Gps

GPS(Global Positioning System)

Presented by

Ashok KumarNarendra Kumar Pankaj

• The Global Positioning System (GPS) is a U.S. space-based global navigation satellite system.

• US military owns it.• Provides reliable positioning, navigation,

and timing services to worldwide users on a continuous basis in all weather, day and night, anywhere on or near the Earth.

Why do we need GPS?

Components of GPS

Space Segment

Control Segment

User Segment

• 24 GPS space vehicles(SVs).

• 20,200 km away from the Earth

• 55o inclination• Satellites orbit the earth in

12 hrs.• Selecting one or more

satellites• Acquiring GPS signals• Measuring and tracking

• Comprises 5 stations.• Satellite orbit, clock

performance• Measure the distances of

the overhead satellites every 1.5 seconds and send the corrected data to Master control.

How GPS Works

How do you use these satellites to calculate your position?

The Global Positioning System is a constellation of 24 satellites that is

used to calculate your position.

Triangulation

• It compares that signal with all the known codes (there are currently 37).

• The receiver determines which satellite it is.

• It decodes the timing information, multiplies by the speed of light to find the radius of the sphere.

• Once it has done that for 3 satellites, it can determine the location.

When GPS receives a signal

• L1 freq. (1575.42 Mhz) carries the SPS code and the navigation message

• L2 freq. (1227.60 Mhz) used to measure ionosphere delays by PPS receivers

GPS Satellite Signal:

• If the clocks are perfect sync the satellite range will intersect at a single point, otherwise not

• The receiver looks for a common correction that will make all the satellite intersect at the same point

Getting Perfect timing

Error Sources

• 95% due to hardware ,environment and atmosphere• Troposphere causes delays in code and carrier• Errors due to Multipath• Receiver noise• Gravitational potential distribution, Earth is not a

perfect sphere • Other heavenly bodies attract the satellite• Not a perfect vacuum in the earth’s atmosphere• Solar radiation effects (largest unknown errors source)

• Poor GDOP (Geometric Dilution of Precision)

– When angles from the receiver to the SVs used are similar

• Good GDOP

– When the angles are different

Errors due to geometry

Error correction

• Atmospheric errors can be removed by– Dual freq measurement (low freq get refracted more than high freq thus by comparing delays of L1 and L2 errors can

be eliminated)

DGPS

• Differential Global Positioning System (DGPS) is an enhancement to Global Positioning System

• Uses a network ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions.

• To Study plate tectonics, volcanoes, and earthquakes• Surveyors use GPS for an increasing portion of their work

Uses of GPS

•GPS is popular among hikers, hunters, snowmobilers, mountain bikers, and cross-country skier

We have GPS receivers operating all over the world.Southern California

Hawaii

Antarctica

Australia

IcelandHolland

• They use codes! Binary codes.• Each satellite has a different code.

How do GPS signals send all this information?

THANK YOU