Introduction to the Global Positioning

System

GPS (Global Positioning System)

GPS, which stands for Global Positioning System, is the only system today able to show you your exact position on the Earth anytime, in any weather, anywhere.

GPS is composed of 24 satellites in 6 different planes located 11,000 miles above the earth that act as fixed reference points.

By measuring the travel time of a signal transmitted from a satellites, a receiver can calculate its distance from that satellite.

When receiving the signals from at least 4 satellites, a receiver can determine latitude, longitude, altitude, and time.

History of the GPS

Developed by Department of Defense of USA

1969—Defense Navigation Satellite System (DNSS) formed

1973—NAVSTAR Global Positioning System developed

1978—first 4 satellites launched Delta rocket launch

1993—24th satellite launched; initial operational capability

1995—full operational capability

May 2000—Military accuracy available to all users

Three Segments of the GPS

GPS Communication and Control

Space segment 24 satellite vehicles Six orbital planes

Inclined 55o with respect to equator Orbits separated by 60o

20,200 km elevation above Earth Orbital period of 11 hr 55 min Five to eight satellites visible from any

point on Earth RS_Vis_14

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

Components of the Satellite

Ground control segment Master control station

Colorado Springs Five monitor stations Three ground antennas Backup control system

There are five master ground stations located at Hawaii, Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs that continually track and correct the satellites for variations in position and time and errors.

US Space CommandColorado Springs

Hawaii

Ascension Is.

Diego Garcia

Ground AntennaMaster Control Station Monitor Station

Kwajalein Atoll

GPS antennas & GPS receiver Position (Lat, Long, Elevation) Velocity Precise timing

Used by Aircraft Ground vehicles Ships Individuals

User segment

Position is Based on Time

T + T

TThe GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is.

Velocity x Time = Distance Radio waves travel at the speed of light, roughly 186,000 miles per second (mps) If it took 0.06 seconds to receive a signal transmitted by a satellite floating directly overhead, use this formula to find your distance from the satellite. 186,000 mps x 0.06 seconds = 11,160 miles

Triangulation

Geometric Principle: You can find one location if you

know its distance from other, already-known locations.

1 Satellite

2 Satellites3 Satellites

3-D Trilateration

1 Satellite 2 Satellites

3 Satellites

Distance to a satellite is determined by measuring how long a radio signal takes to reach us from that satellite.

To make the measurement we assume that both the satellite and our receiver are generating the same pseudo-random codes at exactly the same time.

By comparing how late the satellite's pseudo-random code appears compared to our receiver's code, we determine how long it took to reach us.

Multiply that travel time by the speed of light and you've got distance.

To use the satellites as references for range measurements we need to know exactly where they are.

GPS satellites are so high up their orbits are very predictable.

All GPS receivers have an almanac programmed into their computers that tells them where in the sky each satellite is, moment by moment.

Minor variations in their orbits are measured by the Department of Defense.

The error information is sent to the satellites, to be transmitted along with the timing signals.

Sources of GPS Error

• 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 (Intentional degradation of

satellite position data by

USA)• User error: Up to a kilometer or more

• Errors are cumulative and increased by PDOP.

Line of sight is the ability to draw a straight line between two objects without any other objects getting in the way. GPS trans-mission are line-of-sight transmissions. Obstructions such as trees, buildings, or natural formations may prevent clear line of sight.

Signal RefractionSignals from satellites can be like light. When they hit some interference (air patterns in the atmosphere, uneven geography, etc.) they sometimes bend a little.

Line of sight

Sometimes the signals bounce off things before they hit the receivers.

Multipath

Differential GPS

DGPS is a way to make GPS more accurate.

It works by canceling out most of the natural and man made errors that come into GPS measurements. The process involves two receivers. The rover receiver (the one you have in your hand) and a base receiver (a fixed reference station). The base receiver through the use of precise radar to keeps track of the satellites exact position and altitude and from this computers develop a theoretical model (free of errors) for the signals travel time and location. The station than analyzes the incoming signal and compares it to the model. The difference between the two calculations is an "error correction factor”. Since the base receiver has no way of knowing which of the many available satellites a roving receiver might be using to calculate its position, the base receiver quickly runs through all the visible satellites and computes each of their errors. Then it encodes this information into a standard format and transmits it to the roving receivers via separate radio signals. So the roving receiver can use the data to correct its measurements.

Real Time Differential GPS

DGPS Base Stn

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

DGPS Rover Stn

DGPS HARDWARE CONFIGURATION

Modem GPS

GPSGPS Modem

ModemGPS

Modem

Differential Corrections

Corrected Position

Corrected Position

Differential CorrectionsOnboard GPS SystemReference GPS System

Pilot Display

Central Computer

Post processing / Real-timeBefore

After

Application of GPS Technology

Location - determining a basic positionNavigation - getting from one location to

another Tracking - monitoring the movement of people

and things Mapping - creating maps of the world Timing - bringing precise timing to the world

Private and recreation Traveling by car Hiking, climbing, biking Vehicle control

Mapping, survey, geology English Channel Tunnel Agriculture Aviation

General and commercial Spacecraft

Maritime Defence

Application of GPS Technology

Physics Distance, velocity, time Orbital concepts

Earth Science Mapping Spacecraft

Environmental Science Migratory patterns Population distributions Forest & wild life Conservation

Application of GPS Technology

Thanks for your interest in the

Global Positioning System