The document provides an overview of how the Global Positioning System works using 24 satellites orbiting the Earth to transmit location and distance data to GPS receivers, it also describes how the International Federation of Red Cross and Red Crescent Societies can use GPS for mapping, navigation, and storing waypoints to help with activities like field assessments and finding routes. GPS is generally accurate to within 10 meters but can have some errors from factors like atmospheric delays, multipath signals, and clock inaccuracies.

1. International Federation
of Red Cross and Red Crescent Societies
Global Positioning System (GPS)
Prepared by: Michael SAFRONOV,
IT/Telecom Officer,
IFRC Geneva

2. International Federation
of 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.

3. International Federation
of 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?

4. International Federation
of Red Cross and Red Crescent Societies
How GPS works (2)
 A calculation method called “trilateration” is used.
3D trilateration.swf

5. International Federation
of Red Cross and Red Crescent Societies
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.

6. International Federation
of Red Cross and Red Crescent Societies
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.

7. International Federation
of 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) andthe
For FACT, we use the WGS84 datum, coordinates grid.
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”

8. International Federation
of 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”)

9. International Federation
of Red Cross and Red Crescent Societies
Mapping basics (3)
 Practical Demonstration of Garmin eTrex Legend and use of MapSource.