Global Positioning System An easy to use technology for everyone Source Copyright & Courtesy: Dr. Badruddin, CCC

A very precise positioning system 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 At the cost of 15 billion dollar of your tax money

Although it is a very precise geographic positioning system 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 At the cost of 15 billion dollar of your tax money

Characteristics of GPS Accurate(precise) Free Precise Reliable Anytime & anywhere All weather Unlimited user capacity Almost!

Segments of GPS 1. Space Segment 2. Monitor Station A constellation of 24 satellites 2. Monitor Station A network of earth-based facilities 3. Users & Equipment Source:Trimble

Segments of GPS 1. Space Segment A constellation of 24 satellites Source:Trimble

GPS Monitoring Station

How GPS Works ……… Uses measurements from 4+ satellites Distance = travel time x speed of light Speed of the light: 186,000 mile/sec 300,000 km/sec 3x10^8 meter/sec Source:Trimble

Determining GPS Position Satellite A + 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 B + Next, let us take another measurement from a second satellite, Satellite B Now our position is narrowed down to the intersection of theses two sphere

Determining GPS Position Satellite A Satellite B + Taking another measurement from a 3rd satellite narrows our position down even further, to the two points Satellite C + These points are located where the 3rd sphere cuts through the the intersection of first two spheres So by ranging from 3 satellites we can narrow our position to just two points in space

+ 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 OR Satellite C + We can eliminate one of the two points that gives a ridiculous answer The ridiculous point may be too far from the earth However, GPS receivers use a 4th satellite to precisely locate our position

Determining GPS Position

How accurate is GPS? Time spent on measurement Design of receiver 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 Garmin Mostly used for GIS data collection Mostly used for recreational purposes Mostly used for GIS data collection

Methods of data collection Three methods of positioning Autonomous 10-20 meters Differential 2-5 meters 2-5m 10-20 m cm Phase Differential centimeter

How accurate is GPS? Time spent on measurement Design of receiver 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

Sources of error Multipath PDOP SNR Source:Trimble SNR is also known as signal strength Source:Trimble

Multipath When GPS signals arrive at the receiver having traveled different paths When SA was on, the position could be anywhere on 1st, 2nd, Alpine, or Spruce street. With SA removed, the position is located with in the road width of +/- 15 feet. (road = 35’, the circle is of 20’ radius). Before the location was within 100 meters, that is the point is about a football field away. Now it’s within the 10 meters. Removal of SA made GPS more useful and handy for our day to day use in cell phone, car, watch etc.

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 Strength Indicators

Using Garmin12 Cycle through pages Scroll keypad Quit ENTER Goto PAGE MARK On/Off switch Save a waypoint

Common use of GPS GIS data collection & mapping B. Navigation C. Recreation

GPS for Navigation

GPS for Navigation Are getting popular in car GPS in PDA Are getting popular in car Comes with voice guidance

GPS in recreation

GPS in Farmland Farm equipment capable of producing yield map during harvesting shows how crop yield varies across the field. Allowing farmers to plan fertilizer application for future crops.

Future GPS A. GPS in USA B. GLONASS program from Russia C. GALILEO from European countries

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