International Federationof Red Cross and Red Crescent Societies

Global Positioning System (GPS)

Prepared by: Michael SAFRONOV, IT/Telecom Officer, IFRC Geneva

International Federationof Red Cross and Red Crescent Societies

GPS Basics

The 24 satellites that make up the GPS space segment are orbiting the Earth about 20,000 km above us. They are constantly moving, making two complete orbits in less than 24 hours. The location accuracy is anywhere from 1 to 100 meters depending on the type of equipment used. The GPS is owned and operated by the U.S. Department of Defence, but is available for general use around the world. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.

International Federationof Red Cross and Red Crescent Societies

How GPS works (1)

A GPS receiver needs to know where the satellites are (location), and how far they are (distance).

A GPS satellite is emitting specially modulated signals which pass to GPS receiver the needed data:

Location is determined by “almanac” data, which contains approximate locations of satellites, and “ephemeris” data, which is fed from ground control and contains the exact location. Ephemeris data takes into account orbit imperfectness and other factors.Distance is determined by the GPS receiver by decoding a special timecode in the satellite signal, and knowing the speed of radio wave propagation (approx. 300,000 km/h)

Okay, we know where and how far a satellite is, now what?

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How GPS works (2)

A calculation method called “trilateration” is used.

3D trilateration.swf

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Practical use of GPS (1)

FACT can use GPS for these purposes: Mapping Navigation

One of the most used features of a GPS unit is the ability to store “waypoints”, or coordinates of locations. For example, a field assessment team finds a well in a village, which can be used as source of drinking water – the coordinates of the well can be memorized in the GPS unit. A typical GPS unit (Garmin eTrex Legend pictured) can store 1,000 waypoints with unique names. Another useful capability is “tracking”, or recording of the path you have travelled. This helps to find your way back. Waypoints and tracks can then be uploaded to a PC with mapping software, and/or sent to Geneva for mapping.

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Practical use of GPS (2)

GPS is not perfectly accurate, measurement errors can be a result of several factors:

Radio signal delays in the atmosphere Multi-path reception, when receiver gets the signal directly from satellite, plus a reflection of it from a neighbouring mountain or building. Receiver clock is not perfect. Numbers of satellites “visible” to receiver – generally the more, the better. Intentional degradation of satellite signal – GPS is owned by US DoD, so they can degrade the precision of satellite signal when and where felt necessary.

Typically, one can expect GPS accuracy in the range of 3-12 m outdoors. Most receivers will not work indoors at all.

International Federationof Red Cross and Red Crescent Societies

Mapping basics (1)

Earth is not a perfect sphere, it is an ellipsoid. This fact alone can give positioning error of hundreds of meters, which is not acceptable in many cases. In geodesy, a datum is a reference point on surface against which position measurements are made, plus an associated model of the shape of the Earth. Historically, many countries have been developing their own approximations of Earth’s shape (reference ellipsoid) and coordinates grid. For FACT, we use the WGS84 datum, the latest standard of the World Geodetic System. There are also a few formats of presenting the coordinates, we use the classic latitude/longitude in degrees, minutes and seconds, e.g.:

E 6°12’59”N 46°22’21”

International Federationof Red Cross and Red Crescent Societies

Mapping basics (2)

Map datum is important when paper maps are used. If you’re using the standard WGS84 datum in GPS, and try to map the obtained coordinates on a paper map created for a different datum (e.g. US NAD83), the error can be few hundred meters! Paper maps usually have an indication in which datum they are created. Garmin eTrex GPS can be configured for about 100 different datum, if needed. When sending coordinates to Operations Support Dept. in Geneva, always use:

WGS 84 datum Latitude/longitude as Deg°Min’Sec” (e.g. E 6°12’59”/N 46°22’21”)

International Federationof Red Cross and Red Crescent Societies

Mapping basics (3)

Practical Demonstration of Garmin eTrex Legend and use of MapSource.