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# Indoor Residual Spraying IRS GIS Training and Spray Area Mapping - PowerPoint PPT Presentation

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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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]

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

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.

• 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?

• 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.

• 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.

• 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.

• Atmospheric conditions

• Ephemeris errors

• Clock drift

• Measurement noise

• 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.

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

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).

• 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.

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

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