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Republic of Zambia Ministry of Health. Indoor Residual Spraying (IRS) GIS Training and Spray Area Mapping Basics of Global Positioning Systems (GPS). Training Workshop: 29 February –3 March 2008, Chipata, Zambia Web: www.nmcc.org.zm Email: [email protected] Definition of a GPS.

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Republic of Zambia

Ministry of Health

Indoor Residual Spraying (IRS) GIS Training and Spray Area MappingBasics of Global Positioning Systems (GPS)

Training Workshop: 29 February–3 March 2008, Chipata, Zambia

Web: www.nmcc.org.zm

Email: [email protected]


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Definition of a GPS

A set of satellites in geostationary orbits used to determine geographical location anywhere on the earth by means of portable electronic devices.


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Trilateration: How GPS Determines a Location

GPS is based on satellite ranging:

  • Calculating the distances between the receiver and the position of 3 or more satellites (4 or more if elevation is desired) and then applying some mathematics.

  • Assuming the positions of the satellites are known, the location of the receiver can be calculated by determining the distance from each of the satellites to the receiver.


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How GPS Determines a Location cont.

  • GPS takes these 3 or more known references and measured distances and “triangulates” an additional position.

    Based on this principle, one needs to know the following information in order to compute one’s position:

  • What is the precise location of 3 or more known points (GPS satellites)?

  • What is the distance between the known points and the position of the GPS receiver?


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Determining Current Locations of GPS Satellites

  • GPS satellites are orbiting the earth at an altitude of about 11,000 miles.

  • Paths of the satellites vs. time can be predicted with great accuracy.

  • Orbits, and thus the locations of the satellites, are known in advance.


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Determining Current Locations of GPS Satellites, cont.

  • GPS receivers store orbit information for all of the GPS satellites in what is known as an almanac.

  • Radio waves travel at the speed of light (about 186,000 miles per second).

  • If the exact time of signal transmission and exact time it was received are known, the signal travel time can be determined.


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3-Dimensional Location

  • Four satellites are needed to give a 3-dimensional position.

  • Three measurements can be used to locate a point.

  • Unfortunately, it is impossible to synchronize these 2 clocks.

  • A fourth variable, time, must be determined in order to calculate a precise location.


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Causes of GPS Errors

  • Atmospheric conditions

  • Ephemeris errors

  • Clock drift

  • Measurement noise



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Selective Availability

  • The US Department of Defense has the ability to degrade the Course Acquisition Code at any time.

  • This will cause civilian receivers to miscalculate positions by up to 100 meters.

  • Differential correction will adjust position to submeter accuracy.


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Measuring GPS Accuracy

  • Dilution of precision (DOP) is an indicator of the quality of the geometry of the satellite constellation.


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Using Differential GPS to Increase Accuracy

  • Differential correction requires a second GPS receiver, a base station, collecting data at a stationary position on a precisely known point (typically it is a surveyed benchmark).


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GPS and Canopy

  • GPS receivers require a line of sight to the satellites in order to obtain a signal representative of the true distance from the satellite to the receiver.


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Advantages of GPS Navigation

  • GPS takes navigation to a higher level by providing accurate position and course information.

    • Has implications for health, agriculture, environment, navigation, defense, etc.


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