Introduction to Global Position System

Slide 2

What is GPS

Space Segment

Control Segment

User Segment

The Global Positioning System (GPS) is a Constellation of Earth-Orbiting Satellites Maintained by the United States Government for the Purpose of Defining Geographic Positions On and Above the Surface of the Earth. It consists of Three Segments:

The GPS Constellation

Control Segment

Space Segment

User Segment

Three Segments of the GPS

Monitor Stations

GroundAntennas

Master Station

GPS Satellite Vehicle

Four atomic clocks

Three nickel-cadmium batteries

Two solar panels Battery charging

Power generation

1136 watts

S band antenna—satellite control

12 element L band antenna—user communication

Block IIF satellite vehicle (fourth generation)

Slide 6

Space Segment Description

Very high orbit 20,200 km

– 1 revolution in approximately 12 hrs

For accuracy

Survivability

Coverage

24+ satellites 6 planes with 55°

Inclination

Each plane has 4 or 5 satellites

Broadcasting position and time information on 2 frequencies

Constellation has Spares

Slide 7

Control Segment

Master Control Station

Monitor Station

Ground Antenna

ColoradoSprings

Hawaii AscensionIslands

DiegoGarcia

Kwajalein

Monitor and Control

Slide 8

Control Segment

(5) Monitor Stations

• Correct Orbitand clockerrors• Create new navigation message

• Observeephemerisand clock

Falcon AFBUpload Station

Slide 9

User Segment

Over $19 Billion invested by DoD

Dual Use System Since 1985

(civil & military)

Civilian community was quick to take advantage of the system Hundreds of receivers on the market

3 billion in sales, double in 2 years

95% of current users

DoD/DoT Executive Board sets GPS policy

PLGR

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.

User Segment

ESSC 541-542 Lecture 4.14.0511

How the system works

Space Segment24+ Satellites

The Current Ephemeris is Transmitted to Users

Monitor StationsDiego GarciaAscension IslandKwajalein HawaiiColorado Springs

GPS Control Colorado Springs

End User

Slide 12

Distance Measuring

The whole system revolves around time!!!Distance = Rate x Time

• Rate = 186,000 miles per second (Speed of Light)• Time = time it takes signal to travel from the SV to GPS receiver

Signal From One Satellite

The receiver is somewhere on this sphere.

Signals From Two Satellites

Three Satellites (2D Positioning)

Three Dimensional (3D) Positioning

Slide 17

Triangulation

Satellite 1 Satellite 2

Satellite 3 Satellite 4

Slide 18

Measuring Travel Time

SV Clocks 2 Cesium & 2 Rubidium in

each SV $100,000-$500,000 eachReceiver Clocks Clocks similar to quartz watch Always an error between satellite and receiver clocks ( t) Require 4 satellites to solve for x, y, z, and t

Slide 19

Common Problems - Errors

GPSAntenna

Hard Surface

Satellite

Slide 20

GPS Multipath Errors

Effects of Multipath on the GPS Signal

•Avoid Reflective Surfaces•Use A Ground Plane Antenna •Use Multipath Rejection Receiver

GPSAntenna

Hard Surface

Satellite