MHz

L2 = 1227.60 MHz

These are derived from the system clock of 10.23 MHz (phase quadrature)

• Modulation used is Direct Sequence Spread Spectrum

(code division multiple access - CDMA)

• The satellites transmit as part of their unique Spread Spectrum signal a clock or timing signal.

• The range or distance to the satellite is obtained by measuring how long it takes for the transmitted signal to reach the receiver.

• This is not the “true” range due to clock errors - what is obtained is know as the “pseudo-range.

GPS: How does it work?1. All satellites have clock set to exactly to the same time.

2. All satellites know their exact position from data sent to them from the system controllers.

3. Each satellite transmits its position and a time signal.

4. The signals travel to the receiver delayed by distance travelled.

5. The differences in distance travelled make each satellite appear to have a different time.

6. The receiver calculates the distance to each satellite and can then calculate its own position.

Working of GPS System

Sources of GPS ErrorStandard 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• User error: Up to a kilometer or moreErrors are cumulative and increased by PDOP.

Characteristics of GPS

• Free

• Precise

• Reliable

• All weather

• Anytime & anywhere

• Anytime & anywhere

• Unlimited user capacity

Applications of GPSThe following is the list of some application areas where GPS has been used widely.• Utility industry• Natural resources• Precision farming• Civil Engineering applications• Structural deformations• Airborne mapping• Seafloor mapping• Land & marine seismic surveying• Vehicle navigation• Cadastral surveying

Conclusion• GPS system is actually developed for military

purpose but by finding its enormous use this has been made available for civilian purposes.

• GPS service is offered by US Department of defense(DOD) called NAVSTAR GPS.

• NAVSTAR (Navigation Satellite and Time Ranging) GPS is used widely.

• Russia has a system known as GLONASS or “GLObal Navigation Satellite System” .

• European Union(EU) has also planned its GPSmission called Galelio.

Reference• A text book of SURVEYING III by H.S.Vishwanath.

• Wikipedia

Websites• www.google.com/GPS• GPS World Magazine http://www.gpsworld.com/

Thank you…,