Global Positioning System

An easy to use technology for everyone

By Dr. Badruddin, CCC

What is GPS ?A very precise positioning system

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

• Satellite Based* 24 satellites* 20,200 km high orbit

BUT!Although it is a very precise geographic positioning system

•It is very easy to get youself into trouble•Why?•Because you (probably) don’t understand how it works•And that leads to garbage

Characteristics of GPS• Free• Precise• Reliable

• Anytime & anywhere• All weather

• Unlimited user capacity

Accurate(precise)

Almost!

Segments of GPS

1. Space SegmentA constellation of 24 satellites

2. Monitor StationA network of earth-based facilities

3. Users & Equipment

Source:Trimble

Segments of GPS

Source:Trimble

1. Space SegmentA constellation of 24 satellites

GPS Monitoring Station

How GPS Works ………

Uses measurements from 4+ satellitesDistance = travel time x speed of light

Source:Trimble

Determining GPS Position

• Suppose the distance from Satellite A to our position is 11,000 miles• At this point we could be located

anywhere on the specified sphere

Satellite A+

• Next, let us take another measurement from a second satellite, Satellite BSatellite B

+

• Now our position is narrowed down to the intersection of theses two sphere

Satellite C+

Determining GPS Position

Satellite A

Satellite B+

+• Taking another measurement from a 3rd satellite narrows our position down even further, to the two points

• So by ranging from 3 satellites we can narrow our position to just two points in space

• These points are located where the 3rd sphere cuts through the the intersection of first two spheres

Satellite C+

How do we decide which one is our true location?

Satellite A

Satellite B+

+

• We could make a 4th measurement from another satellite to determine the true point

• However, GPS receivers use a 4th satellite to precisely locate our position

• We can eliminate one of the two points that gives a ridiculous answer

• The ridiculous point may be too far from the earth

OR

Determining GPS Position

How accurate is GPS?Depends on some variables

• Design of receiver

• Relative positions of satellites, technically known as PDOP (Position dilution of precision)

• Postprocessing

• Time spent on measurement

GPS Receivers

GeoXT Versus GarminMostly used for GIS data collection

Mostly used for recreational purposes

Methods of data collectionThree methods of positioning

• Autonomous10-20 meters

• Differential2-5 meters

• Phase Differentialcentimeter

2-5m10-20 mcm

How accurate is GPS?Depends on some variables

• Design of receiver

• Relative positions of satellites, often known as DOP (Dilution of

Precision)

• Postprocessing

• Time spent on measurement

WAAS• Wide Area Augmentation System. • It provides FREE GPS differential correction data for

visible satellites

• Developed & operated by the FAA (Federal Aviation Administration) for flight navigation but it’s available free to GPS users

• WAAS-enabled receivers can provide sub-meter level accuracy anywhere in most locations of the US and southern Canada.

WAAS• Differential corrections are computed from

ground stations and then uploaded to geostationary satellites for broadcasting

• WAAS-enabled GPS receiver automatically uses such correction data to enhance the

positional accuracy

PDOP

Multipath

SNR

Sources of error

Source:Trimble

Multipath

• When GPS signals arrive at the receiver having traveled different paths

What is a PDOP?• Position Dilution of Precision

Good PDOP Poor PDOP

SNR (signal-to-noise ratio)• SNR determines the signal strength

relative to noise

• GPS position is degraded if the SNR of one or more satellites in the

constellation falls below certain range

Signal StrengthIndicators

Using Garmin12

Quit ENTER

Goto PAGE

MARK

Scroll keypad

Cycle through pages

Save awaypoint

On/Offswitch

Common use of GPS

A. GIS data collection & mapping

B. Navigation

C. Recreation

GPS for Navigation

GPS for Navigation

• GPS in PDA• Are getting popular in car• Comes with voice

guidance

GPS in recreation

GPS in Farmland

Future GPS

A. GPS in USA

B. GLONASS program from Russia

C. GALILEO from European countries